Hire the best Gas Fitter

LPG Hot Water Costing - Please help!

Results 1 to 39 of 39
  1. #1
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default LPG Hot Water Costing - Please help!

    Hi All,

    I am in the middle of renovating a 30 year old home in Brisbane that still has the original electric storage hot water system. As you would expect it is in need of replacement.

    I am very keen on the idea of a Rinnai Infinity 26 continuous gas hot water system. The compact size, ability to set the temperature of the water, and the fact that it is only heating water when it is needed really appeals to me. The only thing stopping me from making the purchase is I am having trouble working out how much it is going to cost to run. Natural gas is not available in our area so I will need to rely on bottled LPG.

    My plan is to use a single 45kg bottle with a 9kg for the backup when the 45kg needs to be replaced. This means I will only pay one bottle rental. I have a cheap gas refill service close to me that I can use to manually fill the 9kg bottle when needed. I worked out the following:

    A 45kg bottle holds 2250MJ.
    Running flat out the Infinity 26 uses 200MJ/hr of gas

    The 200MJ/hr figure is at full flow and full heat.

    The shower head flows 9L/min so I would only use 200 x 9 / 26 = 69MJ/hr

    Heating water to 40 degrees instead of 60 reduces this to 69 x 40/60 = 46MJ/hr

    2250MJ / 46MJ = 49 hours of showering.

    Assuming 2 10 minute showers per day (only 2 of us in the house)

    49 x 60 / 20 = 147 days of showering.

    At present a 45kg bottle costs $137 + a portion of the yearly rental works out to be roughly $150

    So basically how water would cost me around $1 per day.

    I went through our previous bills and electric hot water is costing us between $1.10 (summer) and $1.40 (winter) per day so the gas will work out roughly the same and have all the benefits of a continuous system.

    I am very new to all this and would greatly appreciate if someone more experienced with gas could work through my maths and point out any errors before I drop the cash on the system. Another point I should make is that I will be having LPG installed regardless as we have bought a 6 burner gas stove / electric oven for our new kitchen.

    Any help with this would be greatly appreciated.

  2. #2
    3K Club Member
    Join Date
    Jun 2012
    Location
    Sunshine Coast
    Posts
    3,938

    Default

    Quote Originally Posted by vincentvega View Post
    Hi All,

    I am in the middle of renovating a 30 year old home in Brisbane that still has the original electric storage hot water system. As you would expect it is in need of replacement.

    I am very keen on the idea of a Rinnai Infinity 26 continuous gas hot water system. The compact size, ability to set the temperature of the water, and the fact that it is only heating water when it is needed really appeals to me. The only thing stopping me from making the purchase is I am having trouble working out how much it is going to cost to run. Natural gas is not available in our area so I will need to rely on bottled LPG.

    My plan is to use a single 45kg bottle with a 9kg for the backup when the 45kg needs to be replaced. This means I will only pay one bottle rental. I have a cheap gas refill service close to me that I can use to manually fill the 9kg bottle when needed. I worked out the following:

    A 45kg bottle holds 2250MJ.
    Running flat out the Infinity 26 uses 200MJ/hr of gas

    The 200MJ/hr figure is at full flow and full heat.

    The shower head flows 9L/min so I would only use 200 x 9 / 26 = 69MJ/hr

    Heating water to 40 degrees instead of 60 reduces this to 69 x 40/60 = 46MJ/hr

    2250MJ / 46MJ = 49 hours of showering.

    Assuming 2 10 minute showers per day (only 2 of us in the house)

    49 x 60 / 20 = 147 days of showering.

    At present a 45kg bottle costs $137 + a portion of the yearly rental works out to be roughly $150

    So basically how water would cost me around $1 per day.

    I went through our previous bills and electric hot water is costing us between $1.10 (summer) and $1.40 (winter) per day so the gas will work out roughly the same and have all the benefits of a continuous system.

    I am very new to all this and would greatly appreciate if someone more experienced with gas could work through my maths and point out any errors before I drop the cash on the system. Another point I should make is that I will be having LPG installed regardless as we have bought a 6 burner gas stove / electric oven for our new kitchen.

    Any help with this would be greatly appreciated.

    Origin supply me at $110/45kg and $34/year rent

    Phone around for gas and rent prices. I will check Origin to confirm gas price tomorrow but rent is due now at $34 for 1 bottle.

    I,m sure you have a problem with the figures, what is the full flow rate of the Infinity in litres/min when using 200mj of gas ?.

  3. #3
    Golden Member manofaus's Avatar
    Join Date
    Oct 2007
    Location
    muswellbrook
    Posts
    689

    Default

    what about washing?
    Did your costing for the electricity already include the washing or just the shower?
    sorry for the ramble.

  4. #4
    1K Club Member
    Join Date
    Nov 2011
    Location
    Paradise Beach Vic
    Posts
    1,875

    Default

    You don't need a gassy to check your calculations, you need a mathematician.....

  5. #5
    4K Club Member ringtail's Avatar
    Join Date
    Sep 2009
    Location
    brisbane
    Posts
    8,266

    Default

    My neighbour has gas and I have electric. Both work out to a shade over $ 1 per day. Mine is on tariff 33. If I went for a larger HWS and put it on night rate T31 I would come out a lot better off than my gas neighbour

  6. #6
    Golden Member
    Join Date
    May 2012
    Location
    melbourne
    Posts
    624

    Default

    Quote Originally Posted by vincentvega View Post

    Heating water to 40 degrees instead of 60 reduces this to 69 x 40/60 = 46MJ/hr
    Unfortunately, that isn't true. Even though the water is heated up to 60 deg. Hot water is mixed with cold when you have a shower. you use less hot water at 60 than at 40 and assuming you have a shower at the same temperature, you will use the same amount of energy. Heating to 40 just means a smaller heat loss in the pipework, but at the same time, gas water heater is less efficient than electricity. You can pretty much say you'll use the same amount of energy with either gas or electric water heating.

    The calculation for comparing gas or electricity can be simply done by looking at the tariff rate.

    From your information, 2250 MJ cost cost $137. 3.6 MJ = 1 Kwh, So $137 buys you 625 Kwh. that is 22c per Kwh. You can compare this figure with your electricity tariff and work out the percentage saving.

    My guess is there isn't much of a difference. Dont forget infinity means longer showers for some......

  7. #7
    1K Club Member
    Join Date
    Oct 2007
    Location
    Adelaide
    Posts
    1,664

    Default

    Bottom line is that it takes 4.162 KJ to raise a litre of water by 1 degree no matter what the energy source.

    So if you have the shower going for 20 minutes per day, the shower temp is 43 degrees and the incoming water is 20 then you will need 4.79 kWh of heat into the water each day to make that work.

    Allowing for 80% appliance efficiency, this works out at 156 kg of LPG each year for the shower. As I said, that's using those assumptions about incoming water and shower temperatures.

    As an energy source, LPG is almost always more expensive than off-peak electricity "at the meter". The only way you'll save money is if usage is more efficient. That would certainly be the case at, for example, a sports club that uses the showers only on Saturdays but would leave an electric HWS sitting there hot all week. But at home the losses from the tank are a modest portion of total energy use for heating water and off-peak is generally cheaper than LPG.

    If you're going to be buying a new water heater anyway, then the first question which comes to mind is "why not solar?". The price of sunlight isn't likely to go up much in coming years......

  8. #8
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default

    Quote Originally Posted by cyclic View Post
    Origin supply me at $110/45kg and $34/year rent

    Phone around for gas and rent prices. I will check Origin to confirm gas price tomorrow but rent is due now at $34 for 1 bottle.

    I,m sure you have a problem with the figures, what is the full flow rate of the Infinity in litres/min when using 200mj of gas ?.
    Thanks for your reply.

    The flow rate is 26L/min. Specifications are available here

    Rinnai Australia - Products

    I assumed a shower head uses 9L/min and then scaled the gas usage during a shower by 9/26. I am sure this is not correct but it is the best I could come up with.

    $110 per bottle is far cheaper than I have been quoted. I spoke with Origin and Supagas in Brisbane.

  9. #9
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default

    Quote Originally Posted by manofaus View Post
    what about washing?
    Did your costing for the electricity already include the washing or just the shower?
    Our dishwasher and washing machine both run off the cold water supply so I ignored usage outside the shower.

    I figure the incidental hot water useage (laundry / kitchen sink) is so small compared to the amount used showering that I ignored it.

  10. #10
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default

    Quote Originally Posted by justonething View Post
    Unfortunately, that isn't true. Even though the water is heated up to 60 deg. Hot water is mixed with cold when you have a shower. you use less hot water at 60 than at 40 and assuming you have a shower at the same temperature, you will use the same amount of energy. Heating to 40 just means a smaller heat loss in the pipework, but at the same time, gas water heater is less efficient than electricity. You can pretty much say you'll use the same amount of energy with either gas or electric water heating.
    This is one of the advantages of a continuous system that I really like. You dont mix with cold water. You just set the water temperature on the controller in the bathroom and turn the hot tap on. No temperature fluctuation when another tap gets turned on (which drives me mad).

    The calculation for comparing gas or electricity can be simply done by looking at the tariff rate.

    From your information, 2250 MJ cost cost $137. 3.6 MJ = 1 Kwh, So $137 buys you 625 Kwh. that is 22c per Kwh. You can compare this figure with your electricity tariff and work out the percentage saving.
    This info is exactly what i was after! Energy is energy and it makes sense to compare directly like this. I am currently paying 18.5c / KWh for off peak electricity. If I can find a 45kg bottle for somewhere around the $110 mark mentioned above, that is roughly $18c / KWh. Like you said roughly the same.

    Actually a mate said to me the other day "Its just energy, it can't be that much different between LPG and electricity". I guess he was right.

    Thanks very much for this info

  11. #11
    Old Chippy 6K
    Join Date
    Mar 2007
    Location
    Canberra
    Age
    70
    Posts
    6,582

    Default

    Quote Originally Posted by justonething View Post
    Unfortunately, that isn't true. Even though the water is heated up to 60 deg. Hot water is mixed with cold when you have a shower. you use less hot water at 60 than at 40 and assuming you have a shower at the same temperature, you will use the same amount of energy. Heating to 40 just means a smaller heat loss in the pipework, but at the same time, gas water heater is less efficient than electricity. You can pretty much say you'll use the same amount of energy with either gas or electric water heating. My guess is there isn't much of a difference. Dont forget infinity means longer showers for some......
    mmm - the great advantage of this type of instantaneous water heater is that you heat only the amount of water you need and only to the temperature at which you need it. That means you use the controller to set for a shower at (say 39-45) and the hot water supply side only is turned on, or in the kitchen you set at say 50-55 to do the washing up (by hand 50, dishwasher 55) and run only hot water.

    Any system which requires addition of cold water to bring the temperature down is simply wasting the energy used to get to that temperature in the first place. As Smurf said the amount of energy required to get a temperature rise is the same regardless of the energy source, but if you heat only to the lowest temperature needed at the outlet then that's all the energy you use and pay for. If you heat it higher and store it then use cold water to reduce then the difference is energy 'lost' - but you paid for it anyway.

    Solar hot water has a higher capital cost, but then the energy source is free (and that price is never going up - unlike electricity and gas) so heating a storage tank and then cooling by adding cold water makes sense as the net energy cost is the same. With solar it makes great sense to: a) have as large a tank as you can afford (I use double what might be 'normal' for the number of people in the household); b) keep the stored water at the maximum temperature allowed for the tank (65 or even higher if the tank can take it - I store at 85-90), c) makes sure the tank is as insulated as you can make it ie: reduce all losses to air via the tank or pipes; and d) make sure you understand how to best use a solar hot water supply to maximise the solar input (it heats up only during the day [infrared rays so on cloudy days too, just not as much ie: unlike PV which needs light, solar HWS needs heat - many people get confused by this].

    Of course a 6-minute or 10-minute shower will cost more than a 4-minute shower regardless of the HWS type, but again if you are using only HW and not using cold to bring down the temp of stored water then an instantaneous HWS will be cheaper.

    With a hot water system especially (most things really!) you need to look at the lifetime costs including the purchase and any maintenance costs as well as the energy input costs. A HWS should last at least 15 years and often 20 (they can be built to last much much longer, but we aren't doing that right now) and with electricity or gas the running costs will quickly exceed the capital costs, If one looks at the pattern on an X-Y graph then then while the pattern for electricity or gas (note: using fossil fuels) is relatively low capital cost and a steep and consistent increasing/rising use cost. Solar HWS is the opposite ie: a larger capital cost and flat usage costs which makes the line have a downward trajectory (there will be some boost cost for winter and rainy low heat days - but this can be reduced by the actions I describe in the previous paragraph).

    So what does all that mean? Not much unless you have enough cash to pay the higher upfront cost of solar, but if you choose and use an instantaneous HWS then it should cost you less than an electricity or gas storage system (your maths gives the highest possible cost as the HWS will rarely run at full whack in real life use) over its lifetime. If you are renting the place out then of course you don't pay the bills so the cheapest upfront seems best - but I'd be doing a deal in the tenancy agreement to share any savings with my tenants over the life of any more energy efficient system I installed - so we all win. If you are a renter you should seek such a deal with the landlord when any new system is needed.

    BTW - adding extra insulation to any HWS storage tank is one of the easiest and cheapest action anyone can take. Use a piece of hard foam insulation (XPS or other non water absorbing insulation foam) under the tank if you can get it there, but in any case wrap with foil blanket on sides and top (with gas make sure no air intake or exhaust vents are covered and there is clearance around pilot lights etc, with all make sure pressure relief valves are clear and operational) and that all output (HW) pipes are lagged (insulated) as far as you can do so and as well as you can too. Externally especially, but also under house until the pipes go up into the walls. In cold climates input pipes should also be lagged - the cost to you increases when the gap between water input temp and output temp increases.
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  12. #12
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default

    Thanks for that info Bloss! Much appreciated.

    We own this home and are renovating it with plans to be here for another 10 years. You make valid points about solar but the features a continuous system has is really appealing to me, although I know i will pay for it in the long run.

    I just called Elgas and they will do a bottle for $115 with a yearly fee of $33. Looks like this could be a goer.

  13. #13
    3K Club Member
    Join Date
    Jun 2012
    Location
    Sunshine Coast
    Posts
    3,938

    Default

    Quote Originally Posted by vincentvega View Post
    Thanks for that info Bloss! Much appreciated.

    We own this home and are renovating it with plans to be here for another 10 years. You make valid points about solar but the features a continuous system has is really appealing to me, although I know i will pay for it in the long run.

    I just called Elgas and they will do a bottle for $115 with a yearly fee of $33. Looks like this could be a goer.
    Nah, still too dear, let your fingers do the walking.

    Screw them down, they want your business.

    Origin $110 and $34
    A Gas 110 and 25

  14. #14
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by justonething View Post
    Unfortunately, that isn't true. Even though the water is heated up to 60 deg. Hot water is mixed with cold when you have a shower. you use less hot water at 60 than at 40 and assuming you have a shower at the same temperature, you will use the same amount of energy. Heating to 40 just means a smaller heat loss in the pipework, but at the same time, gas water heater is less efficient than electricity. You can pretty much say you'll use the same amount of energy with either gas or electric water heating.

    The calculation for comparing gas or electricity can be simply done by looking at the tariff rate.

    From your information, 2250 MJ cost cost $137. 3.6 MJ = 1 Kwh, So $137 buys you 625 Kwh. that is 22c per Kwh. You can compare this figure with your electricity tariff and work out the percentage saving.

    My guess is there isn't much of a difference. Dont forget infinity means longer showers for some......


    Good enough today is better than perfect tomorrow

  15. #15
    Golden Member
    Join Date
    May 2012
    Location
    melbourne
    Posts
    624

    Default

    Quote Originally Posted by Bloss View Post
    Any system which requires addition of cold water to bring the temperature down is simply wasting the energy used to get to that temperature in the first place. As Smurf said the amount of energy required to get a temperature rise is the same regardless of the energy source, but if you heat only to the lowest temperature needed at the outlet then that's all the energy you use and pay for. If you heat it higher and store it then use cold water to reduce then the difference is energy 'lost' - but you paid for it anyway.
    If you heat up water and store and then mix with cold water, energy is transferred from hot water to cold water to become warm water, the energy is not wasted if you use the warm water to take a shower. What Smurf refers to also works in reverse. When water is cooled, it gives up 4.18 KJ of energy in the form of heat per litre when it is cooled by 1 degree. In fact, this is how heat exchangers work. Many years ago when I was a mechanical engineer working in a hospital, the steam boiler there would generate steam, and some of that steam would be used to heat up domestic hot water through a series of heat exchangers, using the same principle. When 10 litres of 60 deg water is mixed with 10 litres of 20 deg cold water, you end up with 20 litres of 40 deg warm water. 840KJ is given up by the 60 deg water, and 840KJ is absorbed by 20 deg cold water to raise the temperature from 20 to 40 deg. This is different than when 60 deg hot water is allowed to sit in the open and heat is lost to the air which is of course wasted.

    I agree with most of the other things you said. One small point in regards to insulation. When you insulate small bore pipes, the exposed surface area multiply quickly with every cm thickness of insulation and sometimes you might lose as much energy as you are trying to save.

  16. #16
    1K Club Member
    Join Date
    Oct 2007
    Location
    Adelaide
    Posts
    1,664

    Default

    Ultimately, you are adding heat to the water. Energy is energy and there's not a lot of difference between heating the hot water to 70 and mixing it with cold versus only heating it to 40 in the first place. You've still added the same amount of heat overall, just by a different means.

    It's like debating whether it's better to cut a cake into 8 slices or 12. Either way, if you eat the whole cake then you've eaten the whole cake. Energy is energy no matter how you cut it.

    But I do come back to that point about why pay 18c per kWh or thereabouts when the sun is free? At the very least I'd be checking the actual cost of installing solar before dismissing it. Sun prices aren't likely to rise anywhere near as much as electricity or gas in the coming years.

  17. #17
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Smurf View Post
    Ultimately, you are adding heat to the water. Energy is energy and there's not a lot of difference between heating the hot water to 70 and mixing it with cold versus only heating it to 40 in the first place. You've still added the same amount of heat overall, just by a different means.
    Simply the "First Law of Thermodynamics" also known as the "Conservation of energy". There are secondary effects, as mentioned by "justonething" such as higher losses if higher temperature water is in the system.
    Good enough today is better than perfect tomorrow

  18. #18
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    [QUOTE=Smurf;906129

    It's like debating whether it's better to cut a cake into 8 slices or 12. Either way, if you eat the whole cake then you've eaten the whole cake. Energy is energy no matter how you cut it.
    [/QUOTE]

    Or as "Smurf" would have it "First Law of Baking" or "conservation of cake"


    Sorry Smurf; I could not help myself.
    Good enough today is better than perfect tomorrow

  19. #19
    Member Poirot's Avatar
    Join Date
    Oct 2010
    Location
    Brisbane
    Posts
    77

    Default

    One thing that nobody seems to have mentioned is that with an instant gas-heater you cannot get hot water unless you get enough water to chase through the unit. So when you only want a little trickle of hot water, the gas unit will not kick in, whereas an electric (or gas or solar) storage heater will give you hot water no matter how far you open the tap. I must admit I found that very annoying when we lived for a while in a place with an instant gas heater ...

  20. #20
    4K Club Member ringtail's Avatar
    Join Date
    Sep 2009
    Location
    brisbane
    Posts
    8,266

    Default

    You're not the first person I've heard this from. I wont be changing my electric hotty any time soon - or any time before it rags itself

  21. #21
    Old Chippy 6K
    Join Date
    Mar 2007
    Location
    Canberra
    Age
    70
    Posts
    6,582

    Default

    Quote Originally Posted by Smurf View Post
    Ultimately, you are adding heat to the water. Energy is energy and there's not a lot of difference between heating the hot water to 70 and mixing it with cold versus only heating it to 40 in the first place. You've still added the same amount of heat overall, just by a different means.
    mmm, well no that's not so! I am not sure why this is so hard to understand - if you heat water from 20 degrees to 60 (a 40 degree rise) you use twice the energy than you use to heat the same volume (at any particular flow rate) from 20 degrees to 40 degrees or a 20 degree rise.

    If you only wanted the water at 40 for your shower and you add cold water to bring it down from 60 to 40 you have wasted the difference! You bought the energy, added it to the storage tank and then by taking the extra heat out of the hot (by adding cold in the mixer - and yes, it will be the same amount taken out as it took to heat it) you are are letting it run down the drain (and transfer to the air). For a given amount of water use you use twice the amount of energy!

    As to the low water flow being a problem with instantaneous gas - well that's rubbish. True that some older systems did not cope well when flow restrictions were brought in and I wouldn't be installing one in a gravity feed system (but who has those anymore anyway?). There are few times where such a low rate is needed compared to the vast majority of normal uses which are above the quite small minimum flow rate for these systems.
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  22. #22
    1K Club Member
    Join Date
    Oct 2007
    Location
    Adelaide
    Posts
    1,664

    Default

    Quote Originally Posted by Bloss View Post
    mmm, well no that's not so! I am not sure why this is so hard to understand - if you heat water from 20 degrees to 60 (a 40 degree rise) you use twice the energy than you use to heat the same volume (at any particular flow rate) from 20 degrees to 40 degrees or a 20 degree rise.

    If you only wanted the water at 40 for your shower and you add cold water to bring it down from 60 to 40 you have wasted the difference! You bought the energy, added it to the storage tank and then by taking the extra heat out of the hot (by adding cold in the mixer - and yes, it will be the same amount taken out as it took to heat it) you are are letting it run down the drain (and transfer to the air). For a given amount of water use you use twice the amount of energy!
    Yes, you will have used twice as much energy to heat a litre of water from 20 to 60 rather than 20 to 40 that is correct.
    But in order to run the shower at the same flow rate without mixing in the cold you will have to heat twice the volume of water if you're only heating it to 40 rather than 60. As such, the total energy consumed is roughly the same apart from the relatively small difference in pipe losses etc.

    At the end of the day, if you're going to have 100 litres of water at 40 degrees (incoming cold water temp 20) come out of the shower head then it's going to require 2.3 kilowatt hours of heat energy put into that water. Whether you mix water at 60 degrees with water at 20 degrees or just heat it straight to 40 doesn't change the requirement to add 2.3 kWh of heat into the water coming out of the shower head. You're still taking 100 litres of incoming cold water and heating it to 40 at the shower head. That you heated half of it to 60 then mixed it, or heated all of it straight to 40, doesn't change this.

    The only changes in energy use relating to the two different methods are:

    1. Gas versus electric. At the point of use, gas is about 80% efficient (the other 20% being hot air out the exhaust) whereas the electric element is just under 100% efficient (minor losses in the wiring). That's at the point of use of course, upstream production (power stations, refineries etc) of gas and electricity is an entirely different story but that's not the topic here.

    2. Heat losses from the pipework. They will be double at 60 degrees what they would be at 40 assuming the incoming water is at 20 that is correct. But we're only talking about doubling the few % of hot water energy that is lost via pipe heat losses - most of the heat ends up at the shower head not lost in the pipes. So we're saving 50% of something that is only a few % of the total in the first place - it's a saving but it's nowhere near 50% of the total energy used to heat the water.

    3. Heat losses from storage tanks. How much is lost is a difficult question since it depends on several factors but 15% of total energy input is indicative assuming we're talking about a modern tank.

    4. Adding the losses from the gas burner (instant gas), pipework and storage tank (electric) they're a similar % of the total overall. Gas storage HWS have considerably greater losses, hence the gas savings when people switch from gas storage to instant, but in this case we're comparing instant gas versus storage electric.

    So yes, heating water to 40 will use less energy than heating it to 60 and then mixing with cold. But the saving is nowhere near 50% assuming the shower runs at the same flow rate in either case. We're heating water with gas burners or electric elements here then moving it via some pipes to a shower - none of which can actually create or destroy energy. Most of what goes in, goes out the shower. Any potential saving is thus limited to reducing losses - worthwhile but it's not going to save 50% unless we're talking about some ridiculously long pipes or something like that.

    It's like saying that driving at 40 will use half as much fuel per hour as driving at 80. That's true (roughly) but omits to mention that I'll have to drive twice as many hours to get from A to B such that the overall fuel saving is far less. The only real saving is via reduced losses associated with the lower speed - a bit here and there but it's not anywhere near 50%.

  23. #23
    Golden Member
    Join Date
    May 2012
    Location
    melbourne
    Posts
    624

    Default

    +1

  24. #24
    Member Poirot's Avatar
    Join Date
    Oct 2010
    Location
    Brisbane
    Posts
    77

    Default

    Quote Originally Posted by Bloss View Post
    As to the low water flow being a problem with instantaneous gas - well that's rubbish. True that some older systems did not cope well when flow restrictions were brought in and I wouldn't be installing one in a gravity feed system (but who has those anymore anyway?). There are few times where such a low rate is needed compared to the vast majority of normal uses which are above the quite small minimum flow rate for these systems.
    Well, now that is rubbish
    The system I was referring to is now less than 12 months old (Everhot), and I hate running a full blast of water just to get some hot water to shave or brush my teeth!

  25. #25
    Old Chippy 6K
    Join Date
    Mar 2007
    Location
    Canberra
    Age
    70
    Posts
    6,582

    Default

    Ah - I see the light - look at the outlet rather than the source!

    But as to cleaning your teeth etc - as I tried to point out these activities are tiny proportion of hot water use and the usual approach would not be to choose a HWS on the least use need.

    Warning - even greater diversion from thread topic! Smurf - the fuel analogy isn't a good one (although the value of the time vs the energy use is) because fuel use in cars is far from linear. Mainly because above ~60kph the energy used initially (ie: up to around that speed) to move the vehicle weight against gravity changes to mainly trying to overcome wind resistance (roughly drag). Driving twice as fast quadruples the drag and the amount of power needed to overcome that increases by a factor of eight (square cube law).
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  26. #26
    4K Club Member ringtail's Avatar
    Join Date
    Sep 2009
    Location
    brisbane
    Posts
    8,266

    Default

    Quote Originally Posted by Bloss View Post
    But as to cleaning your teeth etc - as I tried to point out these activities are tiny proportion of hot water use and the usual approach would not be to choose a HWS on the least use need.

    Ahhh, but its the little things that count. Those little things that annoy you every single day that ultimately turn into deal breakers

  27. #27
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Smurf View Post
    Yes, you will have used twice as much energy to heat a litre of water from 20 to 60 rather than 20 to 40 that is correct.
    But in order to run the shower at the same flow rate without mixing in the cold you will have to heat twice the volume of water if you're only heating it to 40 rather than 60. As such, the total energy consumed is roughly the same apart from the relatively small difference in pipe losses etc.

    At the end of the day, if you're going to have 100 litres of water at 40 degrees (incoming cold water temp 20) come out of the shower head then it's going to require 2.3 kilowatt hours of heat energy put into that water. Whether you mix water at 60 degrees with water at 20 degrees or just heat it straight to 40 doesn't change the requirement to add 2.3 kWh of heat into the water coming out of the shower head. You're still taking 100 litres of incoming cold water and heating it to 40 at the shower head. That you heated half of it to 60 then mixed it, or heated all of it straight to 40, doesn't change this.

    The only changes in energy use relating to the two different methods are:

    1. Gas versus electric. At the point of use, gas is about 80% efficient (the other 20% being hot air out the exhaust) whereas the electric element is just under 100% efficient (minor losses in the wiring). That's at the point of use of course, upstream production (power stations, refineries etc) of gas and electricity is an entirely different story but that's not the topic here.

    2. Heat losses from the pipework. They will be double at 60 degrees what they would be at 40 assuming the incoming water is at 20 that is correct. But we're only talking about doubling the few % of hot water energy that is lost via pipe heat losses - most of the heat ends up at the shower head not lost in the pipes. So we're saving 50% of something that is only a few % of the total in the first place - it's a saving but it's nowhere near 50% of the total energy used to heat the water.

    3. Heat losses from storage tanks. How much is lost is a difficult question since it depends on several factors but 15% of total energy input is indicative assuming we're talking about a modern tank.

    4. Adding the losses from the gas burner (instant gas), pipework and storage tank (electric) they're a similar % of the total overall. Gas storage HWS have considerably greater losses, hence the gas savings when people switch from gas storage to instant, but in this case we're comparing instant gas versus storage electric.

    So yes, heating water to 40 will use less energy than heating it to 60 and then mixing with cold. But the saving is nowhere near 50% assuming the shower runs at the same flow rate in either case. We're heating water with gas burners or electric elements here then moving it via some pipes to a shower - none of which can actually create or destroy energy. Most of what goes in, goes out the shower. Any potential saving is thus limited to reducing losses - worthwhile but it's not going to save 50% unless we're talking about some ridiculously long pipes or something like that.

    It's like saying that driving at 40 will use half as much fuel per hour as driving at 80. That's true (roughly) but omits to mention that I'll have to drive twice as many hours to get from A to B such that the overall fuel saving is far less. The only real saving is via reduced losses associated with the lower speed - a bit here and there but it's not anywhere near 50%.
    Respect.



    Quote Originally Posted by Bloss View Post
    Ah - I see the light - look at the outlet rather than the source!

    But as to cleaning your teeth etc - as I tried to point out these activities are tiny proportion of hot water use and the usual approach would not be to choose a HWS on the least use need.

    Warning - even greater diversion from thread topic! Smurf - the fuel analogy isn't a good one (although the value of the time vs the energy use is) because fuel use in cars is far from linear. Mainly because above ~60kph the energy used initially to overcome the inertia (ie: moving the vehicle weight against gravity) changes to mainly trying to overcome wind resistance (roughly drag). Driving twice as fast quadruples the drag and the amount of power needed to overcome that increases by a factor of eight (square cube law).
    I know this is way off topic, but inertia is a factor of mass and not weight. You need energy to accelerate a mass (even in space) but this kinetic energy is stored in the car. Once the car is traveling at constant speed you don't need anymore energy to "overcome" inertia. You have confused rolling resistance (mostly tyre rolling resistance), which is dependent on weight, with inertia. Rolling resistance is the dominant user of energy at low speed. At high speed most energy is used to overcome drag.
    Good enough today is better than perfect tomorrow

  28. #28
    1K Club Member
    Join Date
    Nov 2011
    Location
    Paradise Beach Vic
    Posts
    1,875

    Default

    My two bobs worth, some people confuse or don't know there is a difference between continuous flow and instantaneous hot water services.
    Cost of installation with continuous flow when used with nat, gas can sometimes be sky high.
    Waste of water waiting for hot water to come through. [ Bloss hates this argument ].
    Something new I've just realised is gas wastage, when a small amount of hot water is required, that high consuming burner of around 180mj is fired up, costing dollars, and if say only 1 litre of hot water required, you have in effect heated around 5 litres.

  29. #29
    Novice
    Join Date
    May 2011
    Location
    Brisbane
    Posts
    22

    Default

    Thanks for all the excellent info guys. Much appreciated.

  30. #30
    Member Poirot's Avatar
    Join Date
    Oct 2010
    Location
    Brisbane
    Posts
    77

    Default

    Quote Originally Posted by vincentvega View Post
    Thanks for all the excellent info guys. Much appreciated.
    No worries, but I hope you did not get scared by the amount of heat you created (forgive the pun): not all questions wil get this much answers and debate

  31. #31
    Old Chippy 6K
    Join Date
    Mar 2007
    Location
    Canberra
    Age
    70
    Posts
    6,582

    Default

    Quote Originally Posted by Johning View Post
    I know this is way off topic, but inertia is a factor of mass and not weight. You need energy to accelerate a mass (even in space) but this kinetic energy is stored in the car. Once the car is traveling at constant speed you don't need anymore energy to "overcome" inertia. You have confused rolling resistance (mostly tyre rolling resistance), which is dependent on weight, with inertia. Rolling resistance is the dominant user of energy at low speed. At high speed most energy is used to overcome drag.
    I didn't confuse anything - simply used a term with a precise term (inertia) incorrectly when trying to explain something in simple terms - I in fact said what you said ie: that at low speed the energy needed is used to fight (effectively) gravity and the friction between the tyres and the road surface (your 'rolling resistance) and that at a fairly low speed that changes to fight drag (again not just drag, but I was and am trying to give a lay description).

    My main point remains accurate - the total use of energy to move a motor vehicle is not a good comparison with heating because as speed increases the mount of energy needed increases exponentially not linearly (as heating water is). The power needed to push an object through a fluid increases as the cube of the velocity. So for example, a vehicle cruising on a highway at 80 km/h may require only 7.5 kW to overcome air drag, but that same car at 160 km/h requires 60 kW. With a doubling of speed the drag (force) quadruples. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

    To heat water the same amount of energy is needed for each degree of rise for any given volume.

    All of which is still way off topic . . .
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  32. #32
    Member Poirot's Avatar
    Join Date
    Oct 2010
    Location
    Brisbane
    Posts
    77

    Default

    Quote Originally Posted by Bloss View Post
    To heat water the same amount of energy is needed for each degree of rise for any given volume.
    That's all well and good but only if the insulation is 100%. As the temperature of the hot water and the surrounding air temperature increases, so will the amount of energy increase to heat one extra degree

    You started it

  33. #33
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Bloss View Post
    I didn't confuse anything - simply used a term with a precise term (inertia) incorrectly when trying to explain something in simple terms - I in fact said what you said ie: that at low speed the energy needed is used to fight (effectively) gravity and the friction between the tyres and the road surface (your 'rolling resistance) and that at a fairly low speed that changes to fight drag (again not just drag, but I was and am trying to give a lay description).

    My main point remains accurate - the total use of energy to move a motor vehicle is not a good comparison with heating because as speed increases the mount of energy needed increases exponentially not linearly (as heating water is). The power needed to push an object through a fluid increases as the cube of the velocity. So for example, a vehicle cruising on a highway at 80 km/h may require only 7.5 kW to overcome air drag, but that same car at 160 km/h requires 60 kW. With a doubling of speed the drag (force) quadruples. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

    To heat water the same amount of energy is needed for each degree of rise for any given volume.

    All of which is still way off topic . . .
    I was simply pointing out that you were using the terminology inaccurately with which you seem to agree. In my youth (48 years ago) I worked in an aerodynamics laboratory. Back then there were no electronic computers used for engineering. There were not even electronic calculators. We had slide rules and at best mechanical adding machines. All calculation was done by a team of people (including myself). I think that I do have a basic understanding of power required to move a body through a fluid (sub-sonic, sonic and super-sonic).

    PS The lab above was used to design an aircraft that held the world airspeed record in 1956. I also helped develop the worlds fastest helicopter when I was 20 (as a technician not engineer). That helicopter still holds the official air speed record.

    And yes: your main point (that power required to move a car is not linear with speed) remains accurate.

    All of which is still way off topic . . .

    PS It's my birthday and I am now officially an aged pensioner.
    Good enough today is better than perfect tomorrow

  34. #34
    Old Chippy 6K
    Join Date
    Mar 2007
    Location
    Canberra
    Age
    70
    Posts
    6,582

    Default

    Quote Originally Posted by Johning View Post
    All of which is still way off topic . . .

    PS It's my birthday and I am now officially an aged pensioner.
    Happy Birthday -

    I have multiple qualifications in engineering and the sciences (and the arts as happens) - it is so easy to throw in a wrong term when trying explain as I did initially (I edited, but the following posts explain all . . .) - I try to explain to a wider audience than individual posters . . .
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  35. #35
    1K Club Member
    Join Date
    Oct 2007
    Location
    Adelaide
    Posts
    1,664

    Default

    The car wasn't the greatest example, but I think I made the point.

  36. #36
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Bloss View Post
    Happy Birthday -
    Cheers Bloss
    Good enough today is better than perfect tomorrow

  37. #37
    Member Poirot's Avatar
    Join Date
    Oct 2010
    Location
    Brisbane
    Posts
    77

    Default

    Quote Originally Posted by Johning View Post
    PS It's my birthday and I am now officially an aged pensioner.
    Happy Birthday !!
    PS: what were you before your birthday? An under-age pensioner?

  38. #38
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Poirot View Post
    Happy Birthday !!
    PS: what were you before your birthday? An under-age pensioner?
    LOL
    Cheers Poirot
    Good enough today is better than perfect tomorrow

  39. #39
    Senior Member
    Join Date
    Dec 2010
    Location
    Adelaide
    Posts
    199

    Default

    Quote Originally Posted by Smurf View Post

    It's like saying that driving at 40 will use half as much fuel per hour as driving at 80. That's true (roughly) but omits to mention that I'll have to drive twice as many hours to get from A to B such that the overall fuel saving is far less. The only real saving is via reduced losses associated with the lower speed - a bit here and there but it's not anywhere near 50%.
    Quote Originally Posted by Smurf View Post
    The car wasn't the greatest example, but I think I made the point.
    Actually it was a good example. In practice cars (internal combustion) use approximately the same amount of fuel per kilometre at 40 km/hour as 80 km/hour.


    Mpg For Speed - Fuel Efficiency Vs. Speed

    Note 40 km/hour is 25 mph and 80km/hour is 50 mph.
    Good enough today is better than perfect tomorrow

Similar Threads

  1. Process for costing and building a deck
    By Lionel_82 in forum Decking
    Replies: 5
    Last Post: 21st Apr 2012, 01:09 AM
  2. Replies: 9
    Last Post: 17th Oct 2011, 02:15 AM
  3. Council fees costing more than the actual granny flat - is there another way?
    By Coastal in forum Debate & Technical Discussion
    Replies: 8
    Last Post: 21st Aug 2011, 07:04 PM
  4. Costing Advice - Is this right?
    By kahuna71 in forum Doors, Windows, Architraves & Skirts
    Replies: 5
    Last Post: 18th Nov 2008, 09:32 PM
  5. link to costing web-site
    By AndrewPatrol in forum Structural Renovation
    Replies: 3
    Last Post: 10th Jun 2007, 04:32 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •