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Connect evacuated tube solar into a standard gas hottie?

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  1. #1
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    Default Connect evacuated tube solar into a standard gas hottie?

    There are methods available to retrofit a solar hot water circuit into a standard storage hot water cylinder.

    But will this work with a standard gas hot water cylinder? Or are they too inefficient?

    I need to get a new HWS and I have access to a decent solar collector and pump setup. But it would be way better for me to have the storage cylinder as a standard gas cylinder.

    This is because I have limited mains power overhead available for a more normal electric boosted setup. And I don't like the conplexity of an instantaneous gas boosted setup.

  2. #2
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    dont know about the "gas Cylinder"...but why cant you hook up a solar circulation pump back into the Gas Hot water unit.

  3. #3
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    Quote Originally Posted by stumo5 View Post
    There are methods available to retrofit a solar hot water circuit into a standard storage hot water cylinder.

    But will this work with a standard gas hot water cylinder? Or are they too inefficient?

    I need to get a new HWS and I have access to a decent solar collector and pump setup. But it would be way better for me to have the storage cylinder as a standard gas cylinder.

    This is because I have limited mains power overhead available for a more normal electric boosted setup. And I don't like the conplexity of an instantaneous gas boosted setup.
    Standard gas hot water heats the water in the cylinder, adding solar to one will result in the water being heated by gas and circulated through the solar panel. No it will not work.
    If you don’t like the complexity of an instantaneous gas boosted setup why are you even considering the even more complexities of pumping water from a storage tank up into panels fitted on the roof with all the valves and electric pumps needed to make it heat the water, protect it from overheat and frost etc ?

  4. #4
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    I installed an evacuated tube collector and controller to an existing 'standard' 250 litre electric storage tank some years ago, and disconnected the heating element. After watching the tank temperature plummet overnight, I insulated the tank and built an insulated enclosure around it; now the tanks holds its temperature overnight. I do not have any backup hot water heating and we have not run out of hot water for the past few years even during extended cloudy periods (two adults).

    I think the same could be done with a gas storage tank, although because of the flue through the tank, gas HWS tanks probably haemorrhage energy even faster than typical electric storage HWS tanks, which are bad enough. When my tank finally dies, I intend to buy a stainless steel tank with proper thermal insulation, not the rubbish that is sold for domestic hot water storage. My guess is that typically 50 - 95% of the energy put into a HWS storage tank just bleeds out into the neighbourhood.

    I bought the components I needed from Run On Sun: http://www.runonsun.com.au/
    Before you speak, ask yourself: Is it necessary, it is true, does it improve on the silence? - Baba

  5. #5
    4K Club Member Marc's Avatar
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    Quote Originally Posted by stumo5 View Post
    There are methods available to retrofit a solar hot water circuit into a standard storage hot water cylinder.

    But will this work with a standard gas hot water cylinder? Or are they too inefficient?

    I need to get a new HWS and I have access to a decent solar collector and pump setup. But it would be way better for me to have the storage cylinder as a standard gas cylinder.

    This is because I have limited mains power overhead available for a more normal electric boosted setup. And I don't like the complexity of an instantaneous gas boosted setup.
    Hi Stumo, what the other said. Perhaps I can add why this is so, my own way.
    A solar hot water system has, as you know, a solar collector panel, a circulation pump and a storage tank. Because of the pesky habit of the sun to hide at frequent intervals and the hit and miss amount of heat the sun dispenses, you need a booster. The booster kicks in, when temperature in the tank is too low. However it is not that simple. An ordinary electric HW tank has a heating element at the bottom of the tank and when the thermostat flicks the switch, the low element heats the whole content of the tank. In a solar system with electric booster, the tank has the heating element 1/4 from the top, and so heats only 1/4 of the volume of water as emergency hot water before switching off.
    If one would use an ordinary electric hot water for solar, the low lying element would kick in all the time unless you have it on off peak or manually switch off.

    With gas boosting things are more complicated. The heat exchanger in a gas tank is the flue through the center of the tank, meaning the whole tank is heated just like the electric HW with the low element. To save gas, the booster should kick in only when the hot water reserve is low and only to heat up a limited amount of water. Not easy to do, since the burners are at the bottom of a vertical tank. Not that it can't be done, solar with gas booster is old technology and usually works on horizontal tanks.
    My guess is that the system is no good, because the new solar/gas system today are a simple yet expensive combination of solar+instantaneous gas and not solar / gas tank.
    There is an added complication, with solar collector requiring a thermostat regulated circulation pump that has to stop when gas kicks in.

    All in all, it is duable but not easy. I would go large collector and very large tank with proper insulation and forget the booster ... or ... have a dual system, Pure solar and a HW gas on standby that can be turned into service via a manually operated ball valve.
    Science is never settled,
    it advances one funeral at the time.
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  6. #6
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    Quote Originally Posted by stumo5 View Post
    I need to get a new HWS and I have access to a decent solar collector and pump setup.
    This is because I have limited mains power overhead available for a more normal electric boosted setup.
    ..just re-read your post.
    One thing that pops out is why you would have limited overhead mains power to heat water?
    If you have to buy another HWS, may as well buy an electric one specifically for connecting a solar hot water system
    Going gas adds complexity no matter which way you look at it.
    A friend has a setup where he has an electric solar HWS tank but the element turned off and connected up to instant gas on the way out. If the solar hot water water isnt quite hot enough, it kicks in otherwise remains off. Works well but adds complexity and just another appliance to buy/maintain/replace at some point.

    Anyway, others all raise valid points and is the reason why I went all electric.
    The solar hot water panels connect to my electric 315L Rennai HWS that has a circulation pump as part of the HWS...something like this https://www.rinnai.com.au/online/ren...ster-system-7/. The circulation pump on the side has heat sensors that tell the pump when the water is hot in the solar panels to start circulating.

    In hind sight, I would have been better off with a dual element HWS (bottom and top element) and just have the bottom element turned off. The tank I have only has a bottom element and like Marc indicated, when it kicks in, its needing to heat the entire tank. If power supply is a valid concern, you could replace with an 1800watt element. 1/2 the power but twice as long to heat the water and not much more than running a toaster.

    If a new tank is too expensive, keep an eye out on Gumtree and Market place as you can pick up good tanks for ~$100 and sometime less than 10 years old. The element will have a shorter life, replace it when it fails and your still good to go.

    Now 8 years down the track, I've finally got solar electric panels and so glad I didnt buy any gas appliances. With a timer installed on the heating element (not the circulation pump) so its only activated during daylight hours, it will be interesting to see just how much solar power can supply/supplement the hot water in addition to the solar hot water panels.
    Have had a few mornings where the water is cold after going to bed knowing the water is still hot so might have to consider Johns solution of adding insulating around the tank.

  7. #7
    The Master's Apprentice Bedford's Avatar
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    Quote Originally Posted by Bart1080 View Post

    If a new tank is too expensive, keep an eye out on Gumtree and Market place as you can pick up good tanks for ~$100 and sometime less than 10 years old.
    These would need to be checked carefully as a lot of these were decommissioned by drilling a hole through them, and sellers are unaware.
    Posted by John2b, And no, BEVs are not going to save the planet, which doesn't need saving anyway.

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    Besides controlling the circulation pump, most solar HWS controllers have programable (time of day) outputs to control a backup heating element. The element will still only kick in if the water is below the tank's thermostat setting.

    An instant gas (flow-through) heater is the most efficient, common and simple solar hot water backup for inclement weather, and usually can be left switched off except for winter. Instantaneous electric water heaters are also possible but usually draw heavy current which needs to be factored in to the inverter size for off-grid applications.

    With evacuated tube collectors there is still some water heating on fully overcast days. Like I said previously, if you have a large enough storage tank and the heat isn't haemorrhaging out of it, the chances are you don't need a back-up heater for solar hot water anyway. Thinking we would need a backup when converting to solar hot water I put a 15A plug on the heater element in our HW tank for plugging in to the generator, but I haven't done that for several years since I insulated the tank.
    Before you speak, ask yourself: Is it necessary, it is true, does it improve on the silence? - Baba

  9. #9
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    This is the "standard" Apricus setup for evacuated tube SHWS with gas backup, showing all regulatory safety requirements common in Australia:

    Attachment 130000
    Before you speak, ask yourself: Is it necessary, it is true, does it improve on the silence? - Baba

  10. #10
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    Wow thanks for all the replies. I'm starting to realise I didn't think this through lol.

    In response to all the excellent points raised here, I agree and thanks all for your inputs. I will probably just go with electric boosted solar and maybe run a 1.8kW element to limit usage. Am I right in thinking a 1.8kW element can be installed via a normal flex 3 pin plug into an existing GPO? The tank is a proper solar one 315l SS with the element halfway up and solar I/O at the bottom.

    Quote Originally Posted by Bart1080 View Post
    ..just re-read your post.
    One thing that pops out is why you would have limited overhead mains power to heat water?
    We are in a 1950 house that only has a ropey old 6mm2 supply and a 32A MCB for the whole house. I was hoping to also avoid having to add another dedicated circuit for HWS, because we are also at the space limit of our switchboard. With our 2 splittys on full speed we are already using half our circa 8kW limit. So taking half our limit with a 3.6kW booster isn't really an option. We would be tripping the mains every time we put the kettle on.

    To upgrade the switchboard to a bigger one would require our old black wiring throughout the house to be upgraded as well, and the crusty 6mm2 inlet from the street would also need upgraded. So we are looking at minimum $5k before we even actually do anything.

    Quote Originally Posted by Bedford View Post
    These would need to be checked carefully as a lot of these were decommissioned by drilling a hole through them, and sellers are unaware.
    Yes I bought the whole system off Gumtree. The seller had "upgraded" to heatpump and part of the rebates is that the installer has to show a photo of them drilling a hole in the bottom of the tank.

    Pretty damn wasteful on a less that 10 yo full 316 stainless steel solar specific super insulated 315l tank. Fortunately the installer colluded with the seller to fake the drilling photo so the tank is still totally intact. But it definitely needs to be checked. I think drilling a crappy old wasteful tank would be ok because its only got scrap value anyway and should be scrapped. But not a perfectly good and probably more environmentally friendly setup. Luckily sanity prevailed over bureaucracy.

    So I will run with the system as-is. 315l SS solar rated tank into the 18 evacuated tube collector, and try a 1.8kW element to start with. If that doesn't work I will have to bite the bullet and go with instant gas boost.

    Thanks again for all the helpful replies.

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