Solar HWS , wood fire boosted , not working. Hi all, we have a problem with our new wood fire boosted solar HWS. The hot water from the wood stove goes backwards through the solar panels , rather than through the heat exchanger in the storage tank .
The solar system is a new solar edwards lx with a 300l tank. This system has a low pressure loop from the panels separate to the storage tank . Usually this is filled with glycol , however ours is water filled and the hot water thermosiphon loop from the wetback is joined into it.
The solar works perfectly , circulating hot water through the tank, exchanging its heat with the main body of stored hot water . However, when the fire is running the hot water from the wetback will run backwards through the solar panels (opposite to the normal solar thermosiphon direction) losing heat to the atmosphere.
Low pressure side of the system (panels + wetback ) is open vented to the atmosphere, topped up from a small tank filled from the main supply through a float valve.
Woodstove wetback thermosiphon is flowing perfectly , up to the roof mounted solar system , and back down to the stove. A small amount of hot water does move through the storage tank but most goes through the panels .
The entire system was designed and installed by a licenced plumber, who is also the solar edwards recommended installer.
He has no idea what has gone wrong , and i am left trying to solve this very perplexing problem.
Just wondering how you can know for sure it is actually flowing backwards.Originally Posted by smallhouse
Maybe it just appears this way, because most of the heat is absorbed quicky in the heat exchange storage tank.
But any small amount of hotwater that makes it to the solar collector (from the wood wood fire heater) would be insulated and retained in the collector.
I mean is the wood fire heating your storage tank water at all ?
Yes there is some heating of the storage tank water, very little though.
I have spent hours on the roof monitoring the pipe temps at different points and i am convinced that the reverse flow is happening as described. If you go up at night the panels are actually hot .
Water at the lower point of the panels is much hotter than that leaving the storage tank heat exchanger. The only way that this water can be so much hotter at this point is if it is coming from the wood fire, through the panels , and mostly bypassing the main tank.
I'm no plumber but it seems to me that if the thermosiphon circuit of the wood heater is connected to
the thermosiphon circuit of the solar collectors then the heated water will flow through the tank and the collectors
which would effectively cool the heated water at night. It may not be bypassing the tank but flowing through the tank AND the collectors.
You may require some type of non return valve to overcome the problem.
Regards Bradford
Bradford , you have hit the nail on the head. The non return valve does however need to offer a very low resistance to the delicate solar thermosiphon flow.
Apparently it is easily stopped. Anyone know of an applicable valve?
I doubt if there exists a simple non return valve that would have any chance of working on thermosiphon flow.
What about putting a large manual ball valve in - this would allow you to isolate the panels at night for testing purposes at least.
I am still trying to work out why the fire thermosiphon does not work though.
Just wondering do the collecters and fire share a common inlet and outlet to the storage tank ?
That is, are their hot out and cold in pipes joined with a T-piece before the tank ?
Yes they are teed just before and after the tank . http://img855.imageshack.us/img855/4075/photo0176c.jpg
http://img222.imageshack.us/img222/1589/photo0177m.jpg
http://img718.imageshack.us/img718/7530/photo0178mo.jpg
Thanks for those pictures - very interesting.
(I am thinking of getting an Edwards solar roof tank system, and hope to add a fire boost as well)
Maybe your your problem is related to the indirect system with the heat transfer coil in the tank.
I think there is only about 9 litres of water in the solar collectors and tank coil loop.
Compared to 300 litres in the tank itself.
So if a bit of heat from the fire heated water transfers to the collector, it will be very apparent just because there is such a small amount of water in the collectors (2litres).
Perhaps also there is more flow resistance in the long transfer coil in the tank.
Compared to the collector flow resistance.
So some some of the thermosiphon flow from the fire might go in the path of least flow resistance (ie. the collectors).
Just guessing though.
My plumber has done 'identical' setups which have worked. Which has me thinking that its probably some very minor tweak to fix the problem, rather than a more dramatic shifting of pipes.
Some possible differences in my setup : The woodstove pipe run is quite long (6m) and vertically fairly high (4m).This pipe run is lagged in neoprene. The solar system is a new model which may have a different style of heat exchanger.
As mentioned previously, the solar is cycling normally, plenty of hot water. This should rule out any restriction or problem with the heat exchanger.
The wetback is also thermosiphoning just fine. Indeed if there was no flow we would have the wetback boiling fairly quickly , which it isn't.
We will be trying some remedies suggested soon. ( soon as i can get the plumber to come).
Cheers all.
Well good luck with it.
Sorry you didn't get the solution.
One possibilty, for a thermosiphon valve, could be a car thermostat.
They automatically open up wide when the water gets hot.
You could get one to try for virtually nothing and it shouldn't be too hard to plumb in the low pressure solar loop.
But I would try to work out for sure, which way the water runs in the solar collectors, when the fire is going.
You could start the fire, then see which gets hot first, of these 2 :
1/ the connector pipe between the 2 collectors , or
2/ the final outlet from the 2nd collector.
Generally the easiest way to resolve these sorts of issues is to draw system design diagram showing all the connections then superimpose a flow diagram onto that. It is usually very obvious where the problem is occurring. The systems diagram needs to match the supplier of the HWS first then have added components separately shown - each should correctly match the their own system design and then you can resolve the interactions.
There are quite a number of designs that all work, but in slightly different ways, for example I have worked on one installation where the panels are intentionally heated overnight (and sometimes in the day) based on the temperature differential to prevent freezing of panels & pipes - but the warming is only to about 6 degrees and starts at 3. The flow is under a controller and small solar pump though not simply thermo-siphon.
Thermosiphon on a horizontal tank is far from ideal (as is solar HW heating and thermal performance generally too it is a compromise design) I prefer ground based (or ceiling mounted) vertical storage.
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.
Don't know the answer to your problem but the installation looks pretty dodgy to me.
No insulation on pipes, roof penertraion siliconed and a fabricated tee joint.
If it was trully done by a plumber get him to fix those issues.
edit. the roof penetrations could also cause galvanic corrosion
Mate, i have been waiting 3 months for the plumber to come and try to resolve the problem. I don't hold much hope of him redoing existing work. Indeed i am trying to resolve the issue through my own research. Are you sure copper will cause galvanic corrosion with zincalume roofing? .
Yes it is a regulation that there is no direct contact with copper and zincalume, easiest way to find out for yourself is to look at any of the roofing manufacturers' website eg lysaght strathco and look at any of their installation documents.
Also I said "could" because looking at the pic you posted he cut/drilled a small hole compared to the pipe size and there is a bend as it enters the hole. It would be very hard not to contact the 2 at some point.
If you are having so much problems getting him back tell him that if he doesn't fix/resolve your problems you will have to contact the plumbing industry board and being that he was the "solar edwards recommended installer" send them the pics of his quality work. Give him a couple of weeks to rectify before contacting them.
A blob of silicone on a roof penertration is not acceptable and he should have used a dektite with a airgap around the pipe and zinculume roofing.
AFIK the only time you can use a fabricated tee in copper is with hydronic heating and even if it is allowed I'd doubt the flow as it is not a "pulled" tee joint, he probally just drilled a hole on the original pipe and welded the branch from your wood heater to it using silver solder to bridge the gaps.
edit. I can't see how the thermosyphon would work with the panels in the loop when using wood fire as the water in the panels will always be colder than the tank. I would probally suggest puting in a couple of solenoid valves so that you can shut off the pipes to the solar collectors when you have the wood heater fired up, You will however still loose a certain amount of heat due to convection. During the day when the solar panels heat up the solenoid valves would be open and it wouldn't really matter if the water to the wood fire is cold as the thermosyphon works with the hot water higher.
Last edited by ozheat; 5th May 2011 at 07:45 PM. Reason: solution idea
Yes a solenoid would be a more elegant solution than a manually operated ball valve.
Do they work on 12v power?.
I know of a few systems similar to mine which are functioning perfectly, so it is possible, just something has gone wrong in my case!.
Our previous split system solar had a thermostat which triggered the circulation pump when the panels were making hot water. I wonder if a similar thermostat on the wetback might be used to actuate a solenoid closing off the solar panel loop?.
Plumber came yesterday . He decided not to swap pipes as that was going to have the 2 flows fighting each other .
He thinks it's related to the new model system as his previous successful installations were with the older model.
So he put a 20mm ball valve inline in the cold return down to the panels. Not as a permanent fix, more as a diagnostic tool.
Shut the valve, started the fire, got a few big spurts out of the open vent (as some air bubbles were forced out ?), and then...
Well the wood fire thermosiphoned through the heat exchanger alone, for the first time.Hot water leaving the fire, and returning from the HE still warm. We got much hotter water than normal.
So its nighttime , and i tried reopening the valve in the solar loop. Instantly we got stone cold water coming back from the roof to the wetback as all the hot flow went into the panels , lost all it's heat and returned to the fire.
This confirms my belief that the wetback water is cycling through the panels.
It's not fixed yet !
Maybe a small pump in the circuit may help directing the flow in the direction you need.
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