Solar hot water - best way to deal with excess heat?

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Solar hot water - best way to deal with excess heat?

Postby dmshimself » Fri Jan 03, 2020 7:33 am

Hi - I've inherited a solar hot water system and it works fine, but I wondered if it is dealing with excess heat in the best way. The controller stops pumpng to the evacuated tubes when the hot water tank gets too much hot water and then the evacuted tubes on the roof have a steam valve to which allows the water in the tubes to boil off.

As we are on tank water, the idea of wasting water to steam doesn't seem great, but I guess the excess energy has to go somewhere.

I have little practical experience of solar hot water, so any advice appreciated. In particular, are there better ways of dealing with having too much heat comming in?
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Re: Solar hot water - best way to deal with excess heat?

Postby australsolarier » Sat Jan 04, 2020 6:35 am

that is how it works. the pump stops and the bit of water that is in the manifold boils. it is only a very small amount of water. (obviously as the pump stops no more water gets pumped up to the manifold).
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Re: Solar hot water - best way to deal with excess heat?

Postby dmshimself » Sat Jan 04, 2020 7:33 am

A cupful sounds very reasonable. Thanks for the info. Much appreciated
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Re: Solar hot water - best way to deal with excess heat?

Postby davef » Tue Jan 28, 2020 4:48 am

dmshimself,

Not that it helps with the loss of water but my system has a thermal relief valve on the hot water tank that dumps water BEFORE the system gets hot enough for the "steam valve" to actuate.

What happens when all the water in the tubes has boiled off? Does the pump come on to force more water into the collector?
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Re: Solar hot water - best way to deal with excess heat?

Postby australsolarier » Tue Jan 28, 2020 5:04 am

dmshimself
his system works differently.
there is no water in the pipes. only in the manifold.
he would have a over pressure/ overheat valve at the tank as well.
the pump probably turns off at so 80C degrees, that is when the little bit of water boils off in the manifold.

that is how those system work.
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Re: Solar hot water - best way to deal with excess heat?

Postby davef » Tue Jan 28, 2020 7:34 am

austrasolarier,

So, the difference is his pump will not run above 80C whereas mine could keep circulating water even when dumping excess energy via the tank relief valve IF the system told it to turn on.

I shouldn't be surprised that there are different ways to run these system. I think I had the impression that the "steam valve" was a last-ditch attempt to keep the system from damaging itself.

When the steam has all dispersed wouldn't the manifold temperature sky-rocket?

there is no water in the pipes

do you mean there is no water flow in the pipes when this happens?

Dave
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Re: Solar hot water - best way to deal with excess heat?

Postby australsolarier » Tue Jan 28, 2020 12:58 pm

the water is in the manifold not in the heat pipe. there is another fluid circulating in the heatpipe that is independent of the water circulation. there are some heat pipes that are filled though. earlier models.

when the circulation pump stops, the water in the manifold heats up in excess of 140C. the water in the manifold gets turned into steam. the "steam valve" gets warmed up and lets the expanded steam escape.
there are other ways to handle this, like expansion vessels. (they are mostly used when you run the system with glycol, so as not to waste it, but other reasons as well)
the relieve valve at the tank is meant to protect the tank from boiling and over pressure.

speaking for my own case i have 100 heat pipes and 3000l tank for the space heating system. naturally in the summer it is not much used. when the circulation pump is turned off the steam generated in the manifold gets pushed down into the tank. (the system is open vented)
i do however not like this. so i added a cooling loop. so at stead the pump just turning off when 82C is reached it diverts the water circulation through a cooling tank before it gets up to the manifold. the benefits are there is always hot water in the tank and the manifold gets not overheated.
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Re: Solar hot water - best way to deal with excess heat?

Postby davef » Tue Jan 28, 2020 5:00 pm

Thanks for explaining the system. Sounds like it is in a different league to my 30 tube evacuated open (water-only) system, which is just for hot water heating.

Cheers,
Dave
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Re: Solar hot water - best way to deal with excess heat?

Postby dmshimself » Wed Jan 29, 2020 6:49 pm

Very interesting for me too. Thanks for the updates
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Re: Solar hot water - best way to deal with excess heat?

Postby Privatteer » Fri Jan 31, 2020 3:16 am

While the water loss is small one way to prevent the overheat is to use shade-cloth or something similar to cover a portion of the collector in summer.
I've seen them setup with a rope attached to a pully at each top corner so that you don't have to climb roof to adjust.
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