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Frost valves for solar hot water
By PaulJ, Section Remote Living Posted on Sat Sep 11th, 2004 at 05:47:40 AM MST
How well do they work?

I'm planning to buy a solar hot water system, made here in Australia by Edson hot water systems. They recommend the use of frost valves rather than a glycol based antifreeze system to protect the panels from ice damage.
These work a bit like a car thermostat in reverse; when the temperature drops to 0*C (32*F) they open enough to drain the near-freezing water out of the collector panels, replacing it with hot water from the storage tank and then closing when the panels warm up a bit.
The water loss isn't an issue as we're using rainwater collected from the roof the panels will be mounted on, and we will be using a water jacket in the fireplace to provide most of the hot water in winter.
Most winter nights here don't get below freezing. A really cold night woulg be -4*C (24*F?) and if I plan for a worst case of -7 or -8*C (16*F?) I should be ok.
Does anyone here have any experience with frost valves? I guess my main worry is reliability, if they fail to open they could ruin my (expensive) panels ( 3 panels, 4'x8', about A$750, US$550/panel). The supplier says they're fine, and I have no reason to doubt him , I just like to be as fully informed as possible before parting with my hard-earned money.
Thanks in advance for any help, and I'd be happy to go into more detail about the system if anyone's interested.
Paul.

Frost valves for solar hot water | 15 comments (15 topical, 0 editorial)

Re: Frost valves for solar hot water (3.00 / 0) (#1)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Sat Sep 11th, 2004 at 06:03:33 AM MST
(User Info) http://www.greenbits.com/

There are panels with 'soft' tubing that do not get damaged by the frost at all. They can go through 10's thousands of freeze/thaw cycles before stresses cause cracks in the tubing. It's a very elegant solution to an annoying problem, it also takes the fear out of solar hot water. I have yet to find a distributor for those panels here, but once I find one I'll definitely install them.

Some good info on here
www.greenbits.com

Re: Frost valves for solar hot water (3.00 / 0) (#2)
by PaulJ on Sat Sep 11th, 2004 at 06:48:50 AM MST
(User Info)

Thanks for the link, they mention knock/ drain down valves which I assume are the same as frost valves, and claim they can jam open (draining the storage tank) or closed (destroying the panels). I'll have to look into the 'soft' tubing, but if I can't find it here it seems that glycol might be the best option.
Paul.

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#3)
by jacquesm (j@ww.com- I run a whitelist, add 'stjoes' to msg) on Sat Sep 11th, 2004 at 07:04:05 AM MST
(User Info) http://www.greenbits.com/

I have to leave in a few minutes, but when I get back on Sunday I'll dig around a bit and see if I can come up with a brand name for you. Its been almost 2 years since I really looked at hot water (I have an outdoor woodstove now that heats domestic hot water through a heat exchanger, clumsy but it works).
www.greenbits.com
[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#4)
by LEXX on Sat Sep 11th, 2004 at 11:08:37 AM MST
(User Info)

Not a solar panel expert but wouldn't the panels get dirt and gunk stuck in them from the rain water?

Re: Frost valves for solar hot water (3.00 / 0) (#5)
by PaulJ on Sat Sep 11th, 2004 at 08:02:13 PM MST
(User Info)

Possibly, this is another reason I'm leaning towards a closed loop glycol system.
Rainwater is pretty clean here, we also have a diverter system that stops the first bit of runoff (carrying the accumulated dirt off the roof) from entering the tank. It's fine to drink without treatment.

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#6)
by bill541 on Sat Sep 11th, 2004 at 10:35:58 PM MST
(User Info)

This may or may not be an option for you with the panel design you have, but I think a setup called a drain back system actually stores the hot water in a non-pressurized tank and if it gets too cold or the sun goes down, the pump shuts off and the water drains out of the panels using gravity.

I think they also use a heat exchanger to heat the water in the household pressurized system. Any excess heat is stored in the non-pressurized tank.

There are several articles on this subject in Home Power magazine; one of them is in pdf format available here:

http://www.homepower.com/files/olson84.pdf

-Bill-

Re: Frost valves for solar hot water (3.00 / 0) (#7)
by bill541 on Sat Sep 11th, 2004 at 10:51:01 PM MST
(User Info)

This second article covers the drain back system in more detail. Some folks even make their own heat exchangers out of soft copper tubing.

http://www.homepower.com/files/sdhw86.pdf

-Bill-

Re: Frost valves for solar hot water (3.00 / 0) (#8)
by PaulJ on Sun Sep 12th, 2004 at 01:32:11 AM MST
(User Info)

Interesting articles, thanks for the links. I hadn't heard of drain back systems before, they seem like a good solution to the frost problem. Would thermal shock be a problem when the (relatively) cold liquid is pumped into an empty, presumably very hot panel I wonder? Possible long term issues with metal fatigue?
The house we're building is off-grid, so the need for a pump rules such a system out in my case. I really want the system to work without needing any electricity at all.
The plan is to mount a 380 litre (100 gal?) tank as high as possible inside the roof, and use thermosiphoning to move hot water (or glycol) from the panels/fireplace. The fireplace actually complicates things a bit, as the system has to be low pressure due to the risk of boiling and subsequent tank rupture. Apparently pressure relief valves alone can't cope with this in a pressurised system; the thought of 100 gallons of boiling water raining down from the ceiling doesn't really appeal to me, so I'll trust the experts on this!

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#9)
by BT Humble (bt_humble@bigpond.com) on Sun Sep 12th, 2004 at 04:39:50 PM MST
(User Info) http://www.humbletown.org

I'm a bit suspicious of a frost-protection system that uses automatic valves. This is mostly because I have 2 pairs of older (approximately 4'x5') Solahart panels that have been broken by frost when the valves failed (1 pair from Bowraville, the other from Canberra).

They were however quite cheap because of this. ;-)

I'm planning to use a bit of solder to fix the cracked pipes, and then I'll be using a closed-loop antifreeze system through them. My other alternative was to install appropriate (manual) valves so that in the winter I drain the panels and heat my water using the water jacket on my wood stove, only actually filling the panels when the frost danger period has passed. Given that the majority of winter days here are sunny, that seems like a bit of a waste though.

Of course, all of these grand plans of mine will only happen once I've build the house... ;-)

BTH

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#11)
by PaulJ on Mon Sep 13th, 2004 at 03:03:08 AM MST
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"Grand plans ... once I've built the house"- I know exactly what you mean! I've just reached lockup stage, being an owner-builder is not a small task. That's the main reason I'm going for an off the shelf rather than a home made hot water system, there's simply not enough time to do everything. I still have another windmill to finish (it's nearly done), or we'll be stumbling around in the dark with no cold beer when we move in. Can't have that.
You've confirmed my suspicions about frost valves. We're not as cold as Canberra here, but ice is ice and a failed valve would be disastrous. Thanks.
This may not apply to your panels, but all the systems I've been looking at specify that the panels must be filled and hooked up to the storage tank as soon as they're installed to prevent heat damage, even in winter. Apparently an empty panel can get hot enough to shatter the glass, crack the pipes etc. Makes the winter draining option risky, although I have a friend with home-made panels who has done this for years with no problem.

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#15)
by BT Humble (bt_humble@bigpond.com) on Mon Sep 20th, 2004 at 04:33:18 PM MST
(User Info) http://www.humbletown.org

This may not apply to your panels, but all the systems I've been looking at specify that the panels must be filled and hooked up to the storage tank as soon as they're installed to prevent heat damage, even in winter. Apparently an empty panel can get hot enough to shatter the glass, crack the pipes etc. Makes the winter draining option risky, although I have a friend with home-made panels who has done this for years with no problem.

That's interesting, wouldn't it be a bugger if you melted all the solder out of the joints in your copper piping! ;-)

I've just made a mental note that I have to make up some covers for the panels if I ever decide to go down the "winter drainage" route. I'm starting to think that the antifreeze/heat exchanger technique may well be the simplest in the long run.

BTH

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#10)
by bill541 on Sun Sep 12th, 2004 at 11:19:36 PM MST
(User Info)

The pumps can be run off of a solar panel. If there is enough sun to heat the water, there would be enough to run the pump and control system. One other benefit of this type of system is that it can shut down and drain back if the temperature in your holding tank gets too hot. Not sure how gracefully a closed loop glycol system would handle an over temperature situation. At least the glycol under pressure should be able to handle 220 deg F or better.

The solar powered pump does not help the situation with your fireplace heat exchanger though. I suppose a person could have two systems that share the same thermal storage system. That way if one system is shut down, it wouldn't rob heat from the other.

What ever you do, sounds like you are going about it the right way by doing your research before you build.

-Bill-

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#12)
by PaulJ on Mon Sep 13th, 2004 at 05:01:51 AM MST
(User Info)

Yes, it's an obvious solution now that you mention it. Probably the perfect use for a PV panel, although if I recall the pump used was 150W or so which would require a fairly large and expensive panel. There may be ways around this, I'll have to look into it some more.

I found this earlier:
http://www.suburbia.com.au/~mickgg/k4/kernkraft.htm.

He uses a small PV panel to retrofit rooftop hot water panels to a ground level storage tank, quite an ingenious setup.

The over temp possibility does worry me, it could be as much of a problem as ice. In winter, some of the water heated by the fire will be used to heat other parts of the house, which should regulate things a bit. In summer, the panels could get 12 hours of good sunlight on a 40*C (100*F?) day. Then again, some of the neighbours have glycol based systems that seem to work ok, electric grid boosted with no fireplace hookup though.

A pump for the fireplace hookup wouldn't be a huge issue as it woulld just be circulating the water and not lifting it, much lower power needs I would imagine. I still think I prefer the simplicity of a thermosiphon system though.

Thanks again!

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#13)
by PaulJ on Mon Sep 13th, 2004 at 05:06:52 AM MST
(User Info)

Sorry, got the link wrong
http://www.suburbia.com.au/~mickgg/K4/kernkraft.htm.

[ Parent ]

Re: Frost valves for solar hot water (3.00 / 0) (#14)
by PaulJ on Mon Sep 13th, 2004 at 05:11:22 AM MST
(User Info)

That doen't seem to work either; the first link I posted will get you yhere eventually.

[ Parent ]

Frost valves for solar hot water | 15 comments (15 topical, 0 editorial)

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