Solar Hot Water Loop maximum storage temp

6 Posts

Nov 7, 2007 08:42 pm

I have a home with radient floor heat (one large living room tile over cement radiant) and the other carpet over cement radiant.

I have two hot water tanks (40 gals each) which do split duty in the house and are located at opposite ends of the building.

The bedrooms utilize hot water baseboard for heat.

So effectively three water temps running through the house (~ 150 Deg F through the baseboards, ~ 130 from the water tanks and ~100 Deg F to the radiant floor). These all take heat from a central loop which is heated by an oil boiler.

I would like to add Solar to assist with all of this. The ultimate goal would be to have a closed loop solar help support the loop (which runs around 160 Deg F). The though was to install a large storage tank (around 500 gallons) heat with solar amd have a closed loop furnace jump in around 140 Deg F when needed.

Does it make sense to run a solar loop so hot (my guess it would need to be consistently up near 170 Deg F or so. Or would it make better sense to split the system and use the solar for the radiant floor and maybe the hot water tanks. Baseboard handled soley by oil?

I see a lot of generalities out there but no real numbers (temps of loops etc).

Any thoughts would be appreciated.

Larry S.

Dennis Adams

28 Posts

Nov 8, 2007 10:11 am

Re: Solar Hot Water Loop maximum storage temp

Sounds pretty hot to me. How well does your system perform at its existing tempertures? If it is comfortable, there should be no need to increase those temperatures.

Larry Saupe

6 Posts

Nov 8, 2007 08:48 pm

Re: Solar Hot Water Loop maximum storage temp

Thanks Dennis. Yes, it works well now, but is run entirely on oil. To keep the system unchanged and integrate solar I would need to shoot for a 160 Deg F storage tank temp. So... ultimately that is my question. Is a 160 Deg F storage tank temp a reasonable number for solar or should I relinquish the high temp stuff (baseboard heat) to the oil fired boiler and concentrate the solar on low temperature items (radiant and maybe hot water tanks)?

Dennis Adams

28 Posts

Nov 9, 2007 04:27 am

Re: Solar Hot Water Loop maximum storage temp

160ºF is a reasonable temperature coming off solar collectors and appears reasonable for radiant heating systems as well.

My thought was that if you shoot for 170 as a design temperature, it will give you too much heat to reject when solar radiation is strong and require you to shut-down and drain the collectors, or add a heat rejection radiator and/or otherwise complicate the system and its control.

If a little is good, you generally think more is better, but that's only true up to the amount you actually need.

Larry Saupe

6 Posts

Nov 9, 2007 08:15 pm

Re: Solar Hot Water Loop maximum storage temp (and collector over temp control)

Thanks Dennis. This brings up another question if anyone is willing to field it. I often see suggestions to use a shunt to handle excess heat loads during periods of non-use (of generated heat). Why not have a simple system to cover (or partially) cover the collector when temps get too high (or is a shunt just the mother of simplicity and reliability)?

If collectors are drained, are they typically designed to be able to survive without coolant for extended periods in the sun? Seems they would see some pretty high temps.

Thanks in advance,

Larry S

Dennis Adams

28 Posts

Nov 10, 2007 03:43 am

Re: Solar Hot Water Loop maximum storage temp

The equipment should be able to survive on the roof without coolant flows, although its life (esp. of any plastic parts) might be compromised somewhat. Really I'd guess its just much easier to valve some heat off to a rejection point than climb up to the roof every time you want to adjust the water outlet temp.

Michael Radeka

1 Posts

Nov 11, 2007 03:09 pm

Re: Solar Hot Water Loop maximum storage temp

If you are heating your domestic hot water with solar thermal also, drain the panels would eliminate that benefit. Having a shunt or dump zone is (in my opinion) the best option. If you have a damp basement or crawl space, this would be the place to dump it using a space heater. (modine, etc.)

Mike

Tom M

462 Posts

Nov 15, 2007 12:03 pm

Re: Solar Hot Water Loop maximum storage temp

Larry, maintaining a 160 to 170 degree temperature coming from a solar hot water panel is a stretch. Trying to heat up 500 gallons of water to these temperatures is a bigger stretch. You would need a large amount of panels and good sun all day to even come close. Most hot water systems operate on a differential. So anytime that the fluid in the panels is somewhat hotter than the water in the tank or radiant zone, the circulator turns on. This can happen at 40 F as well as 140 F so you may be circulating cool solar fluid if you are not careful.
If you plan on installing a hot water system for domestic and heating, I would suggest that you either independently or series pipe the domestic and heating loops to get the maximum out of the solar loop. Unfortunately your radiant zones aren't the best. The one with cement is okay except for the tile surface. Tile is a resistive and reflective material though it still works okay. Carpeting is also an insulator as is the wood it is installed onto.

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Solar Hot Water Loop maximum storage temp

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