[Greenbuilding] single flat plate collector for household with 29gpd total indoor water use (faucets)

Frank Tettemer frank at livingsol.com
Mon Jul 8 04:35:41 MDT 2013

Hi Reuben,

I see nothing wrong with one flat plate paired up with a twenty or 
thirty gallon tank, for your defined consumption rate. My next door 
neighbor has one flat plate with a twenty gallon tank, here in Ontario, 
Canada.  She has excellent hot water from June to September, which is 
when she drains the system for the winter.

At my house, I have a fourty gallon tank, with two adults in the home, 
and one flat plate. For us, the single flat plate is thought of as a 
pre-heater. The tank water then routes to the fixtures, and passes 
through an in-line propane hot water heater, which fires, or does not 
fire, depending upon tank temperature. I also do not use glycol or a 
heat exchanger, but use tank water to circulate through the flat plate.
Both these homes heat with wood, and use systems within the wood 
appliances to heat the water in Winter, and are similarly setup to use 
gravity circulation in the stove loop.

Having said all that, with a fourty gallon tank, I would like to have 
two flat plates.  Using gravity circulation in the flat plate loop is 
not as "efficient" as having a pump for circulation. Thus two flat 
plates for a fourty gallon tank is great. For homes for other folks, 
we've always installed two flat plates, with a fourty gallon tank, while 
using gravity circulation. One home we've built has a sixty gallon tank, 
matched with three flat plates.

I would guess that a twenty gallon tank would overheat, combined with 
two flat plates, unless you incorporate a heat dump mechanism of some sort.

Frank Tettemer
Living Sol ~ Building and Design
613 756 3884

My interpretation of this very helpful piece on sizing solar thermal DHW 
systems: http://www.builditsolar.com/Projects/WaterHeating/DIYsizing.htm
which takes us here:
In the Northwest, use 1 square foot of collector per 0.75 gallon (2.8 l) 
of tank capacity./

is that we'd be fine with *one *4'x8' collector

32 square feet x 0.75gal = 24gal (storage)

This system will be for my house, and we are not counting on it to do 
much of anything in the winter, when our solar insolation (here in 
Portland, Oregon) is among the worst in the US. The woodstove will be 
relied on to complement the solar collector in the winter.

I found two decommissioned 4'x8' collectors for cheap, but can't 
rationalize using both for my system. Any thoughts or experience on real 
world yield of one-collector systems?
The fellow I bought these two panels from also happened to mention that 
they produced too much hot water. Without knowing anything about his and 
his wife's hot water needs this seemed to confirm my suspicions.

Thanks very much.


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