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Old 08-28-16, 07:00 PM   #2
Steve Hull
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
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Great that you have "new" solar hot water panels. Now what to do.

I have had hot water solar panels on a few of my homes since the late 1980's. All used a drain back system (simple, inexpensive and little danger of leaks).

I rebuilt mine and then added extra high temp foam on the back. Pressure tested each BEFORE rebuilding and putting them on roof.

Orientation is not critical. A due south exposure is "best", but even a SE or SW exposure will work. Most homes have a 3/12 roof pitch (14 degrees) and that is OK. In your northern area, a higher pitch of 25-30 degrees is ideal, but then this complicates the install if your roof pitch is less. If you have a southerly roof with a decent pitch, then just put them there.

I am in sunny and warm central Oklahoma. You are in cool, wet and damp Seattle. Quite a difference in performance.

I feel the best "bang" for your efforts is to create a preheat solar storage water tank. In my case, I preheat a 100 gallon tank with roof top solar water. The output of this goes through a tempering valve to a tankless water heater.

The tempering valve is set to 125 F so that I never get scalding water through the system. The tankless is set to about 120 F. By the time it gets to faucets/showers, the max water temp is about 115 F or so.

My cold water inlet temp is about 60 F. For domestic hot water, I need a rise of about another 60 degrees to get to 120 F. For all spring/summer and much of the fall I get almost all "free" solar water as the tankless unit never fires up (three 4 x 8 foot panels).

In the winter, the storage tank gets typically gets to about 80-90 F or so. Call this a rise from ground temp of 30 degrees. My tankless unit now has to supply the other 30 degrees to get it up to 125 or so. But I have cut my winter hot water bill by about 50%.

Your inlet water temp is a LOT colder - probably in the upper 40s or so. You need a rise of about 80 F to get to a good hot water temp - expensive! With panels, even on a cloudy day, you will boost this cold water inlet temp to about 70-80 F. Solar based water heating is most efficient with this large difference in inlet and output temperatures. In other words, it is easy to heat up cold water - harder to get it to a higher temperature. This makes your solar hot water system more efficient than mine even though you are further north.

Some ideas. Look for a tankless water heater that heats to a specific output water temp - NOT one that just turns off and on.

In my area, the cost of propane per BTU and the cost of electric resistance heating ($/BTU) are almost the same. I bet your electricity costs are low due to hydroelectric.

The cost of an electric tankless water heater unit is very low ($250-$400). The cost of a gas (natural gas or propane) tankless is much higher ($700-1000). Consider the ease of installation of an electric booster tankless heater.

Make sure to use energy efficient circulation pumps (you will need two) for your drain back system. One is a tiny 1/20 HP as it just recirculates water through the heat exchanger into the storage tank only overcoming water pipe frictional resistance. The other one needs to pump water up some height to your roof. Depending on the height, you may need a 1/5 HP water pump.

Lots of information is on the web on constructing drain back systems. No glycol in water loop and simple to make.

Have fun!

consulting on geothermal heating/cooling & rational energy use since 1990

Last edited by stevehull; 08-28-16 at 08:09 PM..
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