Hello,

I own a multi unit 6 plex currently heated by hot water radiant in floor heating.  Each unit about 850 sq. ft.  Does anyone know the cost savings of installing solar panels for preheating the hot water on the roof, and what size of system would get the best return as it would feed into the hot water heater, as well as the radiant infloor system?

These questions have no simple answers. Many things affect the amount you can get out of solar (and how much it takes to support a specific thermal load) such as:

The temperature requirements of the radiant

The latitude.

The seasonal cloud cover.

The seasonal outdoor air temp.

In general, in areas with more than 5000 heating degree days (base 65F) the financial rationale for active solar thermal for space heating is hard to make unless your location is...

...very sunny during the heating season...

...your building is very tight, with R25+ walls...

...and the space heating water temp requirements are quite low (domestic hot water temps at most.)

Without all of the above working for you the size of the solar array necessary to handle a reasonable fraction of the load tends to be unwieldy (larger than the building they're heating.)

But let's start with your latitude, heating season in degree-days (give us a ZIP code if you don't know off hand), how much the building currently uses for heat & hot water, the water temp requirements of the radiant, and the amount of available roof area.

Hello Miles,

I hope you don't mind if I jump in here.  I've waited a bit for your response but perhaps you have unforeseen circumstances to keep you from responding from this very excellent opportunity that we can both learn from.  Thanx in advance.

So Dana, if you are willing, I will provide some data to see what works, if it works.  It is not my intent to become adversarial and I know I can not go toe to toe with you...... but I may have a few questions.

I will reject Miles case for the moment as until we know if he has low temperature distribution the economics are unsure.

Let's say a 2000 sq.ft ranch with ICF's at R-26.  For walls let's use Sips.  I saw 12" at the NESEA show last week but let's go with 8" and call it R-38.  Triple gas windows (not krypton) and a couple of feet of cellulose up stairs.

The distribution would be through the concrete in the basement as well as the first floor.  Let's put this Ranch at either Portland or Gray ME with plenty of roof facing 197 degrees and sloped up lat. + 15.  We'll call this 7000+ HDD (base 65F).

We'll look for 15% coverage on the roof with 10) 2.9M2 panels that produce 31k btu column C sunny.  A good Blu-tec panel should put out about 10K btu less for column D.

For economics we'll say $2000/panel installed price and everybody get 30% off (each state will vary for it's own additional rebate but ME is $1000 currently).

We could use $2.50 oil for comparison but maybe look at $3.00 or an incremental bump.

What does it take for this to make cents?


With Regards.....and some fear and trembling,

Flathead