Solar radiant floor?

Patrick North

Sigh. Dead on- big ol' stone house with lots of beautiful, heat sucking windows. Thanks for the detailed reply. I may still pursue solar as a hw heating option, but otherwise I'll return to the less glamorous fix of insulating and weatherizing.
Thanks again,
Patrick

Patrick North

Cutting costs or corners?

Advance warning: I am just spitballing with this idea and would appreciate some blunt answers before obsessing too much more.
I'm looking into solar powered retrofit radiant floor heating as a means of saving to cut down on the use of our natural gas one-pipe steam system- hopefully saving a few bucks in the process. I'm guessing the retrofit is only really practical on the first floor because it can be accessed by the full basement. Conveniently, the steam system is in two "stages"- one boiler for the first floor, one for the second and third. Some general questions:

Is any part of such an installation reasonably do-it-yourself for the lay person?
Any systems/components that you could recommend, or recommend avoiding?
Anyone know of anyone in my area (Central PA) with experience in the field (a search on this site turned up empty)?

Mike T., Swampeast MO

Old house? Thick wood floors? At best "decent" insulation? Lots of windows? The more you answer "yes" to those questions, the more difficult make the floors operate with reasonably low supply temperatures.

Say in moderately cold weather that you need 12,000 btu/hr average for the downstairs during a day. Say you have 10 hours of good sun. You need 12,000 * 24 = 288,000 btus but you only have 10 hours to collect them. So, you have to collect 28,800 btu/per hour minimum. Certainly doable, but the problem is you have to store 12,000 * 14 = 168,000 btus for later use.

With really high mass floors like concrete you can store quite a bit of energy in the floor itself--essentially by accepting a fair degree of temperature variance between day and night. You don't have that luxury with wood floors, so you have to store it some other way. The typical method is a tank of water. Say you can heat the stored water to 60° higher than you need. In other words you can draw off 60 btus from each pound of stored water before its temperature is lower than required to heat the space.

168,000 / 60 = 2800# of water. 2800 / 8.33 = 336 gallons. That's a very "rosy" estimate that assumes MUCH, not the least of which is 60°F worth of temperature to draw down. In reality, probably something more like 3x that amount of storage...and this still for a fairly small load. In the 70s, people would put large rocks with specific heat higher than 1.0 (water) into the tank(s) so they could store more heat in less space. I presume similar measures are still taken in some cases.

The cost of such a system will be very high and you can't even consider "bare tube" radiant as its temperature requirement will be too high. People here well experienced with solar generally agree that solar heating of domestic water is BY FAR the most cost-effective for existing structures. If you want solar space heating, you really have to design the ENTIRE STRUCTURE with such in mind. I'd say it's nearly impossible that a home old enough to have a one-pipe steam system would be so designed or reasonably modifiable to achieve. Good active solar systems usually start with very good passive solar design...