Dale

I am not an expert, but

At one time, someone wondered about converting the forced air heating system to hot water heat in out (almost) 200 year old Quaker Meetinghouse. While, in general, I would favor hot water heat supplied (when appropriate) by a mod-con boiler with outdoor reset, it seemed to me to be a very bad idea in our case, even though there are a few advantages.



The advantages would be less dust, quieter heating, and probably cheaper heating in a situation other than ours. Our building is two stories high, and heat might go to the top where few people choose to sit. If we put radiant heat in the walls, it might be possible to heat to lower temperatures. Our walls are heavy wood timbers with all the space bricked in. Outside walls are cedar shake; inside walls seem to be plaster.



The biggest problem is that the building is unoccupied for extended periods: often 6 days at a time. One room, with no plumbing in it, we allow to fall to 40F. The other room, with plumbing, we allow to fall to 50F. After that, it takes quite a while to reheat the building. It can take 10 hours with forced hot air because we can produce only 250,000 BTU/hour with our existing furnaces (125,000 from each of two). This much setback means a difficult time during recovery, especially for radiant in slab heating. We would not wish the pipes of the heating system to freeze, and air does not freeze in New Jersey. (In our case, it might even make sense to use a steam heating system.)



For us, the best way to economize on heating is to improve the insulation of this old building, and reduce the leaks, and install better windows. For historical preservation reasons, we cannot put in thermopane type windows, but we have started putting in storm windows (with something like plexiglass instead of glass).

Uh

where are you getting the 35% number?



I could model situations where radiant is LESS efficient than a well designed forced air system. high temp stapleup over unheated basements come to mind...



to the original poster: try contacting the RPA. they have all the final docs available on the subject. there were some poor studies that the building committee might be aware of that are causing the problem.

disagree

good system vs good system and those numbers don't really have to change that much between radiant and forced air. Some, perhaps... mostly due to stratification... and a great reduction in distribution energy usage... but nothing like 35% in an apples to apples comparison.



studies do show that while there may be a theoretical case of lower setpoints for radiant, most people who get radiant don't turn down their systems. I don't recommend using that as a basis for savings claims. It's more of a comfort upgrade, on that issue, then an efficiency benefit.



and in any case, 35% is extreme. you could eliminate infiltration completely and not nix 35% of someone's heat load in most cases.

you should know better

then to argue on the basis of having done "hundreds of losses". I've done a couple thousand easily and I've never undersized a boiler... so that means I'm still oversizing systems. Yet I'm still 20-30% under almost every other heat load I ever see.



Load calcs are important but they sure aren't exact.



I will restate that set point should not change. It does not change in real life, pretending it does only overstates the case.



The only serious additional leakage for forced air is duct leakage if you run it in unconditioned space and don't seal very well. otherwise it's neutral pressure on the house, it pulls as much as it pushes.



another point in the good system vs good system column: good forced air systems don't run ducts in unconditioned spaces.



Seriously. 35% is way out of bounds. That's "you replace a 30 year old poor system with a brand new advanced system" type numbers, not an apples to apples comparison with modern tech.

Paul

Have done it using the ADS many many many times and it is generally a 35% less of a loss heating a space radiantly. The simple solution to the arugment is simply to run the Wirsbo House in the CDAM both ways. Which I think I will do.