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Mixing radiators and radiant floor heat
I asked this over on Gardenweb but realized I would probably get more detailed info on efficiency, etc. here.
We are just starting work on a 800sf addition (2 floors @ 400 sf each) on a 1938 brick colonial in the Washington, DC area. Existing home is 2 floors on a 26x30 footprint so I think about 1500sf finished area. We will also finish the basement (unfinished now) and do some interior remodeling along the way, but I'm not going to worry about heating and cooling for that at this point. The digging for the crawl space for the addition began today and I know I should have already made these decisions.
Currently we have an older gas boiler feeding traditional cast iron radiators for heat. We love the quality of radiant heat (we have central air for the cooling side) and have made the expensive decision to keep radiant heat instead of just toss it all and put in heat pumps. Along the way, we decided to use Runtals for the radiators in the new addition and will probably replace the radiators in the main home with new runtals as well since we have to remove a lot of the existing pipes to clear room for the basement remodel and they take up less space in what are already relatively small rooms. Our HVAC guy also recommended going to all Runtals to keep it simple in the long run and free up space.
One of my questions relates to water temperature for the old rads and the new runtals, which is something I meant to ask the HVAC guy next time we get together but I would love input from all you experts as well. Even though the existing walls in our home are probably only something like R3 (brick, block and plaster) the old radiators only got hot in extremely cold weather and even then you could still put a hand on them and not feel they were too hot to lean on, etc. So maybe they could be run at the same temps since the Runtals will be in rooms with R21 in the walls and R38 in the ceiling. Any thoughts on the value of running Runtals mixed with traditional radiators vs. just Runtals are welcome.
In the last few days, we've also begun considering radiant floors in the addition instead of Runtals and then either Runtals in the main house or just use the existing traditional radiators in the main house. I expect the floors and rads (either type) would definitely have significantly different temperatures. The new boiler spec'd at this point would be a Lochinvar Knight high efficency boiler with an indirect 65 gallon hot water heater.
My questions about radiant floors relate to complexity and efficiency in a mixed system, since there is no way we would take on putting in radiant in the existing house. How much complexity (and cost) does running different temperatures incur? I understand there would need to be a mixing valve. Does this require a lot of fine tunning and expertise?
In a hybrid system of radiant floors and radiators, do you reduce the efficiency of the lower temperature need of the floors since you need to heat the water to the temp for the radiators and then are just "watering it down" for the floors?
Any thoughts are welcome, especially since I know I'll need to make a decision on radiant floors very quickly because that needs to be considered before framing.