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Old 01-10-20, 10:08 AM   #15
BillG
Lurking Renovator
 
Join Date: Jul 2016
Location: Minneapolis
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OK. I live in MN, and agree with the guys that say that a radiant floor is the most comfortable form of heat. That said, with a design day of 40 where you live, I doubt that radiant heat would do you any good. Your body won't be radiating to cold inner wall surfaces, especially since you will be doing a good job of insulating and buying good, low-e windows. Just insulate under the floor better than the usual.

Your idea of a little heat pump in each room is a good one. What you might look into is what is called a "VRF" (variable refrigerant flow) mini-split system, where there is an outdoor unit with a variable speed compressor, and several indoor units. The indoor units are typically small, wall hung units with a fan and a coil, that either heat or cool to their own thermostat. I'd look for the better units which have variable speed fans in the indoor units, because they will often be so quiet that you might not even be able to hear them run.

Regarding the heat gain calculations, the poster above is correct that you must consider heat gain through the walls, as well as solar heat gain through the windows. What I didn't see him say is that it is a little more complicated than that, because those two gains don't happen at the same time. To do it right, you must consider your exposure and time of day. Heat gain through the walls is delayed by maybe 4 hours, but the solar gain through the windows happens immediately. The u factor calculation works for all of the walls based on ambient outdoor temp, but there will be more gain because of the sun heating the south and west walls at different times of the day. Your max cooling load might happen around 4 - 6 PM, and might be based on heat gain from the south wall plus solar gain from the west windows. You might consider mitigating the window gain by shading them, using exterior louvers or retractible awnings.

Regarding the suggestion of radiant hot/cold ceiling panels mentioned above, I'd discourage that. As an engineer, I worked on plenty of them, dating back to about 1980 (more than 1,000 hospital patient rooms) and have worked on a number of chilled beam systems over the years. Yes, they are comfortable until they drip on you, but realize that you will be doing a lot of cooling and reheating for dehumidification, which uses a lot of energy. In my experience, you will just disconnect them.

I'm currently thinking through my next home in northern Wisconsin and coming to the same conclusions. I will be insulating well (R55 walls, R45 under the floor slab, and haven't settled on the roof insulation method yet), heating the floor using a condensing domestic water heater (my design day is a LOT colder than yours), and little mini-split heat pumps in the rooms.
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