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Old 09-15-16, 06:35 AM   #19
stevehull
Steve Hull
 
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
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Let me get this back on track . . .

Oklahoma has a mild winter, with a few windy cloudy cold days. Mostly, it is a sunny winter with days in the 40s and low in the 20s. It is not at all like SE Michigan as I have lived there for seven years (East Lansing) and Michigan has a much colder winter (Lansing, MI ~ 6700 heating degree days). Oklahoma City has 1/2 that or only about 3300 degree days.

The problem is that a "heated" floor, where the floor "feels" heated, requires a floor temperature in the upper 70s to low 80s F. If you do that in a mild winter climate, then you can overheat a well insulated home.

A second issue, with a "heated" floor is the thermal flywheel effect. The home is heated to an air temp of 72 overnight against a temp difference of about 40 F (outside temp in the low 30's). Then day comes and the outside temp goes into the 40s or 50s F with a lot of sun. House heats up as thermal inertia, plus low heating need plus sunshine heats house up to 80 F or so. Too hot.

The above are not conjecture. I have seen several home in the Dallas Ft Worth area (just south of OKC) that have experienced this. Reluctantly, the expensive heated floor system was turned off.

Part of the problems with the homes in the DFW area was that too large a heated mass was used. This was four inches of concrete on top of a crawl space, with 2 inches of foam board below concrete with PEX tubing in the concrete (no ground contact). Despite my recommendations to only use 2 inches of concrete, the builder used four inches. WAY too much thermal mass. Once heated up (they also used high 70's floor temps to "feel" the heat on feet), it took days to cool down. Cooling down the house at mid day meant opening windows in mid winter or running the AC . . . crazy.

Because OKC has a dominant summer, and thus need for air conditioning, there remains the need for that cooling need. Trade offs are present with mini splits vs central ducted heat pump system, but there is still the availability to provide some winter heat with a heat pump with some heat from radiant floor.

So I have decided to potentially use a "moderated" floor where it is perhaps 72-74 F and not in the upper 70s and to potentially use a two inch concrete insulated slab (again above a crawl space or basement). But before this is a well designed envelope with a maximum need for about 36K BTU/hr (3 ton multi stage) heating system. In fact, I already have this 3 ton two stage open loop GTHP system operating.

I am completely remodeling existing house.

The problem is not the engineering of the system, or modulating it so that even and consistent indoor temps are observed - it is the cost! Even with doing all my own labor, it looks like the costs of PEX, manifolds, water pumps, floor thermostats is a couple dollars per sq foot. The cost of a two inch concrete pour is actually small compared to this ($0.70/sq ft for materials).

Bottom line - so it really a good economical situation to do radiant floors throughout a home in the south when you have a short and mild heating system?

Decades of consulting has shown me that well intentioned people will put a perspective WAY ahead of an rational economical solution. I have seen people put in R 100 walls/ceilings (OKC area) because they "think" that this will keep them cool and warm - and yet not pay much attention to air infiltration . . . .

I have appreciated the many posts, but the issue seems to be economics - and the wife. She hates cold floors and she hates wall to wall carpeting (as do I). So it will be a lot of bare clean flooring, but perhaps electric radiant heating in just the bathroom will keep her happy. In there, I can heat that small floor area to 85 F!


Steve
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consulting on geothermal heating/cooling & rational energy use since 1990
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