Radiant floor temperature

Jim_76

I am having an issue with the temperature of a radiant floor. We live in a very well insulated house. Not double studs thick but 2x6 walls, with good air sealing. The radiant floor is not hot but is about 4 degrees above room temp on a moderate day (say 40 out) and heats the rooms fine, even a little too well. We had hoped the floor would be warmer. One person who came over to look at the system said the problem was that the water temp to the floor should be 120F (it is 110F) and should be run from a wall thermostat so it cycles on and off more frequently. I just can't understand how this would make a significant difference as the concrete mass keeps the temp from changing very fast. Currently it is just on a timer that runs it for several hours twice a day since the main thermostat controls a forced air system. He claims his FLOOR TEMP is 100F!! and the room is 70F. I find that very hard to believe but he has been in the HVAC business for some time and seems to be fairly down to earth. He also said he had 2x6 walls, etc for a fairly efficient house.

I have seen stated limits on floor temp of about 85F even for tile. Not sure what this limit is based on (material limits, danger to occupants, or ??). I would like to know...

I did run across this interesting document about the issues of using radiant heat in a very well insulated house, which seems to fit our situation very well... (look about halfway down)
http://www.oregon.gov/ENERGY/CONS/RES/tax/Radiant.shtml

Any suggestions (details) on how to use the radiant as the primary heat and the furnace as backup? I think I need to run the fan at above circulate speed (it is variable speed and very low on circulate) to get decent mixing of upper and lower level air to keep lower level from overheating. Lower level is only one with radiant floor. I understand the basic concept and understand electronics. Could even program a microprocessor to do some custom functions if needed but would prefer off the shelf solution for servicing ease. How do I coordinate the two systems?

I am thinking this is limited by basic heat transfer principles much more than any issue with the equipment. I would love to find out I am wrong though. Any suggestions?

ALH_4

Temps

100F floor temp is very high. Temperature sensitive floor coverings can limit max floor temp. Occupant comfort also limits the max floor temp. Warm feet are nice. Hot feet are uncomfortable.

Rather than cycling the thermostat frequently, I recommend using constant circulation with an outdoor reset control. You can minimize any over or under heating by adjusting the heating curve.

Is there any way to get pipes to radiators on the upper level, perhaps through a closet? Fish some pex through your ductwork? ;-)

-Andrew

Brad White_20

85 degrees

You asked, "I have seen stated limits on floor temp of about 85F even for tile. Not sure what this limit is based on (material limits, danger to occupants, or ??). I would like to know..."

The approximate average exposed skin temperature of a person indoors and at rest or light activity is about 85 degrees. (Of course people all vary just as your healthy normal internal temperature is not necessarily 98.6... )

If you are 85 degrees your body wants to radiate that heat, part of keeping balance and ultimately maintaining core temperature.

If you are facing or surrounded by surfaces that are warmer than 85 degrees, you (the average person) will feel overheated.

The only exception to the 85 degree rule (in my design book at least) is in bathrooms, where I might accept 90 degrees. Buck naked and wet from a shower, I can take the heat, you bet!

Hope this helps.

Brad

Brad White_20

Constant circulation -I agree

plus reset and curve control on the RFH.

If the FHA furnace is kept, the challenge will be in that the upstairs does not have RFH, and I the entire house has FHA including the downstairs. How to balance the heating effect between the two if the downstairs also has RFH? No easy way.

I agree, Andrew, the best course for comfort is to make the upstairs hydronic. Nice panel radiators. Fish the PEX.

Jim_76

specific equipment recommendations?

Thanks for the input. Would like to keep the forced air as it has a humidifier that is needed in the midwest winters but will have to start thinking about other options too.

Any specific recommendations for reset controls, radiant panels, etc that have been recommended?

Brad White_20

Recommendations

This is a great age, Jim. So much technology that was once exotic in industry is now commonplace at a homeowner level, tried and proven.

For controls, Tekmar, Taco, Viega, others... check the ad links on this site and ask around.

For radiant panels: Buderus, Myson, Runtal, others. The former two I listed have an option to come with built-in control valves, two discrete tubes (PEX, no less!) connect to each panel and you are done. I personally have Runtal in my house and valves are exposed, separately installed. I like the look, you may not.

Point is, plenty of options. And as Andrew pointed out, you can snake the PEX via the ductwork to a point.

You do see the value of the air system as a humidification medium, so you are in a very good position to go forward.