Radiant heat/Forced Air question

Discussion in 'HVAC Heating & Cooling' started by joeholms, Oct 28, 2007.

  1. joeholms

    joeholms New Member

    Messages:
    7
    How can I find the happy medium between the forced air furnace and radiant heat floor (Wirsbo)? I have exparimented with the thermostats both digital and dial, but neither is the same. It seems that the forced air takes over when the floor should be heating and when the floor is heating the forced air is running too. It does not seem very efficient. One more thing...which is more accurate a digital wirsbo thermostat or the standard honeywell dial thermostat?? If the wirsbo thermostat reads 68 degrees the honeywell dial can read 63-64 degrees. I have a digital indoor/outdoorthermometer and that is completely different as well! I forgot to mention that the thermostats are fairly near each other on inside walls. Any help or thought would be greatly appreciated. Thanks Joe
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,950
    Location:
    New England
    Well, a thermometer (embedded in your thermostat) should read identically in the same situation. I've got radiant and use forced hot air as a backup - the forced hot air does get the air warmer quicker, but the radiant is more comfortable. Except when coming back from an extended time where the house has been turned down, I use the radiant and keep the forced air thermostat turned down a few degrees below when the radiant is. The Wirsbo thermostats deal with a much bigger heat load...they are trying to heat the structure rather than just the air, so when they turn on and off verses a thermostat designed for hot air could be different. For example, the radiant might turn off before it actually gets to the set temp, thinking that the mass will continue to heat up a little as it migrates from the burried pipes to the room. This would prevent overheating the house.

    Shorter answer, I don't know. I'm surprised they show that different of a reading.
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    Let the thermostat for the floor system get its signal from a sensor in the floor and let the thermostat on the wall take care of the furnace. Otherwise, the slow floor system will always be confused by the quick-response furnace.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,950
    Location:
    New England
    Hydronic systems normally don't use floor sensors, at least the ones I've seen. They use a wall-mounted thermostat. I just set my furnace setting lower than the hydronic, and rarely does the furnace come on.
  5. joeholms

    joeholms New Member

    Messages:
    7
    Well, my biggest concern was the forced air taking over the heating responsibility. My question came about on a cold morning when I noticed that the forced air was kicking on every 5-10 minutes. My furnace comes on often, almost too often it seems. I turned on my air exchanger to get some more circulation going and along with the ceiling fans air must really be moving around because the furnace does not come on as often, but even though it is still set 4-5 degrees below the radiant heat(wall-wall mounted) thermostat it still comes on. Maybe there is no solution to my problem... What seems wierd to me is that the two thermostats could be totally different even thought they are 20 ft apart. Anyways thanks for you input it is very much appreciated. Joe
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,950
    Location:
    New England
    Do you know what temperature water is going to the radiant circuits? ARe you sure it is flowing properly and isn't air-locked? If the radiant isn't capable of reaching your desired inside temp, then the forced air would take over. WHen the forced air does run, how long does it run? Short on/off periods are very inefficient.

    A properly designed system will run continuously on the low-temperature design day (say it was designed to keep the house at 70 when it was -10 out; it would run constantly when at that outside temp, and proportionally less as it got warmer outside). Having a system that is significantly oversized means short, inefficient cycles.
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