I did an experiment with the central heater when I noticed it was blowing slightly cool air. I first let the temperature drop to 67 degrees. Then, I set the thermostat to 68, which triggered the heater to turn on. The air coming from the vents blew mostly cool with a hint of warmth. After letting the temperature level off at 68, I switched off the heater and let the temperature drop back down to 67. When I raised it to 70 degrees, the heater turned back on and the air blew much warmer and eventually leveled off at 70 degrees.

Is there something wrong with my heater? Why does the set point affect how warm the air that comes out of the heater is? Shouldn't the heater always blow with the same warmth regardless of the set point and just shut off when the set point is reached?

  • It depends a great deal on exactly what your "heater" is. Which you have not told us... – Ecnerwal Dec 21 '20 at 13:01

The set point or thermostat is just a switch it has nothing to do with the heat in the system unless a high end one. With that said there are possible multiple speed taps on your blower motor and depending on the setup it could be in a higher speed than needed for heat. Normally we run AC at a higher speed than heating but not always. Some systems are dual stage but a 2-degree difference would not be enough to trigger that if you had it. Providing the model and type of heater you have might give enough information to create a better answer like if it is a heat pump (outside compressor) the air handler inside moves the air a very cold day outside with emergency heat turned off it may not heat as well compared to a gas fired or all electric heater with an air handler. But it is heating, moving air even when warmer than ambient may feel cooler so a thermometer in the duct may show it is a few degrees warmer even though it feels cool. So there can be multiple reasons for your observations both working fine and possibly needing some work depending on the type.

  • 1
    Agreed, Ed. + The OP probably has a 2 stage heating system. The OP said (s)he set it to 70 when temp was 67. For some systems, that's a wide enough gap to trigger 2nd stage heat (whatever that might be in the OP's system), might be aux heat, might just be a higher turn up of a gas furnace, we don't know. And I have found that thermostats "lie" about current temp vs. set point, specifically Honeywell. I don't have space here to get into specifics, but they "lie". – George Anderson Dec 21 '20 at 13:11
  • Oops, one more thing: I have an infloor hydronic heating system and if set point is even 2 degrees off of current temp, the tstat kicks on aux heat. – George Anderson Dec 21 '20 at 13:15
  • Yes George 3 is enough we usually don’t set high unless -3 from set point with a -0 deadband span because it cost so much to run at a tight setting. The hydro is systems I have installed actually are in 1/2 degree zone increments with active cycling because of the lag in hydronic but every one sets there systems up differently I like active loops less variation from the loop length. – Ed Beal Dec 21 '20 at 14:20

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