Heater continues to run past thermostat set point...

Problems related to residential installations.

- Heater continues to run past thermostat set point...

Postby Freon » Fri Oct 28, 2011 2:53 pm

If you have a volt meter and are handy, you can determine what's happening. First, make sure all power is off to the furnace. For there to be a call for heat at the furnace, 25 volts AC is switched to the W terminal from the R terminal (Rh or Rc possibly) at the thermostat and then to the W terminal at the furnace control board via the wiring from the thermostat to the furnace. If the thermostat switch is working then your problem lies at the furnace control board. If the thermostat switch is not working, you may need a new thermostat.

Find where it is easiest to disconnect the wire to the W terminal... either at the furnace control board or thermostat. Then using your meter, measure continuity (resistance) at the thermostat between the R and W terminals as you move the temperature setting on the thermostat. On a call for heat, you should have continuity (zero resistance). When you move the setting to below the room temperature, you should lose continuity showing an open circuit (infinite resistance). Remember, all power off.
Freon
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- Heater continues to run past thermostat set point...

Postby sjlafs » Wed Nov 23, 2011 4:23 pm

Hi, I think I have the same issue, but wanted to clarify the test case to see if I tested correctly.
- heater continues to run past the programmed or manual setting.
Thermostat - 1f80-224

my -
1. power down furnace, and disconnect W wire from furnace.
2. set thermostat to "heat" and set the desired temp a setting higher than current temp.
3. connected voltage meter to the end of the W wire on the furnace end. Reading jumped to 20's.
4. set thermostat to "heat" and set the desired temp to a setting lower than the current temp.
5. connected voltage meter to the end of the W wire on the furnace end. Reading jumpled to 20's.

What does this indicate (if anything at all)? Did I test this correctly?
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- Heater continues to run past thermostat set point...

Postby Freon » Wed Nov 23, 2011 4:40 pm

As long as you measured the AC voltage between the W wire (from the thermostat... verify that the color of the wire connected to the W terminal at the thermostat is the same color at the furnace) at the furnace and the C terminal at the furnace, then when you had the thermostat set to less than the room temperature and still saw 20 volts, AC, between the W wire from the thermostat (measured at the furnace) and the C (common) terminal at the furnace, then it appears your thermostat is calling for heat when it should not be.
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