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I suspected this issue during the winter and confirmed it now in the summer. Regardless of the temp in my home, the blower motor runs non-stop. In the winter this must have been causing my home to got through a permanent heat/cool cycle as the blower would bring cool air in after the home was at the right temp. Then the temp would get low, the furnace would kick in to heat the home and on and on.

In the summer now it seems to be doing the same thing but with the A/C. Once the home is cool enough, the blower keeps bringing in outside air with the A/C off now. This causes the home to heat up and on and on. The adverse effect at night is it's making my home too cold since the outside air is already quite cool.

I check this question. Why won't my furnace turn off?

I can manually turn off the limit switch and the blower stops.

How can I test whether the problem is the limit switch or the thermostat?

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Is there a fan switch on your thermostat, and if so, what's it set to? –  BMitch Aug 29 '12 at 2:54
    
It's set to auto. –  kareem Aug 29 '12 at 3:21
    
When you say the blower is bringing in outside air, how is it connected to the outside? Is there some type of heat exchanger you're referring to? –  BMitch Aug 29 '12 at 11:32
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2 Answers

up vote 3 down vote accepted

If the high limit switch was bad, the furnace should not fire. When the high limit switch trips, the flame/heating element will be shut off and the blower will continue to run to clear the heat. The blower will continue to run until either the system is reset, or the limit switch closes.

I believe high limit switches are normally closed, and they should always fail open. Which means when they go bad, or the temperature gets too high they open the circuit. To test a limit switch, you'll want to check continuity to determine what state the switch is in.

  • If there is a serviceman switch, turn it off. If not, shut the power off at the breaker panel.
  • Open the access panel and locate the high limit switch. It should be somewhere on the hot box.
  • Disconnect the wires from the switch (don't forget which goes where).
  • Using a multimeter, test for continuity between the leads on the switch.

There is an easier test to check limit switches, but it should only be performed by HVAC techs. So I will not discuss it here.

If the circuit is open, you'll want to replace the limit switch.

If the limit switch is adjustable, make sure it is set properly according to the manufactures specifications.

If the limit switch tests OK; or you've replaced it and still have the issue, you'll want to focus on the thermostat.

From your description, it sounds more like a problem with the thermostat. Most safety mechanisms in furnaces do not let the furnace fire if there's a fault, so if you're getting flame I would not suspect a limit switch. I would suspect either the thermostat has gone bad, or it is miswired. If the fan is always on I would suspect either the fan switch has failed closed (is always in the ON position), or the fan wire was incorrectly bonded to the voltage feed line.

Thermostats are basically switch boxes. They'll have one live wire [R] (usually 24v in the US, or line voltage in other parts of the world), one heat call wire [W], one blower fan call wire [G], and optionally a cool call [Y], and/or C wire. When the thermostat wants to warm up, it will connect the R to W and G which tells the furnace the thermostat is cold and needs heat. If it's hot it connects the R to Y and G, to tell the furnace it's hot and wants to be cooled down. If you have a FAN control and you turn it to ON, the thermostat connects R to G and the blower fan turns on.

The easiest way to rule out the thermostat, is to replace it. Troubleshooting thermostats is a pain in the ass, so it's much easier to simply switch it out.

If you've changed the thermostat and you're still having the problem, call an HVAC tech.

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I lucked out. The switch was simply set to permanently on instead of auto. –  kareem Nov 16 '13 at 22:52
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I have an older furnace that uses the fan/limit dial type device and had the same problem running in heat mode. The burners would cut off at the desired temperature in the house but the blower would run continuously, blowing cold air in the house.

I raised the thermostat setting higher than the house temperature to check if the gas burners would turn on while the fan was running. They did. By the burners coming on, that let me know the limit side was alright. I then thought the fan side must be stuck on. I turned the power off to the furnace and tapped the fan/limit device a few times with the handle end of a screw driver to see if it would jar it loose. I Turned the power on and that solved the problem.

This doesn't mean it won't happen again, but I haven't had any more problems. This is a quick diagnosis before having to test with a meter. My wife noticed it first one evening and it was fixed ten minutes later. First problem with the device in 28 years. Most likely a sign it's time to replace. Not hooked up to an AC unit.

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