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Our HVAC Fan only blows in 'fan on' mode while heating. In other words the fan will not blow in heat mode if switched to "fan auto". Both fan on and fan auto modes work when cooling. See image below, any ideas?

UPDATE

I also forgot that I had the blower replaced 1 year back and am now wondering if the wiring of that could be the culprit. See this link for information on the original and new blowers.

thermostat wiring

  • Forced-air units need to have the fan on when in "heat" mode. Are you saying that the "heat" function does not work with the fan in "auto" mode? It should automatically come on whenever the unit goes into "heat" mode (usually after a short period of time while the burner is lit and the unit senses that the burner is lit). – Jimmy Fix-it Oct 30 '15 at 20:24
  • Yes and that is what I'm saying. The furnace turns on and starts burning gas, but the fan to blow the air through does not turn on in auto mode – user1463809 Oct 30 '15 at 20:41
  • Did it work with the fan in "auto" mode previously? If you have the fan running in "fan on" mode today, and the thermostat tells it to "heat", does it work as normal? – Jimmy Fix-it Oct 30 '15 at 20:53
  • What's the make and model of the furnace? – Tester101 Oct 30 '15 at 23:35
  • See the this link for make/model information. I also had forgotten I had the blower replaced 1 year back and am now wondering if the wiring of that could be the culprit. See the above link for information on the original and new blowers. Thank you again for everyone's help! I really appreciate it! – user1463809 Oct 31 '15 at 16:15
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Normally, when the fan is set to "auto", if the unit enters "heating" mode the burner ignites then a heat sensitive device sends a signal to a relay which turns on the fan. The fan pushes heated air into the plenum and through the ducts but it also removes heat from the combustion chamber, so if the fan does not start or fails during operation, the burner shuts off to prevent overheating, damage, fire, etc.

If your fan is not coming on it could be a bad heat sensing device or a bad relay (the fan motor and power supply seem fine as you state that it works under other circumstances).

You did not state whether or not the burner continues to operate without the fan running, if this is the case you may have an additional problem if your system is designed (as most are) to shut down the burner upon fan failure.

  • That's not always exactly true. Most blowers come on after a predetermined delay, but only if the flame sensor (and other limit switches) are closed. – Tester101 Oct 30 '15 at 23:33
  • If the flame sensor doesn't sense a flame pretty quickly, it will shut down the unit. The problem is likely in the blower control circuit, but it's hard to diagnose further without knowing what unit it is. – Tester101 Oct 30 '15 at 23:37
  • OP should be aware of the delay, see previous comments... "should automatically come on whenever the unit goes into "heat" mode (usually after a short period of time while the burner is lit and the unit senses that the burner is lit)" – Jimmy Fix-it Oct 31 '15 at 2:09
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if the fan works on auto only, it doesn't matter what brand. It is the heat sensor that tells the blower to turn on. It is not the blower relay as somebody commented above. If it was the relay, it would not work on any position, auto or manual. Both heat sensor that controls the fan and the on switch at the thermostat that controls it manually, are wired to the the same relay that switches the fan on. So your problem is definitly the heat sensor that turns on your fan when furnace heat chamber reaches the required temperature. It could also be loose wire somewhere, but this is very, very rare.

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I had exactly the same problem on HVAC. 1. The furnace blows hot air only in "FAN" mode. 2. When the mechanical thermometer is switched to "AUTO", I could hear the click sound from the unit, however the FAN does not start blowing the hot air at all.

According to the professional, it seems to be caused by the defective heat sensor/thermostat on the furnace unit. The part itself costs about $150.

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