The blower on my HVAC runs sometimes when it should be off

Question

The blower on my HVAC runs sometimes when it should be off. It does not run all of the time.

The furnace is in the attic and the abnormal running seems to be related to the attic temperature.

Observations:

When the attic is not hot, all functions work properly, the blower is on when it should be on and off when it should be off.

With a hot attic (109 F), the blower was running with the thermostat in HEAT mode, fan in AUTO, and the set point 5 degrees below room temperature.
The blower continued to run when I switched the thermostat to OFF mode. The blower shut off overnight.

With a hot attic , the blower was running with the thermostat in COOL mode, fan in AUTO, and the set point 15 degrees above room temperature. The blower shut off overnight.

Question:

Is this normal behavior? If not, how do I determine what is causing the blower to run when it should be off?

Equipment - Installed 1981

General Electric natural gas furnace model BLH080E936G0

General Electric 5-wire thermostat model AY28X092 3AAT50A2B55I

Honeywell high limit and fan motor control L4064E 1223
Settings: high limit 205 F, Fan ON 120 F, Fan OFF 82 F

@ThreePhaseEel Wiring Diagram

Answer 1

That is actually a Trane/ American Standard furnace I believe. The cause is most likely the limit switch is getting so hot it is opening causing the fan to turn on to cool the limit switch. They often will open around 130-145 and yours may be a little weak.

Answer 2

The behavior of the blower seems to be normal, nothing malfunctioning.

The hot attic is causing the the temperature in the furnace to reach or exceed 120 F. When the temperature in the furnace reaches 120 F, the Fan Switch - Thermal , (FST on the wiring diagram) closes and turns the blower on . The blower continues to run until the attic cools overnight. When the temperature in the furnace falls to 82 F, the FST opens and turns the blower off. The blower will turn on even though the thermostat is not calling for heat. The blower will also turn on with the thermostat in Fan AUTO, or in COOL mode with the set point above the actual room temperature.

The FST is part of the heat mode portion of the blower wire circuit and is in the High Limit and Fan Motor Control device. The device measure the temperature in the furnace. The Fan Motor Control is set to turn the blower on at 120 F and off at 82 F.

Note that the blower could also turn on if something causes the Fan Relay (F on the wiring diagram) to switch to COOL mode. I can not tell from blower speed which mode is powering the blower. This blower operates at the same speed, independent of the mode.

I was able to confirm that that the FST and not the Fan Relay is turning the blower on. I removed the cover plate from the High Limit and Fan Motor Control device and observed the dial movement and blower behavior at various temperatures. I observed that the blower turns on at 120 F and off at 82 F. The Fan Relay makes a click sound when it switches modes. I did not hear any click sounds from the relay when the blower turned on or off.

The high-limit switch, TCO-A on the wiring diagram, is not in the blower wire circuit. This switch is a safety control and is in the Automatic Gas Valve (AGV) wire circuit. The switch is located in the High Limit and Fan Motor Control device. It is set to open if the furnace temperature reaches 200 F. At 200 F, this switch will open and turn off the gas by closing the AGV. In normal operation, the blower will keep the furnace temperature below 200 F.

Answer 3

It's definitely the fan limit kicking in

A metal box inside a hot attic (which is what your furnace is) could easily reach 120°F inside it on a hot day, and when the fan limit switch feels that, it doesn't know that it's the fault of the conditions outside the furnace, so it turns the fan on to cool the furnace's heat exchanger off, thinking the burner must have fired up. With your furnace, this is expected behavior under these circumstances. However, this is also a sign of something: your furnace was never designed or intended to be roasted in a stifling-hot attic the way it is right now.

As to why this is happening -- the fan limit is closing, and then power goes through the first set of normally closed contacts on the fan relay, allowing the second set of normally closed contacts on the fan relay to select heat speed on the fan motor.

Answer 4

I had this same problem - hot air blasting from the vents with neither heat nor cool turned on. I had to break the wire to the fan switch and insert a SPST toggle switch in that circuit. I mounted the switch in the side of the furnace case and flip it off in the spring and on in the fall. The more elegant solution would be to insert some kind of thermostat on the furnace case that opens at about 110 degrees. I guess the furnace designers expect folks to be running the AC anytime the attic temp is this hot. That would keep the plenum at room temperature, not attic temperature.