With the change in weather, i switched my thermostat to Heat last week and my blower was not coming on. After troubleshooting, I found that the furnace kicks on just fine, but when the board tries to kick the blower on, it doesn't start. I tried switching the thermostat to both Auto and On for the fan, and neither worked.

I ended up taking out the control board and replacing two of the relays on the board. One of them, after tearing it open, was definitely burnt out. Based on the wiring diagram and schematic, this relay definitely controlled the 120 to the blower. After replacing the relays and re-installing the board, my blower started working on the Heat setting, as well as when I just want the fan to stay on for circulation. I thought I had fixed the issue.

However, this morning, I've started having the same issue I had last week. It's weird that the replacing the relays got it to work for 3-4 days, but now it won't.

The control board green light is flashing 3 times, which looks to be a Limit Switch Open failure. I think that is just happening because the furnace is kicking on and heating, but the blower never kicks on, so the furnace gets too hot and shuts off, giving me this error.

Looking at the wiring diagram, the AC and Heat wires go through separate circuits to the fan. AC seems to be the high setting and Heat or just keeping the fan on is a low setting. I'm thinking there's something on that low setting circuit that must be causing an issue. If I turn my AC on, the blower kicks in perfectly.

Any thoughts? I was trying to see if I could troubleshoot and/or fix this before having to dish out money for a tech to come out.

Can you post the wiring diagram?

Reposting a different pic:

Reposted.

I'd start with replacing the limit switch. It could be faulty, which is a common enough failure.

It's not part of the cooling circuit.

I don't know where that is. I didn't see anything when I pulled the door off to see the fan, except for the capacitor.

I can't see one from that picture, but use the plug (circled in green) to find it.



Find the red wire, which should be on the bottom left corner in this orientation, and you should be able to follow it to the limit switches. Not sure if it's one module or two or three separate components.

Here is the other side:

This looks like a limit switch circled in green. Looks like it has a manual reset? You might be able to push that red tab on the bottom, or maybe it is a cover for a switch that can be pressed to reset it. If you continued to trace the wiring, you'd find the others.



I'd also check your filter for good measure.

Appreciate the idea. I will check on that tomorrow and see if that fixes the issue.

I replaced the filter 4 days ago, though I waited longer than I should have on that one.

Also, it's not just my heat that won't turn the blower on. The blower doesn't come on when I simply set the fan setting to "On".

DallasTeleAg said:

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Also, it's not just my heat that won't turn the blower on. The blower doesn't come on when I simply set the fan setting to "On".

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This is the hard part of troubleshooting over a forum. Best you can do is eliminate variables, a small number at a time. There are a lot of unknowns in terms of how your circuit board uses those inputs, if the thermostat is sending a signal correctly, what the state of your limit switches are, etc. An actual HVAC guy could maybe tell you how that fan circuit is logically operated, but they would still troubleshoot in a similar fashion.

If I had to guess, the "fan on" setting still uses the heat terminal on the circuit board to run the blower. The easiest, cheapest, and most likely culprit is to check the limit switches, especially given the 3 flash error. There are three wired in series (Burner 1, Burner 2, and Primary Limit). I'd bet all of them have manual resets.

If not that, I'd be checking the "Speed Select Relay (K4)" next. It may not be closing a contact on the heat/fan only speed side of the circuit. It's unknown if the "fan only" setting uses the same contact as the "heat" setting.

I would check to see if your thermostat is sending voltage on the G wire in all three settings (cool/fan only/heat).

If you have a manual, or a model number number to find the manual, they also usually have a good troubleshooting matrix that will tell you the likely culprit based on the symptoms. Otherwise, maybe an HVAC expert will come along with the insider knowledge on a few of these details.

Gotcha.

Yeah, the K4 relay is one of the relays I replaced, and it was definitely burned out. However, it would be strange for the new relay to burn out so quickly, unless the fan is trying to pull too much voltage or something.

DallasTeleAg said:

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Gotcha.

Yeah, the K4 relay is one of the relays I replaced, and it was definitely burned out. However, it would be strange for the new relay to burn out so quickly, unless the fan is trying to pull too much voltage or something.

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That actually could be the cause (pulling too much current, not voltage). If the motor has failed and you have a shorted winding or similar, it would likely blow the relay or at least a fuse somewhere upstream. Not uncommon for blower motors to go bad eventually. I've replaced several.

Yes, current, my mispeak.

That would at least answer why replacing the relay got it working for several days before it started having issues again.

Make sure the small rubber tubing coming from the dP switch to the exhaust duct is not clogged.

Water will condense in the galvanized exhaust duct and oxidize with the galvanized material. This white liquid will run down the inside of the duct, fill up that tube and connection to the proving switch inside the furnace. So the proving switch does not think the combustion air/exhaust fan is on and the safety will not allow the HVAC fan to start nor the natural gas valve to open.

So the combustion air fan will come on, but the Natural Gas valve will not activate and the whole system will just shut down. Then it will try and start again, but the house fan will never come on.

Pull off the rubber tubing and shove a pipe cleaner down the tube. Also, make sure you clean out the bung on the switch inside the unit.

https://dengarden.com/appliances/Everything-You-Need-to-Know-About-A-Furnace-Pressure-Switch

Also, this thread
https://www.doityourself.com/forum/gas-oil-home-heating-furnaces/508759-carrier-weathermaker-9200-error-31-questions.html

3rd picture down, the silver 'disk' looking thing is the pressure switch. There are two tubes coming out. The top one goes to the exhaust duct to sense pressure in the duct and knows the combustion/exhaust fan is running and will allow the system to start.

EDIT TO CLARIFY: It is actually a differential pressure switch. It is sensing a difference in pressure between the area where the gas heats the air and the exhaust duct. It will not allow the system to turn on unless is senses a difference in pressure 'proving' that the combustion/exhaust fan is operating. If it just started and fired up the unit and the combustion/exhaust fan was not on, your house would fill up with waste gas and suffocate everyone.

Just for my info, what is the purpose of the limit switch being discussed, i.e. what is it monitoring? Thinking maybe it's the Blower Door Interlock in the diagram.

The limit switches are a temperature element in this case. Basically a safety interlock if it gets too hot (or not hot enough) in the wrong places. Generally, the term 'limit switch' covers a range of parameters where the contacts change state when a condition is reached (ie pressure, position, temp, elevation, etc.), but you know these are thermal because of the symbology used - a temperature element driving the switch.

It is not likely to be the door interlock, which cuts power in all use cases according to the wiring diagram. This wouldn't allow the blower to work in cool settings.

Yeah, i'm having my HVAC guy come by tomorrow. I think there's a short somewhere on the low motor circuit. I don't want to tear things apart myself to get back there.

BenTheGoodAg said:

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The limit switches are a temperature element in this case. Basically a safety interlock if it gets too hot (or not hot enough) in the wrong places. Generally, the term 'limit switch' covers a range of parameters where the contacts change state when a condition is reached (ie pressure, position, temp, elevation, etc.), but you know these are thermal because of the symbology used - a temperature element driving the switch.

It is not likely to be the door interlock, which cuts power in all use cases according to the wiring diagram. This wouldn't allow the blower to work in cool settings.

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I worked for decades in instrumentation and control systems and based on that background, for me a limit switch is a switch to confirm position (open or closed typically). We referenced temperature switch, pressure switch, flow switch, level switch, etc. for the process variables with high or low limits. Hence the question.