Troubleshooting a Coleman forced hot air furnace limit switch

Let's just get the disclaimer out of the way first: many homeowners may lack the tools, the experience and the ability to mess around with their heating system. I certainly understand that and agree: I leave most things to the pros.

On the other hand, if you wake up in a blizzard and find yourself without heat as I did this morning, there are a few things that you might want to try before you give up and call for repair.

Even if you can't fix the problem yourself, you may be able to determine at least some information about what is causing the malfunction and that information can sometimes be very useful to the people who will ultimately come to fix it.

As I said, we had a nasty snowstorm last night. It wasn't a lot of snow, but what came down was wet, heavy and sticky. Of course the wind was howling too; high winds are what defines a storm as a blizzard.

We lost power several times over night. I know that because every time the power is restored, our bedside wireless telephone chirps "Press Menu for setup instructions". I heard that at least four times last night.

At 5:30, I hauled myself out of bed. The power had come on again, so I walked to the living room to turn up the thermostat. We keep the heat dialed way down overnight; it is programmed to turn back up at 6:00 AM, but I was up earlier as noted.

I turned it up and the furnace did kick in. I went to the kitchen and snagged a yogurt from the fridge. I had barely got to the drawer where we keep the spoons when the electricity went off again. 

Just a few moments later, it kicked in again and shortly after that I heard the furnace start up. I sat down at my computer and started some work.

Perhaps a half hour later, the lights went out again. This time they stayed out until just before my wife got up - possibly the chirping phone setup message woke her up this time. We both sat down at our computers after eating a banana and a few nuts.

A few hours later, my wife noticed that the house was getting cold. I got up to look at the thermostat and saw that it should have been calling for heat, but the furnace was not responding. I shut the thermostat off and turned it back on, dialed down the temperature and dialed it back up, but got no response from the furnace. I could hear the relays click in the thermostat, so I suspected it was not the culprit and moved on to look at the furnace itself.

The covers on these units are a pain to take off and put back on. I have to do that when I change the filters and it is never fun, so my first attempt at fixing our lack of heat was to open the home circuit breaker cabinet, find the one marked "Furnace", and shut it off. I counted to ten, turned it back on, and was rewarded with the sound of the furnace kicking in.

That's good, I thought, and went back to my work. Unfortunately, it was not good: after the long air purge cycle, which is designed to blow out any lingering explosive gases that might have found their way into the ducts, the furnace did not fire.

Obviously I had a partially working furnace, but it did not want to fire.

Furnace manufacturers and home owners both do not like furnaces that blow up, catch on fire, or leak poisonous gases. That's why the units have various safeguards built in, like the air purge before firing off. 

Many furnaces today have some sort of diagnostic indicator that can sometimes tell you what is wrong. For my furnace, it's a flashing green light and the cover of the control box helpfully tells you just what these lights mean.

• One flash - (Ignition lockout)
• Two flashes - (Pressure switch stuck closed)
• Three flashes - (Pressure switch stuck open)
• Four flashes - (Limit circuit open)
• Five flashes - (Gas valve stuck open)
• Six flashes - wired wrong
• Steady green - working properly

But where is this flashing light? Well, you can see it if you step to your right and look at the side of the control box. It's not particularly easy to see, but you do not have to remove anything else to see it. The peephole is just above the blue wires in the picture below.

If you took off the cover of the control box, you could see the light quite plainly.

In a previous furnace failure, the light had been blinking three times, which means that the combustion air switch failed to close. It was a faulty unit. I did not attempt to replace it myself, but I did let the service company know what they'd need to bring. In this case, they had the proper switch on their truck anyway, so it wouldn't have mattered, but of course it could have.

In this case, the light was flashing four times. That means a limit switch was stuck open. I called the service company and told them.

When he arrived, he showed me how to follow the diagram to find the switches. If you look closely at the wiring diagram on the cover of the control box, you can see that an "upper limit switch" and a "lower limit switch" are referenced.

The diagram also shows the colors of the wires that go to these switches. The tech followed the colors and found that the lower switch was a small circular disk that had only the two wires coming into it. When we followed the gray and blue wires for the upper switch, we found something that had a button on it.

Buttons like that are usually reset buttons, so he pushed it.

I then cycled the furnace circuit breaker again and this time the furnace fired off after its purge.

That was a welcome sound! We let it run a bit and then used the thermostat to have it shut off, waited a bit and turned the thermostat up. The furnace worked again, so we were both satisfied that the issue was fixed.

The next time this happens, of course I will try resetting that switch before I call for service.

While I had heating systems on my mind, I decided to change the batteries in my thermostat. It's easy to forget about these - they draw very little current, so the batteries can last for years. It's a good idea to replace them more often than that, though. As we use rechargeables, I just change them whenever something makes me think about the thermostat - that's often enough that I shouldn't ever have a problem.

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