Limit Switch for home furnace

[Jay Luke]

Since I bought the house three years ago, the furnace has always operated as follows:

- kicks in and blows heat for 30-45 seconds - fan stops for 10-15 seconds - repeats this cycle for 30 to 40 minutes, then the fan and the heating stop - 60 to 90 minutes later, starts all over

When I had my furnace vents professionaly cleaned the other day, I asked the technician what his thoughts were, and he stated that this behaviour is due to a faulty limit switch on the furnace. He quoted a price of $300 plus tax to install a new one.

After researching online and verifying for myself that this was rapid cycling, I went down to the local hardware store and found a new replacement limiter switch for $40, so I purchased it. Installing should be straightforward since the existing switch is exposed and already wired, so I just need to set the start/stop limits on the switch.

How are the off/on temperature settings determined? The default settings on the switch (out-of-box) are: Off: 95 On: 135 Limit 210

I can't refer to the current settings on the existing switch because the tech had moved them around before leaving.

Anyone have any knowledge or advice on this?

Cheers in advance - jay

[Stretch]

Jay,

If your fan cycles and not the burner, it sounds like the air flow is set too high. Not a limit problem at all. The duct cleaner is clueless. It may be that the blower shutoff or turn on settings are too low or out of wack. Try settint the On at 135 and Off at 90 to

100. If it still cycles, set the heating blower speed lower.

If it is 60 to 90 minutes between cycles, even when it is cold outside, your furnace is oversized. It should run almost constantly when at design conditions for your area.

Stretch

[Amun]

Forgive me for asking. But why would you trust a duct cleaner to work on your furnace ???

(avoiding the issue altogether that duct cleaning is often a waste of money)

From what you said the old fan limit switch was working fine but might have simply needed a slight adjustment.

It would help to know if the blower ever worked differently, and just "changed" recently.

Or if any work has been done to the heating system before this problem occurred.

AMUN

[SQLit]

I recommend that you get a manual for the unit and see what the manufacture says. Or hire a licensed pro to do the job. I am in a quandry as to why you would wait 3 years to fix this problem.

[Art Todesco]

There is NO reason to get an answer as you did on a forum like this! This is a Q&A discussion board and that's what you asked and here's my personal experiences.

The numbers you quoted, "out of the box" are probably fine. Usually you can't adjust the upper limit .... is that correct on your unit? On my furnace, the limit switch and the fan switch are 2 physical units. As for the on/off temperature, you can adjust it up or down. If you go down, you will get more heat out of each furnace cycle, however, at the risk of feeling a cold draft from the now cooling down heat exchanger. Even 90 degree air moving can feel cold. My original non adjustable switch in the furnace was supposed to be something like

95/135, however, it, and a new replacement unit from the factory, never even got close to those numbers. I was more like 120/155. I then put > Since I bought the house three years ago, the furnace has always operated as > follows:

[Jay Luke]

Thanks for the advice Art. I'm going to install it with the factory defaults and run the furnace through a few cycles. Basically, the blower should kick in a minute or two after the furnace starts the heating process, and should kick out a minute or two after the flames go out. If there's not an uncomfortable cold-ish draft before and after the cycle, then we're in the ball park. Sound about right?

[Jay Luke]

The cleaner was also a technician, so he also did basic maintenance and a 21 point inspection on the motor; I just threw the question out there to get his thoughts after he had finished.

And yes, after the ducts were "cleaned", I wondered if the whole procedure was even necessary. (but agreed, lets avoid that issue for now)

I ran the furnace through a few cycles with the existing limit switch, and it seems the limit switch is indeed faulty. When the burner begins the heating (prior to the blower kicking in), the limit switch dial slowly starts to rotate to reflect the increasing temperature in the plenum. But the On/Off settings on the limit switch's dial don't rotate with the dial. So the "Off" setting may be set to 90, but the dial turns past the setting. Hard to explain, but the end result was that the furnace didn't stop running when it hit 90.

No work had been done to the heating system before this behaviour started, and as long as i've lived in the house (about three years) the furnace has always run this way.

[Amun]

The whole post now is a bit pointless if you already replaced the fan control. As no one will take it back.

But the limits must be set in a certain order, or you will never get it to work right.

Most have basic instructions in the cover, and that's the proper procedure you need to use. And if it came with any printed instructions, read them and do exactly what it says.

From your original post I still think the original control was working fine, but might have needed proper adjusting.

But there are also a lot of other things that have to be working right too, If it was working right but broke/started acting up, at least we all have a chance at offering a probable fix based on the symptoms.

But when you don't know if it ever was right, after 3 years, the problems may be caused by countless things.

Bad parts, poor system installation, poor system design, wrong parts installed, parts installed improperly/not adjusted properly etc.

Only someone there can check/test each part. none of us can fix your furnace by email

Best we can offer is "reasonable guesses" But even guesses have to be based on the system being put together right to start with.

eg, the problem you originally described could just as easily be a circulating fan turning too fast because of wrong motor or wrong drive belt pulleys or a dual speed fan stuck on high

And the "technician" should have been able to test what the cause was before he suggested "trying this" or fiddling with screws.