Hi guys, I have a forced air Tempstar propane Furnace with a weird problem.

It starts pretty normal:

The thermostat tells it to turn on and the main blower for the house vents starts up. Shortly later the exhaust blower starts and the burners ignite as normal. Then it gets weird.

The burners shut off and on every few minutes, but the main house fan is blowing the whole time for what seems like a normal duration of a cycle. But again, the burners go off and on and off and on every few minutes, seems like 6 or more times, until the house is up to temp and finally everything shuts down.

So in other words the whole furnace isn't shutting down, it is blowing into the house through the ducts the whole time. Just the burners are on and off till it is up to temperature. I think it is set for 3 start tries before it will give up, but as I said the on-off happens more than three times.

Is this short cycling?

I had the tech out and he cleaned off the flame sensor (I think) reinstalled, then he jumpered the two wires (Red and White) coming from the t-stat where they connect on the furnace then started up and it did NOT short cycle while jumpered. It worked fine.

We thought he'd fixed it so away he went with only a 15 min call charge. I thought I'd made out great! (I was scared it might be the limit controller $200+!) But then it started acting up again. I should would appreciate any pointers y'all have!!

Scott in Belchertown MA

I read the FAQ and should add this is not a split system and I changed the filter.

Also because the short cycling didn't seem to happen while the tech jumped the R and W wired at the furnace, would that make you think it was the t-stat? Can a t-stat cause a burner to short cycle up to 8 times during a heat cycle?

Can a t-stat cause a burner to short cycle up to 8 times during a heat cycle?

Yes

Thanks Hvac1000! If it were short cycling due to the limit controler, would it only do so three times then shut off, or could it keep going up to 8 times?

I've changed the battery in the t-stat (VERY OLD) and cleaned all connections I could find inside the t-stat, and it is still happening.

How can I be sure it is the t-stat? Will this procedure tell me? The thing is, the furnace starts when it is suppose to, so I know the t-stat sends the signal to start, I am just unsure how to test if the t-stat is causing the short cycling?

If I jumper the red and white terminals- not the wires, but where the wires hook-up inside the blower compartment, (the set screws), and then turn on the furnace and it runs with no short cycling, would that tell me if the t-stat was the issue?

I really appreciate this hvac1000!

I should mention it is a digital t-stat a Hunter Set-n-Save I.

Update: I jumped the red and white wires at the thermostat, turned on the furnace, and it continued to short cycle. Though, it seems random just how long the burners will stay lit before they go out and then relight. Sometime they stay on for only a second, other times, they stay on for a few minutes. And the longer burn usually happens toward the end of the cycle.

I am scratching my head. I should ad again this is not a new install. The furnace has been working for years.

-I don't think it is the roll out as I see no flame roll out to activate it.

-Blower too slow? If that then why does it sometimes stay lit longer than others. I don't hear it slowing down or speeding up.

- Limit Controller? Could that fit my symptoms? (Somewhat random lengths of burner off and on.) I have one of the sealed Limit Controlers, a little black box, so I don't think I can adjust it.

Lastly it probably means nothing, and I mentioned to the tech who was here, but I hear a sizzling sound every so often when I am close to the furnace. As if some one dropped a drop of water on a frying pan. Pretty random intervals.

If you jumped the thermostat out of the circuit the furnace should run all the time. Now that is taking into consideration all the registers are open and the return air filter is clean and not obstructed. This is what should happen on a properly installed furnace.

In your case it is possible the fan and limit control or just a limit control might have become weak allowing the unit to shut down or cycle on and off. If all else is correct then I would feel the limit control might need attention.

Thanks you again! If the fan were getting weak, and it might be, can I up the fan speed. (If not already on high, I haven't examined it.)

Will speeding up a weak fan sometimes fix it triggering the limit controller? Are there any adverse effects from speeding the fan up? (Other than a tad more power consumption!)

On this furnace it is a matter of moving a yellow wire to a higher speed spot.

And the filter is new, all registers are open and the return is clear.

(On a side note, while the fan was running, I pulled out the filter about 5 inches to see if it would help, it didn't, but I was kind of surprised I could even pull the filter out at all while running!)

The furnace limit control is what I am talking about. On older units it is called a fan and limit control since they are in the same housing. If it is weak or out of calibration it will need to be replaced.

Now you might try speeding up the blower motor to see if that helps but usually if the furnace has run for years with the motor speed the way it is then usually speeding up the motor is not the cure.

Very good. Thank you kindly! You are right, the motor has run for years at this speed.

I guess if it can't hurt to speed it up I might try it! But I'd guess it is the Limit Controller at this stage.

I don't know whether to buy a new one and do it myself, or call the hvac to be sure it isn't something else (can it be anything else?) before I spend the cash. Heck it is only like two wires and 2 screws!

Most importantly though, are we in danger from a short cycling furnace like this? Or is it just annoying and wears the parts faster, but not life threatening?

are we in danger from a short cycling furnace like this

Well the furnace could stop working then you could get cold

It is best to replace the suspected part.

Did I understand you correctly that if you jumper it at the thermostat, it continues to cycle, but when you jumper it at the furnace, it runs normally? If that is the case, I would look to replace the old wiring and the thermostat. Seems like you isolated the problem by taking the thermostat out of the circuit. And did you say that when the tech jumpered at the stat it worked normally, but when you did that it failed to work properly. Am I missing something?