HVAC Burner goes on & off

[johnmboy65]

My old Carrier gas furnace heats just fine, but does something I don't recall it doing in the past. Whenever the thermostat has told the system to turn on and provide heat, I hear the burner re-igniting several times while the fan is still running. I assume that since I hear it igniting that it must have previously shut off each time, but have not verified this. It used to be that the burner would just light, fan would then start when heat exchanger reached a certain temperature, and burner would stay on continuously until thermostat was satisfied after which it would turn off and fan would continue for a period, then shut off. What would cause the system to now have the burner re-igniting several times during the run cycle, and is this something to really be concerned about? Thanks for any input.

[Edwin Pawlowski]

There is a high temperature limit thermostat. If for some reason the heat exchanger is reaching the high limit, it will shut down the burner until it drops to a lower temperature. One possible cause for that is reduced air flow over the heat exchanger. Have you checked the filters?

[johnmboy65]

I actually recently replaced the filter, but will take a look at it today when I get home...I'm at work now. Could that high temp limit thermostat just be defective?

[PaPaPeng]

Check your fan belt. It may be broken or slipping. The motor will come on and you hear that running. But the fan doesn't turn whereby the heat exchanger overheats quickly and shuts off the burners.

[HVACTECH2]

Yes, it should be checked as soon as possible. it could be one of several things. waiting could cause a major failure.

[Dimitrios Paskoudniakis]

I had a similar problem in my previous house. I found that my condensate drain line needed cleaning. After that the problem was solved. What caused this to occur, I don't know.

[Meat Plow]

High limit switch. Does the air feel really warm and is it blowing with as much force as it usually does? If you don't know what to look for call someone who does because this could be a portend of something failing.

[Zyp]

Check your air filters, check all supply grilles [be sure they're open.] Check the furnace blower for proper operation. If all of this seems ok, call a licensed HVAC contractor. If the high limit needs replacement, let him do it. Most contractors carry general product liability insurance. If you do it, you won't be insured when something happens.

[M Q]

You didn't say anything about the time interval between relights. That information would be useful to rule out or in various poster's suggestions. You also didn't say anything about the age or type of furnace. Does it have a pilot or other ignition source? Does it have a draft inducer (and pressure switches)? Is it a high efficiency, condensing furnace? Again this information could confirm or deny other poster's suggestions.

[DT]

There are often a number of temperature switches in series on older furnaces, so if any one closes it kills the burner. High temperature plenum, high temperature in the flue, flame rollout switch, etc.

[johnmboy65]

The time interval between relights is something I've not specifically noticed...seems like it's around 3 or 4 minutes, but I haven't timed it. The system is made by Amana ( not Carrier as I originally said), uses natural gas and has a pilot. It's at least 25 or 30 years old, so I'm guessing it's not high efficiency. Don't know whether it has a draft inducer or pressure switches. Service guy is due out to the house tomorrow.

[Stormin Mormon]

Cycling on an overheat limit switch.

Be very concerned. Call for service tech.

[clifto]

My problem a while back was a flaky limit switch. Gas (and of course flame) would cycle on and off during a fan cycle, then eventually the switch just went completely and there was no gas at all.

[johnmboy65]

Sure enough, problem was reduced air flow. The service guys removed the squirrel cage blower and cleaned what must have been about 30 years of dust and dirt from the blades. After reinstalling it, plenty of air flow and no more cycling of burner on and off.