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HVAC system turning on a few seconds after it shuts off

529 Views 11 Replies 8 Participants Last post by  viper
So my heater recently started turning back on for a second or two immediately after turning off when it hits the set temperature. I have a Lux TX9600TS thermostat that was installed about a year ago, I looked around in the settings but don't see a setting for this or why it would have all of a sudden changed. To be clear, when my heater reaches the set heat it turns off, then within a second or two turns back on for a second and then off again. Anyone have any ideas? Thanks for the help
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What heater is this? Model and manufacturer?
Is it getting another call from the thermostat when it does this?
It's a really old Carrier unit, like 25 years old. It still works great other than this weird issue that popped up recently. I can try to find the model number tomorrow of that would help.
Sorry, your question about getting another call, how would I determine this? Thanks for the help.
Once my old furnace started to act funny like yours, and the problem was dust in the wire connectors. I pulled out each wire one at a time from the junction box (if that's the proper name), blew on them, and replugged them: problem solved. Worth a try.
As a temporary test, consider removing the filter one cycle to determine if that makes a change in its function.
Take the thermostat off the wall and jumper R and W to rule it out. if furnace fires up and stays on, replace it.

I say start with that because it's a cheap no-name style stat, not honeywell/white rogers/ecobee.
Is the furnace actually starting the burner again for a brief moment or just cycling the indoor blower?
My old furnace just wore out after 20 years. I tried to revive it but according to my research, I beat the odds by getting 20 years out of it. It would help to know what model your furnace is, is it gas or electric? If your furnace is gas i can help a little in the trouble shooting process.

If gas there are things like burner soot that can be cleaned out, tons of stuff out there, if you plug in your model # there is likely a video of someone trouble shooting that model. Usually if it tries to start and then stops the computer sees a problem somewhere. Look for a blinking light somewhere on the furnaces computer, it should give you a blinking pattern for a troubleshoot code.

First though check the wires at the thermostat and bypass the circuit as described by user_12345a. Make sure it is not the thermostat or associated wiring causing this problem before fixing a furnace that is not broken.
I think it might be the issue of the age already. Since the heater is kind of old, i'd say this is the first sign toward it reaching kaput.
I think it might be the issue of the age already. Since the heater is kind of old, i'd say this is the first sign toward it reaching kaput.
I would second this. I had a 20 year old furnace start acting up. I did my research online and replaced the induction fan, then hired a pro to come clean the burners and do what they could. It never worked consistently I ended up going two winters without a furnace before replacing it this fall. My new furnace is so much nicer, quieter than my old one.
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Being that old, check to see if it has a fan limit control. It will be positioned usually in the center and just above the burner section and usually in a small box. It will have two adjusters on it for high and low limits. If you do, that is likely your issue and what you are experiencing is not the heater turning back on, just the blower because the limit switch is detecting enough heat after a burn to turn the blower back on.

they can get funky after a while. Usually they do this if set too high on the low limit or "cut in" so the blower turns off a little too early. May roll it down just a touch and see how it does.

Either way, this sounds like a heat limit switch issue.

A thermal switch for these applications is typically designed to have a different cut-in vs cut-out, otherwise it will play this game of "am I hot enough to turn on or not". They might cut in at say 100F, but once on, will cut out at 80F. If it tries to shut off at 100F, obviously there would be conflict.
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