Furnace keeps clicking on and off without

Question

I have a Tempstar DC90 furnace connected to a Honeywell thermostat. I’m having an issue where the furnace tries to start and making the normal startup sound, then the thermostat clicks on and off, which turns off the thermostat. This cycle repeats endlessly. In the past I’ve been able to turn off the furnace at the power source and it finally starts up to work after fifteen minutes. Today it took much longer maybe two hours to finally work. Here is a video of it happening: https://youtu.be/LCDfXLDoTTQ?si=c9VFLXCijjvmQ9MV

I’m looking for any assistance in troubleshooting this.

Answer 1

Your thermostat is effectively a switch. When the temp is below the set point it connects the two wires which tells the furnace to send heat.

I had a honeywell thermostat like that and the contacts oxidized in between the lcd and control board portion which caused it to blink that like.

You probably just need a new thermostat or to clean those connections. An easy test if you have two wires leading ( you might have more ) to the thermostat is just to hook those together - this will call for heat. The wires will be live typically with 24v so turning off the furnace and then wire nut them together is the safest option.

If it isn't the thermostat then you typically open up the furnace and it shows you a number of blinking lights on the control board which helps you determine what the issue is.