I have a 25 year old Lennox gas hot air furnace with a Honeywell programmable thermostat that's about 15 years old. The furnace hasn't been coming on consistently and the t-stat display is sometimes normal, sometimes blank, and occasionally every icon on the display is lit up. It's time for a new thermostat. I found that I can trick the thermostat to start the furnace by clicking the mode switch to 'off' and then back to heat. The issue is that the furnace goes into cool down mode and then starts back up again by itself. I can hear relays clicking on and off in the existing thermostat when this happens. I have a new thermostat but won't be able to install it for a day or two. My question is: can the furnace safely cycle on and off for long periods of time?

Should not take 15 minutes max to change it out, why not sooner.

Id replace the thermostat before it causes some other problem that's more costly to repair with the furnace,

Sounds like a T-stat issue. I have a Pro1 stat. Programmable, Alexa, wifi, etc capable. Works excellent!

There's a sensor on the furnace (I think called a thermocouple) that senses the gas flow. This thing gets a thin coating of dirt or ash on it and it malfunctions so the furnace continuously cycles on and off. It can be remove and cleaned with emery cloth OR replaced inexpensively. It seems to occur every couple or few years. Its a common problem so googling your furnace make and model should show you where it is. There's probably even a Youtube video.

Have you tried changing the batteries in the thermostat?

Furnaces cycle on and off due to the heat limit switch.Might have a bad one.Improper cycling abuses the heat exchanger a bit. Not recommended, but in a pinch you can "limp" it by.Change out the T-stat as soon as you can.The old Honeywell chronotherms Sucked!
Also you could have other things going on,hopefully its just the t-stat Goodluck

I had issues with my HVAC (heat pump over 35 degrees, tanked propane under 35) last winter.

Heat flames would fire up (or not) erratically. Replaced heat probe (thermal coupler?) to no avail.

Long story short, I was using the WIFI iPhone app to change HVAC settings.

I abandoned that, and went back to wall mount thermostat (Alexa) voice control.

All good since.

Some thermostats are powered up by a battery. Make sure yours does not have one.

Thermocouple senses the FLAME . . . it generates a small voltage that is used to keep the gas valve open. Yes, the thermocouple does get a coating after years of use . . . and yes, can clean it using emery cloth. However, this sounds more like the thermostat itself . . .

If all else fails, call HVAC contractor ....

Good luck.

If the battery is dead, the thermostat can't signal the furnace, and the furnace won't run.

I agree a dead battery should do nothing. However A low battery will do funny things. When the poster said turning it off and on will fool it for a short time. He should at least verify if there is a battery and if so is it good.

Yes indeed, a battery low on charge does all kinds of strange things when it comes to home heating. If the thermostat has batteries, replace them and see what happens. If there are no batteries, replace the thermostat. Start with the easy stuff.

I have a furnace in my garage. The last couple years, if you bump the temp up more then a couple degrees at a time, the furnace will blow cold air before it reaches the set temp, because it it will trip off a reset on the circuit board. The 1st time it happened, I called the service people. The tech came in, pulled the side panel off, pushed the reset button and watched the furnace cycle through until it shut down on its own, and called it "fixed". I questioned him about it and he told me "As long as it keeps running and makes it to the set temp, after the reset has been pushed, there is nothing that will show up running through the test procedure. I did have him show me where the reset button was. That was a $200 lesson (the cost of the "service call")!

It still does that if I kick the thermostat up more then 2 degrees. I can pull the 6 bolts (I'm down to 3 bolts now) that hold the cover on, shut off the power switch, push the reset, and turn on the power to be sure the furnace burner starts up. After the burner starts, I can put the cover back on, and wait to see if it makes it to the set temp before it starts blowing cold air. If it starts blowing cold air, I redo the reset button. If it has a lot of temp to recover, I might have to push the reset button a couple times. The other day we had to work on my son's plow truck and had to have the garage door open. I shut the furnace off through that process but when we got done, the garage was down equal to the outside temp. It took a little over an hour for the furnace to regain those 30 degrees the garage lost. I had to do the reset button 3 time before it got caught up. When it gets to where it shuts off on its own, it good to go.

I'm sure something is drawing more power then its suppose to, or the circuit board has a problem, but as long as the reset keeps working, I'm not paying $200 to have them tell me the problem won't show up in the test procedure again.

We replaced the thermostat last night and the system is working again.
Thanks to everyone for their ideas.

Happy Holidays to all!!