I was installing a new programmable thermostat, and once I turned the power back on to the furnace it began cycling on and off every ten to thirty seconds. There are four wires, Y, R, W, and G. I used a jumper between Rc and Rh. The wires were labeled correctly because I reinstalled the old thermostat and it worked fine. The fan could be turned on and off manually and if you turned the heat up the furnace would run normally until it reached the desired temperature. Once the temperature was reached it would stop blowing but still cycle the furnace on and off until I cut the power off. The furnace is natural gas.

Any extra/unused terminals on the new t-stat? If so, what are the letter designations? Sorry, but I have to ask - did you read the installation instructions, or just take it out of the package and hook it up?

You may have to set the thermostat to the correct heat cycle rate. Look through your instruction manual and make sure the thermostat is set correctly for your furnace(forced air, steam, gravity, electric, hot water, etc, etc,) you will want yours set to fit your furnace. Also, make sure that the thermostat you installed is compatible with your system. Sometimes a power robbing thermostat will not work with some furnaces. Does your thermostat have batteries?

KC13- There are two unused terminals, H1 and H2. I believe these are for a humidifier and there is no humidifier on the furnace. I did read the instructions and followed them word for word right through all the set-up and trouble shooting sections. I also took the first thermostat back and got a second one to see if this would help. I had the same result's with both thermostats.
Don't worry about asking if I read the instructions because usually I wouldn't but this time I took my time.

mygirlsdad77- The thermostat does have batteries. I tried changing the heat cycle rate a couple of times but nothing changed. I also tried unhooking the A/C which also didn't help. The thermostat says it's for a low voltage system, how would I tell if it wasn't a low voltage system?

Thanks both for your reply's

If you haven't experienced smoke/sparks/flames from the t-stat yet, you have a low voltage system. Still trying to come up with something... that at least SOUNDS smart... :confused:

Can you take some pictures of the thermostat(cover, subbase, etc,)and some pics of the board on the furnace where the thermostat wires hook up. The more pics the better, and the clearer the better. You say when you install old thermostat the furnace works fine. Maybe try using old thermostat again for a day or two and see if it still works OK. Just thinking maybe there is another problem, but it it works with the old thermostat, then we will need to figure out what is happening with the new one. Please let us know what you find.

I was trying to install the thermostat at my mother-in-law's so it will be a few days before I can get any picture's. It's been a week since I re-installed the old thermostat and it still works, it was also working fine all winter.