I have a Honeywell CT500A thermostat. I set it at 68 degrees. the room temperature is 62 degrees. The unit cycles on and the "flame" icon shows on the screen, but then the unit goes off ( the screen goes blank). After a few seconds, the same thing happens. The heater never goes on. Is the thermostat broken?

It may be time for a new thermostat.

sounds like the cycles per hour needs to be lowered in the setup menu. do you have the manual?

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More than likely, the thermostat on the unit is going bad. If the coils haven't been cleaned recently, then I suggest that you unplug the unit, clean it well and then cycle it back on. If it continues, then the thermostat that's located within the unit is going bad. Please refer to the owners manual for proper service and maintenance for your unit. https://www.appliancefactoryparts.com/content/pdfs/140265-1.pdf

Yes there is a one degree tolerance, in fact my heater just kicked on and it is set to 67 and the inside temperature reads 67. It is also a Honeywell.

It's a programmable thermostat. When you change the temperature, it will say temporary until the next programmed schedule starts or a set amount of time. You will have to program the thermostat to the correct temp settings you wish to have, or you can push "HOLD" after changing the temp to make it stay at that set point. Hope this helps!

Here's a manual for your unit. https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-0690.pdf Here's another link to trouble shoot your problem. http://archives.hvacmechanic.com/archives/228

You didn't include a model number, so it's hard to know for sure - but I'll take a stab at this one. Many of these t-stats have a programmable offset to reduce "hunting". This can be as little as 1 degree or more than 2 degrees.

This means if you set the offset to 1 degree and the room temperature to about 65 degrees, the furnace will heat the room until the t-stat registers a rising temperature of 66 degrees. The furnace shuts off and will not turn on again until the t-stat registers a falling temperature of 64 degrees. The average is 65 degrees, and prevents the furnace from starting and stopping numerous times instead of just once with the 1 degree offset.

You can further reduce the number of furnace start and stops by increasing the offset to 2 degrees. This would work like this when the t-stat is set to 65 degrees and the room is cool:

Room Temp Furnace
63 On
64 On
65 On
66 On
67 On
68 Off

The room is now warm and heat or furnace is off. The room begins to cool like below:

67 Off
66 Off
65 Off
64 Off
63 Off
62 On

With a 1 degree offset, the room temperature varies +/-1 degree (about 2 degrees total). With a 2 degree offset, the temperature about varies +/ -2 degrees (about 4 degrees total).

Also, when the thermostat shut the heat off, the heat that was made is delivered to the space instead of wasted by sending outdoors - so the room will get a little warmer even though no fuel is being burned. The same holds true at the other end. When the t-stat turn heat on, fuel is burned, but there's little heat until the source has reached the optimum temperature and then the heat is delivered to the space.

I hope this helps & good luck! Pleas rate my reply - thanks!

what it sounds like happening is the unit is realizing it is falling behind most thermostat s have a 3 degree deadband if they fall 3 degrees or more at a time after running for so long then it will bring on the emergency heat which is located in your inside unit... when it gets really colds and you see this you need to go outside and look is the outside unit still running and is it freezing up on you...also if running check the larger of two pipes make sure the large pipe is hot to touch...based on the description you have given the thermostat is calling for emergency heat but for some reason is not bringing it on or there is a problem with the emergency heat.... hope this helps out some
repost with any additional questions...

You did not make it clear if the thermostat is a programmable unit or not. I am wondering if the thermostat is not set on the correct heat anticipation setting. A heat anticipater is a small heater that helps the thermostat turn off before it over shoots the desired temperature setting. If the thermostat is set for a 5 degree setting, it will turn off at 63 degrees if set at 68. If the anticipater was set at 1 or 2 degrees, it will shut off the heater at 66 or 67. Newer thermostats will only allow the thermostat to cycle 6 times per hour so if you never reach the desired temp, you have to wait 10 minutes before it will allow a call for heat. Check the installation instructions to see if there is a setting where you can get the anticipator temperature closer to your set temp. Good luck.

thats becaue you need more than this zone working like you said about the master/slave this zone is a slave.