Furnace turns off but then back on again briefly

[Francheezie]

I have about a 12 year old Carrier gas furnace and a 2 year old Honeywell digital programmable thermostat.

Recently the system has begun this weird behavior of going through a complete heating cycle, the gas turns off, the fan blows for a minute longer, and then the fan shuts off. All is normal, the house is warm, no problems. But then about 15 seconds after the fan turns off, it turns back on again for about 5 seconds, and then turns off for good until the next cycle.

I suspect the thermostat has to be telling the fan to turn on, so I am tempted to buy a new thermostat.

Any helpful ideas would be appreciated.

[Hube]

You don't say what model of Honeywell thermostat you have, BUT it's more than likely OK anyways.The Thermostat will control the blower for a constant high speed when required,but has nothing to do with your heat cycle fan The Fan, when in the heating mode is controlled by a "sensing" (fan on/off) element witin the heating unit. The setting in this control can be adjusted to eliminate annoying override "on/ offs". Simply lower the setting down a touch Heating fans are best to come ON at approx 125 degrees, and shut OFF when the bonnet temp nears 95-90 degrees. DO NOT adjust the HIGH LIMIT control, it should be around 180 degree setting. Hube

[tom12345]

Hube
I'm having the same problem, furnace turns off then kicks back on for 4 seconds. (Carrier model 58dp075ga)
I just replaced the limit switch(part# HH12zb190. Only $6 bucks so it wasn't to bad of a "didn't work" try). That didn't help.
There are no settings on this limiter switch for off/on(degree settings). It's just a small round part that sticks into the furnace. I do know what you are refering to because I have seen them available for other furnaces though.
My next guess that is in line with the wires for the limit switch is part# hh11az230A. I have no idea what it does but it looks like the next easiet part to replace. I'm thinking it's also a heat sensing device that would kick the motor back on if the temp is to hot in the furnace housing.
Tom

[tom12345]

Well that part(hh11az230a) I was thinking of replacing next is called a "vent safety limit reset switch(230 degrees)". Does anyone have any ideas if replacing this part(if defective) will stop the furnace from cycling on for another 4 seconds after shutdown?
Thanks
Tom

[Hube]

TOM 12345; Unless the limit switch is incorporated (highly unlikely)with-in the fan control, the problem seems like it could be the fan control. The one that cost you (lucky) $ 6 is a 190 degree set HIGH limit. DO NOT change any parts your not certain of, or you will "fix it good" alright. Some units have FIXED settings with no adjustments. Make sure yours are the adjstable type before altering. Read the above reply to Francheezie, it may help you too. Hube

[tom12345]

Thanks Hube..
I'm looking at the paperwork right now that came with the furnace and it looks like there in NO adjustable fan control.
If there is none would that part(fan control) be incorporated onto the control board?
The only thing I can guess is the heating fan rely has a time delay circiut in it???
Geez I hope this isn't going to be a major headache..
Tom

[Francheezie]

Looks like I have a Carrier 58DP050-BA and a Honeywell MagicStat CT3200. I have not looked into replacing any parts yet, but thanks to both Tom12345 and Hube for the feedback so far. It is logical that the furnance is turning the fan back on because it senses a high tempurature. I will look into it further today, focusing on the furnace and not the thermostat.

[Hube]

Francheezie; On the Honeywell model CT 3200 there is a setting for the time adjustment of heating cycles. On the old (years ago) one there is a A and a B screw. TO have a longer more efficient cycle A is out 1 turn, and B is in. On the newer model of ct 3200 there are "dip Switches 1 is on and 2 is off. If you have a manual for the thermostat, check it out. It could solve the problem. Hube

[Francheezie]

I installed the Thermostat set with screw A out and B in originally to go for improved efficiency. Yesterday I tried it with both A & B in, but there was no difference in the fan behavior. The fan coming back on used to be a once in a while thing, not it seems to be all the time and I just heard it come back on two short times in a row. So this problem seems to be getting worse.

[Hube]

As i informed you in my first reply to you, have you checked the setting on the FAN CONTROL? This is the ONLY control that can turn the fan on in the HEATING mode. The only other way is on the the thermostat "FAN ON" slide lever(bottom left side) Post back with info on the control settings. They are inside the unit casing. Hube