Furnace flame and blower alternating on and off

Problems related to residential installations.

Furnace flame and blower alternating on and off

Postby 1harleynut » Tue Oct 29, 2013 3:51 pm

I have a Bryant Furnace 394GAD from about 1991. About 2 or 3 weeks ago, I had the air conditioning unit replaced and since that time, I had a problem begin with the furnace. When I adjust the thermostat high enough for the heat to come on, the flames come on for about 2 minutes. Then the fan kicks on and simultaneously the flames turn off (the pilot remains lit). The blower runs for about 45 seconds blowing heat through the vents and stops. When the blower stops, simultaneously the flames kick back on. This will continue over and over until the temperature set on the thermostat is reached. Until my A/C unit was replaced, the flames stayed lit with the blower running until the desired temperature was met. I called the company that installed the A/C unit and they said that there is nothing that they did wrong and said I can pay them a fee to come out and find out what's wrong with my furnace. I am wondering if there is anything that they could have done to "coincidentally" have this happen. They installed an outdoor Lennox unit with the coil on top of the furnace.

I've replaced the filter. I tried replacing the limit switch since that was pretty inexpensive and there was no change. The limit switch on this furnace has no settings (no dials or anything to adjust the limits). I also replaced the control board this summer, so it's pretty new. I cannot find a flame sensor on my furnace. Is it possible that there isn't one? Is it possible that they ran any wires incorrectly? From what I have read, it sounds like I should be looking for blocked vents, air returns, etc. but I can't find anything blocked and not sure where all I should be looking. Can anyone point me in the right direction? It's not taking that many cycles to heat the place right now, but I'm not confident that this won't be a real problem once the real cold weather season begins. Thanks for any help.
1harleynut
New Contributor
 
Posts: 3
Joined: Tue Oct 29, 2013 3:42 pm

- Furnace flame and blower alternating on and off

Postby Freon » Wed Oct 30, 2013 2:53 pm

The limit switch isn't the issue, as you now know, because if the furnace were overheating, the burners would extinguish BUT the main blower would still run until the combustion chamber cooled to the point the high limit closed, igniting the burners.

You have a very erratic problem. Why did you replace the control board? Was power turned off to the furnace when the board was replaced? If the pilot remains lit when the main burner shuts off, the flame sensor is okay... the pilot is associated with the flame sensor.

Search in Google for the installation manual - ii394h-50-2 addendum.pdf

It will be on the Carrier web site with the word expedio also in the url.

Read the Sequence of Operation. From what you describe it seems something may be bad on the control board. Try setting the blower (fan) to ON at the thermostat then call for heat. Does the main burner still cycle BEFORE the thermostat temp is reached? Check the control board wiring. The AC guys may have messed it up when the installed the AC unit.
Freon
Most Valued Contributor
 
Posts: 713
Joined: Sat Jun 18, 2011 11:26 am

- Furnace flame and blower alternating on and off

Postby 1harleynut » Thu Oct 31, 2013 12:33 am

I replaced the control board when the blower would not shut off. When I looked at the control board, if I pressed on the CFR, I could see sparks, so I replaced the board. It almost looked like a blown fuse in there that was making contact and the sparks ignited if I caused any movement. This was an old model with the glass cover, so I could see the inners of the CFR. The power was shut off when I replaced the control board.

The pilot does remain lit when the main burner shuts off.

The correct manual I believe is ii394g-25-2.pdf for the 394GAD. I read through the addendum you suggested and it was just some information about the filter, so no help there.

I did try setting the blower to ON and there is no cycling. The burner remains on until the thermostat temp is reached.

I am going to check out the wiring as best as I am capable. If I can't find anything there, I may try another control board and if that doesn't change anything, I think I will probably just live with it the way it is for the time being. Luckily, I took a very good closeup photo of the original control board that I can check the wiring with very easily.

Thank you for the input.
1harleynut
New Contributor
 
Posts: 3
Joined: Tue Oct 29, 2013 3:42 pm

- Furnace flame and blower alternating on and off

Postby Freon » Thu Oct 31, 2013 3:40 pm

"I did try setting the blower to ON and there is no cycling. The burner remains on until the thermostat temp is reached."

That is a very telling fact. I would check the blower motor wiring at the control board. Check any kind of DIP switch settings that may be associated with AC and heating blower motor speed. That is one area the AC guys would have mucked around in since they needed a blower motor speed for the AC. You might try a different speed tap for the blower motor and see if it operates properly.
Freon
Most Valued Contributor
 
Posts: 713
Joined: Sat Jun 18, 2011 11:26 am

- Furnace flame and blower alternating on and off

Postby 1harleynut » Thu Oct 31, 2013 11:22 pm

I checked all of the wiring on the control board itself and everything matches up with both the manual and the photo I took of the original setup. All of the connections are solid also. I haven't noticed any DIP switches or maybe I don't know what I'm looking for. I'll check the speed taps maybe tomorrow night and see if changing those does anything different. Thanks.
1harleynut
New Contributor
 
Posts: 3
Joined: Tue Oct 29, 2013 3:42 pm

- Furnace flame and blower alternating on and off

Postby Freon » Mon Nov 04, 2013 6:08 pm

I wish I had an easy answer... I don't. The behavior is not a high limit safety happening... and I wish I knew what the AC guys did. At least for now you can keep the fan set to on and run the furnace normally. Unless there are other electronics in the circuit, I would suspect the control board. I wonder if the AC guys turned off the power...
Freon
Most Valued Contributor
 
Posts: 713
Joined: Sat Jun 18, 2011 11:26 am


Return to Residential