Hello - Just yesterday I started experiencing a constant blowing of cold air from my furnace. So in a sense two problems, the gas is not igniting, and the fan just keeps going, so in my limited knowledge I am assuming these are related problems. I did notice when I was in the basement a couple weeks ago that instead of hearing the usual "click click click" when it was starting, I just heard sort of a muted buzzing sound, but then it started working again.

Anyway, it is a natural gas 394GAW 000125, about 30 years old. About 12 years ago a central air conditioner was added. Currently, I have been unable to turn off the fan at the thermostat, and have only been able to turn it off at the house switch panel. The thermostat was replaced at the time the central a/c was added, it was a chromotherm and is now a Honeywell T8112D1005(1) 0312 PF2B. Based on something I read somewhere (forgot where) I disconnected the green wire at the thermostat, but, the fan kept blowing, so I switched the breaker switch at the panel to off to stop it. I had read that if the fan keeps going with the green wire pulled, this suggests a problem with the fan's relay, or the fan limit control switch. I am not sure where in the furnace either of these components are. I have looked at some pictures/diagrams online, but cannot find them on my unit. I will try to post pics later if that will help. Now, I did find a newer looking limit control switch on the backside of the furnace, it is a White-Rogers 1609-101 Style P1. From looking at things I am all but certain this was added with the air conditioner unit. Could this have simply replaced the old/original limit switch in the furnace, pre-A/C? Or should there be another one still inside the furnace?

Any help here would be greatly appreciated. I am hoping to learn what the likely cause is, things I might be able to test, and/or whether any of this is DIY-able for me. I know the newer furnaces are much more efficient, but replacing the entire furnace for a new one is not really an option for me presently.

 

Sounds like a limit trip. What does the air filter looks like?

Can you get to the evaporator coil to see if it is very dirty?

Some models have a fuse. I would check if you have a popped fuse as well.

 

I did check the filter, and it was pretty full so I changed that. I had forgotten to check it in awhile. For the coil, are you referring to the a/c coil? Would that come into play when trying to heat now? Oh and I am wondering whether the limit switch, the one I found on the outside back of the furnace (the newer one) would have replaced the one inside the furnace which I cannot find now, since the a/c install 12 years ago, or should there still be another limit switch in there and I just need to keep looking for it?

 

I cannot tell you what that is doing in the circuit. It shouldn't be there.

If you stop airflow anywhere in the ducting it will trip a high limit. The grills, the air filter, the blower motor, the blower wheel, the air filter, or the evaporator coil can cause a high limit trip.

 

There are five possibilities with Carrier products when there is blower and no heat.
1. Bad transformer
1. Open limit or rollout switch
3. Open fuse link
4. Bad board
5. Blown fuse

 

Yes that is exactly what I have on the back of the unit, mounted to the side. Since it is newer I am assuming it was added with the a/c compressor was added. But if it shouldn't be there, in my uneducated mind that of course raises lots more stupid questions lol.....

 

I am thinking I need to call a pro, I am not sure how to even find those components.

 

http://dms.hvacpartners.com/docs/100...394G.50.1B.pdf

It would be a great idea to have a 1978 furnace professionally inspected.

 

Yes I agree totally on the inspection. Funny thing now, I had the lower access panel removed so the safety or "kill" switch was open, and after jostling the copper wire from the White-Rogers limit switch, I pushed the safety switch in again for grins and it fired right up. The pilot started, flame started, and within 30 seconds or so the blower started. Turned it back off at the switch panel, re-installed the access panel, and it started right up again. I am wondering if the dirty filter (really dirty, probably 3 years old now that I think of it) caused this failure. I am wondering if there is some other weak link here (besides me, lol).