Furnace blowing cold air, BUT

Ok. The fan limiter switch seemed to be shot, so I replaced it. Even with replacing it, the fan did not work with it plugged in, but if I straight wired the wires, the fan blew. But it blew cold air. With the new and old switch. So then I figured the thermostat was shot.
I replaced the thermostat with a nice Lux digital, and still having this problem. Fan won't blow unless I straight wire the two wires that go to the fan limiter switch, and even so, still blows cold air.

What could this be?

Sorry if it sounds confusing, please ask and I will be happy to answer anything.

 

1) You might have the fan on temperature set too low, causing it to turn on at room temperature.

2) You might have wired the fan into the limit switch. That would cause the fan to run continuously, UNLESS the furnace overheats, in which case the fan shuts off

That can be very dangerous.


So I'd check how the part is set up and wired up with care.

 

I wired the fan limit switch just as it came off. Actually wires are same color so I don't think it really matters...

What do you mean by Fan On temp? On the thermostat? I made sure after I lit the pilot again that the temp was above room temp if that's what you mean.

 

On many fan/limit switches you can adjust the temperature at which the fan is started.

If you have that temperature turned down to 50 degrees, the fan will run anytime the temperature it's in is greater than 50 degrees.


relying on the previous wiring connections as a guide for wiring up a new part is an unreliable means of making connections. You really need to understand what you are doing.


What is the make and model of this part?

 

the one thing I didn't mention is that it's a mobile home. Its a Coleman furnace and doesn't have all of that that you spoke of.

This is my fan switch

No temperature dial.

 

Is the furnace even getting hot enough to turn on the fan? We know nothing other than you saying you replaced the fan-limit switch because the fan did not work unless you hot-wired it. We need to know if the burners are coming on and for how long.

Ha. Just saw your new post. I know all about those thermosdisc thingies from my 25 years of experience with mobile homes. Still need answer to my last sentence above.

Oh. And what numbers are on it? Does it say something like F120-30 or something like that?

 

OK--- you don't have a fan-limit switch which combines two switches into one.

As Ecman suggests, provide the specifications on the switch. If you have the wrong kind that would explain your difficulties.

 

Yes. But the switch that I replaced, matched the same numbers on the old one.

F110-20F

Does that part need to get hot, in order for the blower to turn on?

And the burners do come on. I've let it run for 5, 10 minutes and still doesn't blow air with that switch in.

 

It turns on at 110 degrees, and off again 20 degrees less at 90. So is the furnace burner(s) on, when the fan refuses to come on?

Hmmm. I read the last part of your last post. You could remove it(and take off the wires) and test it with a hair dryer with a thermometer next to it, so the hairdryer blows on both the backside disc and the thermometer at same time. If it does not come on at 110 and higher, then you have a bad new switch. If it does come on, then you may have blockage in the furnace that is not allowing heat to get to the disc properly.

But first, see if you are having 120 volts get to at least one of those wires. You have to have that, for anything to happen. (To test, use voltmeter set to 200 volt scale, and put red test lead on one thermodisc terminal and the black probe to good sheetmetal ground. Do the test for each terminal this way, to find out if you have t least one live wire.) Then what should happen is when the disc gets 110 or more, the switch "closes" and allows the passage of 120 into the other terminal/wire as well. You also could leave off both wires, (!!careful to not let the wires hit the metal furnace, as they are live!!!) and then run the furnace, with the thermodisc switch installed of course, and then test across the terminals with the voltmeter set to low range ohms(200 scale) to see if continuity occurs when it gets to above 110.

Note again that these switches run on 120 ac current, and not 24 volts like a lot of the newer furnace's safety circuit stuff does. So have a healthy respect for what you do.

 

I believe so. When the furnace switch is off, the pilot is lit, (small flame) When I flick the furnace on the burner lights (big flame) lol, don't really know how else to explain it. But with the switch in, the blower does not turn on.

Another thing, is my thermostat. I have central air also, but it's only a 2 wire stat with a heat/cool switch on the furnace itself. So I just wired it in as a 2 Wire heating unit.

 

Ok. I will definitely try this. I thought that MAYBE it could have been a dud new switch but, I didn't want to think that. lol...I will test this out.

 

Make sure you reread my long post earlier, as I added more to the procedures.

Your issue is not thermostat related, unless of course the burner fire goes out prematurely.

 

Just tested the wires going to the switch its at 130... So thats fine. So I'm guessing is its not getting hot enough for the switch to open or close. I turned the furnace on and stuck my finger in there, and theres practically no heat so, is something in there blocking it? How do I go about cleaning or fixing this?

 

Well? Here is something else you could do: If you had one of those thin-rod probe thermometers(1/8th inch diameter x 5 inches long, with 1 inch temp dial on top, that you can buy even in grocery stores, you could put that in there and see what the temp is.

I cannot tell you how to proceed any further, because I do not know how to access furnace parts without seeing the furnace. You will have to try to figure that out on your own. It could be possible that a piece of insulation fell down inside and blocks the passage of heat. Or you have layers of soot/carbon. Or you have a burner pot baffle (if you have a gun-style burner) that is deteriorated and is necessary to direct more heat towards where the thermodisc hole is. Don't know. I'd have to be there. Regarding carbon layer - if you could gain access better to where the fire is, the cleanliness in there may tip you off on if that is a possibility or not. Also, you might have something wrong where your fire is not big enough. Make sure all gas valves are on all the way, including at the meter and gas line serving the furnace.

The thermosdisc is likely mounted on the outside of the heat exchanger, meaning it is between furnace layers where the air going to the thermodisc comes from circulation fan blower air - and if the furnace filter was neglected, you might have layers of caked dust under where the thermodisc mounts. Carefully probe around through that hole, to see if there are restrictions.

 

You can try posting some pictures.

Generally speaking, a furnace is going to get VERY hot if the burners stay on and the fan doesn't operate. I saw one furnace with the outside sheet metal glowing a dull orange when the fan and limit switch weren't wired properly.

How long are the burners operating? Do they get shut off eventually by the limit switch?

Perhaps you are getting poor gas input to the burners, resulting in inadequate heat.

 

Also please post the model numbers I do have a wiring diagram somewhere in my computer once I know your model number I can able post it.

and one of us can able indentify it what the curpit and majorty of Mobile home furance are downdraft unit so there will be at least two thermosat switches in the unit { I know later models have two but earier one were just one next to the blower housing }

Merci.
Marc

 

Its fixed! I took the pilot apart, looked at all the gas lines, and the main gas line had a bunch of residue in it. I cleaned it out, popped it back in, the pilot lit and was much brighter. Furnace is now blowing warm air. thanks for all your help!