tool

I have been noticing the last week or so as the weather has got much cooler here, (It's 12F here) that the temperature has been erratic in my house. (I'm not home much, I rent an older bungalow ((maybe early 70's?)) on a farm site in the country) Seems either too hot or too cold, and the temperature is always much less than what my thermostat is set for.

So tonight at 3AM, freezing cold I go down to investigate the furnace (get this, it is an old "Chrysler" furnace, says 1974 on it) fueled by natural gas.

So I watch it for a bit, the burner comes on, runs a few minutes and goes off but the fan never starts up!

I'm sure this must be the problem. anyone know how to diagnose and fix this??

Thanks for your help.

Geico266

It's called short cycling.

Time to call the furnace repairman. If you don't know what is wrong with it don't try to fixit yourself. Furnaces are not "do it yourself" projects. The money you spend you will save in gas & piece of mind knowing it was fixed right and inspected.

Two people near me were killed last week when the old guy fixed his own furnace.

Maybe be time for a new furnace.

tool

Yeah I hear you Geico, appreciate the advice, but it's pretty dang cold in here! lol

Would love to get it running right ASAP.

Will be calling tommorow for service.

12valve@heart

First off, I am not a furnace repair man nor any other type of appliance repair man so I am not responsible for what you do with what I tell you.

That out of the way, does the fan ever come on? In many applications, the burner will come on and when the temp. sensor reaches a specified temperature, it closes a contact to start the fan. This keeps you from getting a blast of cold air on start-up. After the burner goes off, the fan will continue to run and cycle off and on until the temp. sensor no longer reaches the specified temperature.

In your case, it sounds like the burner is running until the over-heat sensor (not the official term for it) shuts it off because the fan isn't running. It does this to prevent a fire.

The first thing that I would try is to set the FAN setting on the thermostat from AUTO to ON. With the fan running constantly, that temperature sensor may be bypassed and it may allow the burner to continue to run until the thermostat shuts the burner off because of reaching the set temperature.

In the case that I had years ago, I found the temp. sensor and wired around it until I was able to get a replacement. As I recall, the fan started anytime that the thermostat kicked in and shut off when the burner shut off until I got it fixed.

Hope this helps give you an idea of what might be wrong.

tool

Thanks for the advice.

You are right. The fan never comes on, I've watched it do several of these cylces.

On my thermostat (Honeywell) there is no "Fan Setting" only temperature and duration. (It's an old Mercury unit)

I did notice another honeywell box with what looks like a fan blade stamped on it it's about 3"x4" mounted on the furncae right near the burner, there is a white button that says push for manual pull for auto. Neither setting seems to do anything.

berner

Sounds like the motor or fan switch might be shot if it doesn't come on with the switch in the manual position. You could try to turn the motor a bit by hand if there's a dead spot in it. BE CAFEFUL, you don't want to get your fingers caught in the belt if it should start up suddenly.

AlpineRAM

If it's a furnace from the 70s I would go and replace it- modern furnaces are much more efficient.

I second the "Furnaces are not "do it yourself" projects." notion.
Loud "boom" in the night and being handed a harp to play on the cloud doesn't appeal....


AlpineRAM

Chrisreyn

Berner, just in case you didnt know, but most forced air furnaces have a safety switch on the covers.
If you have the cover off the fan WONT come on...its intended to keep peoples fingers attached to their hands..
If you are removing the cover to watch it, and not holding the switch button closed, it will never come on , even if its functional...example, mine is a red bayonet switch that protrudes into the channel the cover sits in..

but I agree with the rest of them, furnaces arent DIY projects at all....

tool

I won't be replacing the furnace unfortunately as it is a rental property and the lease is currently under review. There is a chance I won't be living there soon.

It is actually technically a sublet as the ownder of the property lives in Germany or Italy I believe.

12valve@heart

Keeping in mind my previous disclaimer, here's what I'd do.

On mine, the blower motor is connected to the supply wires with wire nuts. I'd start out with a volt meter to see if voltage is ever reaching the motor. May just be a bad motor. If there is no voltage, I'd unwire and cap off the original supply wires and wire in a plug and hook it to an extension cord to see if the blower ran. (If you don't understand electrical circuits enough to decipher the wiring or the electrical diagram, assuming you still have one, you probably don't want to tackle this.) If it runs, you have a switch or control issue. If the motor is bad, you know what you're dealing with.

For the record, my furnace is an early 70' Coleman and it doesn't have a fan safety switch on the cover. Yes, I know that a new one is more efficient but I'm selling in the next year or so and a new furnace won't improve the value of the mobile home enough to justify the cost.

YMMV. Good luck with getting heat.

12valve@heart

Another quick thought - check for a blown fuse or tripped breaker to the furnace. Lack of voltage on the fan circuit could cause what you describe.

berner

You're right. I had forgotten about the safety switch. Still the same, better safe than sorry.

What I had meant was he could turn the motor by hand a bit and see if it will start up.

Tool, be prepared to replace the furnace. I don't know what the laws are where you live, but up here, if a repairman finds a cracked heat exchanger when servicing your furnace, he's required to red-flag it. You then have a certain amount of time to replace it. If you don't do it, they'll shut your gas service off. Been there, done that when selling my previous house.

tool

Yeah I will check things out witha voltage meter when I get home. Real building boom here now and it's hard to get any service guys out to look at this thing!


If there was no voltage the burner wouldn't cycle would it?

12valve@heart

My understanding is that the burner is controlled by the low-voltage thermostat circuit so theoretically, if the furnace fan and thermostat voltage source were on separate breakers, the burner could work w/o the fan working but I might be wrong. Remember, I'm not a HVAC repairman, just a DIYer. Might also be a tripped reset switch on the motor itself if it has one. Hard to tell w/o getting in there with a meter and figuring it out.

Slice

I am A licenced HVAC person and You need a service call, Way too many variables to consider when dealing with something that is 33+ yrs old. I am not a Diesel Mechanic nor pretend to be thats why someone else fixs mine,..