60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

[SeaKaye12]

Hi all....

A new development. My ancient furnace that's worked faultlessly for decades has now decided to turn itself off.

I can light the pilot in the normal manner...and with the thermostat turned up a bit...the burners light fine. Then; after a short wait...the blower kicks in and the unit runs fine for a few minutes.

Then; the gas shuts off....pilot light and all.

I can re-light the pilot and the scenario will repeat.

Any suggestions? This furnace is very very basic. The house was built in 1961 and I imagine this was the original heater.

Chuck

 

[dockwrecker]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Try changing the thermocoupler first, (the copper tube heat sensor at the pilot light) They run about $10 at most any hardware store. If they get too corroded the pilot light won't stay lit, and when the furnace hits it's upper limit temp and cycles off, the gas valve will shut down.

 

[Redneck_Randy]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

What dockwrecker says.

Bad sensor. My gas heater was doing the same thing and it ended up being the flame sensor. It was really corroded.

 

[BuzzStPoint]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

x2 on the thermocoupler, Also check any high limit switches. They may be running out of spec. But you'd have to test them when the system trips out.

 

[SeaKaye12]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Thanks for the quick replies

The pilot light will stay on indefinitely if the heater is not on. It's only after a few minutes of running (before the house reaches set temperature) that the gas shuts down.

Is that pointing more to the limit switch?

Here's a wiring diagram.... Pretty basic circuit

Chuck

 

[Mntom]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

First of all the thermocouple (not coupler) appears to be functioning properly, but it still has the possibility of being somewhat weak, replace it anyhow. That way you eliminate one of the simple things. Even if the high limit switch is tripping the pilot should still remain lit. Next, watch the flame and see if the flame changes when the blower comes on indicating a cracked heat exchanger. If it is cracked it is done for but I am leaning toward the thermocouple.......

 

[Lion hunter]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

A few times a year I will get dust build up on my fire sensors. It will shut the furnace down right after it ignites. A good cleaning and it's back in business. Don't know if 60's vintage furnaces had those sensors or not.

 

[greenz]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

i do heating and air for a living. Yes thermocouples do go bad so often. But at the same time, i've found just as many pilot assemblies clogged with ash. Remove the whole pilot burner, and look down where the gas comes out, it will more than likely be full of white ash, usually leaving a hole in the middle where gas will still flow, but between that hole and the outside of the burner, is where the air pulls into the burner to mix with the gas, causing it to blow out in the higher drafts of when the main burner is on. Just take a wire brush and clean it really good, when you reassemble and light it, it should be a good deal stronger of a flame, helping it stay lit through up and downdrafts, and keeps that t-couple good and hot.

Just a side note, the high limit switches should only turn off the main burner, leaving the pilot on to relight the main when it cools off. Do a yourself a favor before going through any troubles/wasting money on parts, get a bright flash light and check every inch of the heat exchanger walls you can see. Cracks will look like a scratch on the rusty surface, or it may be blown out. Furnaces only have a life expectancy of twenty years, even if it's still mechanically sound, you would see a quick payback/extremely lower bills, by going with a new high efficiency furnace. Carrier will be coming out with a 98% efficient furnace by next fall, a good ballpark guess on the efficiency of yours is going to be around 65-75% efficient! I see you're from socal, and i've never been close to california, so i don't know the climate there to know how much you use your furnace. Good Luck!

 

[MTboatguy]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

I would say, check the thermocouple connection to make sure it has not come loose, I know I had an old furnace one time, that drove me crazy, only to find out that the screw in on the end of the thermocouple had worked loose, once I tightened it back up, it worked like a charm for many more moons..

 

[dwco5051]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

greenz is right on the money with his advice. One other possibility is that your combustion fan squirrel cage is so clogged with dirt that your main burner flame is out of kilter and is impinging on the whole thermocouple. The pilot flame should only touch the last third or so of the thermocouple for proper operation. It is also possible that the pressure regulator is fouled up and you are not getting enough pressure when both the main burner and pilot is operating causing a weak pilot causing it to trip out.

 

[SeaKaye12]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Thanks again folks...I really appreciate the input.

I would have replaced the thermocouple already; except that I can't get the old one off easily enough. It's mounted along with the pilot feed tube...it looks like I would have to disconnect that tube as well....something I haven't gotten up to doing yet.

I have observed the pilot flame carefully and noticed something:

When just the pilot is on...it shoots out a strong L-shaped flame...one leg of the "L" reaching the thermocouple and the other leg pointing toward the burners. But; after the burners light...it's as if the flame is getting sucked away from the thermocouple and almost all of it is being drawn toward the burners.

Does that mean anything?

Chuck

 

[Tim Frank]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Given the age of the unit and what may be an uneven service/maintenance history, you might consider having a professional inspect the system.
Could be cheap insurance.

 

[Tim Frank]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

i do heating and air for a living. ....
.....
Just a side note, the high limit switches should only turn off the main burner, leaving the pilot on to relight the main when it cools off. Do a yourself a favor before going through any troubles/wasting money on parts, get a bright flash light and check every inch of the heat exchanger walls you can see. Cracks will look like a scratch on the rusty surface, or it may be blown out. Furnaces only have a life expectancy of twenty years, even if it's still mechanically sound, you would see a quick payback/extremely lower bills, by going with a new high efficiency furnace. Carrier will be coming out with a 98% efficient furnace by next fall, a good ballpark guess on the efficiency of yours is going to be around 65-75% efficient! I see you're from socal, and i've never been close to california, so i don't know the climate there to know how much you use your furnace. Good Luck!

^^^^

Best advice so far!!

 

[SeaKaye12]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Fixed; it was indeed the thermocouple. Took about 10 minutes .

The cause of the pilot flame being drawn away from the thermocouple was also simple: It was happening only because I had the access door (for lighting the pilot) **OPEN**. Duh. I checked with the door mostly closed...and that didn't happen.

Where I live; I only use the furnace a few times a year when it's rainy and cold. Maybe that's one of the reasons it's lasted 50 years. I did have the blower motor re-built once; that's it for as long as I've lived here.

Thanks again for all the replies.

Chuck

 

[greenz]

Re: 60's Vintage Custom-Aire TU-75D Gas Furnace Keeps Shutting Down

Glad to hear you have her fixed, i figured in socal you probably aren't using it too much! My personal system is a Carrier Infinity series unit, and this winter alone it has run twice as much as it did during the summer months (the thermostat tells me my run times) here in ohio.

I would still suspect an ash buildup in the pilot though, even with the door for the burner compartment off, it still shouldn't pull away, flicker yes with the initial lightup, but should stay relatively steady, so if it becomes a problem again, i would definitely check that out!