Help with Forced Air Furnace that keeps shutting off

wildrage

I have an older Westinghouse forced air furnace. I think around 150,000 BTU. My main heating source is a steam system, and this one was added along the way to replace some of the gravity fed heating registers in the house. It's not ideal but it works.

When a call for heat comes in the automatic ignition clicks on, the pilot lights. About 10 seconds go by, and then all of the burners pop on. And then they turn right off, within about 5 seconds.

I took out the pilot/igniter/flame sensor and cleaned it with a nylon bristled brush. Didn't seem to help at all. Didn't look like it was gunked up much.

Can anyone help me with what could be causing this?

ayetchvacker

If this furnace uses a circuit board with a flame sensor the issue may be a poor ground. If so, look for a green wire connected to the metal burner bracket or somewhere nearby. If the metal is rusty or corroded a quick clean up should help. If not it is likely the circuit board. But I would get the advice of a trusted professional before throwing any parts at it. Does the furnace have a control board with diagnostic led’s? Model/serial number?

wildrage

ayetchvacker said:

If this furnace uses a circuit board with a flame sensor the issue may be a poor ground. If so, look for a green wire connected to the metal burner bracket or somewhere nearby. If the metal is rusty or corroded a quick clean up should help. If not it is likely the circuit board. But I would get the advice of a trusted professional before throwing any parts at it. Does the furnace have a control board with diagnostic led’s? Model/serial number?

This unit is old old.  I'm thinking around 1980. Pre LED. Just got it started for about 5 mins, then it clicked off. I replaced the fan control about 5 years ago. 

EBEBRATT-Ed

I would remove and clean the pilot ,ignitor and sensor. Some say us a dollar bill to clean the sensor or fine steel wool

wildrage

EBEBRATT-Ed said:

I would remove and clean the pilot ,ignitor and sensor. Some say us a dollar bill to clean the sensor or fine steel wool

Yea I'm thinking it's gotta be that. I just ordered a new one on supply house just in case. Thankfully this furnace is only in use typically when it drops below 30.  Unfortunately it's 15 tonight so it's a little chilly downstairs. Looks like the weather is breaking for the rest of the week so only another 24 hours of my girlfriend complaining . 

wmgeorge

Fine steel wool on the flame sensor, but I would consider a new furnace before I dumped money into something 40 years old.

wildrage

It's a Mercury flame sensor. I sanded it down with fine sandpaper until it was shiney. 

When I restart the system it runs fine for about 5 minutes then shuts down and the pilot igniter starts up again 

The only 2 things I can think of are the fan sensor or the flame sensor.  Since the pilot igniter engages immediately after shutdown I'm leaning towards that. 

I have a new fan and limit sensor as well as a flame sensor coming tomorrow. 

Could it possible be anything else? 

As far as replacing it, I've had it inspected and was told it's in fine operating order.  Living in an old house I've learned that many of the new appliances I buy are inferior to the old 40 year old ones. No pun intended but I've been "burned" too many times before.  Since this unit is used in conjunction with a steam boiler that heats 75% of the house (the forced air unit in question heats 4 large rooms maybe equalling 1000 sq feet), I figured I'd run it as long as I can. 

Jon_blaney

Sounds like a rollout issue. You have rollout or a senser thinks you do. Observe the burn until it shuts off for a rollout condition. Maybe a draft issue or bad rollout switch. Check the switches while they are hot. Check draft.

Also, new is not always better. Keep it.

wildrage

The more I troubleshoot this the more I think it's a bad mercury flame sensor.  On startup the pilot always lights.  Some of the time the main burner comes on for a few minutes , then turns off.  Most of the time the pilot lights then turns off then relights, etc. Seems like the sensor is being wonky. 

mattmia2

Mercury flame sensors do fail. If i recall it is a switch. I also didn't think replacements were available anymore.

EBEBRATT-Ed

Is it a Baso control? Carrier used to love the mercury sensors in their old roof top units

EBEBRATT-Ed

maybe something like this

https://www.grainger.com/product/40LV68?gucid=N:N:PS:Paid:MS:CSM-2295:TVRYAD:20501231&s_kwcid=AL!2966!10!79714726730294!4583314236888913&ef_id=a572c5c11bf31bc50637a7cd03c25baf:G:s&gclid=a572c5c11bf31bc50637a7cd03c25baf&gclsrc=3p.ds&msclkid=a572c5c11bf31bc50637a7cd03c25baf

JUGHNE

Can you post pictures of the gas valve and pilot with sensor?

The mercury switch pilot sounds about right for the 1980's.

I had one fail and checked the price of the mercury bulb.

The simple solution for repair for the 80+ lady was to change the valve to a standing pilot with a thermocouple....simple, reliable, economical etc, but burns gas constantly. You would probably not see any difference in your bill.

The pilot was there, the thermocouple fit where the mercury sensor was.
Was never called back for a problem.

wildrage

Argh. Got the new mercury switch today and promptly changed it out.

Now the pilot lights and stays lit. It wasn't before. It was lighting....staying on for 10 seconds, then turned off to be relit again.

Now it lights, stays on for 10 seconds, and then I hear a humming from the gas valve...and nothing. Waited maybe 2 minutes..smell faint smell of gas then nothing.

I posted a video below. Is this likely a bad gas valve? Seems weird that it was starting just yesterday, albiet for a few minutes. It's providig gas to the pilot, but the main burners aren't kicking on.

(Turn the volume all the way up to listen)

https://youtube.com/watch?v=gjmTjASvDe8