Furnace problems...

[Jeff7181]

My girlfriend's furnace is acting wonky. Occasionally it seems like it will fail to light the burner and the blower just keeps running for hours and hours but it never lights. Sometimes, turning the furnace off on the thermostat for a few minutes and then back on gets it to start up and actually light and work properly for at least that one cycle.

I replaced the filter at the suggestion of her father, who thought maybe there wasn't enough airflow for the burner to light. (the filter needed to be replaced anyway)

Tonight I'm going to watch the cycle and see if I can see anything out of the ordinary, but I'm thinking it could be a problem with the ignitor or the control board.

Any suggestions on a quick diagnosis? I'm not looking forward to another night of sleeping in a <60 degree F room.

 

[edro]

A common problem is a blocked exhaust or vent pipe.
They get blocked with snow, ice or dead animals and trip the heat stat or air flow sensor.
Depending on the age of the furnace, it should shut down the entire system (fan included) and flash a trouble light.
If it's older, maybe the fan would still run as you said.

 

[JM Aggie08]

Could be that the burner is turned up to high, causing it to fail? We had to dial ours back a bit before it would stay on and actually heat the furnace, rather than just blow air through the system.

Might try resetting everything. Switch off power to the furnace, shut off gas(assumption), wait, then re-enable everything and try again.

 

[webdave]

Does the flame come on at all? If so check the flame sensor. There should be a piece of metal where the flames are. Take some sand paper and clean it. I do mine every year. Before I started I had the same issue. The furnace would click on and back off. It didn't think the flame was hot enough.

 

[coaster831]

Another potential culprit is the limit switch and/or fusible link. This is a component that shuts down the burner, but keeps the fan going (to cool the unit down), if the internal temp gets too high. If that component fails, on some furnaces the fan will run continuously without lighting the burner.

 

[Booty]

Does the flame come on at all? If so check the flame sensor. There should be a piece of metal where the flames are. Take some sand paper and clean it. I do mine every year. Before I started I had the same issue. The furnace would click on and back off. It didn't think the flame was hot enough.

I've had this happen with the furnaces in my last two places - definitely check this as it's a super easy fix.

There should be a little window in the furnace you can look through to see a diagnostic LED - count the flashes, then look on the inside of the furnace cover/panel or check online to look up the code. Should give you a pretty solid idea of what the problem is.

 

[Jeff7181]

The problem is intermittent, and when it does light it has no problem heating from below 60 to 70 degrees+ in an hour.

The problem seems to be worse when the temperatures get colder. For example, for the past couple days the lows have been in the teens or higher single digits at night and it was working fairly reliably. About 3 days went by without an issue. Now that the temperature dropped below zero overnight, it had problems lighting all night and this morning. I finally got it to light before I left for work in the morning.

It's a strange cycle that this furnace takes... you hear the click from the thermostat when it turns the furnace on. The blower turns on. If the problem happens, this is the end of it - the blower remains on for hours and hours and it never lights and just recirculates air. When it works properly, it lights, the blower turns off for about 30 seconds and then starts back up and blows warm air until it reaches the set temp and turns off.

 

[DrPizza]

I didn't think the blower kicked on until the plenum reached a certain temperature - that is, the burner burned for a few seconds or so, then the blower kicked on when the plenum reached a certain temperature. Later, the flame kicks off, but the blower stays on until the plenum cools to a certain temperature. I haven't done much work in many years with furnaces, but I'd think it's related to the temperature limit switch or whatever it's called.

 

[Jeff7181]

I didn't think the blower kicked on until the plenum reached a certain temperature - that is, the burner burned for a few seconds or so, then the blower kicked on when the plenum reached a certain temperature. Later, the flame kicks off, but the blower stays on until the plenum cools to a certain temperature. I haven't done much work in many years with furnaces, but I'd think it's related to the temperature limit switch or whatever it's called.

That's how mine appears to operate at home, so I was surprised to hear the blower come on before the furnace was lit.

 

[nickbits]

possible problem with:
flame sensor
hot surface ignitor
backdraft lockout

usually the furnace control board will have a diagnostic led or something that will let you look up why it is malfunctioning

 

[stormkroe]

That's how mine appears to operate at home, so I was surprised to hear the blower come on before the furnace was lit.

Silly question, but are you sure it's the blower motor you're hearing? High efficiency furnaces have smaller blowers on the burner exhaust and they are the first thing to start. It's to evacuate the exhaust of any potential built up gas.
T stat closes, exhaust blower starts, ignition rod heats up, gas valve opens, flame heats flame-proving rod, ignition rod turns off, plenum pre heats, blower motor engages, t stat opens, gas valve closes, blower cools plenum, exhaust fan runs for a while to evacuate exhaust pipe (usually 2" PVC these days).

 

[edro]

What type of fuel?
We had problems with propane flow at sub zero temps.
Propane regulators can freeze up as well, preventing flow.

 

[Blain]

The furnace circuit board should have some type of blink codes that will narrow down your troubleshooting.

One thing that snapped my Trane furnace back from the exact problem you're describing was to disconnect and reconnect every electrical plug on the board.
They all seemed tight to me, but it got the system working again.
About a year later I ended up replacing the main circuit board for $90 from ebay. It's been working just fine every since.

 

[Jeff7181]

I'm sure it's the blower, because air flows through all the vents in the house (condo).

Fuel is natural gas.

I didn't do much troubleshooting because it worked reliably yesterday evening and all night. I did, however, turn it off and back on to watch the HSI. It doesn't look dirty or damaged and glows nice and bright. I suppose I'll just have to wait for it to malfunction again before it's worth troubleshooting further.

 

[Blain]

Posting the brand and model might be helpful.