Furnace kicking on..

Good Morning, I have been having a situation in my own house that is driving me crazy. A few days ago (in the morning) my furnace did not kick on when the programmable themostat called for heat. When you turned the thermostat off and on the furnace then turned on.
The next day it did it and I changed the batteries.
The next day it did it so I changed the thermostat.
It did it again this morning…(new thermostat, new batteries)
It only happens when it goes from the night time setting to the morning setting. When it is running in the afternoon and evening (when I am home) it kicks on every time it calls for heat.

Any thoughts? Am I missing something small or big?


Strange. Next time this happens, check to see if there is 24v present at the R and W terminals at the furnace (where the thermostat wires are connected). (This assumes your furnace uses 24v controls.) If so, then it is not a thermostat issue.

Thanks Michael,
I’ll give that a try, seems odd it only happens in the morning. Maybe I can force it to happen this afternoon.


What a coincidence. My furnace does the same exact thing and it’s only 9 years old.

I figured it out though. When the outside temperatures drop below 10 degrees, my furnace refuses to kick on by itself between cycles. I have to continuously re-set the furnace breaker until the house reaches my desired temperature. When the house reaches the desired temperature, the furnace is absolutely fine.

I had several HVAC techs evaluate this strange phenomenon to no avail. They all keep telling to to replace my thermostat. I replaced it a couple times already and that is not the remedy. So I remedy this situation by always keeping my house at a desired temperature (68 degrees) when I know the outside temperatures are going to drop below 10 degrees. Once the house temperature drops two degrees below what I’m asking for on the thermostat, I’m bumming. I have to keep re-setting the breaker every 25 minutes, until the house temperature catches up with what I’m setting the thermostat at.

Did I Confuse you???

sounds very familiar, but at least mine will run correctly after I reset it in the morning.
I have done some more research, the closest I could find is that it might be the flame sensor. So I took it out and cleaned it with 220 grit sandpaper, it is going fine right now, we will see what happens tonight. It has kicked in three times correctly since I cleaned the flame sensor. I’ll let you know if it is working in the morning, if so you might give it a try, it wasn’t hard (on my Lennox)


Question when was the last time you had your furnace serviced .
We always tell our client to get the furnace serviced and then we try and do our own.

Roy Cooke A happy NACHI Member

This statement implies that there should be 24 volts between R & W on the stat as I read the statement???

If you are going to check for 24 volts at the stat you have to check between R or W and the common side of the transformer as R & W are the same leg of power at the stat.:wink:


David I’m not confused

But it sounds like you have been talking to the wrong HAVC Tech,s

This does not sound to me as if you are having a stat problem. A 9 year old furnace should have a solid state board controlling the unit. It sounds to me by you having to kill the main power to the unit thus resetting the board that you are having a safety locking out the unit and you should probally be looking at your board not the stat.

I use to tell folks that you can not trouble shoot over a phone line and that is still true today???

Tell me what I can do? Do you think that the board replacement is imminent?

David that is a tough question you really need to know WHAT is locking you out what safety. There may not be nothing wrong with the board as it may be doing what it is suppose to be doing locking you out as safeties are installed for a reason Usually if the board dies the furnace will not operate at all. Some of these boards will have blinking lights that corresponds to written trouble shooting somewhere on the unit or the install manual.

Yeah, it does have the troubleshooting blinking light on the board. The clueless HVAC tech told me that the blinking light is normal. I think the HVAC tech just played games with me and actuallly has no idea what the hell he’s doing. Well, I’m selling my house this year anyways, so I think I’ll leave it the way it is and advise or disclose to the Buyers about this situation.

Thank you Charlie.

It might just be getting old. Old furnaces take longer to light, especially when they are colder. Are you sure it’s not trying to light when you say it is not kicking on? My 15 year old furnace takes 15 minutes to start blowing and about 50 sparks from the automatic igniter (there is no pilot light).

No, that’s not the situation at all. When the fan operates the flame operates with no problem at all.

It’s only 9 years old and I’m having cycle problems as I have explained above. It’s working absolutely fine now as the weather is above 10 degrees now.

David, On the newer mid-efficient models the codes for the LEDs trouble shooting guide is usually printed on a label in the lower panel of the unit. They normally have a green or red LED, some have both. eg.
green steady on=normal operating
green slow flashing=initiating stage
red rapid flashing=flame rollout safety switch
red one fast, two slow flashes=flame sensor
NOTE: these are just examples not actual facts…each furnace is different, so check the data sheet.

A furnace guy who happens to be a friend says a lot of his calls have to do with a simple cleaning of the flame sensor which looks like a nail/pin that is located in the middle of one of the burners where the flame shoots out, and when it gets a coating on it, will not allow the furnace to operate. Simply buff or sand with some emery cloth or real fine sandpaper…presto. Most of the time it doesn’t even look dirty or build-up present. It doesn’t cost anything to try and can save a 75.00 service call.

David, if you can give me the following, I’ll try and get an answer about your furnace dilemma. High or Mid efficiency, make, model, serial #, brand and #of gas valve, and if possible the color and rate of light flashes when the unit has tripped or quit.

My buddy is a real whiz with these things and has come across just about everything in his many, many years in the business. I told him of your situation and he has a couple of possibilities that have come up and have nothing to do with thermostats etc. More along the lines of the venting/combustion air and pressure/diaphragm switches. Very tough items to pin point, if you haven’t ran across them before.

Yeah, it does have the troubleshooting blinking light on the board. The clueless HVAC tech told me that the blinking light is normal. I think the HVAC tech just played games with me and actually has no idea what the hell he’s doing. Well, I’m selling my house this year anyways, so I think I’ll leave it the way it is and advise or disclose to the Buyers about this situation.


David if I was sitting in front of your furnace instead of this dumb computer from what you stated that it only kicks off during an extended run time trying to catch up I would first be looking at a limit switch as the guilty party or possibly the roll out switch.

The blinking lights will normally tell you what device failed if you can decipher the codes but you have to observe the configuration of blinking lights before you kill the power to the unit and reset the board.

Isn’t ones own advice hard to swallow… How often have I said, get your furnace serviced regularly…
I’ll see if I have to call in the morning, I cleaned the flame sensor as was suggested and it was working today when I got home. We’ll see what it is when I wake up in the morning.


Thanks Charley and Darrel,

This dilemma only occurs during the freezing temperatures of 10 degrees and below. The furnace is operating absolutely fine within the last few days.

This is what I’ll do…I’ll wait until this auto shut-down occurs again (when the weather gets below 10 degrees).

I will then open the front cover and observe the light pattern. Then I’ll post here (I’ll be saving this post) as to the light sequence and we’ll take it from there. There’s not much I can do until this furnace starts to act up again. I want it to occur naturally verses forcing it to occur, so I can observe the lighting sequence.

This furnace is a 1998 horizontally installed direct vented Concord 80+. It’s installed in a vented closet in my finished attic and receives plenty of combustion air, as the closet attic cavity contains soffit vents, ridge vents, and louvered vents on the side of the sheathed wall.

Let’s wait for colder weather I’ll continue posting here.


Good luck with your issue of re-setting the thermostat. I hope the flame senser was your problem.

Fill us in.

Just as an additional thought about why this stuff happens when it’s subfreezing, if the unit is not exposed to the low temperatures then look to what is.

Gas supply line?

Are we condensing moisture in the gas supply?

As I think Charlie said, you can’t tell anything from here!
When all else fails, change your thinking.
You need an HVAC technician that systematically diagnoses what is occurring, not a parts changer!

Trust me, I’ve had three different HVAC companies over my house to diagnose this continuous problem. Not one HVAC tech (not a parts changer) has figured it out yet. I simply pay them their maintenance and cleaning fee and cross them off my HVAC contact list as being unprofessional.

Maybe the fourth HVAC tech will figure it out. We’ll see.