What is wrong?

Wierd situation…

Inspected a 28 year old functioning Day & Night gas furnace yestday. Gave good heat (~55 degrees between return and supply vents).

Turned it off and let it sit for 15 minutes.

Put Thermostat in Cooling Mode to make sure outdoor unit responded to Thermostat…was only going to let it run for a couple of seconds since it was very cold yesterday.

There was no response from the outdoor unit but the fan inside the furnace kicked on almost immediately.

Turned thermostat to ‘off’ after about 20 seconds but the fan kept going. It wouldn’t shut off unless the serviceman’s switch is toggled off.

Now whenever the serviceman’s switch is turned on the fan comes on immediately and there is no electric ignition to the furnace pilot light…so there is now no heat.

I’ve checked the contacts in the thermostat and that seems okay.

The thermostat was NOT set to ‘Fan’ it was set to ‘Auto’ all the time
There were no other ‘fan’ switches that I could find.

The front cover of the furnace was installed correctly and the safety switch functions as intended. When the cover is off and the safety switch is depressed then the fan comes on…when the switch is not depressed the fan turns off.

Bottom line: Fan runs continuously and there is now no electricity to spark the pilot.

What do you think could be the cause of this?

Could be a bad thermostat stuck in the cooling cycle .


That would be my first inclination…but there are other causes, possibly. Need a HVAC tech…

Most AC thermostats have a 5-15min delay before they will start the unite. that cuts down on short cycling. the furnace runs because the furnace has a timed cycle to run through


I will assume this is the one you called me about today…sorry my HVAC Tech Support knowledge is nothing like my electrical knowledge…:slight_smile:

I usually do not test the AC portion when the temps are below 65 Degrees

I will assume the only circuit going to this is for the ignitor and fan blower…because I was going to say check the breaker panel…just in case but since it is gas probably only have feed to fan and ignitor…

I would suggest you call a local service tech…not to come out but to ASK them what they think it might be…you would be surprised at how they would be willing to help…I get electrical question calls all through the day…even when I am Knee Deep in VOLTAGE…so you may want to ask them…but to me it sounds like it is stuck in a mode…

A heat sensing device is located at the burner chamber to turn on the fan when the gas heat gets the heat exchanger heated up. On some units it is like a mechanical spring and gets stuck.

I think you broke it. :wink:

Thanks Doug… I appreciate the vote of confidence!! :wink:

Yes Paul…same one.

I also don’t test the cooling functionality when it is under 60 degrees…I usually just check to make sure the outdoor unit responds to the thermostat. As soon as the fan comes on I turn it off within a few seconds.

The owner already has a technician coming out since there are a lot of issues (including a disconnected exhaust flue) with this and the other heating/cooling system upstairs.

Thanks to everyone!!



Excellent…what always tends to happen ( or it seems that way anyway ) is you have a unit with an issue…no one messes with it and it works…then you move something or check something and it stops working correctly…NOT because you did it…but because no one messed with the TIME BOMB waiting to happen.

Chances are it is a bad switch or relay and it was just waiting to happen and you just happen to be at the wrong place at the right time…I dont think it is ANYTHING you did my friend…and they know the freakin units should be serviced on a yearly basis anyway and always refer to it being checked and cleaned or serviced on a yearly basis in your report to always save you of any issues.

Remember a home inspection is just a snap shot in time…just happen to be THIS units time…I would say since it is getting colder…you JUST may have saved their pipes from freezing IF it broke down after the sale.

Then again…you are in CA…not sure HOW cold it gets their…where I am from…it is 22 degress right now.

Turning it on for a few seconds can do more damage then running it. Also if the condenser does not have a functioning heater built in you can destroy the unite if it is below 55 deg.

My understanding is if it is just for a couple of seconds it won’t hurt it. Am I wrong? How can it hurt it if it is running for just a couple of seconds?

This is why I don’t run it for more than a couple of seconds (literally!).

NO way do I test below 55 degrees for the past 24 hours and then only if the unit has been turned on so the crank case heater could be operating .

(54 degrees sorry to cold to operate . Please chack with the owner as to condition and when it was serviced ).

. Move on to my next step. Works for me .


You’re exactly right Paul… It was a time bomb…actually faulty wiring…

**I just got a call from the owner. ** The HVAC technician says that there were two bare wires that contacted on the outside of the cabinet and shorted out some board inside the unit. Easy fix but he will have to come back with the part.

I referred to the bad wiring job on the exterior of the cabinet in my report, so I feel somewhat vindicated…even though I’m the one that probably moved the wire causing the contact to be made (like you said).


The inside of the compresser has valves and piston rings just like a car but much smaller and very fragile. at cold out door temp the freon turns to a liquid. when the piston tries to compress liquid it bends the valves or destroys the rings.

Good to know Henry… Thanks for the info.