Hey guys, my furnace and a/c is shut off right now, but my furnace is making short humming noises, clicking/switching noises and the fan starts up and shuts down right away. It does this over and over. I can shut the power off to the unit and turn it back on and it's fine for a while or a day or two. I had the main circuit board replaced two years ago due to failure. Do I have a bad fan relay or a circuit board going out again? Any thoughts?

It could be a number of things. Can you tell where the clcicking is coming from? Is the fan motor free-spinning when its off? Will it work using only the fan switch? Will it run if you manually start it turning by hand? The circuit board may have a blinking light for self-diagnosis that you can check also. Hope this helps.

The furnace and a/c works like it should. You can manually turn the fan on and it runs. The fan does turn free when it's off. The problem is intermitant. As for the red LED flashing and the clicking sound I will have to wait till it starts doing it again. It has been fine for the last day and a half.

The furnace and a/c works like it should. You can manually turn the fan on and it runs. The fan does turn free when it's off. The problem is intermitant. As for the red LED flashing and the clicking sound I will have to wait till it starts doing it again. It has been fine for the last day and a half.

Hi,

I used to do this stuff for a living. Still do for family and friends.
What is the brand of your unit and what sort is it and how old is it?

Heat pump?

Gas furnace w/AC?
If it is a gas furnace, do you know if it is a HSI(hot surface ignitor) or a spark ignition? (If its a spark ignitor, it will make a tick tick tick sound when first starting.

Electric furnace w/AC?

Oil furnace w/AC?

What type of T-stat do you have? Honeywell?

It's a Bryant gas feed furnace with A/C. The unit is 10 years old. The humming noises, clicking/switching noises and the fan starting up and shutting down has been happening while the A/C and the Furnace has been off this last week. All of the noises and the fan starting up and shutting down happen about the same time. The clicking sound seems to be coming from around the circuit board. If I turn the heat, A/C or the fan on everything works like it should. If it's acting up I turn the main power off, then turn it back on and the unit is fine for an hour or two to several days. The model number is 350MAV036080 and the circuit board Number is HD42FZ016.

Are you talkin about the blower coming on, or the vent fan for the furnace? Maybe its possessed (like in that movie "Christine") :p
To be honest, its probably the circuit board again. If I remember correctly, Carriers and Bryants from that time were literally over-controlled (and over-priced), and thus, prone to strange problems like this. Simplicity was not their strong suit.

Programmable thermostat?
Grasping at straws here but change the batteries and see what happens.

It's the blower fan that's coming on, but only for a second then it shuts down and you hear clicking noises at the same time. My 9 year old daughter thinks it's possessed. The dang thing has does it several times in the last week, but not at all in the last 2 days. I'll try changing the batteries in the thermostat just to be sure.

It's a Bryant gas feed furnace with A/C. The unit is 10 years old. The humming noises, clicking/switching noises and the fan starting up and shutting down has been happening while the A/C and the Furnace has been off this last week. All of the noises and the fan starting up and shutting down happen about the same time. The clicking sound seems to be coming from around the circuit board. If I turn the heat, A/C or the fan on everything works like it should. If it's acting up I turn the main power off, then turn it back on and the unit is fine for an hour or two to several days. The model number is 350MAV036080 and the circuit board Number is HD42FZ016.

OK,

When you run the furnace, make sure your fan is set to auto on the T-stat and not 'on'.
The fan coming on could be your ventor motor. This motor just positively removes combustion gases and runs for a minute or so before burner light. It runs a minute or so after the burners shutdown too. Your unit may not have a ventor motor.

If the furnace does not ignite and won't produce heat, the ventor motor could have a clogged exhaust. There are sensors in that area that detect if combustion gases are not being evacuated sufficiently.
Once I had a home owner with a problem like this. The furnace would click and hum as though coming to life to produce heat after the t-stat normally called for heat. It would start its ventor motor and when the motor didn't get enough airflow to depress a diaphram switch, the motor, after running for a minute or two, would shutdown and the unit would sit there for a few minutes. Then it would try again. It would try 3 times and then go into 'lockout'. When the furnace locks out, that is its circuit board disrupting 24VAC to the rest of the control circuits. When you trip the breakers to the unit for longer than a few seconds, the furnace is no longer locked out until it goes through 3 more start processes.
This customer had a bunch of chestnuts in the PVC vent pipe that didn't fully obstruct the flow of gases out of the exhaust but impeded it enough to prevent the diaphram switch from 'making'.
So you may have a furnace trying to light, not lighting and locking out. Which is reset when you reset the breakers.
Or....
Since you have the furnace off, it maybe the low voltage circuits recognizing that the temperature has fallen below where the T-stat is set at and it is going through the motions of calling for heat.

If you know how to take the stat off the wall, try it. Turn the breakers off when you do that, eventhough its only 24VAC to the stat. You shouldn't feel 24VAC but just in case, turn off the breakers.
Taking the T-stat off the wall will remove ANY automatic switching being done. When you take it off the wall, you will see a T-stat sub base sitting on the wall that the stat actually mounts to. On the sub base, there will be different colored wires visible. (Red, white, green, orange, yellow and maybe black)just leave them alone.
Leave the stat off the wall for awhile. If this cures the problem, then you know its just a stat sending errant signals to the indoor unit and you simply need a new stat.
If it continues, its in the unit obviously. (See what I wrote above about venting)
This may be just self checks the furnace is putting itself through. Removing the t-stat from the circuit will both take away any ability to self start and remove all doubt whether or not its the stat or not.
Some programmable stats get weird and start acting like they have a mind of their own. The best in my experience has been the Honeywell T86.

If you do any work on your own unit, make sure the breakers are off. Just to be safe.

Lemme know whatcha find.......

Mike

If you haven't found the problem check the polarity @ the 110v socket .If your neutral leg is reading over 6volts some thing else that is on that circuit is wired wrong .I've come across this several times and just drove me crazy trying to trouble shoot this .

If you haven't found the problem check the polarity @ the 110v socket .If your neutral leg is reading over 6volts some thing else that is on that circuit is wired wrong .I've come across this several times and just drove me crazy trying to trouble shoot this .

Good point.
This is something that can really be a headache.

How's it going? Have you made any progress? Has the temperature started to drop in your area so that you need heating yet?

I haven't been home much this week to know if the furnace is acting up still, but to my knowledge it's been fine. The temps have dropped and the heat works fine. I did change the batteries in the prog. thermostat several days ago. I'll check the voltage on the ground tonight. I'll working through the weeking (tons of overtime), but I'll have monday through thursday off next week. I should be able to tell then if there is still a problems. Thanks for the ideas. I post more later.

Man, you're running your furnace already? :eek:
My wife would kill me if I turned it now. And believe me.... I'd love to.

Ever since I replaced the batteries in the thermostat the problem still hasn't come up. I think the batteries were about dead.

As far as running the A/C I get my way. I work at a paper mill and the temps are quite hot there in the summer so I like it cool at home. As for running the furnace, we have a house that is fairly new that was poorly built and poorly insulated (built by Jerry's Homes also know as Grubb Homes). So we have to run the furnace alittle earlier than what we would like. Don't ever buy a home from this builder. I can tell you horror stories about this builder. The have actully been banned from building in a city here in Iowa. I wish I would have know about them before my wife and I bought our house.