furnace locked out


  #1  
Old 11-30-09, 02:42 PM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
furnace locked out

Hello
I replaced the thermostat in my house, but since then the furnace is locked out.
it's a Bryant gas furnace, 4 year old.
I tried to turn it off and on , but without result.
Do you have any idea ?
Thanks
 
  #2  
Old 11-30-09, 03:24 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Upvotes: 0
Received 1 Upvote on 1 Post
Turn the thermostat down to a low temperature.

Take the cover off the furnace so you can see the burner area.

Turn the thermostat up. This should start a sequence of events that would end with the furnace producing heat.

Describe what you see happening when the thermostat is turned up.

Also, report the model of the Bryant furnace from the rating plate in the burner compartment of the furnace.
 
  #3  
Old 11-30-09, 03:38 PM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
Hello
thanks for your response
I tried what you suggested.the thermostat clicks when I turned it up (temperature set goes over the room temperature) -> the furnace shoud start but it doesn't.
The LED on the furnace still flashes the lock out code.
The model number is 310 AAV

Thank you very much
 
  #4  
Old 11-30-09, 04:03 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Upvotes: 0
Received 1 Upvote on 1 Post
I'd use an AC voltmeter to measure the voltage between the R and C terminals on the furnace, and between the W and C terminals when the thermostat is turned up.
 
  #5  
Old 12-01-09, 10:04 AM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
so I measure the voltage between R and C before turning up the thermostat : 18mV
and between W and C after turning up 320mV
is it the transformer that shot?

thanks
 
  #6  
Old 12-01-09, 10:36 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Upvotes: 0
Received 1 Upvote on 1 Post
What model Bryant furnace do you have? That will be listed on the rating plate for the furnace in the burner compartment.


The 18 volts is unusually low. That might be a problem, although probably not with the transformer.

Did you get the 320 MV read after turning up the thermostat?

With the thermostat turned up to 80 degrees or whatever, what AC voltage do you get between R and C and W and C?
 
  #7  
Old 12-01-09, 10:51 AM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
The Bryant furnace I have is a 310 AAV
I get 320mV between W and C after turning up the thermostat above room temperature, the thermostat says that the heat is on.
I have not checked voltage between R and C when thermostat is set above room temperature.
it should be 24V right ?
Thanks
 
  #8  
Old 12-01-09, 11:01 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by herve View Post
I have not checked voltage between R and C when thermostat is set above room temperature.
it should be 24V right ?
Thanks
It should be, yes.

The 320 MV indicates that the thermostat and/or wiring is not switching the voltage on to energize the W contact and the rest of the furnace.

You can use a wire to connect the R and W contacts directly to find out if the furnace turns on. Making that connection is what the thermostat and it's wiring should be doing when the thermostat is turned up to call for heat.

If that switches the furnace on, I'd inspect the thermostat wiring for any defects to the extent that you can.

Also, try shorting the thermostat wires together at the thermostat to see if that turns on the furnace reliably. If it does, it suggests that the thermostat wires are OK but that the thermostat is defective and not switching on the furnace despite the click and a symbol turning on saying that it is.

But you want to avoid replacing the thermostat only to discover that you have a bad thermostat wire.
 
  #9  
Old 12-02-09, 09:40 AM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
I checked the voltage on the PCB (on the Thermostat terminals) between 24V comm and the ground (G) , I'm getting only 300mV.
There's a 3Amp fuse below the terminals, could it be blown.
what about the transformer ?
If one of them are shot, do you know where I could get the replacement parts ?
The voltage I'm getting at the thermostat are the same that I'm measuring on the PCB.

Thanks
 
  #10  
Old 12-02-09, 10:14 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Upvotes: 0
Received 1 Upvote on 1 Post
Since you have a fuse, it's probably the fuse that's blown.

On furnaces without a low voltage fuse, a short circuit usually burns out the transformer. So you are probably a transformer ahead and a fuse behind.

Remove the fuse and check it --- usually it's visibly burned out if it is, but if it looks good, check it for continuity with a multimeter.

If it's burned out, there usually a reason for that, and about 90% of the time it's a short circuit to ground on the thermostat wiring, which often is caused by the thermostat wiring being cut on some sheet metal it is passing over.

You can use your multimeter to check for continuity from the furnace sheet metal to first one side and then the other of the thermostat wires. I'd disconnect them from the furnace, and it would be a good idea to disconnect the thermostat so that you are testing only the wires for continuity between the wires and the sheet metal of the furnace.

If you do have a short, check the thermostat wiring carefully until you clear the short, then test the wiring again to verify that the wiring no longer is shorted to the furnace cabinet.

Fuses are usually available at auto supply stores. Take the old one with you.

Put the new fuse in and connect a wire between the circuit board R and W terminals to simulate the thermostat and verify that the furnace starts up properly. If so, you can reconnect the thermostat and thermostat wires and you should be done.
 
  #11  
Old 12-02-09, 11:59 AM
dun11's Avatar
Member
Join Date: Feb 2009
Location: Local
Posts: 795
Upvotes: 0
Received 1 Upvote on 1 Post
I agree with Seattle, more then likely the fuse. While all he says about low voltage shorts is true, if you replaced the stat without turning off power thats probably what did it. I would get a few fuses to be on the safe side.
 

Last edited by dun11; 12-02-09 at 12:00 PM. Reason: forgot something
  #12  
Old 12-09-09, 12:58 PM
H
Member
Thread Starter
Join Date: Nov 2009
Location: CO
Posts: 6
Upvotes: 0
Received 0 Upvotes on 0 Posts
thanks guys,
that was indeed the fuse.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: