[Geekfather]

Noticed the other day poor heat from furnace. Being it is 20 degrees heat is nice

It is trying to put some heat out but not able to even hit 60 degrees.

Older Payne 90 gas furnace - no LED on control board for diagnostics. Took the door off and hit the switch manually. I noticed the blower fan (not the inducer fan) would turn on immediately. Normally it should be inducer fan, gas, igniter then blower fan after gas is lit and running.

Is this indicative of a failed thermostat? Probably will need to call a professional in but would like to know up front what I might be looking at and to make sure whomever comes out is straight with me.


Thanks for anyhelp!

Gary

 

[yuri]

Need to check if you are getting 24 volts from W to C on the circuit board with the heat cranked up.

If the fan is running with no burners it could be the limit control is broken from the furnace overheating or a faulty circuit board.

If you have a electrical meter and know how to use it we can walk you thru it. If not call a Pro.

 

[Geekfather]

Have a meter and know how to use it it well.. I also meant to say indicative of a bad control board not thermostat. Can the thermostat possibly not be sending the signal for heat?

Anyways thank you for the assistance


Gary

 

[yuri]

It could be a variety of issues.

However historically when you have no heat and the fan is running on a Payne which is Carrier owned, the limit control is blown open. Circuit board thinks the furnace is overheated and is programmed to run the fan to cool it off.

 

[Geekfather]

Also is there supposed to be a diagnostic Led on this control board? I certainly do not see one. The metal cover that goes over the circuit board shows to use a penny across the self test terminals to test each function.

 

[yuri]

Some of those have a LED but they forgot to put a viewing window in the cover and furnace door.

 

[Geekfather]

I am assuming the limit control is part of the circuit board?

 

[yuri]

It sits above the burners like in this pic.
Brown thingy with a red wire attached to it. Should have a stamp like L170F. which means open at 170F.

 

[Geekfather]

Okay - so what do I need to test measure?

 

[yuri]

Remove the 2 wires on the limit control and check it with an ohmeter. If closed it will read 0 or very little resistance, if open then it reads infinity or OL open line. Or you can crank up the tstat and check if there is 24 volts AC across the terminals. If so it is open. Closed would read 0 volts. However the final test should be with an ohmeter in case you got a bad board or wiring.

 

[Geekfather]

I am about to take off from work to go test. To save time and to report back with my findings. What is considered good bad? So for example if there is not 24 volts at the connector this means what typically (with the heat turned up)

 

[yuri]

If you have 24 volts from W to C on the board with the heat cranked up the tstat is OK. If not it is faulty or you got a broken wire etc.

Then the board sends 24 volts to the limit control and back to the board to start the inducer fan. Some boards have a diagnostic feature which will turn the fan on if the limit is open. Very common with Carrier. If you got no 24 volts to one side of the limit then the board may be bad. If the fan won't stop then the fan relay may be stuck closed but w/o heat that is unlikely.

 

[Geekfather]

Okay just so I understand the procedure:


1. Test voltage between W to C on control board with heat cranked up - does the switch on furnace housing need to be closed? So no 24 volts then need to measure the continuity between w & c back to thermostat correct?

2. If I have 24 volts at limit (one side) but not the other side then most likely the limit is open and bad?

3. I have 24 volts on both sides then good chancethe control board is bad?

 

[Geekfather]

So another quick question. If the thermostat is not sending a signal then will it cause the blower fan to turn on like it is doing?

 

[beenthere]

On Carrier/Bryant, Payne, and other UTC owned brands. The blower will run for 90 seconds when power is turned on while the furnace has a heat call.

 

[yuri]

Check list/procedure:

1) turn tstat all the way down.

2) remove lower blower door and tape the door switch closed with duct tape

3) go upstairs and crank up heat. Like Been~ said it may run the blower for 90 seconds as it thinks there has been a power failure and the furnace was running and is too hot

4) after 90 seconds check for 24 volts AC from W to C on the board. If you have it then the tstat is OK. If the fan is still running after 90 seconds chances are the limit control is open

5) shut off power to furnace and remove wires from limit control and check for continuity across the control terminals

Return here if you need more help as the rest of the checks may not be necessary if the limit is open.

 

[Geekfather]

Alright- On my control board W has a wire and C does not. The voltage measured between them is 24.8 volts which is appears okay. Oddly my limit has 27 volts on either wire.

I am assuming that terminal C is a common ground? So also noticed the system operated properly this time - inducer-igniter - gas- then blower after a pre-warm. I really do not feel any heat still from the ducts - I am assuming it takes awhile to produce heat as it has to push all that cold air from the system. Will update shortly.

I am assuming the temperature set at thermostat is really irrelevant at the furnace. The furnace just keeps running until the temp reaches the desired temp and the thermostat removes the 24 volt signal to control board?

So if after 20 minutes and very to no heat in the air - what can cause this since I can visually see all 3 flame jets working.

 

[beenthere]

Dirty A/C coil or air filter.

 

[Geekfather]

On my system- the return air comes through a hallway ceiling grid - (ranch style house) - comes down into the furnace through an electronic air cleaner then through 2 skinny wire furnace filters (metal screen type) then into the blower. The heat is blown down to the crawlspace through tubing then to each room. Been 20 minutes and still hardly any heat felt through registers. Do gas furnaces use heating elements or something like that? This all happened between Thursday (warm and cozy ) & Friday ( heat started falling off)

 

[beenthere]

The gas flame is the heat exchanger is the heating element.

Check to make sure all of your air filters are clean. That the blower is running at the correct speed. And that your ducts are connected in the crawlspace.