Furnace won't shut off-Reznor F 50

[PharmerB]

Hi there....

I've got a Reznor F-50 Furnace in my garage that is not receiving the signal from the thermostat to shut down when the desired temperature is reached. It works fine turning on but when temp is reached I can hear the thermostat "click" and the digital display 'ON' turns off but the burner will stay on until I turn off the main gas supply. The thermostat is brand new. I replaced the old one with a new digital. Any thoughts?? Thanks in advance

[Jim Draper]

I'm not sure if this will help or not, but I had a similar problem last year. I installed a used furnace in my house and a new digital thermostat. It work fine for a while, but then one day the furnace would not shut off. There is a switch on mine marked "heat, off, cool". I turned that switch to the off position, and it turned the furnace off a few seconds later. I turned it back on a while later, and haven't had a problem since. Also, my father-in-law had his furnace get stuck on, also with a new digital thermostat. He said it was because the thermostat wasn't level. He leveled the thermostat and hasn't had a problem since. I'm new on this forum, but let me know what you find out.

[PharmerB]

Thermostat is level.....turning thermostat to"OFF" does not stop the furnace. I welcome any ideas??

[RegUS_PatOff]

did you have the same problem with the old Thermostat ?

[PharmerB]

Yes, problem started while original thermostat was in. I did repalce it with the same type with manual dial on the front. When that didn't do anything I tried a digital because they were on sale and I thought  it may have better control. 

[RegUS_PatOff]

[user=58004]PharmerB[/user] wrote:

... the burner will stay on until I turn off the main gas supply....

When the Thermostat should turn it OFF, if you then turn OFF the main electrical supply, then back ON,

does the Flame come back ON ?

[PharmerB]

When the thermostat is turned off and I light the pilot, the burners flare open as soon as I turn the gas knob from "PILOT to ON" even though the thermostat is not calling for heat. Could it be that the gas valve responsible for suppling gas to the burners is getting stuck open. I don't know the mechanics of it so I don't know if that is even a possible situation.

[RegUS_PatOff]

try it with the power disconnected (house circuit breaker)

[PharmerB]

When I switch the breaker off, it only kills the furnace fan. the gas burners are still going full open. Thermostat is 24volt so not connected to the house 120V.

[RegUS_PatOff]

Isn't the 24v Transformer on the same Circuit Breaker ?

 

[PharmerB]

Thermostat is 24V, 2 wire to the furnace. It is powered by AA batteries with a lithium back-up. There is no power going into the thermostat from the house

[RegUS_PatOff]

ssoo.... the lithium battery is 24v ?

[PharmerB]

good question? There are no power wires going to the thermostat and when I kill the breaker it only affects the fan on the furnace. The thermostat still has battery power and the gas burners still burn.

[jb8103]

The thermostat should only function as a switching device for a 24 volt control circuit in the furnace, powered by a 120-24 step down transformer in the furnace. The batteries in the thermostat are probably to power the digital display. Leveling the new t-stat should not be critical because there is no mercury switch in the new digital t-stats.

The main gas valve should be controlled by a relay energized by the 24 volt control circuit. When the 120 volt power is shut off, the transformer will be de-energized, and the relay will in turn de-energize, and the gas valve should shut off.

I am only a first year student in HVAC and not familiar with this furnace. But here is a link from Reznor with the F-50 installation and operation manual listed:

https://www.rezspec.com/index.php?pageid=000000000014&mod_files[currpos]=000000000002

Find the wiring diagram for the F50 on page 26, Figure 30, if I am not mistaken.

Hope this is not a bum steer, because I'm green. I'd suspect that gas valve relay, but the old hands here can set matters to right if this is the correct manual and schematic.

Best of luck.

[jb8103]

Well, the link did not resolve all the way for some reason, but should take you to a page with an "installation manuals" link. Click that, and find Models F&B in the list.

Hope I'm right about this.

BTW, can anyone tell me why the 120V fan control closes on rise? It should open on rise...

[PharmerB]

OK.... When furnace is running and I flip off the breaker and unplug the unit from the wall (I know it shouldn't matter but want to make sure no power going to furnace)the fan stops but the gas bunners are still going full on. It even seems that the flames get bigger with lots of orange flames. I don't know if this is just because the fan is not on?? If this means the relay is not "de-energizing" and shutting the gas valve off would I have a defective relay or would the gas valve be the culprit?

[RegUS_PatOff]

sounds like a stuck Gas Valve..

while the power is OFF, give the Gas Valve a small rap with a screwdriver handle...

and/or see if you can disconnect the 24v connections at the Gas Valve ..

[PharmerB]

Tried that...same thing. When I light the pilot and turn the knob to 'on' the burners either flare open right away (with thermostat off) or on the rare instance then don't open then I am able to turn on the furnace with the thermostat but when desired temperature is reached I hear the stat 'click' off and the furnace 'click' but the flames are still burning strong. I will see if I can get a pro to install a new gas valve.

Thanks to every-one for their sugestions and comments

I will report back if that solves the problem.