Furnace help needed

[Bill B]

Here's the story, Tuesday night/Wednesday morning our power went out at least once. Yesterday the furnace was running as I was leaving for work. As I am want to do I reached over and turned the furnace off with the thermostat as I was walking out the door. Later when my wife came home she thought the furnace was on because the main blower was running. After a while she realized the furnace was not on at the thermostat, and she turned it on at the thermostat. Needless to say, even though the blower was running, no heat.
After I came home I started digging into it. I turned off the thermostat and removed it from the base and turned off the furnace at the switch by the furnace. I waited a few minutes then turned on the switch. The main blower came back on. I opened the furnace and lightly tapped the relays on the circuit board, thinking it was a stuck relay. I closed up the furnace and turned everything back on. It worked for one start up cycle, and then the main blower stayed on. I said the heck with it and went to bed. This afternoon I got a new circuit board and replaced it. After replacing the board I had to cycle the gas valve to get the burners working, which I'm pretty sure is normal. Everything was fine for one cycle. I had to cycle the gas valve in order to get the furnace to light for the second cycle. Now the darn exhaust fan won't turn off. The burners and main fan cycle fine.
Any help would be appreciated.
Oh, the furnace is an older Armstrong sx80.
Thanks.

 

[sig-guy]

Here's the story, Tuesday night/Wednesday morning our power went out at least once. Yesterday the furnace was running as I was leaving for work. As I am want to do I reached over and turned the furnace off with the thermostat as I was walking out the door. Later when my wife came home she thought the furnace was on because the main blower was running. After a while she realized the furnace was not on at the thermostat, and she turned it on at the thermostat. Needless to say, even though the blower was running, no heat.
After I came home I started digging into it. I turned off the thermostat and removed it from the base and turned off the furnace at the switch by the furnace. I waited a few minutes then turned on the switch. The main blower came back on. I opened the furnace and lightly tapped the relays on the circuit board, thinking it was a stuck relay. I closed up the furnace and turned everything back on. It worked for one start up cycle, and then the main blower stayed on. I said the heck with it and went to bed. This afternoon I got a new circuit board and replaced it. After replacing the board I had to cycle the gas valve to get the burners working, which I'm pretty sure is normal.

Nope.

Everything was fine for one cycle. I had to cycle the gas valve in order to get the furnace to light for the second cycle.

What's the valve number? SV95**?
Brown turn knob with one plug on the top and one in the side?

Now the darn exhaust fan won't turn off.

Even with no call for heat?

The burners and main fan cycle fine.
Any help would be appreciated.
Oh, the furnace is an older Armstrong sx80.
Thanks.

Model would help. Or a photo of the inside of the furnace.
I'm assuming this unit has the Honeywell Controls System with the SmartValve?
Board would be a ST91**
Valve would be a SV95**

Are you sure you wired everything correctly?

Where are you located?

 

[Bill B]

The furnace model # is GUJ125d14-1b Serial #8494k20291
The board is a R40403-003 which is the replacement for the ST9120A that was in it.
The igniter is an 8600c
The gas valve is VR8204h 1097 brown knob, single three wire connector
I will admit that it is possible two wires from the thermostat may have gotten switched. I will recheck those, but everything else is wired correctly.
I'm in Hammond.
Thanks for helping.

 

[churchmouse]

You did not need a board. Does the burner lite on the second cycle?
Are you able to pull a code from the function lite on the board?....There may be a small observation port to allow you to look in at the board. The function lite should give you a code in a series of flashes. There will be a list of failure codes on the furnace somewhere.
Is the filter clean?...seems like an obvious thing but you would be surprised...

If the blower is running and no burners that is an indication of a safety trip or some other limit stopping the burners from firing.

 

[Bill B]

The old board had no leds on it.
As it stands right now the furnace is working fine except that the exhaust blower will not shut off. I will go check and see if the light on the new one is flashing a code and will cycle the furnace to see if it lights on first or second try.

 

[churchmouse]

The old board had no leds on it.
As it stands right now the furnace is working fine except that the exhaust blower will not shut off. I will go check and see if the light on the new one is flashing a code and will cycle the furnace to see if it lights on first or second try.

If the exhaust blower is running and you wired the furnace properly it is calling. If no heat call something is amiss.
I hate trying to diagnose long distance by keyboard. Might take 10 whole minutes to do if looking right at it. Problem is I am a long way south of you.

 

[Max Volume]

Most likely one of the safeties is indicating an overheating situation which causes the blower to stay on.

 

[churchmouse]

If it lites and goes back out in 3 to 5 seconds you have a flame rod issue.
I have also seen HSI (hot surface igniters) lite 1 time and fail on the next cycle.

 

[Bill B]

Ok.
1) the thermostat is wired correctly
2) the furnace lights on the first try
3) there is no light on the new board either.

 

[churchmouse]

Does it cycle on the second try???
If not it is overheating one of the safety's in the series. Could just be failing and need replaced. Take a pic of the schematic and post it up please.

 

[Snapdragon]

I know you may have done this too, but... Lots of stuff here https://www.google.com/search?q=fur...fficial&client=firefox-a&channel=np&source=hp

especially How to Fix a Gas Furnace: Furnace Blower Does Not Turn Off Video - About.com - YouTube

and Furnace Blower Won't Shut Off (Revisited) - DoItYourself.com Community Forums

 

[Bill B]

Does it cycle on the second try???
If not it is overheating one of the safety's in the series. Could just be failing and need replaced. Take a pic of the schematic and post it up please.

Yes. The only abnormal function is the exhaust blower stays running.
I will try to post a picture of the schematic.

 

[churchmouse]

On most modern 80+ furnaces the blower staying on will lock out a second or sequential cycle due to proving switch not opening and allowing the board to reset.

 

[Bill B]

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[churchmouse]

That diagram is showing the components and the safety series but not how they interact. The pressure switch usually has to be cycled to reset the safety circuit it is wired in.
Was this same problem the one you 1st saw before the new board was installed????

 

[Bill B]

This is the same as the other diagram on the furnace:

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The pressure switch opens and closes fine, I checked it with a meter for both continuity and voltage.
the high limit and roll-out switch are closed.
I just triple-checked the wiring, it is wired correctly.
I have also removed the thermostat from its' base.
The exhaust blower kicks on as soon as I close the interlock.
Does the "blower control center" tell the ignition module that the blower is running and it's ok to start, or does the ignition module tell the blower control to start the blower?
Thanks for all the help and suggestions so far.

 

[Bill B]

Just to clarify: The furnace works fine, as in it maintains the temp. set at the thermostat, but the exhaust blower runs constantly.

 

[Bill B]

That diagram is showing the components and the safety series but not how they interact. The pressure switch usually has to be cycled to reset the safety circuit it is wired in.
Was this same problem the one you 1st saw before the new board was installed????

Before the new board was installed, the main blower ran continuously and the furnace would not attempt to fire.

 

[churchmouse]

It sounds like you have the exhaust motor wired hot. It should only run on a call for heat. Those assembly's are not engineered for continues duty.

 

[Bill B]

It sounds like you have the exhaust motor wired hot. It should only run on a call for heat. Those assembly's are not engineered for continues duty.

You are a genius.
They had wire tied all the wiring into one huge bundle. I cut all the damn ties, separated all the wires and re-checked all the wires. It turns out that I had one of the several black wires on the wrong terminal. Now I feel like a total dumbass.
+10,000 to you and everyone that helped.