Armstrong SX 80 Blower Issue

[Jaks69]

I am trying to fix a furnace for my neighbor, problem seems very obvious but hit a couple bumps.

 

Armstrong Ultra SX-80- (Vintage 1996)

 

Let me explain- Furnace calls for heat, system goes on triggers the exhaust fan, igniter lights, flames come on 30 seconds later the blower is suppose to turn on but does not turn on until the high limit triggers. He has been running the system for the last 2 years with the system in "FAN ON" mode instead of auto.

 

With all that said my attempt to repair was to replace the main board and since the blower motor was making noise and going bad I replaced that too. Installed all that plus a new 24v transformer. Put the system in Auto, call for heat, same thing, exhaust fan turns on igniter on, fires, hits high limit, flames go out then blower goes on. What am I missing here....

 

To add to the clues, I was told the gas valve was changed out for some reason a few years back but I could not identify if the existing valve was the exact replacement or not. Is there a possibilty that the gas valve controls the blower on after it calls for heat? The gas valve appears to be functioning properly but I am thinking it may not be the exact replacement losing that function.

 

Any insite on this?  

[Daveb1972]

Short answer is the board needs to know when to start the timer. Different manufacturers accomplish this from 2 main methods, internal to the board, or external. The first think I'd do is check the furnace w/o thermostat connected. Connect the w and r terminals to see if the issue is furnace or control, next I'd look to see if there is a connection specified on the schematic for gas valve, and how many wires are scheduled to be there. Blower timers are sometimes set for 90 sec or more so check to see what the blower timer is set for, and time the burner run time from ignition to limit cut out. Also look for blower to start immediately when limit is tripped which is a common attribute for most controls. Ill try to download and review the schematic to see if I can find any more details that may be helpful.

[Jaks69]

The Delay on is set to 30 seconds

The delay off is set to  140 seconds, both are default settings (I have tried the others) From what i have found the gas valve has a terminal for EFT (electronic fan timer) that is supposed to energize when the gas valve opens and de-energizes when the gas valve closes.

 

I havent measured there yet to see if that is working, that is probably the culprit.

Edited January 28, 2013 by Jaks69

[Daveb1972]

Is the fan controlled by the main board, or is it a separate timer control?

[Jaks69]

From the schematic (Page 6)  it looks like the board and the gas valve both have EFT's on them, or the energizing of the gas valve is what sends 24v back to the board to activate the blower on relay.

 

https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-1270.pdf

[Daveb1972]

Yup. That is what signals the fan timer that flame is on, start countdown. I didn't realize you had s smart valve. Should fix it.

[Jaks69]

Measured the EFT terminal, no voltage. Changed out Smart Valve (Gas), everything fired like it should. All Fixed! Thanks for the input on this.