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Old 01-24-2011, 09:35 PM   #1
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Open pressure switch on furnace


I have an Aire-Flo furnace that is approximately 7 years old. The model number is AF 80MPA100B5A. I returned home yesterday to discover that the unit was not heating. A breaker was tripped in my panel box. After resetting the breaker, I got 3 flashes on the smart gas valve that indicated the pressure switch was open. I blew through the fube on the switch and did not find anything clogged. I also checked the switch with an ohm meter. It appeared to open after the infuser started running. I checked the fan board and discovered a larged black spot on the back of the board. The old board was a Honeywell ST9160B 1076. I got a new board Honeywell makes to replace this along with several other boards, followed the instructions on wiring the new board and turned the unit on. The same thing is happening -- the infuser runs and then shuts off and I get 3 flashes on the gas valve indicating an open pressure switch. The ignitor does not glow and nothing else happens. I tried to jump the two wires to the pressure switch after the infuser started running, but get the same error. Do you think that whatever burned up the old board also burned up the electronics on the smart gas valve?

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Old 01-24-2011, 10:17 PM   #2
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Open pressure switch on furnace


More then likely the smart valve is shot.

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Old 01-25-2011, 07:51 PM   #3
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Open pressure switch on furnace


Okay, now I am really baffled. I replaced the smart valve and am getting the same error code -- open pressure switch. I bypassed the switch and then turned on the power to the unit. After going through the cycle, I got the same error code that the pressure switch was open. I should have gotten an error message that the pressure switch was stuck closed, but I didn't. I also completely disconnected the flue vent from the furnace so that it was blowing directly into the attic and still got an error code that the pressure switch was open. I tried sucking on the tube on the pressure switch and could hear it click when I put a vacuum on the tube.

So, I have changed the fan control board and the smart gas valve, bypassed the pressure switch, disconnected the flue vent so that there could not possibly be an obstruction in the vent and each time I still get an error saying that the pressure switch is open. Any ideas?

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Old 01-25-2011, 08:08 PM   #4
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Open pressure switch on furnace


Check the nipple on the inducer where the hose for the pressure switch hooks up. Might be some blockage there. Use a paper clip or small drill bit to clean the hole out. Still doesn't expain why the fault shows even with the pressure switch jumped though.
Afraid you might be money ahead to call for service on this one. Next step is going to be testing all the circuits and safeties with a meter.
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Old 01-26-2011, 11:35 PM   #5
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Open pressure switch on furnace


I finally discovered that one of the wires from the pressure switch routed through a temperature limit switch located on the squirrel cage housing. There was one limit switch on the bottom and one on the top. I bypassed the top switch and everything worked fine. So, the trouble code that indicated an open pressure switch was actually an open limit switch. Installed a new limit switch and we are all warm tonight!

I'm posting this just in case someone runs into the same problem.
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