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About a year ago our heater stopped coming on. Through lots of research and trouble shooting we were able to figure out that the pressure switch was stuck closed. If we unhooked one side of the switch and blew into it, the heater would magically work (Of course the heater was disconnected from power during the actual work). We replaced the pressure switch and had no problems with the furnace for the rest of the winter.

Now we move on to this fall. We tried to turn on our heater, and once again, the pressure switch is stuck closed. I got a new pressure switch and installed it, but the heater is still flashing two lights, which means the switch is still stuck closed even though it is a new switch.

I have now come to the conclusion it isn't the switch at all, but what else could be causing the same LED error message?

Did you test the PS to see if it is in the closed position ?


Does the booster/combustion/draft inducer motor come on when the t-stat calls for heat ?


Thanks,
Robert

When I blow into the PS it clicks, but as soon as I stop blowing it clicks again. When the thermostat calls for heat there is a slight humming. The heater is definitely trying to do something. After a few minutes the lights start flashing twice again.

I hope this answers your question, if not please let me know. Thanks so much for your help!

Hi,

Do not blow into the switch, moisture will ruin it. See if the small booster motor is actually running. Is the humming from the motor running or trying to start ?


The motor has to run, has to run fast enough to develop enough rpm's to reach correct water column pressure at switch, make sure tubes are connected tight, not kinked and not clogged, air pipe to motor is clear, 24vac at one pressure switch terminal and the switch itself.


Best way to only buy what is needed is to call an hvac tech out to properly diagnose.



Thanks,
Robert