Goodman GMPNO-80-4

[laxdef69]

Kind of a weird one...the vent blower (A made up word, I mean the pump that induces the draft in the PVC exhaust pipe) is strong enough to make the pressure switch close...I can clearly hear it...but occasionally the furnace won't light.

If I jumper it out, it lights immediately.

There doesn't appear to be any cracks in the vacuum tube from the blower to the switch, and the switch shows continuity when tested under vacuum....

Any other ideas?

Under advice from a contractor I know, I checked all the drain lines,and they are clear (still not clear why i checked the drain pan and exhaust stack lines, but whatever).

Should I just replace the pressure switch and be done with it?  Although I HATE the idea of just changing a part without understanding why it failed in the first place.

Thanks!

[hvacdrd]

It is possible to have bad contacts in the pressure switch, even though the switch pulls in and clicks. To test you will need a vaccum guage of some type or manometer to verify that there is the proper pressure (negative) at the swtich.

The switch is rated at -0.85"w.c. (water column is a measurement of low pressure- 28"w.c. = 1psi)

Switch part#  B13701-50

[laxdef69]

Thank you for the fast response!

I don't have a vacuum guage, I just sucked in a little bit on the vaccum hose and tested for continuity.

I'll replace the part and hope...thats again!

[dkpd1581]

You checked the exhaust PVC flue because if it settles over time and the proper drain angle is not kept, the moisture will build up and cause the exhaust pressures to fall.  This in turn cause the pressure switch to change state.  There is a prescribed fall of X inches per Y feet of horizontal run.

[laxdef69]

Tons of slope in the flue pipe...so that's not an issue.

Thanks for the info though...always glad to add to the knowledge bank...

Changed the pressure switch, and all is good now!

Thanks again!