Welcome! Here are the website rules, as well as some tips for using this forum.
If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Pressure switch problem really sucks
Have a 10 year old 80% eff Lennox gas furnace that will not start and flash code and low voltage testing show a pressure switch fault. Switch will not close with exhaust vent fan running. The tech guy at Lennox says the switch should close at around a negative .4 inches of water column and bench testing (sucking on tube) with manometer and multimeter shows it does. When connecting the manometer to the pressure switch port on the exhaust blower it shows a solid and steady neg .7" of water column. With the manometer connected to the suction line with a tee it shows zero inches of column when the vent blower is running and if I pinch off the tube going to the switch the meter shows the .7" again but when released the suction drops back to zero. Have tried two different switches, both new, one from Lennox and a Robert-Shaw genetic and all behave the same way, not working when hooked to the furnace but fine in the sucking bench test.
The venting is not blocked and I've used new tubing to the switch. I've even taken the original switch over to the neighbors house and connected it to his furnace (a York) and it works fine, his blower shows a neg .7" also. I can jumper the switch out after the vent blower starts up and the furnace will light up and work properly until the t-stat shut it off. However it will not restart for the next cycle with the jumper in place due to a safety built into the control. Don't want no sticking switches. Lennox guy says it must be the exhaust blower (doubt it) and to replace it. Anyone else had this happen. Thanks.