I'm having a problem where the furnace is not firing. The inducer motor runs for about 20-25 seconds then shuts off.

I'm troubleshooting all the safety items to see if something there is causing the failure to fire.

The pressure switch has two vacuum hoses going to it. One goes to the condensate box and the other to the burner enclosure. Should the pressure switch hold a vacuum/pressure from both connectors? I can suck/blow into the hose going into the condensate side of the switch and it holds vacuum/pressure. When I do the same on the burner enclosure side of the switch I can hear an air leak in the switch and it does not hold vacuum/pressure. Is this normal operation or is the diaphram shot in the switch on the burner side?

Thank you.

Scott

 

there should be a control board on that furnace that has a diagnostic light. STOP blowing/sucking on the pressure switch you will ruin it! The ONLY way to PROPERLY check a pressure switch is with the use of a manometer. Check for the diagnostic light and a there should be a legend somewhere on the furnace showing you what the light flashes stand for. 95% of the time it is NOT the pressure switch at fault even if you get a pressure switch code.

It is normal for the pressure switch to not hold pressure. There is a vent hole on the switch's hose connection.

 

This unit is from 1990. It does not have any diagnostic LED's on the control board.

 

some had led lights then too..............regardless you are in dangerous territory and need to call a pro.
http://www.docs.hvacpartners.com/idc...i398a-40-4.pdf
The sequence of operation is on page 10.

 

I have an HVAC friend coming in to get this fixed for me. I just thought I could do some of the legwork for him. Thanks for your help. I'll pass the document on to him.

Scott