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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22678
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I'm working on my sons furnace. He had a HVAC Company out

Customer Question

I'm working on my sons furnace. He had a HVAC Company out who replaced his pressure switches and a blower motor. When they completed, it did the same thing. Now they want to replace the control board. They charged him full price for the repair and now want more to see if the control board fixes it. I went out and looked at it, turned unit off and on. Reset Errors and it worked ok. After awhile it flashes 3 reds (Pressure switch stuck open). The schematics on the furnace show one switch normally open and one normally closed. Using a multimeter they both test as normally open. Does the normally closed switch show as being closed while bench testing?
Submitted: 1 year ago.
Category: HVAC
Expert:  BillyHvac replied 1 year ago.


on this unit there are up to 3 switches in series with the pressure switch.

Have they checked the inducer limit?

This is commonly a venting issue...too small pipe or too many 90's etc.

The air switch should bench test open...the limit and inducer limit in the same series should test closed.

Customer: replied 1 year ago.
I don't know if they checked it or not. I told my son not to continue to pay for something without any assurance it is going to fix. I don't know what the original switches looked like but the ones in there now are both normally open. The schematics shows that one is supposed to be closed. This is the one that has a hose from the burner box.
Expert:  BillyHvac replied 1 year ago.

The only open switch is the pressure switch...once the draft motor starts te air flow created by the exhaust sucks air through the tube and closes the switch.

This allows ower to go on through to the the next 2 is a high liit normally closed...this opens if interior of furnace gets to warm (like plugged filter, too slow blower)..the next switch is also heat activated but is if the inducer housing gets too hot.

These last 2 switches can open and close on their own accordiing to temps,

The only open switch should be the air switch and it should close on start up.