differential pressure switches

[handyandyshvac]

Hello all,
I am hoping someone can enlighten me as to the method for testing a differential pressure switch.
I am working on an Armstrong furnace which has a differential pressure switch set at .7.
I know how I was taught to verify differential pressures, but I am getting some funny readings at the burner box.
Thanks

[handyandyshvac]

***90% furnace
better way to phrase question, what pressures should I be getting from the burner box with a functioning inducer, no cracks in heat exchanger, well sealed system with intake piping disconnected?
Any help here is much appreciated. Ive got one here that has me scratching my head.

[pacnw]

just dealt with a Lennox and they have all the test procedures and desired minimum for the unit in the manual.

try contacting mfr for the info.

if you have a dual manometer, connect it and get your reading.

if single, connect it to each port and do the math.

can't get too specific here.

[rundawg]

***90% furnace
better way to phrase question, what pressures should I be getting from the burner box with a functioning inducer, no cracks in heat exchanger, well sealed system with intake piping disconnected?
Any help here is much appreciated. Ive got one here that has me scratching my head.

The pressure sensed by the switch is relative to the pressure in the burner box. In order for the furnace to operate, the larger negative must always be on the combustion air inducer side of the switch.

You can not use one set of numbers for each manufacture.

As pacnw mentioned, it's good to have the manufacturer data for each furnace you work on.