7 year old Heil furnace, started blowing cold air yesterday and the status LED is doing 3 blinks which means "pressure switch open when should be closed". I removed the tube off the switch and used a straw to suck/blow a few times very gently and heard the switch engaging normally. I then blew into the tube itself and reconnected and it works immediately. Briefly (20-30 min). Then cold air again. I blow into the tube again (didn't touch the pressure switch again), and it works again. Briefly. Repeat.

Are we thinking this is crud somewhere blocking the airflow from the tube and I'm dislodging it temporarily when I blow into the tube? Or does this sound like a bad pressure switch? Thanks.

 

The pressure switch verifies that combustion air to burn the gas is free to enter, burn the gas and exit up the vent pipe.


You don't want to blow into the pressure switch, since that can cause the delicate diaphragm to get out of calibration.

One fairly common problem is that the fitting on the inducer motor housing that the hose from the pressure switch fits to can get plugged with rust or corrosion. You can use a small diameter drill bit to clean out any debris --- be sure the drill bit goes all the way into the inducer motor housing.

Find the rating plate in the burner compartment of the furnace and post the make and model of your furnace.

 

It's Heil, Model H9MPD075F12C1. I followed the rubber tube from the pressure switch to the other end, removed it, and used small drill bits to poke around - it was clean and I didn't sense any debrs or corrosion, it was a plastic fitting.

Thanks for your help, it's greatly appreciated.

 

I've got two of these same furnaces so I was able to swap the pressure switch from one to the other. The "bad" furnace worked, for 30 minutes. Now it's again blowing cold air.

Any ideas on what I can try to fix this? Thanks. The switch seems to be fine. But everytime I disconnect the tubing and reconnect, it magically works again briefly.

 

It's not the switch. Something in the exhaust/flue system is incorrect. The pressure switches are very rarely defective. I have yet to replace one.

I use a manometer. It's a vacuum meter that connects in the pressure line. Based on that reading I can make changes and watch the effects.

I have to look at your service manual and see if anything pops up.

You could make one from clear plastic hose.

 

Ummm.

Generally it takes a more sensitive manometer than a simple vertical water filled manometer to measure the pressure in a furnace inducer assembly with an adequate degree of accuracy.

A slope manometer that uses oil rather than water is much more sensitive, and will accurately measure parts of an inch water column pressure.

An example is at:

Precision Inclined Manometer - Draft Gauge for Low Precise Measurement


Also there are mechanical pressure switches which will do this.

I used to use a Dwyer slope manometer which cost probably $50 or so.

 

PJmax - I saw a similar thread here recently, and your advice showed a picture of the blower and the port that could be clogged. My furnace doesn't look like that.

Pic of my furnace:


Which port(s) should I be trying to unclog? Thanks so much. Again it's a Heil Model H9MPD075F12C1