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majorapus 10-23-2009 07:30 PM

Pressure Switch, Draft Inducer, Burner Box, Problem
Starting last week, sometimes when I come home I hear that my furnace is not running properly. I have a york 92 efficiency furnace. The draft inducer just runs and nothing else happens. The LED indicates "Pressure switch stuck open".

Using a volt meter I measure 25 volts from one terminal to ground, and 0 volts from the other terminal to ground. There are two tubes connected to the pressure switch. One connects to the draft inducer. The other to the burner box.

I disconnected the hoses from the draft inducer and the burner box. If I suck on the draft inducer hose I hear the pressure switch click and the volt meter reads continuity through the switch. If I blow on the burner box hose the pressure switch clicks again. The pressure switch appears to be working.

When I walk outside (while the draft inducer is running) I can feel suction on the intake pipe and a draft out of the exhaust pipe. I removed the curved PCV prieces from the intake and exaust pipes so I could look down them with a flashlight. The pipes look clear as far as I can see (8 ft) before they bend.

The exhaust pipe is made up of 2 sized pipes. The smaller diameter connects the furnace up to the larger diameter which is outside. The larger diamter has become home for a pool of condensation. But because the pipes are different diameter, the water level is lower than the smaller pipe, and the smaller pipe is dry.

I have checked my condensation hose, it appears clear. I've checked the draft inducer pressure tube and the burner box pressure tube, those are fine too.

If I turn off the furnace and turn it back on the draft inducer immediately starts up. If I remove the tubing from the draft inducer, leave it connected to the pressure switch, and suck on it, the LED blinks green (I then shut the furnace off to prevent the ignition cycle). If instead I leave the draft inducer pressure tube connected, and disconnect the tube connecting the burner box to the pressure switch, the LED blinks green (no blowing in the tube required.)

When the draft inducer is on I can feel suction from the pressure tube. I can feel the burner box blowing air out of its pressure tube.

Everything by itself seams to be working, when I put it all together it wont turn on.

Is the pressure switch too stiff? Is the draft inducer not producing enough draft? I'm not sure what the burner box tube is for exactly, what does it indicate that removing it solves the problem?

Plumber101 10-23-2009 07:47 PM

The problem here is that just blowing into a hose and feeling a suction on a hose really does not mean anything.

These pressure are measured in inches of water column.

What you may be looking at is the inducer is operating fine but your heat exchanger may be cracked.

The best way is to measure the actual positive and negative pressures with a manometer

I would ask you to call a pro because checking a exchanger is not a DIY project

You may just have a bad pressure switch but with a dual pressure switch it is best to actually measure the pressures

yuri 10-23-2009 08:30 PM

And now that you sucked and blowed on the pressure switch it is damaged/no longer accurate or safe/diaphragm is stretched or damaged. You need a new pressure switch for safety reasons and the unit checked with a manometer to measure the DIFFERENTIAL pressure across that switch. Rating for the switch in "WC must be known also. The pressure switch measures and proves the draft thru the heat exchanger needed for proper combustion. Not enough draft and you can get CO poisoning and overheat and warp and damage the heat exchanger/flame rollout and other very dangerous events. Your heat exchanger may be damaged/cracked and the whole unit needs checking by a York tech or seasoned Pro.

Houston204 10-23-2009 11:03 PM

A condensate furnace will lockout on pressure switch when the drain is restricted. You should never suck or blow into a pressure switch, it is probably damaged now.

majorapus 10-24-2009 01:26 PM

A service man came out to look at my furnace. He showed me that the draft inducer was creating 1.5 in h2o pressure. He said it should be creating at least 3. So he is going to order a new part and bring it out when he gets it. He also said that the pressure switch was okay (I told him I blew into it).

Does all of this sound fair?

yuri 10-24-2009 01:43 PM

Pressure switches are never okay to blow into as they are delicate items. Someday it will fail (Xmas or New Years Day) and you may be sorry. Lots of people blow into them as they don't know what can happen, your choice whether to keep it. It is also a safety device and if it fails to shut down the burner dangerous problems can occur. LOTS of techs don't think about safety or LIABILITY but I sure do. Without knowing what part he is ordering we cannot say if he is correct. I assume it is the inducer fan itself.

majorapus 10-25-2009 06:10 PM

Should the draft inducer be generating more than 1.5 in h2o pressure? He said it should generate 3.0 in h2o, does this sound right?

yuri 10-25-2009 06:14 PM

If it has a "WC rating stamped on the pressure switch read it. Otherwise it would have to be read from a service manual.

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