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ryansturmer 03-12-2008 06:51 PM

How do I test my draft inducer?
I've isolated the draft inducer/pressure switch as the source of a problem I'm having with my furnace, but I don't know if it's the inducer or the switch! Is there some sort of test I can perform to determine which is bad? The draft inducer seems fine (it spins up anyways, seems to run at a reasonable clip) and the pressure switch is ok just by manual inspection. Is there some sort of vacuum gauge I can hook up to my hose to see if I'm losing pressure?

biggles 03-12-2008 07:43 PM

with all hooked up just pull the 2 wires off the switch,and read it with ohms with the ID motor off should be open 0.00,then start the ID motor and read your ohms if the switch proofs the air from the ID...the switch should close and read 1.0 or at least bounce to anything but 0.00.if the unit is old the hose might be weak/leaking or the switch might be sticking.just blow into it(hose) and see if the switch makes and breaks in your hand with the meter hooked up.

ryansturmer 03-12-2008 09:46 PM

I've blown into the switch and it seems OK. not sticking or anyhing as far as I can tell. The failure I'm seeing is when the unit heats up, the switch starts to read intermittent, and then eventually open, even though the blower is still running. The blower still seems to be going at top speed, though, so I'm not sure what's up.


Mainah 03-13-2008 05:08 AM

a service tech would put a draft gauge at the proving switch (on the end of the tube where it connects to the switch)- it needs at least -.02 to close. Typical readings are much higher. The most common problem is the tube gets plugged affecting the reading at the switch causing intermittent problems and then eventual failure. Just disconnect and blow through the tube, there should be no restriction at all. If the tube is clear and there is plenty of neg. draft at the switch just replace the switch and all should be fine-
do not bypass this very important saftey switch!!

I'm assuming this is indeed an inducer and not a powerventer so make sure your chimney is clean, no blockage in your smoke pipe from the furnace to the chimney, especially where the pipe enters the chimney- if all this is fine but you have no draft at the switch it's likely the fins on the blower wheel have started to fail, causing weak draft-

ryansturmer 03-13-2008 10:05 AM

Is there a preferred source for purchasing a draft gauge? A quick search doesn't turn up much. Is there something else that they're called that I can be looking for?


tnthvac 03-21-2008 07:46 PM

The meter is called a manometer, if you buy one make sure it is a negative and positive, reading meter. They are quite expensive around 100 bucks!

statman 03-21-2008 09:28 PM

If the motor is running fine and the squirrel cage is clean with no obstructions in the chimney (check this....I have come across dead birds in some vent pipes), then I would replace the pressure switch..probably no more than 30 bucks.

1001miles 10-19-2008 08:15 PM

Did this problem ever get resolved? I too am having the exact same issue so if ur's got fixed that would be great if you would share!!!!! :)

dac122 10-20-2008 09:31 AM


Originally Posted by Mainah (Post 107195)
a service tech would put a draft gauge at the proving switch (on the end of the tube where it connects to the switch)- it needs at least -.02 to close. Typical readings are much higher.

What are the units for that .02? Is that IWC, or PSI from the hose, or resistance on the switch? Would a vacuum gauge measure draft off that hose? Conversely, would a vacuum pump work to test that pressure switch?

Was diagnosing some trouble on mine this weekend and was also wondering.

tnthvac 10-20-2008 07:41 PM

I don't think a vaccum gauge would tell, unless it read water colum!

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