Gas furnace pressure switch question

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Old 11-22-16, 01:43 PM
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Gas furnace pressure switch question

Hi,

Our 4 year old Trane XB90 is not firing. It is showing a two-flash error light on it's board.

The problem started early Monday morning then went away when I vacuumed about two feet into the exhaust and intake PVC pipes and reset the furnace. It then came back about 5 hours later.

After checkng around, I was able to get an exact brand new OEM replacement switch and was wondering if it is OK to install as is. It was only $20 so figure it's worth it if the problem ends up being the switch and not something else.

Do these switches have to be calibrated or adjusted after installation or are they set up by the factory?

Thanks
 
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Old 11-22-16, 01:58 PM
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There is no post installation calibration required.
If it's the same exact part number just go ahead and swap it.

I doubt you'll find that it helps the problem.
Did you check the the hole in the inducer blower where the hose connects ?
Many have a tiny hole that gets clogged with rust.

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Old 11-22-16, 02:14 PM
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Thanks Pete

I'll put the new switch in and see what happens.

Before that, I'll check to see if the hole to the inducer motor is clogged.

I'll let you know how it goes.

Thanks again
 
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Old 11-22-16, 03:03 PM
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Every year, railway carloads of perfectly good pressure switches are replaced....

Unfortunately, often enough the wrong pressure switch is installed and makes things WORSE!

Lets suppose that 95% of the time the pressure switch is actually OK but that it's replaced for $20 and it takes a week to get the part. That means that $400 is spent for parts and the furnace is out of use for five months for every time the pressure switch is actually the problem.


Heh, heh! A few railway carloads of flame sensors are replaced that are perftly fine, too!

Oh, well....
 
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Old 11-23-16, 08:27 AM
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Heat's back on

OK, first off, thanks for the replies to the original post and for the comments.

Pete was right, the new pressure switch did not solve the problem it did, however, lead to the fix. Here's how:

Our gas furnace replaced an old oil burner that had a flue that ran through our crawlspace (about 15 ft.) to outside. When we switched to gas, the installers used this same exit path to vent and supply the new furnace.

So we have two runs of PVC pipe that travel through the crawlspace, side be side, and exit the house by the AC unit.

Both of these PVC pipes are open when they get outside. There is not a screen or grate on them at their end points outside. They do, however, point away from each other.

Since the fresh air intake tube is open, I thought it might hav an obstruction. So I cut the air intake tube on top of the furnace, and saw that the intake grate in the furnace was blocked with leaf debris. I vacuumed it out and the furnace came back and has been working fine.

Here's the plan re going forward:

I'm going to reconnect the cut PVC using a fernco coupling--Let me know if you guys think this is OK to do....

Next I'm going to get or make screens to put over the outdoor ends of the both pipes. Hopefully, this will keep debris and critters out so we don't have this problem again.

Thanks!
 
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Old 11-23-16, 10:12 AM
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I have never had to replace a pressure switch.

The fernco should be fine.
I like to point the pipes downwards using 45 degree fittings.
A screen is..... ok....... but now it's a maintenance point that needs to be checked for clogs.
Hardware cloth.... metal screen with 1/4" square holes would probably be ok.

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