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Old 01-15-2012, 07:37 AM   #1
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Strange pressure switch problem


I have a furnace with a "pressure switch stuck open" (3 red blinks) problem.

Here's what I have:

The draft assist blower is working and the draft vent is unobstructed

the pressure switch has 2 hoses, 1 to draft fan and 1 to "heat exchange box" (probably not the right term).

When first switched on, the Furnace starts into the cycle, but stops when the pressure switch fails to close.

Using an ohm meter I can see that the switch does not close, unless I remove the hose from the furnace box.

If I start the cycle with my finger over that hose on the end normally attached to the heat exchange box the switch will not close when the draft fan is on, but as soon as I remove my finger, the switch closes.

I ran a paperclip an inch or so into the inlet in the heat exchange box, and it did not seem to be obstructed at all.

I understand the pressure switch verifies the draft fan is on, but what is the other hose doing. Is it checking for some specific pressure condition in the heat exchange box? I'm not sure how to proceed.

BTW, is this too much info for starting a topic, or does it save back and forth time asking for info I didn't give.

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Old 01-15-2012, 08:14 AM   #2
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Strange pressure switch problem


Not sure what furnace box means.

Got a pic?

I am thinking burner box or heat exchanger cover.

if it's wha t I think it may be a plugged drain.

What brand and model? is it 80 or 90% ..like I said..pics help.

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Old 01-15-2012, 08:17 AM   #3
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Strange pressure switch problem


Post make model and serial # and some pics with both doors off. It is a "differential" pressure switch which is checking the draft thru the heat exchanger. Determines flow/draft by the differential pressure from in to out of the exchanger which is where those ports are. Check your venting outside for blockges.
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Old 01-15-2012, 08:53 AM   #4
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Strange pressure switch problem


Thanks for the quick responses.

It's a York Affinity 9.S (not 9.5) . Still looking for SN.

I meant to say when I remove the hose from the burner box (that's what the pressure switch lable calls it).

Right after the cycle starts and the draft inducer starts, if I remove the hose running from the pressure switch where it attaches to the "burner box" (see pic), then the switch closes. Makes sense to me know after reading yuri's response. there must somehow not be enough pressure in the burner box to allow the differential switch to close in response to the vacuum from the draft inducer.
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Old 01-15-2012, 09:14 AM   #5
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Strange pressure switch problem


I forgot to ask hvac5646, 80 or 90% what? Also, which drain are you thinking might be plugged?
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Old 01-15-2012, 09:21 AM   #6
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Strange pressure switch problem


in the second picture, were the tube connects upward to the burner box........remove it and ream out metal tube with unfolded paper clip..........let me know if that fixes it.
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Old 01-15-2012, 09:27 AM   #7
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Strange pressure switch problem


you have a 90% furnace. Since the hose goes to the burner box( not the collector box as i had speculated) it is not a drain blockage prb.

Your combustion air pipe may be plugged . Check out side for blockage at the pipe opening...pull the burner off to see if you have any debris where the pipe
connects to the burner box. Also check the exhaust for same just tobe safe.

The pipe connected to the burner box terminates out doors?

Last edited by hvac5646; 01-15-2012 at 09:29 AM.
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Old 01-15-2012, 10:37 AM   #8
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Strange pressure switch problem


That inlet (pressure switch into burner box) is wide open.

I have a pretty good stream of air coming out the exhaust (outside) when I first start the cycle. I visually checked the air inlet (from outside into the furnace, and I hooked it up to a shop vac (with the burner box open), and easily pulled air through it.

The only new info is, the drain trap (small black box in between 3 different drain hoses in the furnace that leads to drain) was pretty clogged with bugs and junk. I cleaned it out, reassembled, and still same problem.

Is there often some type of clog in the tubes that lead from the burner box, through the heat exchanger (maybe not the right term) to the exhaust fan?
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Old 01-15-2012, 12:05 PM   #9
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Strange pressure switch problem


OK, LETS DO SOME SIMPLE DIAGNOSTICS.............sorry for the caps.......lol remove the burner compartment door and see if that cures the problem..from your pics it looks like 4 screws hold it on...let me know
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Old 01-15-2012, 12:10 PM   #10
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Strange pressure switch problem


Now thats just cheatin . You are supposed to use a manometer. LOL.

For all the other readers NEVER run a furnace w/o that door on for more than a minute as that creates an unsafe condition. Hate to see someone use that as a "fix".
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Old 01-15-2012, 01:29 PM   #11
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Strange pressure switch problem


Yes, if the burner compartment door is removed (or even just cracked open a bit) the differential pressure switch goes to it's happy place (closed).

With a shop vac on the outside vent (exhaust) I can hear air (a lot of air) being sucked in through the open burner compartment door. Which leads me to believe there are no significant blocks in the exhaust line. Also, with the burner compartment door in place, the shop vac still moves plenty of air. Which leads me to believe that the air intake line from outside has no significant blockage. And yet, it seems clear that somehow there is enough of a pressure drop in that compartment to offset the other side of the deferential pressure switch.

And yes yuri, I do understand that all these devices (burner doors, pressure switches, exhaust fans) have a safety purpose and should not be bypassed for other than brief testing. But it is worth mentioning, thx.
Also the only manometer I have is for blood pressure.
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Old 01-15-2012, 02:11 PM   #12
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Strange pressure switch problem


U may have a bad pressure switch and enough draft. there may be a inches wc rating on the side of it. then you need a Pro differential manometer and check. w/o it U/we are guessing. UEI makes them an U can order one online. I use this baby.

http://www.ueitest.com/product-em201.html

may B cheaper than calling a Pro and U get 2 keep the tool. if there is no rating then U are hooped and need to get that info from York or try a new switch.
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Last edited by yuri; 01-15-2012 at 02:15 PM.
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Old 01-15-2012, 04:06 PM   #13
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Strange pressure switch problem


Quote:
Originally Posted by yuri View Post
Now thats just cheatin . You are supposed to use a manometer. LOL.

For all the other readers NEVER run a furnace w/o that door on for more than a minute as that creates an unsafe condition. Hate to see someone use that as a "fix".
I said lets do some diagnosticts........never told him to run it that way did I ?
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Old 01-15-2012, 04:07 PM   #14
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Strange pressure switch problem


Quote:
Originally Posted by jones814 View Post
Yes, if the burner compartment door is removed (or even just cracked open a bit) the differential pressure switch goes to it's happy place (closed).

With a shop vac on the outside vent (exhaust) I can hear air (a lot of air) being sucked in through the open burner compartment door. Which leads me to believe there are no significant blocks in the exhaust line. Also, with the burner compartment door in place, the shop vac still moves plenty of air. Which leads me to believe that the air intake line from outside has no significant blockage. And yet, it seems clear that somehow there is enough of a pressure drop in that compartment to offset the other side of the deferential pressure switch.

And yes yuri, I do understand that all these devices (burner doors, pressure switches, exhaust fans) have a safety purpose and should not be bypassed for other than brief testing. But it is worth mentioning, thx.
Also the only manometer I have is for blood pressure.
You still could have a drain issue..... you should remove all the rubber tubes and clean them out just like you did with the trap.

Last edited by harleyrider; 01-15-2012 at 05:43 PM.
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Old 01-15-2012, 09:03 PM   #15
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Strange pressure switch problem


Okay. Ran a snake through both intake and exhaust pipes from furnace to outside. No debris in either. Ran pipe cleaner through all drain tubes, no debris. removed draft inducer fan and turned it on, it moves a very large amount of air. Finally, I adjusted the set screw on the differential pressure switch. After that, the furnace seems to work fine. My concern is, the set screw is out quite a bit. It is probably only 2 or 3 turns away from coming out, but it is a tight plastic fit, so I don't think it could possibly vibrate out.

My plan is to run the furnace only while someone is home, and awake. I'll do this for a few days, just in case I'm missing a huge safety issue somewhere, and the furnace goes super nova. I imagine this is highly unlikely, but not being an HVAC guy, I figure what I don't know might very well hurt me.

Does this sound like a decent plan? Should I order a new switch?

This is my first day using DIYchatroom, and I am impressed. Thank you to all three of you for your time and suggestions.

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