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Old 12-19-2012, 01:53 PM   #1
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Help with checking pressure switch


I contacted Coleman Tech Support about the pressure spec for a pressure switch. The reply was:

"Current part number for replacement switch is 324-35972-000 set point is .1 inches of W.C. Booster fan needs to create a minimum of at least -.2 in W. C. as measured at the upper tap on inducer."




I believe to check for the -.2 in. W.C, I would tee into the upper hose with a manometer, but don't understand what the set point is all about.

Why are there two flex hoses instead of just one? Second tap is to allow air to enter the other side of the diaphragm, this enabling movement of the diaphragm?

This switch isn't bad - just need to learn how to confirm proper pressure is reaching this pressure switch and others.


Last edited by justplumducky; 12-20-2012 at 06:46 PM.
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Old 12-19-2012, 02:46 PM   #2
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Help with checking pressure switch


It is called a differential pressure switch and it measures the pressure thru the heat exchanger or thru the inducer fan or thru an orifice in the fan. If you study fluid mechanics/physics you know that pressure difference across an orifice can give you a flow measurement. You need a VERY accurate professional grade manometer to do small fine measurements like that. Google those terms for more info.

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Last edited by yuri; 12-19-2012 at 02:55 PM.
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Old 12-19-2012, 03:24 PM   #3
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Help with checking pressure switch


Thx for your reply Yuri.

Yes, it is in a mobile home (mine), but no problems with it. I do home repair for the residents in the park where I live, and need to learn how to confirm proper pressure is acting upon these switches I encounter. I don't have a manometer yet, but have been looking at UEI, Fieldpiece, Testo and Dwyer. Also need a manometer for checking gas valve manifold pressures. If you have a recommendation, happy to have it.

You mentioned something about special problems with the plastic fans on these combustion air (inducer) motors?

Last edited by justplumducky; 12-19-2012 at 03:38 PM.
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Old 12-19-2012, 03:43 PM   #4
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Help with checking pressure switch


I had problems with a Miller or Intertherm unit with a plastic inducer fan. Not sure if Coleman is in cohoots with Miller. York bought them out. Mobile home units can be a pain in the arze due to the venting with the combustion air coming in around the exhaust pipe and those inducer fans are cheap, weak and cheesy and can wear out quick and lose capacity/oomph. With that very low differential they trip easily and you get nuisance issues. Most furnace use .4 to .5 " WC but because those units are very small in size I guess they can get away with less flow/draft. Those are all good brands and I find UEI to be good value for your $$ and I have one. Testo is more expensive and I have one for delicate work and use the rubber cased UEI for rough work/everday handling. I have a EM201 and check it out at the UEI site.
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Old 12-19-2012, 04:37 PM   #5
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Help with checking pressure switch


Glad to hear you have a UEI manometer for the less delicate stuff - I watched a tech use one the other day (have read online about a few others with UEI and no problems with them) - I especially like the magnet-in -case feature. I bought a Fieldpiece HS35 with the hanging magnet and don't really like it much. Too many times when I use the hanging magnet, the meter is hanging at an angle not so easy to read. Guess I could get used to it - learn how to hang it better, but still prefer the simpler "stick it flush against the cabinet" -type. Better than no magnet at all (for my money), but much prefer being able to just set the meter flush against the cabinet (especially on the edges of the cabinet side walls), like with the UEI. I used a Klein CL1000 for a short time (loved the magnet built into the back of case), but didn't like waiting for the auto-ranging feature to get around to my range, when checking capacitance - took it back. I have two other meters (Fluke voltage probe and Fluke clamp meter), so I may dump the HS 35, unless I could put a permanent magnet on the back of the HS35 case? Would that interfere with the internals of the meter?

Quote:
Mobile home units can be a pain in the arze due to the venting with the combustion air coming in around the exhaust pipe
I understand about the concentric arrangement of the exhaust/combustion air pipes - how does that cause problems? Debris coming in with the combustion air?

Quote:
and those inducer fans are cheap, weak and cheesy and can wear out quick and lose capacity/oomph.
Yepper, I've replaced a few of them, and I get those where the fan blade shaft will lock up after setting thru the winter.

Quote:
With that very low differential they trip easily and you get nuisance issues.
The sensitive pressure switch is subjected to erratic combustion air intake, which can cause the switch to cutout and shut down the furnace?

Thx so much for your help.

Last edited by justplumducky; 12-19-2012 at 04:46 PM.
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Old 12-19-2012, 05:08 PM   #6
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Help with checking pressure switch


Quote:
Originally Posted by justplumducky View Post
Glad to hear you have a UEI manometer for the less delicate stuff - I watched a tech use one the other day (have read online about a few others with UEI and no problems with them) - I especially like the magnet-in -case feature. I bought a Fieldpiece HS35 with the hanging magnet and don't really like it much. Too many times when I use the hanging magnet, the meter is hanging at an angle not so easy to read. Guess I could get used to it - learn how to hang it better, but still prefer the simpler "stick it flush against the cabinet" -type. Better than no magnet at all (for my money), but much prefer being able to just set the meter flush against the cabinet (especially on the edges of the cabinet side walls), like with the UEI. I used a Klein CL1000 for a short time (loved the magnet built into the back of case), but didn't like waiting for the auto-ranging feature to get around to my range, when checking capacitance - took it back. I have two other meters (Fluke voltage probe and Fluke clamp meter), so I may dump the HS 35, unless I could put a permanent magnet on the back of the HS35 case? Would that interfere with the internals of the meter? I don't know but would suspect it would and would not try it. That meter has to be 100% accurate or U could kill someone with a bad gas pressure setting etc. I LOVE that magnet. Can tow a trailer with it.


I understand about the concentric arrangement of the exhaust/combustion air pipes - how does that cause problems? Debris coming in with the combustion air?Yes, and they can ice up in real cold climates and birds nest there etc etc and they can seperate internally and the exhaust circs back into the intake


Yepper, I've replaced a few of them, and I get those where the fan blade shaft will lock up after setting thru the winter.


The sensitive pressure switch is subjected to erratic combustion air intake, which can cause the switch to cutout and shut down the furnace? Ideally U want to have a few inches of WC to play with so when it slows down or you get a bit of blockage it is not so hair trigger sensitive to cause a problem. Resi furnaces can easily produce 2-3X the draft it takes to prove the switch so U got room to play with. No room when it is at .1"WC

Thx so much for your help.

.....
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Last edited by yuri; 12-19-2012 at 05:11 PM.
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Old 12-20-2012, 10:34 AM   #7
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Help with checking pressure switch


I may dump the HS 35, unless I could put a permanent magnet on the back of the HS35 case? Would that interfere with the internals of the meter? I don't know but would suspect it would and would not try it. That meter has to be 100% accurate or U could kill someone with a bad gas pressure setting etc. I LOVE that magnet. Can tow a trailer with it.

Looks like I gave you the impression that the HS35 is a manometer - my bad - it's only a dual ranging multimeter. Noper, I wouldn't attempt to add a magnet to a manometer, and really wouldn't attempt it on anything else without manufacturer approval. Truth is, before I do something like that (adjusting manifold pressure), I would probably call out a pro first (to do it) - watch him and pick his brain (I have someone who allows me to do that).

However, I truly am grateful for the "would not try it", and along that line... I assume you're talking about an incorrect manifold pressure setting resulting in excessive CO emissions and overheat conditions? Well, don't want to wear out my welcome here - I've learned that HVAC is a highly technical and complex field, and you can only answer "so much" at a time.

I just read the thread about the new Trane unit that was set (by a tech) to 4.0" W.C. instead of the factory spec of 3.5". You (Yuri) and beenthere had a lot to say in that thread. Realizing more and more, now, the value of searching here in DIY~ and googling for info.

Last edited by justplumducky; 12-20-2012 at 11:36 AM.
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Old 12-20-2012, 11:56 AM   #8
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Help with checking pressure switch


Well it sounds like you have a good mechanical background AND learning attitude so I don't mind helping guys like you. I get annoyed with the people who are too damn lazy to google anything B4 asking or just want entertainment but if a guy wants to help himself and work with us we will help U. Very few new guys these days (and I have trained 100's of apprentices) have a good learning attitude so come back if you need more info but only after googling first. There is a very good book "Fundamentals of Gas Utilization" by Dutton or Dossat which they use in the gas course here and I would highly recommend you get it and read it. Will make your life a LOT easier.

Good Luck

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Last edited by yuri; 12-20-2012 at 11:59 AM.
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