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Furnace Works Only When +ve Pressure Tubing Disconnected from Pressure Switch

Furnace Works Only When +ve Pressure Tubing Disconnected from Pressure Switch

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  #1  
Old 10-16-11, 03:52 PM
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Question Furnace Works Only When +ve Pressure Tubing Disconnected from Pressure Switch

I have a Keeprite DC90 Condensing Furnace about 10 years old in my cottage. I use the furnace only a few times a year and found it not working last week, the first time I try to use it this year.

The inducer fan keep running but the flame and the blower fan never started. I measured the pressure switch with an Ohmmeter and it is OPEN.

The pressure switch is a Tridelta PPS10009-2174 and has 2 connections for rubber tubing, with a ‘-’ and ‘+’ sign on the left and right connection respectively. The left ‘-‘ connection goes to the inducer fan and the right goes to the combustion chamber.

I disconnected both tubing and confirmed that they are not blocked. I also discovered that the pressure at both tubing are negative (sucking). I also found out that when only the tubing (‘+’) to the combustion chamber is disconnected, the furnace works normally. When furnace is burning, I saw water dripping from the drain pipe, which suggested that the condensation trap is not blocked.

I don’t think I can leave the tubing disconnected, so my questions are:
1. Should the right connection tubing (‘+’ to the combustion chamber) have positive pressure instead of negative?
2. If yes, what caused the negative pressure? I checked that that the PVC pipe ends outdoor were not blocked
3. If no, what is wrong? Does it mean the pressure switch is not working? I take it out and verified that it closes when I blow (with my mouth) into the + end and suck at the ‘-‘ end.
4. I also noticed that the exhaust PVC pipe outdoor is tilted up at that end such that rain water can be flow into the furnace – it is OK?

Any help or suggestion will by very much appreciated!
 
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  #2  
Old 10-16-11, 04:19 PM
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most likely your venting is blocked by leaves or an animal or a nest. check all your venting (intake and exhaust) to be sure it is clear!
 
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Old 10-17-11, 05:34 PM
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I did check with a fish tape and did not feel any blockage, I'll check again this weekend. Do I need to cut the PVC pipe(s)? Thanks!
 
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Old 10-17-11, 06:52 PM
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something as small as a bees nest could block the air enough to cause the pressure switch to not make. You may have to cut and then repair the flues to ensure they are free and clear. Best to call in a pro with the proper tools to determine the cause.
 
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Old 10-17-11, 07:45 PM
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I'll try one more time this weekend and call a pro if no success.

I did tried to contact my furnace installer before I try to fix this myself but they seem to be too busy and just didn't call me back even I left message and also talked to the receptionist.

Thank!
 
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Old 10-17-11, 11:40 PM
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I presume this has two PVC pipes. If so, the one pulling air inside is the one likely to be plugged. Pull that pipe off at the furnace and see if the furnace will light up. Check for any leaves or debris where the pipe disconnects from the furnace or along the length of the plastic pipe.
 
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Old 10-18-11, 06:17 PM
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Thanks to everyone for the advices. I'll let everyone know about the result after this weekend.
 
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Old 10-23-11, 09:29 AM
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99% sure this is not a blocked pipe, tested as follow.

The intake and exhaust are both a inverted L, 4ft up then 7 ft across. I can hear and feel strong air flow/suction when motor is on.

From outside at the PVC pipe ends
1. Wrap around the intake with a supermarket plastic bag (the transparent ones that used to hold produce) and it was suck in immediately if I didn’t catch it
2. Wrap around the exhaust, it inflated and flow away immediately.

At the furnace, connect multimeter to 2 ends of pressure switch
3. In normal connections (both ‘+’ and ‘-‘ tubings connected, switch remain open
4. when only ‘+’ is connected, switch remain open
5. when only ‘-’ is connected, switch closed in a couple of seconds
6. with ‘-‘ connected, when the ‘+’ tubing is bring closed to the switch, switch become open.

Can anyone tell it is normal to have the ‘+’ tubing giving a negative pressure? I think I I need to buy a switch to try it out.

Thanks!
 
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Old 10-23-11, 09:37 AM
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buy the switch. still can not say for sure the problem is that. You need a manometer to properly test the pressure switch. 1/10 of an inch of water column of pressure will make the switch not make contact. Call a pro or spend less money than a pro on a new switch. 99% of the time it is not the switch at fault, but there is still the 1% of the time that it is. take your chances...
 
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Old 11-03-11, 06:45 PM
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Hvac

You have exactly the same pressure switch as I have and maybe the same furnace NUGM075EGB.
You also have exactly the same problem that I have.
With regard to your question of negative pressure at the + side of the switch. The right side marked + means that it is a higher pressure than the left - side. Now both the left (inducer fan) and the right (combustion chamber ) have negative pressures. The left on our furnace should be at about -2.5" (new furnace condition) and the right should be about -0.1". In this case the differential would be 2.4" All this info is in the "technical Support Manual " which you may have got when you bought the furnace.

The reason that the switch works when you remove the right tube from the switch is that it increases the differential across the switch by about 0.1" and that can be just enough to make the it operate. I had the same situation. My problem is that the induction fan is not creating sufficient suction (neg. press.) to operate the switch.

You may have the same problem. You would need a manometer to tell you this.
If I find the problem on my furnace i will let you know.
 
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Old 11-06-11, 03:05 PM
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Hi Huge5, thanks for the detailed explanation.

I bought via online a pressure switch (it take more than 10 days to ship from Indiana to Ontario, Canada). I installed it this weekend and it works right the way.

I would not go through all these trouble if my installer can be a little bit more flexible on his schedule. He refused to work on weekend. He also said that if he do not have the parts with him such that he has to come again, it will be addition charges for the 2nd trip (and I have to take another day off!). I am not sure if other contractors are the same but this already triggered me to start the process myself.

Thanks everyone again for all the information!
 
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Old 11-07-11, 06:35 PM
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good news. can you give me the source for your pressure switch and how much it cost please i am thinking of getting one at a lower pressure setting.
 
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Old 11-08-11, 04:05 AM
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Don't post that info here. Send it through a pm!
 
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Old 11-08-11, 02:17 PM
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My recommendation is to install ONLY the pressure switch specified by the manufacturer.

The pressure switch is a VERY IMPOTANT safety device and making substitutions could well be hazardous. In particular, intentionally installing a pressure switch that closes differently than that designed by the manufacturer is a VERY BAD and UNSAFE idea.
 
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