# Pressure switch problem??



## burchis

Good morning, this is my first post. I feel fortunate to have found this forum and hoping to find an answer to my problem.

I had a Heil furnace (#GDE100F14A2) installed into my 30 year home just over 5 years ago. (Funny how problems always occur after the warranty ends). After noticing that the igniter was continuously cycling in and out I contacted the professional for a service call.

His remedy was as simple as blowing into the pressure tube to clean it out. Quickest $65.00 that fellow ever made. The furnace continued to work fine for about two weeks then the same thing happen. But, I was too savvy to shell out another $65.00 so I blew into the tube and all was well for about another two weeks. Well it's been about 6 weeks now and I have learned to just leave the lower cover off of the furnace to make it easier for me to blow into the tube.

Can anyone comment on what they feel could be the cause of this. Since the problem only occur after several days of satisfactory operation I doubt that there are any mechanical failures. And since the furnace worked fine for the first 5 years I doubt that heat vent/cold air returns balance is a factor. Thanks in advance for any and all suggestions.


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## NickTech

when you blow in to it, what comes out? is it water? check to see if there is a natural trap in the tubing holding the water. trim it back as to keep it as straight as possible so any moisture will drain away. ***if the you can see the ignitor energize, then it is not the pressure tube. the pressure switch and tube must first work properly in order to allow the ignitor to come on. i am willing to bet it is the flame sensor located on the burner. it is the single rod that senses if you have flame present. if this rod is dirty or not engulfed in the burner flame then it will shut the system down,then retry. these can be cleaned gently with steel wool. also check to see if the whole burner lights. the flame rod may be located on the opposite side of the ignitor as to ensure the complete ignition of the burner manifold. this may be a gas pressure issue, related to improper gas piping and "volume theft" from another gas appliance making an intermittent problem. the reason why it worked after you blew into it may be. because when you blow into the tube, you first shut off the power. shutting off the power will reset the module to allowing it to relight.when this happens again don't shut off the power immidiately. look throught the lower compartment door thru a peek window and observe a red LED light. count the blinking sequence. then turn off the power and remove the bottom panel. behind the door is a diagnosis chart of blink codes and problems. let me know what happens. :Thumbs:


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## burchis

I don't have to turn off the furnace to blow into the presure tube. The lower door doesn't have the saftey switch, only the top door does.

Keep in mind that the furnace works fine after blowing into the tube. Could the problem be a slighty clogged or restricted flew vent. The piping is at least 25 years old. Could soot and other debris cause too much restriction in the flew whereby the pressure switch is sensing inadaquate venting? However, if this seneraio is correct then why does the furance work fine for about two weeks before having to repeat blowing through the pressure tube?

Maybe condesation is gathering in the tube. But what is causing the condensation? The furance worked fine for the first 5 years?


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## NickTech

perhaps you have a sticking pressure switch. before blowing it out again try jumping the 2 leads at the p-switch. if it starts the burner sequence then thats what it is. well if you have a hi efficiency furnace it would be designed to condense liquid in the flue pipe, the flue pipe being PVC. however if your your unit is a medium efficiency unit it would not condense water, being that it is about 400 degrees and it's made of sheetmetal pipe. goodman had that problem with moisture collecting in the p-tube and not allowing the unit to come back on.


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## burchis

Nicktech, I really appreciate your replies. I am concerned however. Why do you feel that if bypassing the pressure switch and all works well that the problem is the pressure switch? The pressure switch must have a function, ie- to detect sufficient pressure in the system. If I were to simply bypass this switch there is no guarantee that the pressure is adaquate. Just trying to understand the logic here. Please explain, thanks


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## NickTech

If You Are To Jump The Pressure Switch, It Is Only To See If The Ignition Begins, It Is By No Means A Permanant Fix. The Jumper Is Not To Be Left In The Circuit. Most Furnace Modules Will Detect The P-switch By-passed And Won't Let The Ignition Start Anyway.
This Is An Intermittent Problem You Have Where You Need To Utilize A Process Of Elimination. You Already Know The Venter Motor Works Properly, And That It Draws A Sufficient Draft. Unless There Is An Intermittent Problem With The Inducer, The Other Thing To Look At Is The P-switch. A Restriction In Your Flue Pipe Would Most Likely Be Permanant Until Cleared And Not Intermittent. If Your P-switch In Sticking Open The Ignition Will Not Start. If He P-switch Is Stuck Closed, The Furnace Module Will Not Allow The Burner To Start. This Can Be Diagnosed By Using The Blinking Codes On The Module Itself. A Digital Manometer Can Be Used To Test The Negative Pressure Draw From The Venter. It Should Be A Ballpark Of -1 To -3 "wc.


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## burchis

NickTech,

I have determined (99%) that my problem is condensation in the pressure tube. I have positioned the tube in an upward arch to prevent moisture gathering. I will have to allow the system to function for a few days before I can be certain that my problem is corrected.

What causes this condensation in the pressure tube? I understand that when warm air rapidly turns cold the net result is moisture. My furnace is located in my non-heated garage but it never gets below freezing. The flu vent is about 20 feet long with a couple of bends.

The furnace is in its 5th season now and this problem never occurred before. Why do I now have condensation settling in the pressure tube? Thanks for any further input.


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## Marty4164

What size is the old vent?
Did they run a liner down the old chimney if needed?
LOW TEMPERATURE RISE = LOW FLUE TEMPERATURE = CONDENSATION

Could be a number of things :
underfired furnace
oversized vent pipe/chimney
low temperature rise/high CFM across the heat exchanger.
Check the TD rating on your furnace. If its lower than the rating you should lower blower speed.
The volume of air is high. The temp rise is low, the BTU transfer from the flue gasses to the air stream is high. So the flue temp is low. Problems: cool,drafty air discharged from the registers. Condensation of flue gasses causing corrosion and damaging flue pipes and heat exchangers. Water in pressure switch hoses.

This info was taken from a Feb. 2000 service bulletin from Heil. Hope it helps.


One more question. What type of ignition does your furnace have?
I know Heil likes to use the smart valve which they had a problem with. They had a bad connection internally and if you wiggle one of the plugs it would work. Could you be moving one of the plugs when you're working on the pressure switch?? 
Just a thought.


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## burchis

Could a colder then normal attic cause condensation? Could this cause the flu to get too cold and thus cause condensation down at the furnace? Just trying to eliminate hidden possibilites.


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## Marty4164

What type of chimney do you have? Is it 6" B-vent going through the attic? 
Is it a brick chimney with a tile liner? Tell your service company they didn't fix your problem and have them look into your venting problem.
Are you sure you don't have an older model SMART VALVE?


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## burchis

Thanks Marty for your help. I'm pretty sure that it's not a smart value ignition.

The flu is 6" duct pipe going thru the attic, not a chimney. I have replaced the pressure switch tube with a clear tube and positioned it in an upward arch as to avoid any condensation accumulating and causing a problem. This has seemed to corrected the problem for now. I still see condensation forming in the tube but there has not been sufficient enough to emulate the original problem.

I read your reply about Heils bulletin and a lot of it made sense. I am curious about the tempature problem in the attic or the oversize of the flu vent. Is a 6" vent normal? I'm sure this would of been normal for the old Williamson furnace that was originally installed 30 years ago. But the 5 year old Heil furnace, although not a high efficienecy furnance, may not require this size of flu vent. Again this is the fifth year of service and the problem never accured until now.

Thanks for all who has addressed my problem on this fourm.


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## NickTech

well HEIL sells a retrofit door kit for units not using a liner. your case may be similar, not enough air volume/velocity. by code the vent pipe shouldn't be more than 2 nominal sizes from the size of the vent connector at the furnace. if the connector is 3" the max size vent should be 5" and the min should be 4". is the flue pipe in your attic single wall or type B vent (dbl wall)? the out side diameter will be off by more than an inch since the nominal size goes by the inside diameter. to see what you have check the connections of pipe, are the rough cut/crimped the put together. or are the special fittings that lock together like type B vent?


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## burchis

The flu vent exiting the furnace is 3" diameter. The adjacent water heater flu vent is also 3". The two separate vents enter a "y" pipe and then into a double walled 6" pipe that goes up thru the ceiling, thru the attic and out the roof. Photo of furnace and water heater piping included.


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## burchis

When determining flu vent size, would I want to consider the shared venting of the water heater? ie- 3" water heater vent + 3" furnace vent = what size of vent after the 'y' pipe?


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