Trane high efficiency gas furnace works intermittently due to fresh air inlet pipe problems

smithpp

We have a Trane gas furnace located in a basement furnace room with input air and exhaust gas lines running horizenally to the exterior of the house. For several years the furnace worked fine. About the last 3 years the furnace has sometimes failed during startup. We have finally pinned down that the cause of the gas furnace refusing to start up is due to the air flow through the inlet pipe into the furnace from the outside the house, but we don't know why it is happening. The furnace is vertical with 2 holes on the top, one for exhaust (which appears to be working fine) and one for inlet fresh air from the outside. The furnace worked fine for about 3-4 years. It is 7 years old. Both the inlet pipe and the exhaust pipe currently use ASTM D2665 DWV PVC pipe schedule 40. The inlet air pipe is 2 inches and the exhaust pipe exits a 2 inch furnace pipe and immediately expands into 3 inches. If you remove the inlet pipe from the hole on top of the furnace and allow air to enter the furnace directly from the furnace room, the furnace will always come on and act normally. We have probably checked this 20 times. It may then continue working ok one more time, days, weeks or several months, but it always eventually stops coming on. Typically, it works for a few days or a few weeks. Multiple technicians have not been able to tell us what is wrong with the air flow (they usually say it is the pressure switch, which has been replaced twice). We think there is some problem with the air pressure of the air coming in that is keeping the pressure switch from closing. But why would it be intermittent? The manual says that the inlet pipe diameter should not be smaller than the exhaust pipe diameter, and ours is smaller. The installer just did it that way. But again, why would it be intermittent, and it doesn't explain why it worked fine for 3-4 years. If there is an expert in this area on the forum we would really appreciate any explanation of why this is happening. Should we replace the 2 inch pipe with 3 inch pipe? If so, can we use SDR 35 ASTM 3034 pipe instead (which is labeled sewer pipe). It is much lighter and would be easier to strap. The furnace room has an opening in the wall where the 2 pipes terminate to the outside. The opening is much larger than the space taken up by the pipes. Could we simply put a 90 degree connecter (to protect from debris) at the top of the furnace inlet hole and not extend the inlet pipe through the wall to the outside? Very much appreciate any advice we can get.

HVACNUT

Can you post a pic of the termination outside? From like 15 ft away if you can.

The intake is definitely free of obstruction?

Has the inducer motor been amped out? Don't know what degree of high efficiency, but a PSC inducer motor will draw less than 1 amp. 

Has the inducer assembly been removed and checked for birds in the fan housing? It happens. 

Also excessive condensate can build back up and trip the pressure switch or even backup into the inducer housing. All the drains were checked?

smithpp

The intake is free of obstruction. Pretty sure one or more of the technicians measured the amps but not positive. The motor sounds OK. It doesn't look like there is any way for birds to get in. Openings are screened. Externally the ends of the two pipes are 20 inches apart.
hope the pic uploaded

smithpp

all the drains were checked. NO backup.

ratio

I had a similar call, a several-year-old Trane condensing furnace wouldn't light reliably. The flue/intake was did not meet specs, so I corrected that, but still no go. I ended talking to Trane tech support. After verifying the pressure switch calibration etc., tech support condemned the heat exchanger. He said that if the flue & intake are installed per the manual and the pressure switch is working correctly, it can only be the HX. At that point, we handed it over to the original installer for warranty repair as we would have had to charge them to do it, so I don't know the final outcome; but I expect that fixed it.

smithpp

The furnace doesn't start, i.e., there is never any gas entering the furnace or igniting. So there is never any heat exchange although the input air flows through the HX. Also, why does it start when we just let the input air be the air from the furnace room? There is a window with a screen with a small mesh that lets outside air into the basement furnace room.

smithpp

Is it ok to use SDR 35 ASTM 3034 pipe for the furnace air inlet pipe. It is much lighter than the ASTM D2665 DWV PVC pipe schedule 40. It would make the strapping much easier.

ratio

The airflow though the burner, heat exchanger, flue, & intake all must be in the right range for the pressure switch to make. Of those four, the only one that gets clogged in a not-necessarily-visible way is the heat exchanger.

Although, I have heard of the intake ingesting a white grocery bag. It stuck to the side of the pipe due to static, but when the inducer came on it would balloon out & restrict the intake. My Old Guy said that was pretty hard to find!

Teemok

Bellies in the intake pipe? Water could be sucked in and collect creating a blockage. I like an exhaust up and out with intake facing down config. That would get exhaust out of the walk way. Tisk, tisk.

Teemok

Bugs in the the tubing related with the pressure switch maybe? I've found dead spiders and bees that caused intermittent pressure switch behavior.

PRR

> birds to get in
My last house, I found a bird in the stonework chimney, and a squirrel in the radon fan. The bird was cooked dry, but the aged squirrel about made me toss my lunch. I found the bird because the horizontal smoke-pipe was half clogged with crumbled masonry (unlined chimney from 1830). Found the squirrel because the radon stopped blowing when the squirrel tried to turn-around inside the fan.
Here my external vents get oversized screens. We don't get the big grey squirrels from NJ, but our cute red squirrels are an excellent fit in 2" PVC.

smithpp

pressure switch was replaced twice. Did not solve the problem. The intake pipe is completely clear and outside opening is screened. The intake hole in the furnace is clear. The furnace has started up every morning for the last 3 weeks, but it will eventually fail to start up. There is no pattern to the failures. Sometimes fails right away and sometimes lasts for 2 or 3 months. You can always "fix" it by removing the pipe from the hole at the furnace and then just replacing it back in the hole at the furnace again. Then it usually lasts for days/weeks/or a few months.

Teemok

The bugs (real bugs) might be in the small rubber tubing involved with the pressure and the narrow fittings that the tube attaches to. Especially at the heat exchanger port.

But.....lets back up. Model number would help.
The furnace will give a fault code. Flashing led sequence or how ever that unit communicates. Once you know what the control is telling you you can prove or disprove the existence of that condition. It gives you a place to start. Control boards do lie and intermittence is there game sometimes. Witnessing the process and seeing when it hangs up helps.
Since the pressure switch has been changed twice let assume it's ok. The draft inducer blower must prove it works by activating the pressure switch at the right time with negative air pressure. Inducers generally work or they don't but maybe the fan has a bad capacitor or bearings and it's not spinning at full spec. Maybe the blower wheel has a problem. Logic check where the fan pulls from and look where the pressure switch attaches. Could there be a blockage between them. You pulling the intake pipe may not have anything to do with the intermittence and it might. If the intake pipe is clear and short I don't see how it makes much difference but weird stuff happens. Condensate drain would be one of my checks. If it builds up it can cause intermittent problems. 1/2" -3/4" Hose at heat exchanger bottom drains to a trap. I confirm it drains. A small amount of water can be pored down the flue, it should come out of the drain pipe with little delay. Till more info.

EBEBRATT-Ed

Change the intake pipe to 3" Why debate this? The manual tells you they have to be the same size, Just do it. debating why it ran for years the other way does not help.

Maybe when you take the 2" down you will find something in it.


Your PROOF is that the furnace runs when the air intake is disconnected. If the exhaust pipe is clear and the air pressure switch is good and the blower and heat exchanger are not obstructed there is nothing else to look at.

JUGHNE

I had an issue with a cond furnace that was as head scratching as yours.

It had done well for 2-3 years but would lock out on pressure switch shut down.

The piping was 2" pvc in and out. It was just within the specs of length and fittings for 2" operation.

After a long dance with this thing.....switches....fan etc. I changed the outlet drain tee which came with the furnace. It was a 2" pressure tee with inlet on the branch, exhaust on the run going up and the drain on the run pointing down.

I installed a DWV tee with the (reduced) branch pointing down for the drain, the exhaust just ran straight thru the runs on the tee. Then a street 90 ell to go up and out the wall.
This improved the pressure readings more than enough to satisfy the switches.
Previously the exhaust gases had to turn a sharp 90 corner and now they have a smooth 90 to turn the corner.

It would have been better to build a 90 with 2 45's giving a longer radius to the 90 sweep but I was limited by room.

I solved another similar problem with this method also.

Another thing is if there is a "periscope" outside on the exhaust piping. The cold air will settle into that riser and fill the horizontal portion with heavy cold air. Some of these inducer fans are "just barely" powerful enough to move the air in thru the inlet and then push out thru exhaust. That cold air will be an added burden on the fan.

Teemok

Without knowing the model, fault codes, the quality of pitch and equivalent length of the flue and intake it's hard to say anything more. I've installed lots of Trane and American Standard furnaces with long but in spec flues and never found the inducers lacking. Heavy cold air is nothing I've ever heard any hvac tech or rep. worry about. We don't know if these pressure switches/switch are looking for positive, negative or both pressures. This is not how to diagnose. Chase the facts not theory's.

smithpp

Thanks to everyone for information about this. I have attached a picture of the furnace and the pipes connecting to it. The Trane model number is UC1D120A9601A . The length of the horizontal portion of the two pipes is about 13 feet. The pitch on the pipes is 1/4 inch per foot. The vertical distance is about 3 feet coming up out of the furnace with a 90 degree into the horizontal pipe. Distances are within the specifications in the table in the manual for this furnace. The fault code has three red blinks. Which the manual identifies as the pressure switch but we know the pressure switch is OK. There have been 3 pressure switches, the original and two others. None have made any difference with this issue. We have checked the condensate drain, along with repair techs, and it is working properly.

pecmsg

When the switches were replaced was the pressure tested?

smithpp

yes, it was tested. was ok. why would it only be OK part of the time, since the furnace works most of the time?

Teemok

From the manual that I found: 2" pipe is never aloud period, for UC1D120A9601A. With 3", If you count up all 90's as 5' of pipe plus both the in and out pipe you might be pushing it. 16ft x2 = 32' + 5'x6 (90's) =30' 30+32=62' max is 60' This is broader line with 3" pipe. I estimated 6-90's and you probably have less so you might be good with 3".
Also from the manual: "The inlet pipe can be of a larger diameter, but never smaller than the vent pipe"

Teemok

Teemok

Look at the fitting equivalence. A 3" short radius 90 is 10' of pipe. Do the math. 60ft max for both runs added. Each pipe (intake and vent) can't be longer than 30' equivalent pipe length. Every ft. and fitting counts.

pecmsg

smithpp said:

yes, it was tested. was ok. why would it only be OK part of the time, since the furnace works most of the time?

What was the reading?
how close to the trip point of the safety?

Teemok

@pecmsg you disagree with the manual? This is a 120,000 btu furnace.
@smithpp Change the intake pipe and be done with it.

EBEBRATT-


Ed said:

Change the intake pipe to 3" Why debate this? The manual tells you they have to be the same size, Just do it. debating why it ran for years the other way does not help.

Maybe when you take the 2" down you will find something in it.


Your PROOF is that the furnace runs when the air intake is disconnected. If the exhaust pipe is clear and the air pressure switch is good and the blower and heat exchanger are not obstructed there is nothing else to look at.

pecmsg

Teemok said:

@pecmsg you disagree with the manual? This is a 120,000 btu furnace.
@smithpp Change the intake pipe and be done with it.

EBEBRATT-


Ed said:

Change the intake pipe to 3" Why debate this? The manual tells you they have to be the same size, Just do it. debating why it ran for years the other way does not help.

Maybe when you take the 2" down you will find something in it.


Your PROOF is that the furnace runs when the air intake is disconnected. If the exhaust pipe is clear and the air pressure switch is good and the blower and heat exchanger are not obstructed there is nothing else to look at.

I didn't disagree I asked if pressure readings were taken or the code said bad safety, so they blindly changed the safety without checking the pressure.
That code only points to a direction Where to look!

smithpp

Again, thanks for the advice. Sorry, we don't have the pressure reading data detail. It was quite a while back. When we installed this furnace we relied on the installer to use the right size pipe and since all worked well we didn't read the manual at that time to double check the installation. Lesson learned. There are two 90 degree joints on each pipe, one near the furnace for the vertical/horizontal connection and one at the terminus to the outside. We think the equivalent length is probably OK for the 3 inch exhaust vent. Our plan is to replace the 2 inch intake pipe with a 3 inch 90 degree elbow at the intake hole on top of the furnace. Repair contractors have told us that we don't really need an intake pipe to the outside since the furnace room has a 2 1/2 foot by 1 1/2 foot screened louvered opening to the outside (no glass) and the room gets plenty of fresh air through it. The two pipes currently extend through this opening by cutting the louvers slightly to allow passage to the outside. We have put a 90 degree elbow (only) into the intake hole at the top of the furnace and it works but we may have to wait for some time to see if it is a permanent fix. We could replace the entire 2 inch pipe with 3 inch pipe but it isn't clear it would be any better than just using the 90 degree elbow. The furnace room is closed off from the rest of the house with a door, so essentially all the air in the furnace room is outside air.

Teemok

The only reason to have two pipes (direct vent) is to avoid using the air in the room you just heated as the air for combustion. There are combustion air opening rules in the code. You must account for all gas burning appliances in the room to properly size the openings needed. Separate high and low openings are often required. You might be just fine with a 90 on the furnace but a professional with eyes on the seen can tell you, not me. 3" installed correctly and it will be done. Support those horizontal vent pipes every 4ft. Glad it wasn't super natural.

Teemok

@pemsg? We're cool, I just didn't understand.