It is well below freezing. A high-efficiency furnace PVC exhaust pipe with an elbow outside the house got blocked with an ice clog. I'd like to remove the clog. How to do it? Shall I pour hot water?

There are several questions that need to be answered about what type of system you have, but i found this posting that may help:
As your gas furnace runs, is there water leaking at the base? Is the furnace's air filter wet, too?There are a few things that could be happening here. But if you have a high-efficiency furnace, here's the most common cause: a condensation leak.
We'll explain why a furnace creates condensation, and what's causing it to leak out of your furnace.
Note: You'll know if you have a high-efficiency furnace if:

• The yellow energy guide tag says your furnace's efficiency is 90% or higher AFUE.
• The vent/flue pipe is white PVC as opposed to metal.

Why a high-efficiency furnace creates condensation
How in the heck does a high efficiency furnace create condensation/water anyway?
It's all about how the furnace extracts heat from combustion gases.
A lower-efficiency furnace extracts some heat from combustion gases and then quickly vents them out the flue pipe.
But a high-efficiency furnace, to extract more heat, has 2 heat exchangers. These allow the furnace to extract heat from the gases for a longer period of time. This causes the combustion gases to cool and then condense.
That condensation then exits out your home through a drain. But if that water is pooling around the furnace, there's a problem preventing it from draining properly.
Causes of a condensation leak around a high-efficiency furnace

• Clogged condensation tubing
• Clogged condensation drain
• Breaks in the condensation line
• Issues with the condensate pump (if you have one)

You'll need a furnace technician to diagnose which of these is the issue and then fix it.
Other causes of leaking water around a furnace
If you have a conventional, standard-efficiency furnace...then its flue pipe may be incorrectly sized.
If you have a whole-house humidifier connected to the furnace...there may be an issue which is causing the humidifier to leak into your furnace. Your humidifier should be getting annual maintenance from a professional to prevent this.

When natural gas or propane burns, water vapor is given off within the flu gasses so a bit of condensation in, off and around the flu discharge is quite normal. If it is a high efficiency furnace with the pvc flu pipe, they must be pitched properly BACK TO the furnace so the moisture will drain backwards and out a drain tap at the bottom of the riser. That style of furnace will have TWO drains...one up high for the air conditioner coil and one about 16" up from the bottom of the furnace off of the 'condensing coil' that the flue gasses pass through to extract a much greater amount of heat from the burned gas.

Many flu pipes are installed incorrectly which causes water to drip outside instead of back to the furnace and this usually results in huge icicles hanging off of them in the winter and eventually the partial blockage causes the flu's pressure switch to open.

If you have regular galvanized flu pipe, you have less than a 90% high efficiency furnace and the resultant flu gas temperature is much higher with a much greater quantity of water vapor in it. A little condensation is somewhat normal on the top of the flu cap as the hot moisture laden vapor comes in contact with the cold and dry outside air. If the moisture is all over the galvanized flu pipe in the basement, you are possibly venting into an oversized and unlined masonry chimney. IF that's the case, the chimney should really be lined with a flexible liner and properly connected to the furnace and water heater (if gas fired) with the proper connections and configuration.

Any furnace providing heat where it should and has little to no heat from its exhaust is doing a GREAT job ;-)

Our high efficiency gas furnace was installed with a PVC pipe as a 'chimney' which bothered me to no end until I found the surface temp of that plastic pipe only barely warm to the touch- that is a home run!

I'm surprised the building inspector allowed the condensing furnace you described draw combustion air from the garage !! I'm not familiar with any manufacturer that recommends that as normal practice. Virtuall ALL two pipe condensing furnaces require both pvc pipes to be terminated OUTSIDE the structure with specific heights and distances apart. There is also a limit to the length of both the flu and the combustion air piping and each 90 degree pvc elbow adds the resistance equivalant of roughly 10' of straight pipe. If re-routing is required, you have to bear in mind the direction of 'fall' in the piping usually goes back towards the furnace and remember two 45's may give you a greater variance of change in the piping and still be equal to the equivalant of one 90.

Personally, I would never suggest terminating the fresh air (combustion air) intake between floors of a structure.

Water can leak from any furnace for a couple of different reasons.
If it is in AC mode, the AC evaporator could be freezing due to lack of refrigerant or airflow (check your furnace filter). Or the AC drain could be clogged normally a hose or PVC pipe coming from the evaporator.
If it is in heating mode could be high efficiency furnace not draining properly or leaking humidifier.

First, check the drainline from the furnace. You have a 90% gas fired furnace. Where 80% furnace has 1 heat exchanger, yours has 2 heat exchangers. And the second heat exchanger will cool the exhaust enough, that it ends up below the dew point, and your furnace outlet/exhaust will condensate. This drain is probably stopped up. Now, I've installed and service many of these systems and I've only seen the piping at the roof you described once. And that was when the roofers replaced the roof, damaged the furnace flue pipe, and tried to repair it themselves. Normally, and this may not apply to yours, we tie in the exhaust/supply lines about 2-3 ft. above the furnace. Then 1 pipe goes through the roof. Thus 1 pipe is a kit that has an inner liner & outer pipe(2 pipes in 1). Now I'm not sure if this is a problem or normal for your system, but it may be worth looking into. You should be able to find it in the installation manual, or call a local parts house that sells your brand, or as your tech. And if it's just your drain, forget all about the piping I mentioned. Hope this helps!

Hello Jmranes,
It sound to me like your getting alot of condensation on your exhuast pipe and a weep hole was drilled to "BAND AID" the issue. These is not how 90% plus furnaces are installed. If you are referring to the outside intake and exhaust PVC piping, then there shouldn't be any weep holes. When the furnace is properly installed, the furnace comes with a condensate kit (refer to owners manual) that is mounted on the side of the furnace and all the condensation that collects overflows and should be piped directly into a floor drain. As for the intake and Exhaust for side discharge application, the Exhaust PVC pipe should come out through the side of house and immediate goose neck up (90 degree) with a straight piece of PVC pipe about 2' to 3' or so in length, then a final 90 degree PVC fitting on that so the exhaust blows out. As for the intake, that also comes out the side of the house about 2" to 3" inches parallel to the Exhaust but should only be a 90 degree PVC fitting facing down. The idea here is making sure the furnace doesn't suck in exhaust air. Also, making sure proper size pipe was used. Both outside pipes should be open to atmosphere. I hope this helps.

Generally, the furnace must be at least 90+ efficiency to be able to use PVC pipe. In addition, high efficiency gas furnaces require a condensation pump, to pump excess moisture out of the furnace. If yours has always been venting up a vent pipe, then NO, you can't use PVC pipe.

Hope this helps you.

i have fixed almost every one of pc9 -----we have installed in our company,code three{flashes} at first new presure switches installed, same thing,york knows about this,induction motor bearings,could be bad.our local york dist, has boards,inducer, ***.motor, dual press.switch,free.as is known as the only way to fix.after all else is ruled out.one local competator called us if we wanted to join class action suit . good luck.furnaces work good at first, season or two then bam

code 31 indicates pressure switch did not close or reopened during the heat cycle....1st thing to check is the inducer motor,,this is a small motor that turns a small blower wheel that draws the exhaust gases through the furnace and blows them into the vent pipe....1st locate the inducer assembly,,most easily identified by the small plastic fan blade that cools the inducer motor as it runs..1st turn off tstat and turn off furnace power switch..then turn on furnace switch,then set tstat to heat,set the temp above room temp by 5 degrees...remove the furnace front cover,locate the inducer motor,,if it isnt turning,but is warm or hot after a minute or so,likely the inducer motor is bad..if it isnt turning and isnt warm after 1 minute,then likely the main control board is bad,,not turning the inducer motor when it should,,,,,if the inducer motor is running,,then the pressure switch isnt closing,, could be a bad pressure switch but more likely there is a problem with the venting...some possible problems that would cause the pressure switch not to close in an 80% eff furnace [metal vent pipe] are: [1] a blockage in the vent pipe,base of chimney,dead bird at the furnace where vent connects to furnace or down inside the inducer assy,accessed by removing vent pipe from furnace....if all clear the chimney could be blocked up inside,,is the draft good when vent is removed from base of chimney?..if furnace is a 90%+ type [pvc plastic vent pipe],,then check the intake pipe at the furnace by removing it where it attaches to top of furnace..is the intake clogged with anything?..then check the intake and exhaust pipes outside for blockages,,including ice/frost/snow at the exhaust pipe outlet..