Tag: vent

This is a recent call I ran that clearly had an interesting past. It had a condensate pump to pump the water just a few feet up to a drain that went to a common drain. The “cleanout” after the trap is supposed to be a vent, not a cleanout, which means it needs to be uncapped and vented higher than the indoor drain pan so that if the drain backs up the water goes back into the system and trips the condensate switch.  With the current position, the vent could not be raised because it would come right up in front of the filter.

My best guess is that when the installing contractor installed a new, larger air handler it no longer had the fall they needed to get the existing drain so they capped the vent (seen in the photo above before it goes into the wall).

I’m using this system as an example because it has every possible drain don’t you could come up with

  • No Cleanout
  • No float switch (condensate switch) to prevent pan overflow
  • Vent Improperly placed or causing filter obstruction
  • Using a pump when one wasn’t originally needed
  • Using a common drain with no vent
  • No insulation on the condensate drain horizontal runs

We did rectify as much of it as we could here which included

  • Adding a condensate switch
  • Repiping with a proper cleanout and vent before the pump
  • Insulating the horizontal portions

Unfortunately, we couldn’t eliminate the pump and I forgot to take “after” photos

This is an example of a drain properly pitched, insulated, trapped and vented with a system and secondary pan switch installed. Two things I do notice on this one is the pan doesn’t look like it overlaps 3″ at the top of the unit, The cleanout would be better right above the trap and I can’t tell what’s supporting this drain to ensure the pitch stays correct.

It’s amazing how much there is to the simple, humble condensate drain.

— Bryan


Double traps are no good. End of tip. Ok, here is some detail.

Anytime your drain goes up and down more than once you have a double trap UNLESS you place an air vent between the two traps that vents ABOVE the drain inlet.

The double trap causes drainage issues because air becomes trapped between the two traps and air is lighter than water. This causes the air to want to travel up as the water flows down resulting in NO DRAINAGE.

A vent allows the air to move instead of becoming trapped. This is why you vent a drain after the first trap if there is another trap or the potential of another trap.

This is also why you vent a drain after the first trap and before a common drain if you are connecting more than one drain. It helps prevent the possibility of a double trap and thus prevents a nasty backup.

In general just pitch the drain properly, install only one trap and don’t interconnect (unless required) and you will have no problem.

That was easy!

— Bryan

This article was written by senior furnace tech Benoît (Ben) Mongeau. Ben hails from the frozen tundra of Ontario, Canada where high efficiency gas furnaces are commonplace.

While some codes and practices may be different from the US I find that most of it is common sense and translates pretty well. One glaring difference between Canada and the USA is the requirement in Canada for specifically certified PVC or CPVC vent pipe. Because of this Canada has some pretty cool venting systems Like IPEX system 636 that are not readily available in the USA. I’m leaving all this in because there is already talk about making the change in the US so I bet it’s coming.

Venting for High efficiency Gas Furnaces –  Assembly

Here are some good venting practices.  (This is mostly stuff I learned during a training session from IPEX, one of the major manufacturers of plastic piping, with a little of my personal experience and tips)

First of all, venting must be planned in order to be sized properly.  Depending on the BTU rating, length, number of elbows in the run, the size will vary, typically in residential from 1½ to 3-inch pipe.  Every manufacturer has its own vent sizing charts.  Read the manual, don’t guesstimate.

Use the proper tools when installing plastic venting.  !!!Avoid using a sawzall or hacksaw to cut your lengths!!!: it creates a multitude of statically charged shavings that will stick to the inside wall of your pipe.  Once that condensate starts flowing, it will bring all those shavings to your drain and trap, blocking all those narrow passages and causing water backups, furnace failures, all kinds of things to piss off your service colleague who’s on call that night.  I highly recommend using a proper pipe cutter.  It is the best way to achieve a clean, straight cut.  The straighter and neater the cut, the more joining surface you have once you’re cementing it together.

vent pipe cutter and chamfer/deburring tool (REED venting solutions kit, which I highly recommend purchasing 

It is recommended to dry-fit the whole vent system before actually starting to cement joints together, just to be sure your lengths and angles are good.  Also, as mentioned in my condensate drainage tip, make sure the vent is sloped towards the furnace for the whole length.

Before applying cement, prepare the pipe ends by cleaning them up (wipe off any obvious dirt) and, most importantly, reaming them.  Use a proper reamer / chamfer tool (pictured with the cutter above).  This is a crucial step: if the pipe end is not reamed/deburred, the edge actually tends to slightly stick outwards, especially when cut with a proper cutter, ironically.  This will cause the pipe to push (I like to call it ‘’snow plowing’’) the cement at the bottom of the joint instead of letting it slip around the pipe, leaving large uncemented gaps in the structure of the joint and often causing leaks.  See comparative pictures of chamfer/un-chamfered pipe ends below.


Cut, not reamed /chamfered 636 PVC pipe


The same pipe end, reamed / chamfered and deburred  

Next, once all pipe ends are reamed and clean and ready for assembly, it’s time to start cementing.  Apply primer first if required, then apply the cement.  Don’t be shy, apply a generous coating around the whole joint surface of the pipe and fitting (yes, cement is applied on both the pipe and the fitting).  I recommend going around the pipe/fitting 3-4 times with the dabber/roller/brush to ensure a full coating.  Once both ends have cement applied, quickly (before it dries!) push them together, straight and all the way to the bottom of the joint, and as much as possible try to give the fitting a quarter-turn while assembling the joint to further evenly coat the entirety of the joining surface.  Very important: once you hit the bottom of the joint, hold the pressure for about 30 seconds (or longer) so the cement has time to set!  If you let go immediately, the still wet fitting and pipe will naturally pull back from each other and this can easily lead to leaks.  Wipe off any excess/runoff cement if necessary and proceed to the next joint.

Once assembled and when the cement has dried, as I mentioned before, the two pieces are basically welded together.  You cannot take them apart, so make sure your angles are correct, or you’ll have to cut it out and restart.

Support the pipe as necessary, per local codes/guidelines.  Support spacing usually varies depending on pipe size.  Avoid creating too much tension on the venting as it can lead to leaks/cracks.


Other tips:

  • Don’t leave your cement cans open longer than necessary.  The solvent part of the cement is quite volatile (evaporates easily) and as it evaporates, the viscosity of the cement will increase and it will get more difficult to use.  Once your cement has gelled (i.e. has a consistency very reminiscent of that of Jell-O) throw it out.  It is no good.  Keep an eye on your cement’s viscosity.  It should always be liquid, although with various degrees of thickness depending on the type, but NEVER jelly.
  • If a reducing fitting is used on your venting, always install it on a vertical portion, never horizontal, otherwise it will allow for condensate to pool in the vent.  Remember… slope for drainage!
  • Respect local guides and regulations and manufacturer’s specs regarding clearances when choosing where to terminate the venting outside.  Also, terminate the exhaust higher than the air intake (usually about 1ft minimum) if you are installing a sealed combustion system, to avoid recirculation of combustion products which can be quite disastrous.  Typically on a sidewall termination the air intake will be terminated with a downward-facing elbow and the exhaust will be snorkeled up, i.e. elbow up, 1ft pipe, then elbow out away from the wall.  There are also manufactured termination kits (concentric, for example) that are available and sometimes easier on the eye.  Make sure it’s certified, though!  Again, manuals will tell you if there are termination kits available and certified for use with the product.
  • Be careful to read install manuals for any specifics regarding the furnace you are installing.  There is often specific procedures for attaching the vent pipe to the cabinet’s internal exhaust fitting/flue collar etc. and it will vary from one manufacturer to the other.

— Ben

You can read the full IPEX 636 install instructions HERE

This article was written by senior furnace tech Benoît (Ben) Mongeau. Ben hails from the frozen tundra of Ontario, Canada where high efficiency gas furnaces are commonplace.

While some codes and practices may be different from the US I find that most of it is common sense and translates pretty well. One glaring difference between Canada and the USA is the requirement in Canada for specifically certified PVC or CPVC vent pipe. Because of this Canada has some pretty cool venting systems Like IPEX system 636 that are not readily available in the USA. I’m leaving all this in because there is already talk about making the change in the US so I bet it’s coming.

Venting for high-efficiency gas furnaces – Materials

Due to the condensing nature of a high-efficiency furnace, its venting must be made of a material that is resistant to corrosion. In a great majority of cases, plastic piping is used to vent high-efficiency equipment. It is classified as “Type BH” venting. The lower temperature of the exhaust gases also mean that the natural draft effect observed in conventional metal chimneys (heat rises) does not occur at a significant level. Which means those exhaust gases have to be forced outside. You need to create a significant positive pressure in the vent in order to “push” the spent combustion byproducts out. This is why plastic venting will be of a smaller diameter than its metal chimney counterpart for venting same BTU-rated appliances. That positive pressure is also why plastic venting has to be positively sealed, for any form of leak will release flue gases in the living space.

There exist many, but mainly three types of plastic are commonly used for high-efficiency appliance venting: ABS, PVC, CPVC.

(Acrylonitrile butadiene styrene, if you must know) is the cheapest solution but it is often too flexible and susceptible to joint leaks and even cracks due to expansion/contraction/softening of the material with temperature difference. Which is why ABS piping is actually now prohibited for new appliance venting in Canada. Never use primer on ABS.

(polyvinyl chloride) is what is most commonly used nowadays. There are different types/grades of PVC on the market and some of them may not be allowed for use as flue gas exhaust. Always check your local/state/province codes and regulations. For example, here in Canada Schedule 40 PVC DWV (drain PVC) may not be used. Only FGV (flue gas vent) PVC certified to a specific standard (ULC S636) may be used.

Note from Bryan: In the USA schedule 40 DWV pipe (the usual stuff) is still the standard, there is talk this may change soon so stay tuned.

(Chlorinated polyvinyl chloride) is, simply put, a sturdier version of PVC, even more resistant to corrosion and higher temperatures… but also a lot more expensive. It is more often seen on high-efficiency residential boilers, where, in some applications, even PVC is not sufficiently resistant. For easy recognition, vent/drain piping is usually color coded. Most often, ABS is black, PVC is white and CPVC is gray / tan. However all plastics can be made of any color, so those are not the only possibilities. Be extra careful about that especially when it comes to certain fittings supplied with the furnace. A prime example would be the vent flange on new Carrier/Bryant/Payne furnaces. It is black, but actually is made of CPVC. Which means you may not use ABS (or PVC) cement to attach it to your venting.

Note From Bryan: Read the manufacturer’s instructions

Those plastic piping systems are joined with a cement, which most people will incorrectly call glue (it’s okay, I usually say glue too). It is not glue. It is not an adhesive. Cement is basically the plastic you are working with, dissolved in a solvent. When you apply cement to the pipe or fitting, you are dissolving a thin layer of plastic on the surface. Once the joint is assembled, the solvent part of the cement evaporates, leaving only a continuous piece of plastic that is now basically part of the pipe and part of the fitting. The two pieces become as one (how poetic!). They are basically welded together. Always be careful to use the adequate cement. PVC cement will not bond properly to ABS or CPVC. An exception I know of would be the IPEX System 636 CPVC cement, which is certified for joining both PVC and CPVC pipes in any manner (PVC to PVC, PVC to CPVC, CPVC to CPVC) . Always use the correct cement, made by the same manufacturer as the pipe you are installing, since it uses the exact same plastic “recipe”, if you will. It is the only way to ensure a proper bonding (again, in Canada they utilize certified systems).

In addition to the cement, there is also primer, which is nearly pure solvent. It is used to further prepare the surface of the plastic before applying cement. Note: In practice it not necessary to always use primer on DWV pipe (UNLESS IF SPECIFIED BY YOUR LOCAL CODES). Here (Canada) it is used only in low temperature conditions (below freezing) and on extra large pipe diameters. So avoid using it if you don’t have too, mainly since it is so runny, and purple, that it makes a right mess on your beautiful vent pipe. Also, CPVC and ABS do not require a primer (according to Oatey)

As always, READ the manufactures instructions on the furnace / boiler being installed as well as the pipe / cement being used to ensure that you are using the correct
materials for the job. In part 2 we will cover more specific vent fitting tips.

— Ben

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