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Venting for High efficiency Gas Furnaces – Part 1 Materials
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 for specifically certified PVC or CPVC vent pipes in Canada. Thus, 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 the article because there is already talk about making similar changes 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 means 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 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 the 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.
Many types of plastic exist, but we mainly use three types of plastic 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. That 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 are rumors that this may change soon, so stay tuned.
(Chlorinated polyvinyl chloride) is, simply put, a sturdier version of PVC, and it's even more resistant to corrosion and higher temperatures… but it's 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, or 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 type of 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, but it has been dissolved in a solvent. When you apply cement to the pipe or fitting, you dissolve 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 one (how poetic!). They are basically welded together. Always be careful to use the adequate cement type. 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 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 to, mainly since it is so runny and purple; 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 manufacturers' instructions on the furnace/boiler being installed and 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.