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Plastic: Myths, Realties and Responsabilities


Polypropylene flue gas venting

White or gray? PVC, CPVC or another plastic?


By André Massé

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Since the publication of the ULC S636 standard, we are routinely asked about the various plastics provided for venting flue gas and especially about their limitations. While many manufacturers, agents, distributors and installers have their own interpretation of it, please remember that only local regulation applies. What is it, really?

With the force of Québec regulation behind it, code B149.1 is the main guiding reference for natural gas and propane equipment installations. This code also refers to a variety of standards, codes and manuals, such as those from manufacturers. However it’s important to remember that in case of contradiction between the code and other guidelines, the code has precedence.

An approved system like type BH vent is defined according to code B149.1 as: “A vent complying with ULC S636 and consisting entirely of factory-made parts, each designed to be assembled with the others without requiring field fabrication, and intended for venting gas appliances.”


Code B149.1 and venting in plastic

Among other things Article 8.9.6 from code B149.1-10 specifies that:
“Vents constructed using plastic piping shall be certified to ULC S636.”

At first glance, a plastic vent bearing the ULC S636 specification should be sufficient. Let’s look at the standard more closely.





PVC evacuation vents

a.    Class I: venting systems suited for gas-fired appliances producing flue gas temperatures of over 135°C (275°F), but not more than 245°C (473°F).

Even if this article does not cover metal venting, note that this category does cover stainless steel venting AL 29-4C. This vent type is also Class II approved to cover temperature below 135°C.


b.    Class II : venting systems suited for gas-fired appliances producing flue gas temperatures of 135°C (275°F) or less.

i.    Class II a) : specifically for temperatures up to and including 65°C (149°F).

PVC (polyvinyl chloride) used as flue gas venting is specifically covered by this classification. For PVC the suitable temperature limit is 140°F, however the ULC S636 standard allows manufacturers to exceed this temperature for flue gases and not liquids such as used for tests.

ii.    Class II b) : specifically for temperatures up to and including 90°C (190°F).

CPVC (chlorinated polyvinyl chloride) used as flue gas venting is specifically covered by this classification. For CPVC the suitable temperature limit is 180°F, however the ULC S636 standard allows manufacturers to exceed this temperature by 10°F for the same reasons given above.

iii.    Class II c) : specifically for temperatures up to and including 110°C (230°F).

PP (polypropylene) used as flue gas venting is specifically covered by this classification. Two manufacturers could be listed for this standard limited to 230°F.

iv.    Class II d) : specifically for temperatures up to and including 135°C (275°F).

No plastic product seems to be currently listed for this classification.




Even though ABS (acrylonitrile-butadiene-styrene) is more heat resistant than PVC, it has been excluded from the ULC S636 standard, because the cement used does not result in melting between the various parts of the venting system. In fact, the use of the cement-glue for ABS will crack over time. This means that, even if the manufacturer’s manual states that ABS is suitable, this material must not be considered in Canada.


PVC and PP in buildings classified fire-resistant and high-rise buildings in Québec

Among the ULC S636 approved plastic flue gas venting, only CPVC meets the building code requirements for flame spread index over 25 and smoke generation index over 50.
PVC System 636 meets the flame spread requirements for fire-resistant type buildings which ARE NOT high-rise. When pipes are used in a plenum ceiling, both the flame and smoke indexes must be met, so CPVC is a necessity.
It is therefore fundamental to determine the building type for which it is being bid before selecting the material.



To get a perfect joint between the materials, cements choice is important. As an example, I chose IPEX which has the largest market share in Québec. Keep in mind that other manufacturers also have their own cements. It’s important to understand that plastic venting meeting the ULC S636 standard is approved as a complete venting system. That makes it impossible to mix and match parts or segments coming from different manufacturers in the same venting system. Otherwise approval does not stand.

CPVC evacuation vents


Remember that small or short radius elbows are designed for plumbing. Plumbing elbows should therefore be excluded from any flue gas venting system because their restriction is greater. Several furnace manufacturers confirmed to us that the equivalent lengths given in their installation manuals, which refer to short radius and long radius, should instead refer to IPEX long radius and extra-long radius. So for example, for some manufacturers, an IPEX 90° extra-long radius elbow would have 5 inch equivalent lengths as compared  to 7 inches for the long radius elbow. For 45° elbows the equivalent lengths are 2.5 inch for extra-long and 3.5 inch for long radiuses.



Heat from the flue gas leads to thermal expansion and causes all venting material to expand. So for very short venting fitted with a single 90° elbow it’s a good bet that it will exert undue pressure on the venting which could even cause it to break if this venting is permanently attached in the sidewall. In the same way, a relatively long vent rising vertically through the roof could in the midterm break the seal of the product used to seal the roof resulting in water infiltration.

A plastic finishing plate slightly larger than the vent itself could be adequately sealed to the wall or roof. After that the evacuation vent could be inserted into the finishing plate. Any infiltration could be blocked by using a finishing plate used on electric masts which pass through the roof or a Furnco coupling between the largest conduit and the venting installed outside. Rubber allows some amount of expansion/contraction movement therefore undue pressure on the vent is avoided. This method or another is not required, but this kind of precaution can help to avoid annoyances.



What do the appliance manufacturer installation manuals have to say – the ones which specify that materials such as ABS, PCV, CPVC and/or PP can be used and do so without any restriction?

It’s important to understand that appliance manufacturers have their equipment approved according to a standard specific to each type of equipment. Unfortunately, these standards are not uniform and, in some cases, may not reflect other standards in effect in Canada such as ULC S636. However, remember that the current B149.1 code has precedence over the installation manual in case of contradiction, as stated above.

We should also mention that some manufacturers install a sensor or device in the flue gas outlet to prevent reaching the temperature limit for a PVC venting system. To execute this function when needed, these devices cut-off the gas supply forcing the burner to stop. As a result, several complaints have been received because the device stopped operating when demand was greatest.

IPEX cements and their use



Unlike standard or mid- efficiency appliances, condensing appliances must cool the flue gas to recover the latent heat; making them high-efficiency appliances. To do that, they must recirculate the air (for a furnace or unit heater), heat-carrying fluid (for a boiler) or just the water (for a water heater).

Air-cooled equipments are normally at room temperatures of more or less 80°F. In this way, flue gases will be rapidly cooled to temperatures more than acceptable for PVC.

Equipment with a heat-carrying fluid, like a boiler, is cooled by the cooler, returning water. Radiant floor heating, snow melting, low temperature radiators or chilled beam applications are adequate for the water return. Returns with a temperature above 140°F could have difficulty cooling the flue gas and are consequently unacceptable for PVC.

Finally, the water heater for domestic hot water uses city water which enters it at temperatures varying seasonally between 35 and 70°F to cool their flue gases. Clearly, at these return temperatures, the flue gases could be acceptable for PVC if the setting temperature is around 140°F. In commercial applications in contrast, it is not uncommon to see settings adjusted near 180°F. With the volume of water reaching this limit, the flue gas will exceed the PVC tolerance level. Furthermore, if applicable, a recirculation system will greatly reduce the cooling function of the water from the water supply system. PVC should not be considered for this type of installation.



a)    Whatever the application of a gas-fired condensing boiler, the factory-installed temperature limiters are set between 180°F and 200°F. Because it is possible for the temperature of this kind of appliance to reach these limits, it is likely that the water returns will not be able to cool the flue gases to temperatures suitable for PVC.

b)    Tests with specific equipment

The measured temperatures are for information only. They can definitely vary with changing conditions and brands. They were not provided by the manufacturers and do not constitute an official version of the data.

- Residential water heater with forced venting: temperature measured at 175°F

A rubber roof finishing plate protects the seal while allowing the vent to expand and contract.

- Condensing boiler: 7°F hotter than the water return

1.    110°F on a radiant floor system
2.    160°F on a cast iron radiator system
3.    187°F on a fin radiator system

- Furnace: 135°F

- Condensing commercial water heater:

1.    130°F without recirculation
2.    185°F in recirculation mode with a 180°F set point

- Suspended unit heater: 106°F


Why not use the flue gas temperature measured during the appliance’s efficiency test at startup? This test, under adequate conditions, could give you reliable information on the temperature and material used. Unfortunately, this test can only be done once installation is completed.



Some conditions of gas appliance use can lead to scale deposits on the exchanger because of mineral precipitation from the water under the effect of heat. In such a case, this condition will reduce the flue gas cooling effect resulting in an increase in their temperature. So, despite efforts to remain within acceptable PVC parameters, in these cases, gas temperatures could suddenly become unacceptable for PVC.  Without being alarmist, a very severe incident occurred in Aspen, Colorado in 2008 caused by the absence of cement in the PVC flue gas venting system for a gas boiler feeding a snow- melting network.



Without being alarmist, a very severe incident occurred in Aspen, Colorado in 2008 caused by the absence of cement in the PVC flue gas venting system for a gas boiler feeding a snow- melting network.



In any installation, it is fundamental to refer to current installation manuals, local codes and code B149.1. Each installer must refer to them. For example, Article 8.9.5 of code B149.1 requires the first 3 feet of a plastic venting to remain easily accessible for visual inspection. Failure to refer to current standards and codes could result in a code violation, or even guilty negligence...



Unfortunately, the use of PVC and CPVC is usualy a question of competitiveness. Don’t forget that people’s safety, the client’s trust in your company, your reputation and image and that of the whole industry are involved. The same goes for your responsibility as an expert installer. So, let’s be vigilant... and thorough.



André Massé is Director Laboratories and Training, Master Group L.P.

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