There are many options available today for boiler flue venting, but not all solutions are created equal. Each has its advantages and disadvantages in terms of cost, installation, performance, and perhaps most importantly, safety.

Plumbing Engineer recently discussed venting with Keith Page, technical product manager with Selkirk, a leading manufacturer of chimney, venting and air distribution products for the commercial and residential HVAC and hearth industries for more than 80 years. Page is responsible for the company’s Heat Fab AL29-4C stainless steel boiler flue product, as well as Polyflue, a polypropylene venting system. He shared his perspective on the different trends and realities in the venting market today – most notably, the rise of PVC pipe for condensing boiler venting.

PE: What is your reaction to the rise of PVC as a boiler flue venting material?

KP: PVC has made real inroads in the 3-inch and 8-inch boiler flue diameters, especially 4-inch and smaller. But, it has a lower threshold of safety. PVC’s threshold of softening is around 150°F, and most condensing technology runs around 140°F. Now for condensing furnaces (air exchange heating equipment), it’s not too big of a concern, especially because furnaces have been using PVC for much longer than boilers without any issue on record. Condensing furnaces probably run around 110°F or 120°F flue temp. However, boilers can run a lot hotter, and they’re right at PVC’s threshold without any safety factor.

PE: How prevalent has PVC become for venting?

KP: For condensing furnaces, it’s pretty much the entire market. Condensing boilers probably started switching in 2008 and 2009. Stainless steel used to be a major part of those boilers in the early and mid-2000s. But around 2008 and 2009, boiler manufacturers and customers started looking for something that was lower cost.

PE: How is stainless steel different?

KP: When it comes to condensing boiler flue, or really any condensing technology from a flue gas standpoint, I will always recommend our stainless product. I also want to speak to polypropylene because there are benefits there as well. But from an ultimate safety and reliability standpoint, it’s AL29-4C stainless steel. In this marketplace, it has the most credibility from a use and application standpoint. It is ultimately the safest because it is noncombustible. So, in theory, I don’t have to worry about over-firing issues that can happen with a boiler. Any boiler has the potential to over fire, not to mention that sometimes, building personnel mess with the controls. The next thing you know, you have hotter flue gasses than you planned for.

PE: Is cost the primary factor that moves people away from stainless?

KP: Yes, there is a cost tied to that and doesn’t come close to PVC. But, now we come to polypropylene, which has good credibility in Europe and has been in use for 20 years. It does have a temperature limit, and we’re not shy about showing that limit in any of our material and literature. It’s ULC listed operating point is around 230°F, which provides a greater degree of safety than PVC does at around 150°F. And, in the U.S., there are no vent testing standards that PVC venting has to go through. It’s a different story in Canada, where there is testing. But here in the U.S., many are using plumbing grade PVC pipe, so the testing is on the OEM that’s doing it. Now granted, the OEMs may do their boiler flue test, but it is not a venting standard, which has safety and integrity criteria. A few OEMs have steered away from plastic and have not permitted PVC, but most have allowed it because everyone is after the lowest cost installation.

PE: Is this an example of where the market got ahead of testing and standards?

KP: If you look at the fuel gas code, it says “plastic pipe to be installed per the manufacturer’s instructions.” The code body shed its liability right there, other than making sure the OEM approves PVC venting.

PE: Are OEMs comfortable with that?

KP: The furnace guys are. They don’t really have an issue of record. They typically don’t run any hotter than they’re supposed to. But with boilers, it’s a different scenario. It’s a different situation with how the heat is transferred and everything else. There are safety electronic controls in place, but electronic controls can fail, so if the boiler over fires, there can be a big safety concern. Many OEM boiler engineers don’t like it [the use of PVC in boiler flue venting], but everyone has to be competitive and some have gravitated toward it and some have not. Some use PVC vents, and some use CPVC for the first 5 or 10 feet [CPVC has a softening temperature of approximately 195°F] and then transition to PVC. They still show stainless as an option, so they haven’t displaced it, but it’s just another option. Canada’s code requires listed vent. It doesn’t matter what the material is, it just has to be listed to their ULC S636 standard. After showing these comparisons, we’ve converted some specifying engineers to go back to stainless, so they’d mandate in their specification that they wouldn’t permit PVC on the boiler installation, even if it is allowed.

PE: Did you start to offer the polypropylene product as a way to get into the lower cost space but still have some level of temperature tolerance?

KP: It is our best chance at getting competitive to PVC venting, and polypropylene also offers many advantages in terms of installation time. There are no toxic glues or primers and there is no set time. If you truly read primer and cement instructions, you are supposed to let it set up for several hours. But, unfortunately, some installers push it together for 20 seconds, do the rest of the installation, fire up the boiler and go. [Our polypropylene product] doesn’t require that. We put the pipe together with locking bands and it’s ready for use. It also field cuts easily, like PVC does. You can field cut single wall stainless steel, but it is much more difficult. We also have a warranty, and PVC doesn’t have a warranty. Another item polypropylene has that PVC doesn’t have, is a flexible liner. So, you can reline a masonry chimney or unused vent, or you can take a flex liner and run it up a vertical shaft, since it has zero inch clearance to combustibles.

PE: You mentioned that polypropylene has long been in use in Europe, but is still making inroads in the U.S. Is there a reason the U.S. market has been more hesitant to it?

KP: I think it’s still price driven, which is the main factor. Once guys get their hands on it and use it, they like it. They build it into their overall system cost, which is approximately 15 or 20 percent over PVC. And, if you look at the overall system, flue pipe is just a fraction of what the overall boiler system costs. I know people are always trying to put as much money in their pocket as they can and you’re always trying to compete with the next guy. But, if you’re selling a whole system, the building owner or decision maker should not be looking at the vent pipe. They should be more focused on the equipment. That’s where most of the costs are.

PE: So, would you suggest shifting the focus from cost to things like safety and performance?

KP: Ideally, I’d like people to look back to stainless AL29-4C just because that has the most credible corrosion resistance plus zero flame/smoke spread and over-firing boiler concerns. The next best thing is going to be polypropylene if someone is more budget conscious and at least that has gone through a venting test standard. Plus, the larger diameter PVC gets a lot heavier to handle vs. stainless steel. Stainless steel will always look better too. Boilers have become better looking over time, so why ruin the look with PVC pipe. I’ve seen some showcase-like mechanical rooms and the stainless flue venting tops it off.