Hello everyone, 

   I encountered a propane water heater with an assistance fan right on top of the unit, vented with Poly(Vinyl Chloride) (PVC) Plastic Pipe, Schedules 40. The vent is obviously hot and it shows color discoloration... I need verification that this is a correct or incorrect way of venting this water heater since I cannot just go by feeling. So far I found on the web the normal stuff, that it is used for pressure rated for water situations. Any documentation out there that you could share please?


Tags: PVC, heater, venting, water

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After further research, it turns out this is a combi single wall heat exchanger, power vent (funny because it looks just like a water heater) so it is providing DHW and heating for the second floor. The manual clearly states that schedule 40 can be use to vent this unit, but after looking more carefully, there are black scorched marks at the beginning of the vent and the color is now yellow instead of the sharp white on the rest of the pipe.  Manual also says that the vent can be horizontal up to 40 feet with an elbow, which it is the case in here. My gut tells me this is not safe even though it seems installed according to the manual. 

Any ideas?


Something is wrong - bring in a technician that knows the equipment & get it serviced

Use your flue gas meter - is it in speck?   Of getting to hot some thing is wrong, did you or some one check LP gas presser?   Both running and other things burning gas presser?   

Read the instruction manual closely. You should find limits for the size, length and number of bends allowed.

You may want to look inside the pipes, small animals have been known to crawl in and die.



Discoloration is not unusual; The blower fans get dirty so the vent gasses slow down. HVAC technicians usually clean them with high pressure air (nitrogen bottle), problem remedied. Plumbers will replace the pipe (or everything). There is a vent pressure sensor that will shut the burner off if the flow slows too much or there is a counter-pressure cause.

It can likely be PVC just make sure it is not cellular core PVC.

Hello,  I'd take some photos and send them to the manufacturer, along with info on their unit. Let them tell you what's right or wrong here. They should help you get to the cause of the overheating. 

Yours,  Larry

I agree with Sean to bring in a technician that can do some diagnostics on it.  I would also want to know if it is set up for propane and not natural gas.  Even so propane does run hotter which could lend to the discoloration, however you are right in not wanting to leave to chance.  Also make they check for the combustion make up air and how it is getting that.  

make sure the gas modulator / controller valve has propane orifices (jets), if it came from factory with orifices for natural gas (probably did) it will burn hot as hell on propane. it's a pretty common miss.

This has been a topic of discussion for 20 years now. It is true that appliance manufacturers often sanction PVC (solid, not cellular core as Charles Buell noted), as venting material. This keeps cost down and market share up and there are thousands of installations which operate without problems.  Any system can fail or be poorly installed, that is understood. I have had trouble finding evidence of specific failures where the material was installed according to manufacturer's specifications, although the discoloring you describe is disconcerting.

BUT, a few things to consider:

1. PVC has a specific temperature limitation, (140 degrees F. with no pressure),  printed at regular intervals on the pipe. Combustion temperatures, especially when making domestic hot water and over-riding low temperature heating operation, will often exceed this.  This alone, to me, would disqualify the material.  I have received some counter-arguments that because the flue gas is in gaseous form, the surface temperature does not get as hot as it would with flowing hot water.  (I do not by that on its face.)

2. PVC has excess chlorides which can leach out with combustion condensate and if so pitched, can flow back to the appliance and heat exchanger. Even stainless steel can be pitted and degraded by this and aluminum core exchangers fare the worst of all.

3. In light of the above, some PVC manufacturers are shying away from sanctioning the use of PVC as a venting material. They recognize the temperature limitations and I imagine do not want to be associated with the potential danger in an application sanctioned by others. You may start by asking such manufacturers across the industry. I do not want to list names as I cannot speak for them.

4. As a matter of discussion: The use of polypropylene and CPVC, with the polypropylene being more highly favored, are also recommended by manufacturers and show a higher degree of temperature resistance, at 180 degrees, vs. PVC at 140 degrees. Those temperatures alone disqualify PVC based on direct application, in my opinion. My office practice (engineering), is to specify only metal-based venting systems (e.g. AL294C) where also sanctioned by the appliance manufacturer. In commercial work, we see the risk of allowing PVC can violate other codes such as fire codes, where such vents may pass through fire separations and might not be picked up during construction for example.  Some systems such as polypropylene might be an exception but that is a detail for this discussion's purposes.

4. Here in Massachusetts, the state plumbing board, which lists and approves all plumbing and gas appliances within the Commonwealth, has and is taking the position that if a manufacturer sanctions the use of a venting material, they have no exception, but they are also seeking broader opinions from across the industry. In other words, at the official code level, the jury is still out.

Not a definitive answer, but it shows that you are not the only one asking this question.

I will leave you with an anecdote, a housing development on which I was called in to solve some potential design issues.

The primary issue was that the local ordinance mandated Massachusetts' "stretch code", the use of a more stringent but not yet officially adopted version of the energy code. This required that the appliances, (water heaters used to heat hydro-air as well), meet certain efficiencies.

The engineer of record had specified units that were vented with PVC. From that it was assumed that the efficiency was in the appropriate range, but was not. (88% vs. 95% by memory).

Resolution aside, what I found from this is that the draft inducing fan also would, by design, draw in ambient air into the vent stream and thus cool the vent gases below 140 degrees.  This would seem to recognize and possibly solve the temperature limitations. But, it also had the effect of thwarting combustion testing if this were not known. The sampling port was downstream of the fan and there was no place for a sampling port upstream where undiluted flue gas was flowing.

Thank you all for such amazing information because now I have enough information for an educated "reporting" of the problem... as I was reviewing the photos of the energy audit, I did noticed that both pipes, the fresh air and the exhaust vents located close to the ground. 15 inches of snow would cover those vents.... not that it would be the explanation of what is happening, but certainly  it is clear that something is happening and I wanted to have enough information to be able to carry an educated conversation with the homeowner when it comes to this unit.  I like to thank everyone of you for such great content because this helps me be a better auditor. 

Amazing! and happy "late" Earth Day! 

If you decide to replace or change out the vent pipe,  it might be quite interesting to save it and have it sectioned where the area appears to be discolored from heat.  If it was indeed heat the discoloring is likely to be all the way through the PVC.   It would then be perhaps useful to get some good pictures and see if there is someone in the state that would like to see the pipe.  If the pipe was installed per manufacturer - the heater was installed per manufacturer and it is over heating -- its also possible the vent pipe itself is at fault.   Or you might discover discoloring is surface only and not caused by the heat....


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