How are you venting? PVC Flues Are They Safe?

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A few years ago I was asked to inspect the mechanical systems in some university housing units at a university in the Midwest. The school had hundreds of apartments in numerous buildings that had combination water heater/heating hot water units that provided heating hot water to a coil in the adjacent A/C unit. The units were about 10 years old. There were also reported problems of not having enough hot water and some units were getting calls for not enough heat in the peak winter months. 

A brief site visit was scheduled and an inspection of a couple of unoccupied units was done. The inspection revealed combination water heaters with a coil to provide heating hot water to the adjacent A/C unit.  The flues had purple/ brownish PVC pipes with yellowish/tan colored PVC pipe fittings used as flue materials. The flue pipes were obviously deformed from heat and they were sagging.  A maintenance guy for the University said that there had been some really bad ones that had broken or cracked in the fitting and sagged like they were melting. They said the carbon monoxide alarm had gone off prompting a maintenance call. They replaced those pipes and then requested me to investigate the system.  The combination water heater did not have a high flue temperature safety cut-out switch to shut off the burner if the flue gas temperature got too hot.  The manufacturer’s name and standard number were on the pipe and it was Schedule 40 PVC pipe.  The water heater installation manual we obtained had language that recommended using PVC pipe as a flue material. The Water heater had scaled up due to minerals in the water supply and the scale build-up caused the flue gas temperatures to rise causing the noted problems. I called the PVC pipe manufacturer and they indicated they receive these calls from time to time with someone complaining to them about problems with the PVC pipe on a flue application.  Their standard answer was that their pipe is not made to be used as flue pipe materials.

I found it interesting that PVC Pipe manufacturers do not promote the use of PVC vents for combustion gasses in their technical manual. In fact they have specific warnings stating that the pipe is not intended to be used for venting combustion gasses. The manufacturer of the PVC pipe was quite familiar with this issue and he pointed to the temperature limitations in the PVC pipe installation manual.  He e-mailed me a link to their technical manual and then we went over all of their temperature limitations and warnings against using the pipe for such applications.  He noted all of the physical and temperature limitations of the PVC piping.   He said they do not recommend PVC pipe for use in venting fuel fired appliances. He went on to point out that there is no listing for PVC pipes used for venting flue gasses for the product temperature limitations in the installation manual.  There is a loop hole in the form of an exception that was added to the International Fuel Gas Code that allows plastic flue gas piping to be used if the appliance manufacturer tests his equipment with plastic flue piping and certifies the plastic flue pipe for the installation.

502.1 General. All vents, except as provided in Section 503.7, shall be listed and labeled. Type B and BW vents shall be tested in accordance with UL 441. Type L vents shall be tested in accordance with UL 641. Vents for Category II and III appliances shall be tested in accordance with UL 1738.
Plastic vents for Category IV appliances shall not be required to be listed and labeled where such vents are as specified by the appliance manufacturer and are installed in accordance with the appliance manufacturer’s installation instructions.

503.4.1 Plastic piping. Plastic piping used for venting appliances listed for use with such venting materials shall be approved.  

(IFGC Definition: [A] Approved – Acceptable to the code official or other authority having jurisdiction.)

503.4.1.1 Plastic vent joints. Plastic pipe and fittings used to vent appliances shall be installed in accordance with the appliance manufacturer’s installation instructions.
Where a primer is required, it shall be of a contrasting color.

The plastic vent exception is a ticking time bomb! There currently is no requirement for a safety switch to shut down the appliance if the temperature of the flue gas exceeds the rating of the plastic flue pipe. This is where it is important to understand that a new installation (which is what all of the appliance testing uses) will have the lowest flue gas on the first day of operation and over time as scale and sediment build up on the heating surface the heat transfer is less efficient and all boilers, water heaters and furnaces will have a flue gas temperature rise over time. Furnaces increase in temperature when the air filter is dirty and the airflow is reduced to limit heat transfer. As the heat does not transfer the flue gas temperatures will rise. In PVC flues, the thermal expansion and contraction during the burner cycling weakens the pipe and fitting with expansion and contraction stresses. It is not uncommon for the pipe to change colors and then the expansion stresses can cause a failure of the fitting near the base of the PVC socket. I have asked numerous boiler and water heater manufacturers to show me a test with their appliance that tests for deflection of the PVC pipe at temperatures well above the PVC temperature limits.  A real world test simply does not happen unless they have the temperature limit switch. They do not test the units with a fouled heat exchanger. Now some manufacturers have voluntarily added a temperature limit or safety switch in the flue outlet of the appliance so that if the flue temperature exceeds the limit switch temperature/PVC pipe temperature limit, it cuts off the fuel to the burner to prevent problems with melting or cracking the flue. Not all manufacturers of high efficiency equipment have the safety switch installed on their equipment, because it is not required by code, so those manufacturers are playing Russian Roulette.  Are they gambling that they will probably sell enough units to pay for a few lawsuits when someone is killed or left incapacitated from carbon monoxide poisoning?  Some manufacturers have simply chosen to use stainless steel flue materials in condensing applications. This is OK. Some have chosen to allow PVC flue materials but they install a safety cut-out switch in the appliance flue outlet.  This is OK. Others have chosen to use less expensive, high temperature plastic materials such as Polypropylene with or without temperature limit switches. With high temperature plastics is better than using PVC but a safety concern and with high temperature plastic flues and temperature limit switches is OK.

One of the PVC piping manufacturer’s technical manuals has the following information:
Using Plastics for Combustion Gas Venting
The piping manufacturer recommends that inquiries about the suitability of plastic piping systems for venting combustion gasses should be directed to the manufacturer of the water or space heating equipment being installed.
As stated in the International Code Council’s International Fuel Gas Code 503.4.1.1:
Plastic Pipe and fittings used to vent appliances shall be installed in accordance with the appliance manufacturer’s installation instructions.
Furthermore, several of the ASTM standards applicable to PVC plastic pipe and fittings that this manufacturer, manufacture’s their pipe to include the following note:
This standard specification for PVC pipe does not include requirements for pipe and fittings intended to be used to vent combustion gases.

There is no standard referenced in any of the codes in the United States for a plastic flue vent for combustion flue gas piping.  Many water heater and boiler manufacturer’s recommend PVC Flue vents for combustion gasses.  In Canada there is a Standard, ULC S636 but it has several flaws in that the standard allows flue gas temperatures that exceed the temperature limits of the pipe material manufacturers.   The maximum temperatures listed in the ABS, PVC and CPVC pipe manufacturer’s technical literature are as follows: 

Maximum Operating Temperatures For Various Piping SystemsPiping System                          Maximum Operating Temp – Degrees Fahrenheit

ABS – Sch. 40                                                 160 F
PVC – Sch. 40                                                 140 F
CPVC – Copper Tube Size                           180 F
CPVC – Sch. 80                                               200 F

In the manufacturers literature, the maximum operating temperature of the system is shown in the above table.  Any temperatures above the rated temperature will allow the pipe to melt, sag and possibly collapse or pull apart.  There are serious consequences of carbon monoxide asphyxiation and fire that should not be ignored.

Generally for a new condensing water heater or boiler the stack temperature will be about 20 degrees higher than the water temperature. The design of the unit and the efficiency of the unit along with several other factors including water quality will affect the stack temperature. If a water heater is set to store water at 140 degrees Fahrenheit to minimize Legionella bacteria growth as per the new ASHRAE Guideline 12 standard then the flue gas temperature will be about 160 degrees Fahrenheit or higher when the heater is new.  As scale builds up and the heater efficiency falls off, the flue gas temperatures can easily increase to well over 350 degrees Fahrenheit.  Some water heater manufacturers pull room air into the combustion gasses to mix with the combustion gasses to help cool down the combustion gasses and many of them do not publish the airflow being pumped up the flue which needs to be added to the combustion air requirements for the equipment.  This also adds to a negative pressure in the building which can lead to moisture intrusion issues.  Even if someone had their water heater set at 120 degrees Fahrenheit, with scaling and fouling of the heating surface the flue gas temperatures can rise well above 300 degrees Fahrenheit.  Boiler thermostats or burner controls measure the water temperature and are generally limited to 200 degrees Fahrenheit.  The flue gas temperatures must be much higher in order to get the water to that temperature.  Commercial Water heater thermostats or burner controls are generally limited to 180 degrees Fahrenheit and they can overshoot by several degrees.  Residential water heater burner controls are limited to 160 degrees Fahrenheit and they can overshoot by several degrees. In all of these cases the flue gasses can easily be well over 20 degrees higher than the operating temperature when the unit is new. As scale builds up on the heating surfaces, the scale insulates the flue gasses from the hot water in the system and it causes the flue gas temperatures to increase.   Some Boiler and Water heater manufacturers have offered stack or flue gas temperature gauges as a way to see if the unit is scaling up and losing efficiency. A temperature gauge is a good idea to monitor the flue condition.  A temperature sensor or probe that is inserted into the flue at the flue connection to the boiler of water heater with a high-limit control that shuts off the burner if the flue gas temperature exceeds the temperature rating of the flue pipe could be a possible solution.  I now have a patent on such a control. I think temperature gauges and high-limit temperature probes in the plastic flue pipes would be an answer to the dilemma of cost verses safety. A standard will be needed for plastic flue pipes as a system that should include a temperature gauge and a high limit probe that shuts off the burner if the temperature exceeds the temperature limit of the pipe material. Then PVC, CPVC and Polypropylene flue gas piping can be safely used on high efficiency boilers and water heaters. 
Without a standard for proper use of these safety devices in combination with plastic flue gas piping or without the use of stainless steel flues, plastic flue materials can melt as flue gas temperatures rise.  Not only are you losing energy when scale builds up, but the flue gas temperatures rise and can melt the plastic flue piping and can cause flues to become blocked or disconnected. A blocked or disconnected flue can be a carbon monoxide danger and/or a fire danger.

Recently a professional woman in the Pacific Northwest who worked as an executive for a large company was permanently disabled when she was exposed to carbon monoxide gasses when the PVC fitting on her two combination water heater boiler units had identical failures on adjacent PVC flues. The water heater manufacturer recommended using PVC pipe as flue material in that case. 

Apparently the cycling of the burner on and off caused expansion and contraction of the two PVC flue pipes. The PVC fittings which were injection molded cracked at the point where the expansion and contraction occurred at the base of the socket on each elbow. The separation gap was approximately a quarter of an inch. The separation allowed carbon monoxide under pressure to be forced into the home. The lady that lived in that home suffered debilitating brain damage from long term exposure to carbon monoxide.
Another incident occurred where a family of four died when they were overcome by carbon monoxide when the PVC flue pipe sagged and pulled apart. The investigation found that the PVC pipe joint was not glued, but it melted, sagged and came apart. The water heater manufacturer recommended using PVC pipe as flue material in that case. 

I have heard arguments by many contractors and a few manufacturers that do not believe PVC flue venting for combustion gasses is a problem because they have not witnessed a problem.  I have seen melted and discolored piping in many of my investigations and I have seen numerous cracked and separated fittings, so I know it is a problem. If you have photos of plastic flue pipes that have failed. Send them to me at the following e-mail address and I will post them on my website.  My e-mail is and my website is
I also see the proliferation of recommendations for the use of PVC piping products for use as corrosion resistant combustion flue venting for high efficiency, condensing boiler and water heater equipment manufacturers. There are manufacturers of High Temperature Plastic flue materials and there are safety switches that can be added.  We should strive to move away from PVC.  The use PVC pipe approach seems to be a way to lessen the initial cost of installing a high efficiency boiler, furnace or water heater.  High efficiency equipment will cost significantly more than less efficient models, so there seems to be a movement by some manufacturers to promote the cheaper and less appropriate PVC materials even though they are combustible and  not listed for combustion flue gas venting applications.    The other options is for the equipment manufacturers to recommend code approved and listed stainless steel flues which are corrosion resistant or high temperature plastics.  But then the cost of their equipment will seem even more expensive. 

I have not seen any testing data or an independent test report from a boiler or water heater manufacturer that shows that PVC piping has been tested and approved for the conditions it will likely see in a water heating or boiler installation.  Any testing should include the extreme conditions when scaling occurs and flue gas temps rise near the end of the equipment’s service life.   I would like to see the safety cut-out witch or a test with a half inch of scale on the heating surface.

There is a Canadian Standard for Venting Combustion gasses.   The ULC S636 Standard is a Canadian Standard that covers the design, construction, and performance of gas venting systems intended for negative or positive pressure venting of gas-fired appliances producing flue gases having temperatures under the following:
(1) Class I venting systems are suitable for gas-fired appliances producing flue gas temperatures of more than 135 degrees C (275 Degrees F) but not more then 245°C (473 Degrees F);
(2) Class II venting systems are suitable for gas-fired appliances producing flue gas temperatures of 135°C (275 Degrees F) or less;
(3) Class II venting systems are further classified into four temperature ratings as follows:
(A) Up to and including 65°C; (149°F)
This temperature limit was intended to allow the use of PVC Pipe for use as a flue gas material.  The published temperature limit for PVC pipe is 140 degrees Fahrenheit and the allowable temperature in the ULC S636 standard exceeds the temperature limits set by many PVC pipe manufacturers.     
(B) Up to and including 90°C; (190°F)
This temperature limit was intended to allow the use of CPVC pipe for use as a flue gas material.  The temperature limit for CPVC pipe in many of the pipe manufacturer’s technical data is 180 degrees Fahrenheit.  The ULC S636 standard appears to allows the material to exceed the temperature limit for several CPVC piping manufacturers by 10 Degrees Fahrenheit.
(C) Up to and including 110°C; (230°F)
This temperature limit was intended to allow the use of Polypropylene (PP) pipe for use as a flue gas material.  There is currently one manufacturer listed to this standard, but the potential for the flue gasses to exceed the 230 degrees Fahrenheit is still there. 
(D) Up to and including 135°C. (275°F)
I am not aware of any plastic pipe manufacturer’s that meets this sub-section of the standard. The potential for the flue gas temperatures to exceed the 230 degrees Fahrenheit is still there. 

Boilers and water heaters that allow any plastic flue venting materials should have a high temperature limit switch to shut-off the boiler or water heater at the rated temperature of the flue material.  They should provide warning labels warning of the dangers of melting or collapsing flues if the heat exchanger is fouled.  When the heat exchanger fouls the flue temperatures will rise.  

I would like to see the exception for plastic venting removed from the code and replaced with a standard similar to the ULC636 standard or a requirement for a safety cut-out switch in the appliance flue outlet.  It will be interesting to see which way the industry goes on this issue. There are forces pulling each way and I believe a significant change will be coming within the next few years. The industry can develop a standard to allow low cost, high temperature plastic materials in conjunction with safety controls to shut down the boiler if the temperatures get too high or they can simply recommend using stainless steel flues which are already approved. 

About the Author: Ron George is President of Plumb-Tech Design and Consulting Services LLC.  Plumb-Tech provides plumbing, piping, HVAC and fire protection system design services. They also provide 2D and 3D CAD services, Building Information Modeling, construction coordination detailing services, contractor fabrication drawings, code and standard consulting services, Investigations of plumbing & mechanical system failures, litigation support and training seminars. Website: e-mail: Phone: (734) 322-0225.

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