Some information on PVC & CPVC

Tim McElwain · October 2010

VENTING TODAY – A COMPLEX SUBJECT

 

Plastic Piping

 

National Fuel Gas Code specifies that plastic piping used for venting appliances listed for use with such venting materials shall be approved.

Before the introduction of high-efficiency (90 + percent efficiency) gas utilization equipment, plastic piping was prohibited as a vent material. High-efficiency (Category IV) appliances reduce vent temperatures, resulting in condensate formation. As accumulation of condensate can become a source of corrosion of metal vents, plastic piping became the preferred material. Paragraph 12.5.2 of the National Fuel Gas Code requires that plastic vent materials be used for listed gas utilization equipment only when specified in the manufacturer's instructions. 

 

Section 12.5.3 of the National Fuel Gas Code Special Gas Vent 

 

Special gas vent shall be listed and installed in accordance with the special gas vent manufacturer's installation instructions.

 

All special gas vents are listed vent materials. Special gas vents are listed in accordance with UL 1738, Standard for Venting Systems for Gas-Burning Appliances, Categories II, III and IV. Installation instructions for special gas vents include limitations on operating temperature, categories of appliance to be used with each vent, clearance to combustible materials, types of fittings and joint sealant to be used, and vent termination requirements. 

 

Special attention should be given to the following areas: 

 

 

1. Proper support for the special gas vent to prevent sagging and to allow for expansion, contraction, and condensate drainage 

2. Proper cutting and cleaning of joints and fittings, and the use of recommended joint sealants (substitutes are not usually permitted) 

3. Construction of a condensate trap (see the appliance manufacturer's instructions for special requirements) 

4. Wall penetrations (the pipe should not be secured at a thimble, because the pipe must be allowed to move to accommodate expansion and contraction) 

5. Insulation [the vent pipe or the fittings of the inside of a wall thimble must not be insulated when polymeric (nonmetallic) vent materials are used]

 

Product Recall 

 

More than 15 years ago, a class of special gas vent known as "high temperature plastic vent" (HTPV) was introduced to the market for use with mid-efficiency appliances. Field experience has shown that these vent systems are prone to failure. The failure may occur because of improper installation practice and/or corrosion from acidic condensate. At this time an active product recall is still under way, with the cooperation of the U.S. Consumer Product Safety Commission, appliance manufacturers, and the vent pipe manufacturers. The product recall covers furnaces that are horizontally vented, as well as all boiler installations. Those who encounter one of these vent systems should call (800) 758-3688 for information on how to proceed. This number is operated by the product manufacturers and will be in operation until the recall is substantially complete. If the number is not in operation, questions can be referred to the furnace, boiler, or water heater manufacturer. 

 

To determine whether the installation has an HTPV pipe system that is subject to this program, the vent pipes attached to the natural gas or propane furnaces or boilers should be checked. Vent pipes subject to this recall program can be identified as follows: 

 

· The vent pipes are plastic. 

· The vent pipes are colored gray or black. 

· The vent pipes have the names "Plexvent," "Plexvent II," or "Ultravent" stamped on the vent pipe or printed on stickers placed on pieces used to connect the vent pipes together.

 

 

The location of those vent pipes should also be checked. For furnaces, only' HTPV systems that have vent pipes that go through the side walls of structures (horizonta1 systems) arc subject to this program. Other plastic vent pipes, such as white PVC or CPVC are not involved in this program. 

 

The Effects of Temperature on PVC Pipe

 

Polyvinyl Chloride (PVC) is a thermoplastic, and as such, its physical properties change with temperature variations Dimensions, pressure capacity, and stiffness are all affected by temperature changes. The published dimensions and performance ratings for PVC pipe and conduit products are usually applicable only for 73°F. The following will help to explain how PVC pipe and conduit products are affected by operating temperatures other than 73°F. 

 

Dimensions

Like all materials, PVC expands with increasing temperatures and contracts with decreasing temperatures.

 

 

The coefficient of thermal expansion for PVC is:

 

 

3.0 x 10-5 in/in/°F

Because the length-to-diameter ratios of PVC pipe and conduit products are generally very large, length change from temperature variation is the most noticeable. A good rule of thumb in design of PVC pipe and conduit systems is to allow 3/8" length variation for every 100 feet of pipe for each 10°F change in temperature. (This rule is independent of pipe size.) Table 1 can also be used to determine the effects of temperature changes on the length of PVC pipe and conduit. 

 

NOTE:

 

THE MAXIMUM RECOMMENDED OPERATING TEMPERATURE FOR PVC PRESSURE PIPE IS 140° (f).

 

FOR CPVC IT IS 200° (F)

 

This is reason enough to pay very close attention to flue gas temperatures on a lot of 90+ condensing equipment.

More on Plastic Piping

As I continue my research on venting materials and venting issues I find more and more a concern of the use of plastics for venting. This discussion was to be only a few articles when I started but has surely grown as I look further into this issue of Special Venting. 

 

I solicited some comments from Glenn Stanton the Manager of Technical Development for Burnham Hydronics and U.S. Boiler Co., Inc. His comments are included here as to PVC venting.

The concerns expressed lie with several factors. The first of these being that PVC and ABS Pipe and fitting manufacturers do not necessary express approval for using these products for venting gas byproducts and the ASTM standards hold no mention regarding the use of venting and PVC. One or two companies that manufacture PVC pipe and fittings have put out statements to the effect that they do not recommend using it for venting gas combustion byproducts. The second issue is that we, you or nobody else can guarantee that the pipe your distributor stocks is solid core or foam core. Some distributors stock one or the other based on primary demand and if they did stock both that is no guarantee that they will have solid core every time you order or that what is shipped to you is solid core. There is a very strong likelihood of foam core pipe ending up in many of these applications due to those reasons. The third issue is that PVC is certified and approved for 140°F temperature for "pressure applications". Mod/Con boilers can in fact exceed these temperatures during high fire conditions with Domestic Hot Water demands. The boilers have flue gas sensors that know this is happening and will turn down the input or modulate to protect the pipe. Reducing the boiler input when it needs to recover the heater will lengthen that DHW recovery period. To what extent that happens is a function of boiler sizing and demand? I too have seen a couple of manufacturers that approve CPVC for the first several feet. But the real question is.... How many distributors do you deal with that actually carry 3" and 4" CPVC pipe and fittings in stock? The fourth factor is there is more and more discussion these days about this topic and the trend of the governing agencies uses verbage referencing "Vent Systems". We see this with AL29-4C, Polypropylene and Ipex. They are indeed "vent systems" that have been tested and certified for venting applications. As mentioned, Canada has taken action to discontinue the use of PVC and ABS and that any material used for venting must be a "system" that conforms to their standards. Our stance with our Freedom ™ CM and CHG boilers is to use either AL29-4C or Polypropylene vent systems. These systems are tested and certified for venting purposes and have temperature tolerances far in excess of what you would expect with these boilers. That's my story and I'm sticking to it..... and I will be willing to bet that many other equipment manufacturers will begin to see it that way too as thing progress in

Canada.

Then some recent information from Viessmann President Harald Prell add to the discussion.

 

Venting of Residential Viessmann Gas-Fired Condensing Heating Boilers

At present, there are industry discussions in regards to venting of gas-fired condensing heating boilers. The following represents the opinion and position of Viessmann Manufacturing in regards to venting Viessmann gas-fired condensing heating boilers, series Vitodens 200 and Vitodens 100.

Since the mid 80's, we manufacture and sell gas-fired condensing heating boilers; now sold in more than 30 countries, including North America. In almost all countries, PPS (polypropylene) is mostly used in coaxial vent-type applications or stainless steel for single wall venting systems. PPS is suitable for a steady flue gas temperature of 250° F (121 ° C) and for short-term exposure up to 280° F (138° C). Stainless steel vent pipe is suitable for 550° F (288° C); typically, an SA240 316 L material is used or the higher grade in North America AL29-4C.

Due to the fact that gas-fired condensing boilers are being vented, not only the temperature of the flue gas becomes an important factor but also, in combination with the extreme high moisture content, the associated acidity (pH level) of the flue gas condensate and the extreme temperature exposure to outdoor conditions need to be considered when selecting materials.

PPS, as well as stainless steel venting systems, have successfully been in use for many years and carry independent certifications for venting these types of heating boilers properly.

CPVC material is certified for 90° C (194° F) and PVC is certified for 65° C (149° F) according to ULC-S636 Standard for Type BH Gas Venting Systems. The Viessmann gas-fired condensing heating boiler Vitodens 200 and 100 series are approved for use with listed CPVC material.

IPEX (the manufacturer of CPVC venting systems) informed that this material is now readily available. Should there be supply issues, please contact your Viessmann sales representative or Viessmann directly- (in Canada at 1-800-387-7373 or in the U.S. at 1-800-288-0667).

My Comment: This next information I find very interesting and certainly does not only apply to Viessmann equipment only.

 

The flue gas temperature exiting a gas-fired condensing heating boiler depends on a number of factors; some impact more than others:

1. The maximum allowable supply water temperature rating on the heating boiler or the maximum adjustable aquastat or limit settings. The Vitodens 200 is limited to a max. supply water temperature of 75° C (167° F) and the Viessmann Vitodens 100 series is limited to a max. water supply temperature of 80° C (176° F), plus cut in and cut off differential.

2. Venting with a coaxial vent pipe system, where fresh outside air moves around the PPS pipe, preheats the combustion air and cools flue gas temperature further.

3. The heating boiler utilizing one separate vent pipe and a separate fresh air intake pipe.

4. Boiler utilizes standard room air for combustion air or outside air directly.

5. Flue gas velocity within vent system (vent length and restriction).

6. Possible vent restrictions (partial icing or blockage of vent terminal and/or air intake).

7. Excessive wind and pressure impact on terminations.

8. Possible partial heat exchanger flue gas passageway blockage.

9. Cycling frequency pending on control strategy, system water flow, zoning, etc.

10. Use of boiler for indirect DHW.

After all of the above is considered, is there a safety margin left? After all, reliability and dependability for heating comfort are key factors in our opinion.

The main factor influencing flue gas temperature is however the return water temperature to the heating boiler. This is the primary influence on how high the flue gas temperature will go and exit into the actual flue pipe.

Both models of Viessmann heating boilers are certified to ANSI Z21.13 CSA 4.9 Low Pressure Steam and Hot Water Heating Boiler Standard by CSA. The test procedure within this particular standard calls for a boiler water supply temperature maintained until the limit control functions ± 3° C (± 5° F). When the boiler is tested under this criteria and a very low return temperature is selected (by the manufacturer), it will drive the flue gas temperature extremely low. Typically the flue gas temperature on both Viessmann heating boilers is between 5° C (9° F) and 15° C (27°F) above the return water temperature; therefore, for example, with a low return water temperature selection of 27° C (80° F) into the boiler, a flue gas temperature of 42° C (107° F) would be the net result. This flue gas temperature would not pose a problem in general for any type of PVC or ABS material; however, this test with a very large temperature differential of 55° C (100° F) between supply and return is not realistic. Also, at that temperature differential, the flow rate through the boiler would only be 20% of the actual required flow for a typical 99911 ° C (20° F) hydronic system design temperature differential; again, not realistic in an everyday install.

Example under full input - design condition:

If the boiler water supply temperature would be 82° C (180° F), provided the boiler is certified to that temperature, then one would typically assume a temperature differential of 11 ° C (20° F) and therefore the return water temperature would return back at 71 ° C (160° F) to the heat exchanger. The dew point of natural gas is 57° C (135° F) at sea level.

The boiler would not condense anymore and the stack temperature would certainly be higher than the return water temperature of 71 ° C (160° F). It would probably reach the 85° C (185° F) to 88° C (190° F) mark.

This operating condition now clearly shows flue gas temperatures higher than what the limit is on standard PVC, CPVC and ABS. Even if the heating boiler has a limit at 71 ° C (160° F) set for the boiler water supply temperature and an 11 ° C (20° F) spread to the return water temperature, the return temperature would still be 60° C (140° F). the flue gas temperature could exceed the maximum listed temperature limits.

Especially when heating boilers are utilized to provide domestic hot water through an indirect fired domestic hot water storage tank, return temperatures back to the boiler, when the tank temperature reaches 60° C (140° F), will rarely be less than 60° C (140° F) due to obvious reasons and higher flue gas temperatures will be the result again.

Some information on PVC &amp; CPVC

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Some information on PVC & CPVC