Using PVC for condensing boiler venting. Will it melt? 1

[cdxx139]

Has anyone dealt with the issue of using PVC (or CPVC) for a condensing boiler flue vs AL29-4C S/S?

Almost every boiler manufacturer IOM allows PVC, CPVC or S/S flue material. I specified S/S venting and the contractor came back and wanted to install PVC, I rejected it, and it started this debate. I have done some research, and this is what I have found.

AGAINST PVC (my original position).

- PVC manufacturer material sheet states PVC is not tested for venting. The melting point of PVC is something like 160 degF. The flue of a 140 degF HWS condensing boiler is worst case 50 degF over HWST (a manufacturer gave me this rule of thumb). Im still not sure how to calulcate this, if anyones can tell me where to find the formula, it would be most appreciated.
- As the sysytems is used it will develop scale in the heat exchanger and reduce its efficiency, increasing the flue temperature higher than originally calculated.
-Just because the manufacturer says PVC can be used, does not mean it will work in my application.
- A recent article explains most of this:

http://www.plumbingengineer.com/may_11/code.php

FOR PVC (im starting to lean inthis position)

- The boiler manufacturer IOM states PVC is allowed. The boiler is 3rd party certified, which means the boilers are tested at their highest ratings (above 180 degF), and the venting allowed is acceptable to them. (If a 3rd party says its ok, shouldnt the engineer accept it?)
- From a code stand point, the inspector will allow any venting material acceptable to the manufacturer. (if the inspector is ok with it, should the engineer overrule?)
- Rumor? The article is written by a guy who makes money for expert witnessing and is drumming up business. The example in the article did not explain the problem was the incorrect vent connections. Even if S/S was installed incorrectly, there could be a carbon monoxide issue. (would make sense?)

Curious if engineers would allow PVC venting in these situations?

knowledge is power

 

[317069]

you don't base your decision on online.com website, you have to base it on standards and codes.

 

[RossABQ]

I believe that rule of thumb would be 50 deg above HW Return temperature, would it not?

But I agree, Code and local law govern.

 

[KiwiMace]

If you have specified SS, and the contractor has bid on that spec, then you have paid for SS. If you pay for SS you don't have to waste your time listening to the contractor whine about other options. That is the purpose of the specification.

 

[DRWeig]

Kiwi got to the heart of your current issue. If the contractor wants to go PVC and you agree, he owes the owner some money back, amount subject to your approval. The owner has already paid for stainless if the contract has been awarded.

I can't really speak to the temperature issues and whether PVC is good or bad in this application, but as a controls guy, I have seen dozens of condensing boilers installed around my area. PVC everywhere, never seen a metal one.

Carbon monoxide troubles with which I've been involved (only 2 in my 30 years) have both been related to improper burner / combustion air tuning caused by neglect at installation time. The theory that scale can make flue temperatures rise above what PVC should be used for seems to make sense, though.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[willard3]

You specified SS, get SS.
Let the contractor put PVC on his own boiler.

 

[cdxx139]

Based on standards and codes, it is OK to use PVC.

Based on my initial engineering sense, it was NOT OK to use a venting material that melts at 160 degF, and has a gas that is 190 degF run through it for potentially long periods of time.

Maybe it is a heat transfer topic I dont understand?

As for the 50 above HWST vs HWRT, Im not sure. But what if the system has an 80 degF delta T? Shouldn't the flue gas always be greater than the HWST? You need something warmer than 140 degF to heat water to 140 degF.

And as for specifying S/S and the contractor must follow the specification, you are right. But the contractor is on site everyday, the engineer is not, so the contractor has the ear of the owner. He tells the owner that PVC will work, the engineer is overspecifying the project (implying i am wasting the owners money), and he'll give a credit for S/S (buts never the actual credit. When its a change order its 150% of the actual price, but when its a credit, its 50% the actual price.)

But either way it becomes a discussion, which turns into phone calls, and a potential site visit and more meetings. All time wasted on a budget that is already very tight.


knowledge is power

 

[RossABQ]

A condensing boiler is essentially a counterflow heat exchanger. The leaving flue gas is condensing because it is contacting the entering (HWR) water, so the delta-T required for heat transfer is relative to HWRT. Note that the drains on the flue ducts are likely PVC or CPVC even with SS flue material.

I went thru this exact same scenario with the contractor and owner on a job to add an identical boiler to a battery of two existing boilers. It was repeatedly emphasized in both the specs and the pre-bid walk-thru that the 3rd boiler was to be installed exactly as the existing (which included SS). The contractor did indeed offer a deduct for using PVC. As I recall, the contractor also proposed eliminating insulation as part of the deal.

 

[MisterDonut]

Codes might allow PVC, but that does not mean PVC is acceptable or in the ownedr's best interest in every situation. Your engineering judgement can be used to require more than the code allows. The boiler manufactuer might say that PVC is acceptable to them, but they won't take responsibility if the PVC flue gets hot in an upset condition and begins to warp or sag.
I would not specify PVC on any elevated-temperature service. State your case to the contractor and owner and that should be the end of it. Let the owner know that additional time spent on the subject is an additional service.

 

[PEDARRIN2]

There some very good articles on this topic by Ron George,

http://plumbingengineer.com/may_11/code.php

and

http://www.plumbingengineer.com/may_12/code.php

My current position is to not allow for PVC vent piping on a condensing gas burning appliance.

 

[cdxx139]

Thank you Ross, that makes sense. I would still love to know if there is a formula out there to determine flue temp, if anyone knows where to find it? Also, did you allow the contractor to install PVC instead?

Mr Donut, that was my thought exactly.

Peddarin - Good articles. But even if the boiler is CSA certified to ANSI test standards with PVC? The test runs the boiler at the hottest temp is allowed, not just the operating temperature. Not sure the duration, but if a 3rd party testing accepts it, shouldn't the engineering community accept it and allow PVC. (Im playing devils advocate)

knowledge is power

 

[RossABQ]

Also, did you allow the contractor to install PVC instead?

Hell no!

 

[PEDARRIN2]

CDXX139

I don't think so.

The PVC pipe manufacturers do not recommend using it due to the temperatures. And there is no standard for the use of PVC for flue piping.

The flue gas will be at least as hot as the coolest temperature of the water in the heater/boiler. With water heaters being set at 140 F for legionella, the flue gas will be at least 140 F at 100% efficiency, which will never happen. And water heater efficiencies will go down due to fouling from hard water so flue gas temps will go up even more.

As the article states, the test only looks at deflection of the pipe.

As the articles states, the tests that are performed do not mimic real world use.

And given the safety aspects of using PVC as a flue material - I would not want to be the engineer who has to explain to the company lawyer that he specified it because it was cheaper and the manufacturer recommended using it, when there are articles out in the open about the concerns.

 

[317069]

when a code allows somthing, that is set. it is allowed and we have to go with it.
owners don't have to pay extra money just because mr engineer wants them to do.
As an engineer, you are responsibile to protect public then client property which is an HVAC system in our case.
protecting client from overspending is also your responsibilty.
when you reject or accept an item in your project you have to proof with professional reason supported by professional resources why you are rejectin or accepting this item, again not based on an article on XXX.com
I am not aginst or with the PVC but I am talking about the way to make a professional decission.

 

[willard3]

I will point out as I do to my clients that codes are the minimum required. Engineers are hired not to do just the minimum and my interest is the client's interest, not the contractor's.

Professional decisions that are driven only by cost are not professional decisions, they are cost decisions.

 

[317069]

Professional decision is not driven by one factor only, there are many factors help to build such a decision.
Cost is an important one among other important. But the cost is not only the number you see it in contractor invoice, you can buy a cheap car but later you will pay the difference.
Every business including engineering is based on the cost and benefit; you can not change it by such fancy words like professional, controllability, obviously....etc
it is right that code set the minimum, you go beyond the minimum (code) when client’s requirements are beyond the minimum.
like our case her, if client told CD that he want to use PVC because it is less expensive, allowed by code, then when CD say NO, he has to tell client why, he can not tell him because I read an online article written by Mr. 317069 saying that it is not recommended.

He could tell him for example ok if you use this PVC now, then after two years you will have to change it, you have to pay at that times extra work to change existing vent, but if you use what I specified then this will last 20 years.
Finally, a question: if PVC is approved for this installation, then what is the difference between this case and a case that if your client told you I want to use Carrier furnace instead of Trane.

 

[PEDARRIN2]

317069,

Comparing use of PVC on a flue to whether a Carrier or Trane furnace is provided is comparing apples to oranges. If the furnaces are equal - then price and maybe the actual dimensions are about the only thing to consider - not any safety issues.

Use of PVC pipe indescrimately can be (and has been documented to be) a potential safety issue.

Engineers belong to professional organizations where other engineers who have more experience can share their experience. Ron George is such an individual. The article was from Plumbing Engineering which is the magazine of the American Society of Plumbing Engineers. While directly related to water heaters, it also applies to boilers. I agree that engineers cannot just look at (and quote) any source and say that it is acceptable, but there is a level of competence that is understood to be in trade magazines.

I regularly post questions on this site and also question more senior engineers in my company. I recognize that postings on this site may or may not be reliable - but that is an engineer's judgement. I look at who is posting, how many posts they use, references they use, and also how many other engineers agree with them. If I have a question or concern with their advice, I ask the question and let them explain. Then I make a decision about whether to follow their advice. I do the same with more senior engineers in my company

I would have no problem telling a client there are references in an established trade publication from engineers respected in the field that PVC piping has had issues and that is why I do not recommend using it. If the client wants to still use it, I make sure I document that decision so when/if there is a fire or fatalities, the lawyers do not look at me or my company because I show I recommended not using it and why.

 

[317069]

Pedarrin2

- it is not apple and orange, it is the same, I said ( IF PVC is approved by code for this installation) this means if a code accept PVC then we don't have safety issue (another fancy word)
- we are mixing here between two things, engineering practice and accepted materials,
- if Mr. 317069 is a member of ASHRAE( you can be a member, just pay fees), and if Mr. 317069 wrote an article in ASHRAE magazine, that does not mean his article can replace ASHRAE handbook.
- You said "If the client wants to still use it, I make sure I document that decision so when/if there is a fire or fatalities, the lawyers do not look at me or my company because I show I recommended not using it and why."
ARE you sure your company is an engineering company

 

[RossABQ]

The Fuel Gas Code doesn't have blanket acceptance of PVC or other non-metallic materials for flue gas ducting, it says IF the boiler manufacturer allows it, it needs to be installed in accordance with the boiler manufacturer's requirements. The Code covers a wide range of applications, and PVC may be acceptable for some of them, others not. And regardless of the manufacturer's requirements, the engineer needs to evaluate the potential risks based on the specific installation and operating conditions.

 

[willard3]

Any Engineer can see and understand the difference between PVC and SS flue pipe.

He specified SS.......get SS.

The cost argument always boils down to what is cheaper, not how it functions. The cheapest heating system is a bonfire in the middle of the floor.....doesn't make it a good system.