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Engineering vent systems


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The most common and difficult problem I’ve been running into lately is improper venting of gas & oil fired appliances. This has ranged from orphaned chimneys, to deteriorated flues to improperly sized connectors to just plain poor designs.

In order to further my knowledge in this area, I attended a seminar offered by the NJ DCA and presented by Julius Ballanco, president of the American Society of Plumbing Engineers.

The seminar was quite informative yet confusing. It’s pretty clear on what causes problems but it can be difficult to determine what’s properly designed and installed and what’s not.

For instance, if a new furnace (fan assisted) is installed, it’s possible (even probable) the water heater connector may have to be made bigger. In his seminar, he stated the following:

Fan-Assisted Appliances

Multiple Connections-

1) Strange impact to water heaters.

2) Vent connector may have to be double wall.

3) Rules of thumb:

a. Majority of water heaters require vent connector increase in size by 1 inch

b. For masonry chimneys- majority of time chimney has to be lined.

So, how is a home inspector supposed to ‘inspect’ venting of appliances? I had one house late last year that had a furnace & water heater in the basement and a furnace in the attic, all connected to the same B vent (I believe I posted this problem on TIJ but can’t find it now).

During the exterior inspection the B vent cap was iced over. When I went into the attic, you could see where condensation was dripping from the B vent, a chunk of ice had formed on the plywood floor under the B vent elbow. Condensation stains were visible at most of the vent connections in the attic. The buyer had three HVAC companies come out before one finally did some calculations and re-designed the vents.

So, what’s a home inspector to do when ‘experts’ can’t figure it out? During the seminar, Julius worked thru several examples of venting and ran the numbers thru a software program that went step by step thru the process of sizing. The software took about 5 minutes while trying to do manually out of the book would take hours. When actually designing the vent system in the field, the hardest part would be measuring the distances.

My question is; when does engineering overtake inspecting?

When I see another venting problem, would it be easier to purchase the software, calculate and offer the results to a HVAC contractor to review or just simply state a problem exists, have a HVAC contractor review the vent system for problems (knowing that most HVAC companies are clueless when it comes to venting multiply appliances?

Here are 2 recent pictures; one is 45 year old chimney, the other is a 4 year old B vent.

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Originally posted by RickSab

I would state the concern and let the customer take it to a HVAC guy. I'm not smart enough to take on that liability too.


Recently inspected a house where the water heater vent was orphaned into an exterior masonry chimney. (the heating system was replaced with a direct vent unit). I found backdrafting at the vent/connector. The drafting improved after the water heater burner was on for about 5 minutes. Contractor comes in and says that using a draft measuring device that the draft is OK.

I said to the buyer (a friend who's a builder) that it's 65 degrees outside now. What happens to the draft when it's 30 degrees outside and that exterior chimney is stone cold? He was thinking of the relining the chimney, but I told him not to waste the money and just put in a direct vent water heater to solve the venting problem.

Darren, you're right, if the HVAC company can't even get the clearances and firestopping right, how would they address the venting problem?

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Yes, it's difficult sometimes to know who to trust.

This new furnace was installed by an HVAC contractor, inspected and approved by the local AHJ, and soon thereafter red-tagged by the local gas utility because the vent didn't terminate 18" above a dormer wall within 8' of the vent.

Nevermind that the primary and secondary condensate drains discharged directly into the crawlspace.

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I couldn't tell you how many times I write up the stains on the attic section of the B-vents. I do spend a little time explaining to my clients what has been happening and there's a blurb in the report. But I don't expect that the contractor will understand it and I don't expect that the problem will be corrected. I expect that clients just say the heck with it and let it go.

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Letting it go may result in a death. Attached is a picture of an attic installed vent (single wall vent in attic is not allowed); 6 years old. Oh yeah, house was inspected about 18 months prior.

I believe this is a serious problem that we call out but will probably never get properly repaired because no one will do the proper calculations. With the software, all that is needed is to plug in the numbers. As Neal pointed out above, it will even ask the question if the masonry chimney is interior or exterior.

Somewhat depressed in NJ

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