Boiler venting question

[klarenbeek]

Can someone help settle a difference between two inspectors here?

We have two 1.5 MBH boilers (replacement) in a church. They are listed as category III vented appliances. The contractor vented them into an existing 75' tall, 24" round clay lined chimney in good condition. Table 504.3 and section 503.6.9.3 say cat. III appliances shall be vented in accordance with manufacturer's instructions. Section 501.6 says vents under positive pressure shall be designed for positive pressure. The manufacturer is OK with the vent system and says with the size of the stack the vent will be in a negative. Would this be acceptable because the manufacturer approved it or is it not acceptable because a clay lined chimney is not designed for positive pressure?

Help!

 

[globe trekker]

Can you have the manufacturer submit something in writing to cover that

installation, ...that they will cover / warranty the "negative pressure"

chimney, ..."totally"? Simply "saying that they will approve the install"

would not be acceptable IMO. [ Some ] manufacturers will promise a lot

to sell their product.

.

 

[TimNY]

As many here have told me.. get it in writing. They say a lot on the phone; when i ask for it in writing nothing ever happens. I have also spoken to two different reps and gotten exact opposite answers.

I would think that if it is listed as a cat iii you can't negative vent it because it wasn't tested that way. At the very least I think they are going to take an efficiency hit as the heat is exhausted out the vent rather than being transferred.

 

[Dr. J]

As stated, get it in writing, from the manufacturer, not the local rep.

From an engineering standpoint, there are three issues: 1) will the clay tile be degraded by the vent gasses; 2) will the clay tile leak flue gasses to an occupied space; and 3) will the boiler work properly with a negative pressure if it is listed as a positive pressure appliance.

1) Category III means no condensation, so this is not an issue. If anything, a lined masonry vent keeps the flue gasses warmer than a double wall metal vent.

2) If it shown that the height and diameter of the vent is sufficient to create a natural negative pressure, this would not be an issue. Besides, if it is in good shape, it probably can withstand a slight positive anyway. I handled 3 million btu/h before right?

3) Only the manufacturer can address this. Some boilers (usually old-school scotch marine type) require a positive pressure at the flue collar so the flame behaves (too much draft can cause the flame to separate from the burner). Other boilers (usually the modern high tech types) are listed as Cat III just to allow direct or horizontal venting without a draft inducer, but work just fine with a negative pressure. Several are listed as Cat III for direct/horizontal, but Cat I for vertical. Many times, for vertical common vent installations, the manufacturer requires a barometric damper to control the draft. This verifies that the boiler is expected to see a negative pressure at the flue collar.

Bottom line, assuming this is a modern medium efficiency boiler, I would tend to believe the manufacture that this will work (but would still want thier written assurance - trust but verify).

 

[klarenbeek]

We do have an e-mail from the president of the boiler company stating that venting into a sufficiently sized clay lined stack is acceptable. He also gives guidance (somewhat vague, but doable) for overfire draft control. These are modern high tech type boilers, but they are not dually certified for both cat I and II. I believe the manuf. has also provided a more clarified letter addressing the stack size specific to this job, but can't be sure of that right now because I have to get going on today's inspections.

 

[TimNY]

Thanks Dr.J for that insight.. good stuff.