Help with chimney sizing

Mike_Sheppard

I’m out of town for the rest of the week working and don’t have my fuel gas code book with me to check on this. Would someone mind giving me a hand?

I have a boiler here that is dumping exhaust out of the barometric damper when it runs. Positive draft. I’m being told this has been happening for 3 years now since the boiler was installed. They say when it gets really cold outside it will draft better.

Cast iron sectional, input of 5,155,000 btu/hr.

The vent connector is Schebler model PA, 16 inch. Picture attached. It has two 90s in the vent connector to the chimney. From the connection to the chimney to the top of the chimney is about 30 feet. From the vent connector at the boiler to the connection at the chimney is about 6 feet. So total combined height of 36 feet. Total lateral of 8 feet.

Vent connector is all 16 inch. Two 90s. The chimney liner is rectangular, 13 inches by 17 inches.

Liner area is 221 in/sq
Connector area is 201 in/sq

Can this vent the boiler properly?

STEVEusaPA

Is it also a make-up air issue?
I'd say no if it follows the same rules as smaller residential.
Traditionally, the chimney connector should be no more than 75% of the chimney height.
36 total feet X .75=27 usable feet
8 lateral feet + 2-90° elbows (28 feet/elbow-I think)= 64'

Mike_Sheppard

@STEVEusaPA i checked make-up air. It isn’t a problem. They have plenty. There are two identical boilers next to this one. Different chimneys. Same height. But they have plenty of draft. The only difference. the chimneys are larger and they only have one 90 in the connector instead of two.

I’m trying to remember, but I recall the code book accounting for two 90s in it’s charts. But don’t remember if it accounted for two 90s in the lateral or not. That may be the problem right there. The one extra 90 is enough to kill the draft.

STEVEusaPA

I'm no expert, but technically you have 2 elbows and a tee. I think that tee is working against you also, creating turbulence that's popping out the baro.
A tee @ 14" has 89' of equivalent pipe length.
Ideally coming straight out of the flue then 90, then 45 right by the chimney would be better, with the draft regulator close to the chimney, but this isn't a simple try with off the shelf pipe.

And obviously I assume you checked the entire connector for blockage.

How about what's happening where it exits the building? Is that stack against the building wall where maybe the others are more interior.
Sorry I don't have much more than that.
edit: Does it immediately go negative where you have your analyzer when you stop the burner?

Mike_Sheppard

@STEVEusaPA i was just thinking about the tee as well. That slipped my mind. Exits the boiler straight into a tee, then two 90s after the barometric. The other boilers also enter a tee directly out of the boiler but they are drafting fine. I’m not sure why the installed used tees right out of the boiler like that. You’re right, a 90 would have been much better.

Almost looks like they could have 45’d out of the boiler then 45’d into the chimney.

Exhaust termination on the roof is good. It’s only a 2 story building. Flat roof. No walls around the termination.

I didn’t check specifically for it going negative as soon as I shut it off, but I never saw it negative at all. The only time it’s going negative is when the boiler is off. As soon as the blower comes on it goes positive. Positive while firing.

Picture attached of negative draft while burner off. Barely anything. And that’s with barometric damper completely closed.

No blockages in connector.

Mike_Sheppard

Called a local rep that does venting and exhaust fans. Described it to them and they said it sounds like too much horizontal equivalent for the short chimney. I made a drawing and sent it to them. They are going to run some calculations to see if it can be fixed by removing the tee and reducing the fittings or if it will need an exhaust fan.

STEVEusaPA

Draft booster may do the trick. Would still like to see the T go, and the baro moved closer to the chimney.
But then you may need a ladder to adjust it.

T was probably there for easy access to clean/inspect. Always nice with oil...hold a bucket there, hold your breath, drop it open, hope you don't end up looking like a chimney sweep.

EBEBRATT-Ed

My gas code book is on a flash drive and I can't find it

I don't think you need a clean out tee when venting into a chimney. Tees are a killer. If it is a pressure fired boiler you may not need the barometric...unless you have too much draft. That's a Weil 88 it should have a damper in the smoke box outlet, maybe it is partially closed or the linkage broke?? I am sure you checked to see if the boiler is plugged.

Mike_Sheppard

@EBEBRATT-Ed the boiler is spotless. The smoke box damper is set currently to maintain a positive 0.1”wc overfire.

I think you are right about the tee. The only time I see them used like that is at the bottom of a stack. Or “metal chimney” whichever you call it. I don’t think a tee where it is now serves any purpose.

Mike_Sheppard

Maybe this is the answer. Picture attached. Chart from the manual. For balanced draft (requiring positive overfire but negative draft and barometric), vent / liner should be 24 inches. That’s a tad larger than 16 inch vent connector and 13x17 chimney liner.

It even says forced draft (positive venting) should be 18 inch. The current vent connector is 16 inch and the chimney liner is less area sq/in than an 18 inch round vent.

It’s looking like they either need a new vent connector and chimney (lol) or an exhaust fan system installed.

Edit: I still haven’t had a chance to sit down and fully absorb vent/chimney sizing out of the code book. But I’m looking under chimney size. Have to interpolate for a 13x17 lD liner which is roughly equal to the area of a 16 inch vent. According to what I am looking at in the code book the chimney size based on height and lateral is definitely too small.

EBEBRATT-Ed

@Mike_Sheppard

maybe down firing it will be the only practical solution

Mike_Sheppard

@EBEBRATT-Ed it is a modulating boiler. Low fire input is 1,700,000 btu and it is still partially dumping out of the barometric. Manual wants a 24 inch chimney. We’re sitting at 16 inches. And much more horizontal equivalent length than vertical. My recommendation is going to be an Enervex exhaust fan on top of the chimney. Expensive solution, but probably the only solution.

STEVEusaPA

@Mike_Sheppard any update?