Flue Sizing - Based on the Unique Variables

[RedOakON]

As a practical thought experiment, what diameter of class A chimney would be advisable based on these variables?

The appliance has a firebox with the dimensions:
32in Long
19in Wide
20in High

The appliance can burn wood, coal and oil. The rated BTU outputs for the fuel types are:
Wood: 135,000 BTUs
Coal: 155,000 BTUs
Oil - Max Setpoint: 175,000 BTUs
Oil - Min Setpoint: 120,000 BTUs (this is the setpoint it will be exclusively fired at when buring oil)
The appliance will primarily be fired with oil at the 120K BTU setpoint and the wood/coal as an alternative and backup heat source.
The appliance has an 8in diameter collar.

The Chimney will be plumb dead straight.
26.5ft from the appliance to the top of the stack and located on the interior of the structure except for the 4 ft of stack above the roofline.
The class A chimney will have an exceptional R-value for its type class.
The building has a positive air system (which I cannot confirm is relevant to the question at hand, but I thought it worth including).
Also the building is at 1000ft above sea level.

The chimney installer/inspector is very concerned the appliance will be over drawing, but is adamantly opposed to placing a barometric damper in the flue.

Given all of the above, what diameter of chimney would be best suited to these conditions?

See attached for a diagram. Thanks in advance.

 

[maple1]

I would maybe start by getting a second opinion from another 'installer'. Is he both an inspector and installer? BDs with wood burners can be a topic of discussion but IMO are a necessity when burning oil. What is the appliance? What does it spec? Likely no matter what it is, that chimney arrangement will most certainly present more draft than wanted. In which case a damper will be needed of some kind. I much prefer BD over a key damper. But that might cause creosote issues depending on the appliance and how dirty it burns. I have a similar arrangement, with a 7" stainless chimney. Have never burned coal but my old unit was a wood/oil combo. With both key and baro dampers.

 

[sloeffle]

Forced draft or natural draft ?

 

[RedOakON]

I would maybe start by getting a second opinion from another 'installer'. Is he both an inspector and installer? BDs with wood burners can be a topic of discussion but IMO are a necessity when burning oil. What is the appliance? What does it spec? Likely no matter what it is, that chimney arrangement will most certainly present more draft than wanted. In which case a damper will be needed of some kind. I much prefer BD over a key damper. But that might cause creosote issues depending on the appliance and how dirty it burns. I have a similar arrangement, with a 7" stainless chimney. Have never burned coal but my old unit was a wood/oil combo. With both key and baro dampers.

Hi Maple1,

I'm working on getting a second opinion today. Installers are few and far between out here.

Yes, typically the WETT inspectors here wear both hats of being the inspectors and dealer installers of the brand(s) they choose to represent as there just would not be enough business for them to live on if they were only certified inspectors.

The appliance is the Glenwood 2950 from my signature line. It specs 8in diameter chimney to match with the 8in diameter collar on the appliance, but even the manufacturer said a barometric damper is likely necessary given the circumstances.

I'm not personally opposed to barometric dampers only because I often and throughly sweep my chimneys. This inspector's bias against barometric dampers is focused solely around the point that if a chimney fire were to occur it would give the fire all the oxygen it needed to runaway.

 

[RedOakON]

Forced draft or natural draft ?

Hi Sloeffle,

It is forced draft, but there is no prevention from it naturally drawing across draft blower. At this point in time, I don't really want to modify it with a shudder flap due to warranty becoming void.

 

[maple1]

Hi Maple1,

I'm working on getting a second opinion today. Installers are few and far between out here.

Yes, typically the WETT inspectors here wear both hats of being the inspectors and dealer installers of the brand(s) they choose to represent as there just would not be enough business for them to live on if they were only certified inspectors.

The appliance is the Glenwood 2950 from my signature line. It specs 8in diameter chimney to match with the 8in diameter collar on the appliance, but even the manufacturer said a barometric damper is likely necessary given the circumstances.

I'm not personally opposed to barometric dampers only because I often and throughly sweep my chimneys. This inspector's bias against barometric dampers is focused solely around the point that if a chimney fire were to occur it would give the fire all the oxygen it needed to runaway.

Not sure about that one. Your following post indicates it might draw all it wants anyway - just whether it would be drawing thru the furnace or not. I suspect the ignition of most chimney fires comes from draft drawing through the firebox and pulling the ignition source into the chimney. Either way - running an oil burner on a chimney like that without a baro is not something I would do.

 

[sloeffle]

I'm with maple1 on this one, I wouldn't put a chimney that size in without a baro. On windy days the baro on my 6" chimney ( shorter height, and roughly half the square inches ) would sometimes be wide open and the draft would still be outside of what the manufacturer called for.

Sound like an interesting setup for how the appliance drafts. On most appliances the fan should be sucking or it should be a natural draft, not a mix of both. I see that manufacturer doesn't give an EPA rating. They are marketing it as a commercial appliance to skate around the EPA regs. Most appliance that burn "two things" don't really do either of them very well from what I've heard from other folks.

 

[brenndatomu]

I'm not personally opposed to barometric dampers only because I often and throughly sweep my chimneys. This inspector's bias against barometric dampers is focused solely around the point that if a chimney fire were to occur it would give the fire all the oxygen it needed to runaway.

Clean the chimney often and well, burn truely dry wood, not this "seasoned" stuff everybody talks about (I season my food, not my firewood) and if you have a quality wood burner (not a smoke dragon) there shouldn't be anything in the chimney to burn.
What does the OEM say about controlling draft...if they say to use a baro to control high draft, (and it will be high with that chimney) then I don't see how this inspector can have a say in the matter (other than making sure its installed properly)

 

[brenndatomu]

From the manual. Tell the inspector to buzz off.

 

[bholler]

Hi Maple1,

I'm working on getting a second opinion today. Installers are few and far between out here.

Yes, typically the WETT inspectors here wear both hats of being the inspectors and dealer installers of the brand(s) they choose to represent as there just would not be enough business for them to live on if they were only certified inspectors.

The appliance is the Glenwood 2950 from my signature line. It specs 8in diameter chimney to match with the 8in diameter collar on the appliance, but even the manufacturer said a barometric damper is likely necessary given the circumstances.

I'm not personally opposed to barometric dampers only because I often and throughly sweep my chimneys. This inspector's bias against barometric dampers is focused solely around the point that if a chimney fire were to occur it would give the fire all the oxygen it needed to runaway.

They also greatly increase creosote buildup making the risk of chimney fire much higher. I really hate to see them but some furnaces do need them.

 

[RedOakON]

Update on the project:

I spoke with a second WETT inspector this morning in detail about the appliance, its requirements and the overall conditions present at this site. While he came across as a more reasonable guy than the first inspector, he too, is also hung up on the barometric damper as being a no go issue for him.

The first inspector had suggested the idea of purchasing and installing an 8in diameter chimney measuring the draft (testing it to indeed prove the draft would be too strong!) and then running a 6in liner down the same 8in chimney and retesting it with the manometer to see if the draft would come down to an acceptable range. In the end this would leave me with an 8in chimney with a 6in liner running the course and still no barometric damper for the oil, but a passed inspection from the first inspector. What are your thoughts about the consequences of this approach?

 

[RedOakON]

Not sure about that one. Your following post indicates it might draw all it wants anyway - just whether it would be drawing thru the furnace or not. I suspect the ignition of most chimney fires comes from draft drawing through the firebox and pulling the ignition source into the chimney. Either way - running an oil burner on a chimney like that without a baro is not something I would do.

And to add to your point, there is the matter of the existing positive air system running constantly in the space which will only further push air though the unrestricted feed port.

 

[brenndatomu]

Did you show them the statement in the manual where OEM calls for a barometric damper?!
They don't wanna let you do the baro, but they'll let you undersize the flue! Which by the way, won't lower the draft much, if any...might even raise it...but it will cause smoke rollout while you are reloading.

 

[RedOakON]

From the manual. Tell the inspector to buzz off.
View attachment 307978

Funny enough, yesterday I photocopied and highlighted this exact page from the manual as a point of proof to email to both inspectors to have on record simply because it is so crystal clear that a barometric damper is needed.

To the credit of Maple1, Sloeffle, Brenndatomu and Bholler I agree with all of you; a barometric damper is needed in this chimney regardless of the diameter simply as means to control the draft and flame characteristics of the primary fuel this appliance will burn which is oil... I just need a licensed inspector to agree lol.

 

[brenndatomu]

Generally these guys live and die by what the OEM specs out...

 

[RedOakON]

Did you show them the statement in the manual where OEM calls for a barometric damper?!
They don't wanna let you do the baro, but they'll let you undersize the flue! Which by the way, won't lower the draft much, if any...might even raise it...but it will cause smoke rollout while you are reloading.

Yes, both inspectors have the manual and that particular page you posted highlighted in their email now. Maybe one of them will have a change of mind over the next day or so. Either way, I'm going to call another inspector in the morning and go from there.

There is a sort of maniacal obsession though among both of these inspectors that revolves around the fact the appliance happens to be able to burn wood and that all else about the appliance is irrelevant to the best practices for wood burning and venting rather than looking at it holistically and taking the overall best approach.

Why do you believe it may draw more with a 6in chimney? I'm just curious because my rough calculations for this setup seemed to suggest to me that a 6in chimney would be the most balanced given all the factors.

 

[RedOakON]

I'm with maple1 on this one, I wouldn't put a chimney that size in without a baro. On windy days the baro on my 6" chimney ( shorter height, and roughly half the square inches ) would sometimes be wide open and the draft would still be outside of what the manufacturer called for.

Sound like an interesting setup for how the appliance drafts. On most appliances the fan should be sucking or it should be a natural draft, not a mix of both. I see that manufacturer doesn't give an EPA rating. They are marketing it as a commercial appliance to skate around the EPA regs. Most appliance that burn "two things" don't really do either of them very well from what I've heard from other folks.

Just for fun, tomorrow I will measure the static cold draw on the old 7in oil furnace chimney on the other end of the basement. It's only about 2/3 as tall as the one that is being discussed here, but it may still serve as compelling evidence to both of the inspectors for just how necessary a barometric damper is on the new chimney.

 

[brenndatomu]

I will measure the static cold draw

That means nothing really...it is when the stove starts to waste heat up the flue that you get draw (draft)
Its a bit like looking at a cars speedometer that reads up to 120 MPH to say how fast it will go...while its still parked in the driveway...pretty much meaningless.
There are so many things that affect static draw...wind speed/direction, outdoor temp, indoor temp, which floor the appliance is on, the chimney height, the chimney type and placement, the insulation and air sealing level of the house, other appliances that may be placing a positive or negative pressure on the house

 

[brenndatomu]

Why do you believe it may draw more with a 6in chimney? I'm just curious because my rough calculations for this setup seemed to suggest to me that a 6in chimney would be the most balanced given all the factors.

Everything else being equal, a 6" chimney will have more velocity, and tend to stay warmer.
I'm sure there are situations where either one could have more draft (a 6" or 8")

 

[maple1]

Everything else being equal, a 6" chimney will have more velocity, and tend to stay warmer.
I'm sure there are situations where either one could have more draft (a 6" or 8")

That was my feeling too. Have always assumed smaller flue could mean more draft, all else being equal.

Would like to see Bhollers input on that one.

 

[brenndatomu]

That was my feeling too. Have always assumed smaller flue could mean more draft, all else being equal.

Would like to see Bhollers input on that one.

I believe that when you are penciling out theoretical draft, the larger diameter will have more, but I still think going smaller, on a machine meant to have a 8" pipe, will not achieve lower measured draft when in operation...and could cause smoke rollout during loading.

 

[PassionForFire&Water]

you need to follow the manufacturers manual.

What is the flue collar size on the boiler?
You can t go smaller then that

 

[RedOakON]

That means nothing really...it is when the stove starts to waste heat up the flue that you get draw (draft)
Its a bit like looking at a cars speedometer that reads up to 120 MPH to say how fast it will go...while its still parked in the driveway...pretty much meaningless.
There are so many things that affect static draw...wind speed/direction, outdoor temp, indoor temp, which floor the appliance is on, the chimney height, the chimney type and placement, the insulation and air sealing level of the house, other appliances that may be placing a positive or negative pressure on the house

Fair enough; it's one less thing on the to do list for me. I was just thinking to use the reading as a starting point to show how strong the draft will be with the appliance hooked up and running with a hot flue let alone cold.

 

[sloeffle]

In my feeble brain it's no different than a water pipe. We all know a 6" pipe can carry about twice as volume as a 8" pipe. Why would it be any different for a chimney,

All other things being consistent, wouldn't a 8" chimney have more draft due to it being more square inches vs a 6" chimney ? Isn't that why some of the vendors for these products are so adamant about lining a standard chimney with a 6" flue, a standard terra cotta flue has too much draft.

 

[RedOakON]

That was my feeling too. Have always assumed smaller flue could mean more draft, all else being equal.

Would like to see Bhollers input on that one.

Very interesting indeed. I too would be curious to see Bholler's input on this idea as well.