How Is Draft Affected by Flue Diameter and Flue Height?

I have a ranch house with a full basement. It has a centrally located hearth, where the main floor fireplace is about 16 feet from the bottom of the fireplace to the top of the main floor flue, and the basement fireplace from the bottom of the fireplace to the top of the flue is about 25 feet. I had been shopping wood fireplace insert and a coal fireplace insert, and have already decided on the wood insert. Originally I planned on installing the coal insert on the main floor, and wood insert in the basement. However for two reasons, one of which is flue height, I have chosen to reverse that decision. (Other reason doesn't apply to this thread.)

The wood fireplace insert installation manual says the "Ideal performance is with height of chimney between 14-16 feet measured from the base of the appliance." The insert requires a 6 inch round liner that has 1 inch insulation the full length. It repeats this elsewhere in the installation manual with the following warning "Chimney heights exceeding 25 feet from base of appliance may create an over-draft situation. Overdraft condition may create over-firing. Over-firing may ignite creosote and/or damage appliance and chimney."

Of the coal fireplace inserts / hearth appliances I have read up on so far, I have not seen any mention of a draft / flue warning like the wood fireplace insert installation manual has warned. I realize that creosote is not as much of a problem with anthracite coal, but would the draft strength still be an issue with coal? I have not decided on which coal manufacturer / model to purchase, but assuming two coal inserts where one has a 6 inch round flue, and the other has an 8 inch round flue. Would a 8 inch diameter flue of the same length as a 6 inch diameter flue have a stronger draft?

One thought I had in deciding on which manufacturer and model coal fireplace insert is to limit my options to ones with a 6 inch round diameter flue, that way in case I were to decide to later switch the inserts so the coal insert was on the main floor and the wood insert to the basement, I could ensure the flue length during installation was long enough to accommodate either insert, and not have to buy new liners. What might I have to consider about draft in a 6 inch diameter flue on a coal fireplace insert with a flue height of 25 feet (measured from the bottom of the unit)? Could a strong draft cause the unit to over fire even with the dampers nearly shut?

Depending on the height and size of the pipe/chimney, it can affect the draft.

With a coal unit, you typically put in a Barometric Damper that automatically limits the draft in an overdraft situation by opening and closing to maintain a constant draft on the stove/insert. Also, the only by product of coal burning is fly ash which is non combustible, so no chimney fire or anything.

If your chimney is in good shape with a liner or not, a coal insert would work either way. most of us just use a standard block chimney or with a good liner depending. The exhaust on the coal units are not that high, 200-400 deg (if that).

I'm reading from your response that the height is not an issue.
On a fireplace insert, I'm not sure how a barometric damper would attach, along with a flexible liner.
Maybe on a hearth appliance like Alaskastove offers enables a barometric damper to be attached, but I have not read their manuals yet.

I plan to insulate the liner for the basement flue, which I would be installing the coal fireplace insert to. That way if did switch the units around, I would not have to take the liner back out to insulate. The three coal insert installation manuals I have read so far, I do not recall a requirement for insulation. My understanding for why the wood fireplace insert requires insulation on the liner is to prevent creosote buildup. Would an insulated liner be bad for a coal appliance?

Do you plan to run both appliances on one chimney flue, both at the same time?

One big advantage you have is that the fireplaces are internal to the house, rather than running up an outside wall. At least that is how I understood your description.

The problem with inserts among others is, how do you clean the chimney with the insert in the way? Consider that before making decisions. They also need fans to get the heat out which means running 24/7 and burning electricity and maintenance of those fan motors and fan blades. Not good in power failure.

If your flues are internal to the house, I doubt you need any liners, and I would install without and worry about a liner afterward if needed. Coal will rot out a liner in time. The liners are not cheap either.

The best installations are free standing stoves with the pipe entering the chimney above the mantle. Doing your homework and reading up is good, but don't assume the makers know what they are talking about, which most don't. Surprisingly the best stoves in many ways are the best of the antiques.

Draft is linear to combined flue + chimney height. If 15 foot tall flue + chimney 'A' is yielding 0.03" of water column draft for a given nominal internal chimney temperature and a given outside temperature, then 30 foot tall flue + chimney 'B' under the exact same conditions (including chimney cross sectional area) will yield a draft of 0.06" of water column draft.

lsayre wrote:Draft is linear to combined flue + chimney height. If 15 foot tall flue + chimney 'A' is yielding 0.03" of water column draft for a given nominal internal chimney temperature and a given outside temperature, then 30 foot tall flue + chimney 'B' under the exact same conditions (including chimney cross sectional area) will yield a draft of 0.06" of water column draft.

i feel this discussion is for the most part not given near enough attention and as a result ALOT of people struggle with heaters that are perfectly good but don't seem to function as desired.

in many many cases there is simply not a chimney of placement, construction or condition to supply enough support for a heating appliance.

neither are draft increasing caps and other non powered measures given enough consideration.

i myself am lucky in this department, I have an excellent chimney in this house. though that wasn't always the case in every house I've burned coal in.

even for people who already have stoves of some kind installed and aren't have the success they hoped, I would highly recommend exhausting every avenue to improve and or upgrade the chimney as a first step toward remedy.

that's not always popular due to house hold ascetics etc. but, then one needs to weigh the need, desire ratios of having a solid fuel heater in the first place.

personally, I have and will again change the whole interior of houses in order to have a functional chimney so that I can have heat the way I want it.

best of luck to all who have these types of issues,
steve

Gardener, If you go with a wood burning insert get a good liner(get a heavy wall SS not the thin one's, the one I had was a Homesaver 304 heavy wall thats was reinstalled in another house 5 years ago after I used it for 20 still good as new) and hook it up directly to the stove makes cleaning a breeze just sweep the liner and everything falls right into the stove plus you don't get creosote leaching into the mortar joints of the clay liners if there compromised in anyway,as far as insulating it I would but my SS was not, I think a warm flue is better then a cooler one especially with wood. I would not use a SS liner for coal I have not had luck with the two different ones I used but just do a search on that subject and you will see! Franco b is right about inserts I know I have one but its not setup like a insert its more like a free standing stove as it's hooked right up to the 8" clay with a Barometric damper in the 8" stove pipe, without a convection blower the heat stays in and around the stove and not dispersed into the room so if your power is sketchy then thats something to really think about my power is really good,I have a 13000 watt gen with 38HRS on it since 2004 plus I run it once a month just to give it some exercise. If I were to get a new stove I would get a rebuilt antique stove or something without a blower,I'm getting tired of the noise from the blowers plus if I go with a freestanding stove an old one would be the only one I would put in front of the opening but thats just a look thing for me.
I have changed my setup three times before I finely get it to work right,I didn't have any luck with the large flue for the fireplace so I relined it with 8"round clay liners,used Perlite to insulate the air space and added 2 more feet(I now have 4' over the peek)to the chimney hight= one hell of a drafting chimney. As far as the flue size I would not go over 2x the cross section for the stove outlet but just read the manuals for the specific model especially with the new wood stoves and all the EPA crap. I like to stay as close to the stove outlet as possible but I'm kinda old school in that regard.
Keepaeyeonit

KingCoal wrote:

lsayre wrote:Draft is linear to combined flue + chimney height. If 15 foot tall flue + chimney 'A' is yielding 0.03" of water column draft for a given nominal internal chimney temperature and a given outside temperature, then 30 foot tall flue + chimney 'B' under the exact same conditions (including chimney cross sectional area) will yield a draft of 0.06" of water column draft.

i feel this discussion is for the most part not given near enough attention and as a result ALOT of people struggle with heaters that are perfectly good but don't seem to function as desired.

in many many cases there is simply not a chimney of placement, construction or condition to supply enough support for a heating appliance.

neither are draft increasing caps and other non powered measures given enough consideration.

i myself am lucky in this department, I have an excellent chimney in this house. though that wasn't always the case in every house I've burned coal in.

even for people who already have stoves of some kind installed and aren't have the success they hoped, I would highly recommend exhausting every avenue to improve and or upgrade the chimney as a first step toward remedy.

that's not always popular due to house hold ascetics etc. but, then one needs to weigh the need, desire ratios of having a solid fuel heater in the first place.

personally, I have and will again change the whole interior of houses in order to have a functional chimney so that I can have heat the way I want it.

best of luck to all who have these types of issues,
steve

Believe that, the chimney will make or break your setup.

A cap isn't going to fix a weak chimney.

lsayre wrote:Draft is linear to combined flue + chimney height. If 15 foot tall flue + chimney 'A' is yielding 0.03" of water column draft for a given nominal internal chimney temperature and a given outside temperature, then 30 foot tall flue + chimney 'B' under the exact same conditions (including chimney cross sectional area) will yield a draft of 0.06" of water column draft.

I don't think that works for an outside chimney where the flue gases are loosing heat as they ascend.. does it?

Lightning wrote:

lsayre wrote:Draft is linear to combined flue + chimney height. If 15 foot tall flue + chimney 'A' is yielding 0.03" of water column draft for a given nominal internal chimney temperature and a given outside temperature, then 30 foot tall flue + chimney 'B' under the exact same conditions (including chimney cross sectional area) will yield a draft of 0.06" of water column draft.

I don't think that works for an outside chimney where the flue gases are loosing heat as they ascend.. does it?

No, but as I stated all things being equal it is linear. If things other than height (including nominal top to bottom flue temperature) are equal it will work.

Thank you tumbrddl, I will be reading (mmm, studying) the Rumford page on draft.

The dealer I spoke with did not think the height of my
basement flue was an issue. He no longer performs or contracts installs, but only
resells bagged coal and inserts/stoves/furnaces. For this reason, I am not comfortable
to rely on his advice, hence all my questions on this forum.

Lightning asks:
"Do you plan to run both appliances on one chimney flue, both at the same time?"

The main floor fireplace and the basement fireplace have separate flues, which are 12 x 12 clay tiles.
I intend to only operate one appliance at a time, including the dryer and bathroom exhaust vents,
that way they do not compete for air.

franco_b wrote:
"One big advantage you have is that the fireplaces are internal to the house, rather than running up an outside wall. At least that is how I understood your description."

correct

"The problem with inserts among others is, how do you clean the chimney with the insert in the way?"

A full liner bypasses the clay tiled flue entirely. ???

"I would install without ... a liner. Coal will rot out a liner in time."

I have not read or heard that before. The dealer I spoke with said a liner was optional, but I thought
he said a liner as spec'ed by the manufacturer would eliminate most issues with draft, since the
clay flues in our hearth / stacks are 12 x 12.

When I was talking with HVAC sales people about installing a gas furnace, I was insistent to know
why a high efficiency furnace requires PVC venting, which I later found the explanation is that
the exhaust produces carbonic acid that quickly eats away anything metal it comes into contact with.
I have not seen anything mentioned of burnt coal exhaust deteriorating a metal reline, though
I have not specifically searched that subject, I will now.

"best installations are free standing stoves with the pipe entering the chimney above the mantle"

Agreed. Though because the brick/hearth/chimney structure is partially supporting the house, and unless I can
find someone who knows how the thing was constructed, I am wary to make any alterations.
I believe it is possible to remove the hearth and lower the fireplace floor to level with the
basement slab in order to fit a stove rather than an insert, but it is not a risk I want to take.
I have an idea of how it is constructed from spaces where the brick veneer is not applied,
but finding someone who can interpret it with authority versus opinion??? Again a risk I am not
comfortable with.

Keepaeyeonit wrote:
"I relined it with 8"round clay liners,used Perlite to insulate the air space and added 2 more feet"

What is your total flue height now? (from the base of the stove)
Also, are you saying you used perlite on the flue for your coal stove? or the wood burning insert/stove?
Anyway, how did you plug the flue in order to keep the loose perlite in place?

coaledsweat wrote:A cap isn't going to fix a weak chimney.

a cap of this type does and will increase draft on any otherwise functional flue. granted, if it's rusted in half or broken thru half way up nothing is going to help but I doubt we were talking about that.

i've used this style of cap in 3 applications they did what was represented and over came otherwise deal breaker conditions.

https://www.famcomfg.com/chimney-caps-1/round-bas ... nless.html