Will This Chimney Draw Well ?

[Quincy Fivelos]

Top of firebox to top of Chimney opening is 31'8"

 

[bholler]

You really only need a 12 or 13" round flue for that size. And 2 11" or 12" vents should be enough to supply air.

 

[bholler]

At that height a 12" flue will be fine and i would do 2 12" vents. Inless you like dumping money up the chimney.

 

[Quincy Fivelos]

Thanks. That seems like a gob of passive air to be bringing directly in to the firebox? And then the issue of where to bring it in? Vents along the front edges?

 

[bholler]

Thanks. That seems like a gob of passive air to be bringing directly in to the firebox? And then the issue of where to bring it in? Vents along the front edges?


View attachment 225394

It is that is why it is rarley done right. But that is the only way you will get a fireplace that doesnt suck air you already paid to heat out of the house

 

[Simonkenton]

My fireplace opening is a little bigger than yours, mine is 42 x 38
I have 98 square inches of outside air intake to feed that beast, that works pretty well.

 

[maple1]

So, 800 CFM up the chimney means we'll need 800 CFM of combustion/MUA. Right? You can do two 6" passive intakes directly in to the fire box (unheated). I believe a 6" passive is good for 75 CFM (@5pa differential to outside)? So two is 150 CFM leaving us needing at least 650 CFM MUA in to the house?

In MN make up air is nearly always brought in to the return air duct. Outside air could be as low as -30°f. HVAC installers here consistently say that the heat exchangers in residential gas furnaces should never have air below +40°f. Theoretically this is due to the potential for creating extreme condensation. I have never seen anything from a manufacturer saying anything similar though.

So, the air being brought in from outside is filtered, pre-heated to between 38-50°f before being introduced in to the return air duct from where it will enter the furnace, be heated to room temp and then distributed throughout the house.

I'm not an engineer of HVAC person so mostly just spitting out what I've read and been told by HVAC folks and builders.

I still can't get my head around this. If fresh make up air is getting fed right to a return duct, that would mean an equivalent volume of heated air is not getting into the return. So where is it going? Outdoors? Have never seen or heard tell of this before.

 

[WiscWoody]

When I first started to build my lake home up here I thought I’d have a open fireplace and I had bought big one at 54x42 for the chase and spent $1300 for it. Well it sat here for years while building went on around it but then I found out how inefficient they were and I ended up putting it on Craigslist for free and gave it away. I bought a wood stove for the place and never looked back, well except for now lol.

 

[Ashful]

I still can't get my head around this. If fresh make up air is getting fed right to a return duct, that would mean an equivalent volume of heated air is not getting into the return. So where is it going? Outdoors?

Up the chimney.

 

[maple1]

Up the chimney.

Right. Which again makes no sense to me. Make up air should be going right to the combustion source, then up the chimney. That seems like a lot of wasted heated interior air.

At least that's my take - and I have been off base before.

 

[Ashful]

Right. Which again makes no sense to me. Make up air should be going right to the combustion source, then up the chimney. That seems like a lot of wasted heated interior air.

At least that's my take - and I have been off base before.

Ideally, yes. But as the OP already noted, the CFM requirement makes meeting that need entirely at the source a little impractical.

Fuel economy is always a partial consideration, but not everyone’s end goal.

 

[bholler]

Ideally, yes. But as the OP already noted, the CFM requirement makes meeting that need entirely at the source a little impractical.

Fuel economy is always a partial consideration, but not everyone’s end goal.

It is not impractical at all. In fact it is the most practical solution.

 

[Quincy Fivelos]

It is not impractical at all. In fact it is the most practical solution.

How best to accomplish this with the Isokern?

A 12" flue is 452 sq inches which would require 2 8-9" intakes to be equal in sq inches. A flue is effectively 'powered' though so will exhaust much more air than the equiv passive intakes so I'd estimate that you'd need at least 2 10" (314" ea) fresh air intakes to have equal air flow? That's a lot to try to integrate in to the firebox somehow (and I'm not sure that'd be enough) and these are some pretty big holes to put in the house envelope from an energy standpoint.

These also need to be done in a way that when not used that they do not introduce excessive energy loss and dew point problems?

 

[bholler]

How best to accomplish this with the Isokern?

A 12" flue is 452 sq inches which would require 2 8-9" intakes to be equal in sq inches. A flue is effectively 'powered' though so will exhaust much more air than the equiv passive intakes so I'd estimate that you'd need at least 2 10" (314" ea) fresh air intakes to have equal air flow? That's a lot to try to integrate in to the firebox somehow (and I'm not sure that'd be enough) and these are some pretty big holes to put in the house envelope from an energy standpoint.

These also need to be done in a way that when not used that they do not introduce excessive energy loss and dew point problems?

If you want an open fireplace you will need lots of makeup air. So yes they are big holes to put in the house from an efficency syand point. But an open fireplace is horrible as far as efficiency .

 

[Blazingembers7749]

like others have said a different style firebox with a nicely parged breast and smoke chamber will allow you to decrease your liner size and wont rely on so much makeup air if using too much makeup air is an issue. Ive had good luck with the precast ahreins fireboxes save your money and do the smokechamber yourself. they make a stainless one but i dont think its worth it. As long as you realize the firebox is going to asthetically look different and your ok with that they work really well. The firebox will be shallow compaired to traditional. A 12" round will give you plenty if using a traditional firebox. If you can do a fresh air intake do it. any amount will help from drawing room air. Im a insert person myself but a well designed open fireplace with a good smoke chamber and liner all sized correctly will give off more heat and use less air then i think some realize. Also an open fireplacwe will always have a asthetic apeal to some people and i can understand that. will it beat the efficiency of a moderstove? No. But i dont think anyones going to debate that.

 

[Blazingembers7749]

 

[Blazingembers7749]

How best to accomplish this with the Isokern?

A 12" flue is 452 sq inches which would require 2 8-9" intakes to be equal in sq inches. A flue is effectively 'powered' though so will exhaust much more air than the equiv passive intakes so I'd estimate that you'd need at least 2 10" (314" ea) fresh air intakes to have equal air flow? That's a lot to try to integrate in to the firebox somehow (and I'm not sure that'd be enough) and these are some pretty big holes to put in the house envelope from an energy standpoint.

These also need to be done in a way that when not used that they do not introduce excessive energy loss and dew point problems?

How do u get your sq in? 12" round is 6*6*3.14= 113.04sqin. 12 square is 12*12=144sqin. I apologize if i missed somthin.

 

[bholler]

How do u get your sq in? 12" round is 6*6*3.14= 113.04sqin. 12 square is 12*12=144sqin. I apologize if i missed somthin.

Good catch i didnt notice that

 

[Simonkenton]

"How do u get your sq in? 12" round is 6*6*3.14= 113.04sqin. 12 square is 12*12=144sqin. I apologize if i missed somthin."

Yes, sir. pi r squared

 

[edyit]

A 12" flue is 452 sq inches which would require 2 8-9" intakes to be equal in sq inches

yeah as others have noted your math is a little off, it happens to the best of us, i know i get distracted when i think about pie

 

[Ashful]

Yes, sir. pi r squared

Around here, pie are round.

Sorry, my 7th grade nerd humor instinctively kicking in, again.

 

[Simonkenton]

Yes, I heard that one in seventh grade, our math teacher told it to the class, He must have laughed for 5 minutes, he was a nerd too.

But, I tell you, I hear a lot of people talking about "all the crap we learned in high school...."

Well, it wasn't crap. You can figure out how big of a duct you need if you remember pi r squared.

Also, I took Geometry in 10th grade, this was in 1965. I loved that course for some reason Geometry really resonated with me ti was the only math I ever liked.

And so, thirty years later, I was building a log cabin, needed to figure the length of the rafters, I used the Pythagorean theorem, a squared plus b squared equals c squared. The wisdom of brilliant Greek intellectuals from 2,500 years ago, and, thirty years after I studied it in high school I used it to figure rafter length.

 

[BrotherBart]

Geometry was the only class in high school I failed. Also in 1965. I think it had a lot to do with Rebecca Barrier that sat across the table from me. I reminded her of that on Facebook recently. She said she failed it too but that we had fun.

 

[Ashful]

And so, thirty years later, I was building a log cabin, needed to figure the length of the rafters, I used the Pythagorean theorem, a squared plus b squared equals c squared. The wisdom of brilliant Greek intellectuals from 2,500 years ago, and, thirty years after I studied it in high school I used it to figure rafter length.

Interesting fact, the Pythagoreans never knew the Pythagorean theorem. It was named after them, because they were well-known to have enormous (and pretty accurate) tables of right triangle ratiometric dimensions. But their tables were developed by literally drawing accurate triangles on the ground, and measuring them, not by the theorem that was later given their name.

 

[begreen]

I've read that the basis for the theorem was known by the Babylonians long before Pythagoras. Chinese and Indian scholars also understood this. Some give original credit to ancient Indian Vedic mathematicians who wrote down the basis about 1000 yrs prior to Pythagorus.
https://link.springer.com/article/10.1057/jt.2009.16

https://en.wikipedia.org/wiki/Pythagorean_theorem