Flue Sizing - Based on the Unique Variables

[spuldup]

That furnace size and stack height is very similar to my setup, which I have had the experiance of running with both 6" and 8" chimneys. The 6", which came with the home, was too restrictive and had smoke rollout constantly. The 8" works great. Make sure your stove is tight, new ropes, doors adjusted, no leaks. I would not follow the 6" liner in 8" advice for cost reasons alone. That is a great way for the installer to tell you he has no idea what he is doing but would still like your money.

For oil or coal usage, yes you should install a baro damper per the mfr. specs. When burning wood you could close it to keep stack temp higher, but not fully necessary.

 

[RedOakON]

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 not 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.

As Brenndatomu said, the velocity/pressure of the flue gasses is an important consideration to the total flow/draw of the chimney. But I also agree with your point Sloeffle because it was that very reasoning that lead me to thinking 6in diameter was the more balanced size to match with the factors in play here.

But to the contrary of your point and mine, it could be said in water pipe terms that a 6in pipe can flow more than and 8in pipe if the pressure was increased enough. So now and to sound very much the arm chair quarterback about this; does the diameter of flue pipe determine the draw or does the velocity of flue gases up the pipe determine the draw up a height of chimney held constant in both.

 

[RedOakON]

That furnace size and stack height is very similar to my setup, which I have had the experiance of running with both 6" and 8" chimneys. The 6", which came with the home, was too restrictive and had smoke rollout constantly. The 8" works great. Make sure your stove is tight, new ropes, doors adjusted, no leaks. I would not follow the 6" liner in 8" advice for cost reasons alone. That is a great way for the installer to tell you he has no idea what he is doing but would still like your money.

For oil or coal usage, yes you should install a baro damper per the mfr. specs. When burning wood you could close it to keep stack temp higher, but not fully necessary.

I really appreciate you sharing this; especially since you say your factors are quite similar. Very good to know.

And yes, it is very unlikely that I will be using the first inspector/installer for anything to do with this or any other chimney related projects.

 

[salecker]

Any reason you can't install the Baro after the inspection?

 

[brenndatomu]

Why would it be any different for a chimney,

Because it's not just about the max CFM's possible...the only time max CFM's come into play would be maybe when the loading door is open...the rest of the time that big ole chimney is breathing through a straw, so to speak...and since chimneys work on temperature differential, keeping the chimney warm means a lot, which is much easier to do with a 6" than the much larger 8".
That's being said, if that furnace is designed for a 8" chimney, then that's what it needs...I've heard there are a few models out there that are 8" and still work OK on 6" but no idea if this is one or not...that probably would be different on each individual install.

 

[brenndatomu]

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

Me too...just FYI, if you put a @ symbol in front of their user name, their name should pop up (often in a list of possibilities) as you type it out, then you can click on it to imbed it in the post, that will alert them the next time they get on...like this @bholler

 

[RedOakON]

Any reason you can't install the Baro after the inspection?

Between us friends here, the thought may have crossed my mind.

 

[brenndatomu]

A non-commercial service in support of responsible home heating with wood - How Chimneys Work

The Wood Heat Organization was formed to support the public in the responsible use of wood energy in the home. We fulfill our mandate by providing reliable information, by conducting research into wood heating-related issues and by representing the public interest in discussions of policies that...

www.woodheat.org

 

[RedOakON]

A non-commercial service in support of responsible home heating with wood - How Chimneys Work

The Wood Heat Organization was formed to support the public in the responsible use of wood energy in the home. We fulfill our mandate by providing reliable information, by conducting research into wood heating-related issues and by representing the public interest in discussions of policies that...

www.woodheat.org

Great resource. I really appreciate that this website's chart confirms my 45.9 Pa calculation was not far off from the truth.

It also gave an explanation for why the first inspector suggested to use a 6in chimney at his discretion given all the factors.

The most compelling reason I have right now for sticking with an 8in Chimney is @spuldup 's findings with his own and very similar set of factors.

 

[spuldup]

On mine, we bought the home with 6" and used it with that for about a month. It had 2 sections of pipe that had fallen off the top. The chimney company would not install new 6" extension because the furnace collar was 8". So we bit the bullet for about $4700 and have the 8" installed the whole way up. With the 6" we had to baby it or risk smoke rollout. It needed the air almost fully open. It was also difficult to start a fire without opening the ash door to pull in enough air. With the 8" it drafts way better and the air is only set about 50% open for most weather. The stove pipe has (2) 90s , into a thimbile to the Class A chimney, then the outside tee, then (2) 30 degree offset fittings, so there is some restriction but being so tall compensates for that. Had the 6" been a straight shot I suspect it would have drafted better.

 

[RedOakON]

As an update on the matter, I was fortunate to get in touch with an oil inspector who has a what may be the most balanced opinion of all the inspectors thus far.

The summary all of his requirements are as follows:

1) A barometric damper must be installed on this chimney.
No surprise there.

2) A 45 degree Y-pipe and not a Tee for the inclusion of barometric damper should be used.
This one I don't quite consider so critical, but he says it helps the barometric damper to function better.

3) He suggests to put a key damper after the barometric damper to mostly close it off for when firing with solid fuel.

4) According to his calculations, if I go with an 8in chimney diameter he says I will have to operate the oil burner at the higher end of its firing range (165,000+ BTUs) to avoid condensation in the flue. Otherwise he recommends a 6in diameter chimney and a maximum oil firing rate of 140,000 BTUs. Or alternatively I would have to buy a variable speed air handler and decrease the air flow over the heat exchanger from 1,700 CFM to around 1,400 CFM to keep the exhaust temperature high enough to prevent condensation.

5) He can only sign-off on the oil part of the appliance functioning properly and its requirements having been satisfied. A solid fuel inspector still has to sign-off on the other half of the matter.

All said and done, I have nothing but the best to say about this guy. Hopefully there is a WETT inspector out there with same practical experience who will sign-off on the other half of this. More to follow.

 

[RedOakON]

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.

@bholler what are thoughts on this concept. Thanks much

 

[brenndatomu]

) A 15 degree Y-pipe and not a Tee for the inclusion of barometric damper should be used.
This one I don't quite consider so critical, but he says it helps the barometric damper to function better

Never seen one...bet that will be impossible to find.

He suggests to put a key damper after the barometric damper to mostly close it off for when firing with solid fuel.

Close off just the baro?
I wouldn't bother...if you have a chimney fire those key dampers don't fit tight enough to do squat.

 

[bholler]

@bholler what are thoughts on this concept. Thanks much

I honestly don't know enough about your furnace to make any judgement. With stoves some work better downsized others better upsized. There are just so many variables it's hard to say

 

[bholler]

Never seen one...bet that will be impossible to find.

Close off just the baro?
I wouldn't bother...if you have a chimney fire those key dampers don't fit tight enough to do squat.

Yeah I would just pull the head and cap the hole

 

[RedOakON]

Never seen one...bet that will be impossible to find.

Close off just the baro?
I wouldn't bother...if you have a chimney fire those key dampers don't fit tight enough to do squat.

I typed a 1 in place of a 4. I just corrected it though. The product line he uses calls them Boot Tees or 45 Degree Tees. See attached. In short, you're probably right that they are expensive, hard to find and only provide marginal benefit.

I'm going to try and sell the key damper idea to the 2 WETT inspectors and see if they accept it.

 

[RedOakON]

Yeah I would just pull the head and cap the hole

That would work too. I'll certainly mention it to both inspectors and see if that makes the difference.