Effectiveness of a Fresh Air Kit pipe reduced in diameter at the hearth?

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DuaeGuttae

Minister of Fire
Oct 26, 2016
1,639
Virginia
Our family is in the process of having a new home built next to my mother’s house. Part of the build includes a Blaze King Sirocco 30.2 woodstove. The stove is fairly central to the house, and the installer ran four-inch pipe from the back of the hearth through the basement to the outside. The run is perhaps sixteen feet, and so he used four-inch pipe for better air flow per Blaze King’s recommendation.

The stove was installed some time ago, but the fresh air kit was not connected at that time. In talking to the supervisor from the builder about it last week, I finally understood that the installer was missing the Blaze King Fresh Air Kit. I was able to show the part number to the supervisor in the manual, and he or his hearth shop ordered one. I know the supervisor was aware of the four-inch piping, and so I don’t know where things went wrong, but the kit that arrived last night had a three-inch faceplate and flex pipe rather than the four-inch we had expected. I was there when the supervisor saw the box this morning, and he was pretty unhappy.

I’m not sure what decision-making went on between the builder and the stove installer who was scheduled to come out first thing this morning [there was time pressure here because a county inspection had been scheduled for today as well, though I’m not sure that happened], but I was surprised to find this evening that the three-inch kit had actually been installed. It may be that because the four-inch vent pipe makes the run from the outside up to the hearth, and the reducer and the three-inch pipe occur only above the hearth to the back of the stove which is installed at minimum clearances from the wall that it will work just fine. I make no claims to understand much about physics or air flow dynamics, so I’m wondering if you folks who have a better understanding than I do on this matter can help me know if it’s fine to leave it the way it is, or if I should ask to have it corrected. They did a nice job on the install, and if it will work, I’m happy not to have a larger pipe in view from the side [the pedestal blocks it from the front, but the side is still quite visible because of the layout of our house. I plan to camouflage it a bit by storing some soapstone bed warmers on the hearth ], but most of all I want the best performance for the stove. I’d welcome any feedback. Thanks.
 
If your outside air path is smooth wall rather than flexible (vacuum hose or drier exhaust type) I think you'll be fine for sure.

I think it's worth trying anyway because the impedance will already be less along the full length , and the constriction near the stove will add a bit of impedance but I don't think it'll be that much. The problem is if the full long length is lower diameter.

If it doesn't work you can change it later?
 
We make a 4" FA adapter for runs longer than 10-12'. Easy swap to use the 4".

BKVP
 
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I swapped the 3” adapter to a 4” when I rebuilt my hearth and used 4” metal ducting. Here’s how small a 4” duct looks behind the pedestal.

[Hearth.com] Effectiveness of a Fresh Air Kit pipe reduced in diameter at the hearth?
 
Why not spray paint it gray or black?
 
Why not spray paint it gray or black?
I did paint the rigid duct but the aluminum flex can look really bad if the paint flakes off. I have to stretch the flex to install it so that could do it.

Plus, it’s in the back and this stove has the pewter door and rails. It’s fine to leave the bare aluminum.
 
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I got thinking about this more. I doubt the connection size at the stove matters much since you have run 4”. It must be restricted somewhere between the hookup and the firebox and that restriction is likely much greater that the 3” at the OAK connection. Add an increaser and see how it performs. I’d be more concerned with the 16’ lengths and the screen at then opening than the 3” connection. If I’m understanding everything correctly. Offering a 4” kit is a matter of convince IMO unless BK has some data that says otherwise. I absolutely think 4” makes a big difference when used on long runs.
 
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Exactly. That was my point above.
 
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BK wouldn't make a 3" if it was too small...and larger pipe on the long run is exactly what you need...sounds like you are all set.