Outside air restrictive

[letmebegus]

First, thank you all for your many contributions that have answered my questions over the last 5 years.

I have increasingly successfully been using a Jotul 500. This year I added an outside air intake system using a Jotul adapter and connecting a 3" foil flex duct. This stove sits on the hearth of a wood fireplace, so the flex duct runs down the large ash dump with insulation around it, the ash door removed and screen covering the door to keep critters out.
The outside air system significantly reduces the amount of intake air and chokes down the stove.

The stove has to sit on short legs so there is a mildly tight bend (not crimped), then 18" of run, a 90 bend down, then 28" of drop, a 90 bend , then 6" of run then open to the outside air
When the chimney cleaner came this fall to clean and inspect, he said it looked like it should run fine, but so far it doesn't.
I have disconnected it and it runs like old times, reconnect and chokes down, currently disconnected and burning well.

When disconnected, there is a good strong draft pulled through the intake hole on the bottom of the stove, but in my feeble mind its not more than can flow through this duct.
But then research happened:
A decent fire going and no cold air intake hooked up shows 730 cfm entering the intake hole according to my fancy flow meter.
Maximum cfm according to the interwebs through a 3" flex duct is about 400 cfm

So now what?

Thank you in advance for thoughts or advice

 

[Todd]

Is your house super tight? If not I wouldn’t bother with an OAK. If you really feel you need it maybe you can increase the pipe to 4”?

 

[letmebegus]

Its not super tight, but it is getting more so as I replace more and more windows.
The thought is, as I burn 5-700 cfm of air that means 5-700 cfm of cold outside air is being vacuumed in through leaky windows and door seals. That seems like a problem worth solving

 

[Todd]

Lots of debate out there about whether to install an Oak or not. Personally I prefer a little fresh air exchange and never really noticed a difference when I had an OAK in a previous home but then every home is a little different too.

 

[GG Woody]

I would increase the F/A to 4"
Area of 3" pipe is 7.06sq"
4" is 12.56sq" (nearly double)

Since you like research and the 3" isn't really working for you, and is likely trash, try drilling some holes in the flex pipe close to the stove, you'll get some outside and inside air.

I work in HVAC, we don't use 3" for anything other than some sawdust collection connections to equipment. 3" with some bends is getting quite restrictive. The smallest we will even use for bath fans is 6". You have 4x the area of 3" pipe in a 6" pipe.

 

[letmebegus]

I would increase the F/A to 4"
Area of 3" pipe is 7.06sq"
4" is 12.56sq" (nearly double)

Since you like research and the 3" isn't really working for you, and is likely trash, try drilling some holes in the flex pipe close to the stove, you'll get some outside and inside air.

I work in HVAC, we don't use 3" for anything other than some sawdust collection connections to equipment. 3" with some bends is getting quite restrictive. The smallest we will even use for bath fans is 6". You have 4x the area of 3" pipe in a 6" pipe.

Thank you, what I guess led me down the 3" avenue is that the Jotul adapter to connect an OAK is 2 3/4" diameter. So I will set up a 4" pipe and reduce to the adapter as close to the stove as possible and report back on levels of success.

"try drilling some holes in the flex pipe close to the stove, you'll get some outside and inside air."

My concern with this is: when the stove is not running cold outside air can come in through those holes.

 

[EbS-P]

What year was the house build? And do you have any high volume exhaust fans?

I don’t think an OAK is needed. There is no way that stocve moves over 500 cfm. None. Maybe 50 possibly 100. My whole HVAC system runs about 700 on stage 1.

It’s not really been cold enough to get a good draft. Yet. But that doesn’t reall Matter. When you wanted to use the stove it did not burn well with the oak. Simple answer is don’t use.

Any heat you would be saving can easily be made up with by burning not that much more wood.

The math point is any cold air enter must be heated up and given the same fuel load the exhaust temps and stove temps will be lower because the air was colder coming into the stove.

OAK should make the stove run better not worse. If the latter is true they weren’t needed in the first place.

 

[begreen]

Has a brush been run through the OAK duct to be sure nothing in it is restricting flow? Is the intake on the windward or leeward side of the house?

 

[letmebegus]

What year was the house build? And do you have any high volume exhaust fans?

I don’t think an OAK is needed. There is no way that stocve moves over 500 cfm. None. Maybe 50 possibly 100. My whole HVAC system runs about 700 on stage 1.

It’s not really been cold enough to get a good draft. Yet. But that doesn’t reall Matter. When you wanted to use the stove it did not burn well with the oak. Simple answer is don’t use.

Any heat you would be saving can easily be made up with by burning not that much more wood.

The math point is any cold air enter must be heated up and given the same fuel load the exhaust temps and stove temps will be lower because the air was colder coming into the stove.

OAK should make the stove run better not worse. If the latter is true they weren’t needed in the first place.

House built in 1995 2/3 are still original windows, no high volume exhaust fans, what exhaust fans we have are rarely used.
I did check and my flow meter measures f/min not volume, so you are correct that the cfm is much lower than I thought. Still there is a ton of restrictive friction on the airflow the way it was set up.
Thank your for correcting my cfm mistake and your thoughts of efficiency exchange. Since its only about $20 more invested and time spent testing will be enjoyable, I'm going to give it another attempt with a less restricitive OAK system.

Has a brush been run through the OAK duct to be sure nothing in it is restricting flow? Is the intake on the windward or leeward side of the house?

It was a brand new system put in place this fall. Vent is on West side of home usually where the wind comes from... usually

 

[begreen]

It was a brand new system put in place this fall. Vent is on West side of home usually where the wind comes from... usually

It shouldn't be an issue, but sometimes odd stuff happens. Is there a critter screen on the intake? The OAK sounds like it is being restricted in some form or another. Can you post a picture of the 'mildly tight bend'?