OAK Install for an NC30 in the basement

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dboone

Member
Nov 22, 2008
45
Western Missouri
I am strongly considering installing a homemade outside air kit for my NC30 in my basement. It will be somewhat of a challenge since I would estimate about a 16' run with several bends. I also will have to core through a foundation wall in 2 different places. I have a suspended concrete porch with an interior concrete wall in the basement. I plan to run the pipe through the first concrete wall and into the storage room and then take it up just above grade and go through the concrete wall again. I do have access to HD hammer drills at work that could put a 2" or 4" hole in the concrete. I believe the NC30 has a 3" intake pipe coming out of the bottom. Here is my first dumb question. What if I tried to reduce this 3" pipe down to 2" or actually 1-1/2" since it has to make it through the hole? I hear that drilling a 2" hole in concrete is a lot easier because it significan't reduces the chances of hitting rebar with the core bit. I would have to think this would reduce the air flow. What do you think? If it is really going to hurt my air flow then I will go with a 4" hole & a 3" pipe. My reasons for wanting an OAK. 1. I just like the theory of pulling cold air from the outside vs warm air inside 2. Occassionally I have had smoke billowing into the room due to a negative pressure situation 3. I often in the off season get that foul & moist soot smell...particularly when it rains or when I run the attic fan. I would appreciate any of your thoughts on my plans to do this and in particular about the pipe sizing question. (Below is a crude diagram of the install)
 

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First off, I love the drawing.

4 inch pipe area [ 2 inch radius ] = (2*2) * 3.14 = 12.56

3 inch pipe area [1.5 radius] = (1.5*1.5) * 3.14 = 7.065

2 inch pipe area [1.0 radius ]= (1*1) * 3.14 = 3.14

BIG DIFFERENCE !!!!

Will most assuredly effect airflow.
 
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Given the length of the run, I would go with 4 inch. I would put a positive shutoff somewhere in-line. In a below grade install, it is often advised to include a P-trap loop to reduce some of the "push" of heavy cold air rolling down hill but in your case, the temperature in the storage room and the amount of insulation on the pipe will determine if the loop would be desired or necessary. The cold outside air will absorb some heat from the storage room en route to the stove. You will want to have adequate insulation to prevent frost and condensation from forming.
 
I love the drawing too.

Agreed on pipe size--definitely go for 4", especially for a long run.

Question--might it be best to install a bypass valve on his OAK, to allow him to suck input air directly from the basement when he wants? My understanding is that the effective chimney height is the difference between air input height and exhaust output height, so my concern is that the OAK is reducing the effective height of his chimney, and so he might at least want the option of bypassing the OAK on startup, when the chimney is cool and overall draw is minimum.

Thoughts?
 
RenovationGeorge said:
Thoughts?
No, the effective height wound be the base of the stove since the heavy cold air from the OAK would be pushing. It's easier to envision if you view the cold air as water.
 
LLigetfa said:
RenovationGeorge said:
Thoughts?
No, the effective height wound be the base of the stove since the heavy cold air from the OAK would be pushing. It's easier to envision if you view the cold air as water.

I'm not sure that's correct.

Hot air rises in a chimney. If the inside end of the OASupply is warmer than the outside end--which it should be when the stove is needed--it will draft upwards, away from the stove, like any chimney, and just like the actual flue.

So in effect, you have two chimneys pulling against each other--the chimney and the OAS--though the flue should win easily once the stove is heated up.

This normally isn't an issue in an above-ground installation, where the OAS entrance is lower than the stove, and the OAS's upward draft is towards the stove, increasing the effective chimney height rather than decreasing it.

For that reason, I suggest a value allowing the OAS to be bypassed on startup.

Thoughts?
 
Are you factoring a cold stove and startup or an operating stove? There is a concern WRT startup, that the draft not reverse and your assertion of "effective flue height" has some validity for that brief time. If you mean that the effective height is affected during operation, I contest that notion.

Compare the size of the flue to the size of the OAK. Consider the velocity in the OAK. Consider the heat source on the flue. The OAK should and would be insulated along the long lenght and in theory should not pick up much heat/boyancy. The small diameter means that the cold will push through faster and not leave room or time for convection.
 
LLigetfa said:
Are you factoring a cold stove and startup or an operating stove? There is a concern WRT startup, that the draft not reverse and your assertion of "effective flue height" has some validity for that brief time. If you mean that the effective height is affected during operation, I contest that notion.

Compare the size of the flue to the size of the OAK. Consider the velocity in the OAK. Consider the heat source on the flue. The OAK should and would be insulated along the long lenght and in theory should not pick up much heat/boyancy. The small diameter means that the cold will push through faster and not leave room or time for convection.

Good! The take-away is that we agree that in this case the OAS might make startup more difficult. So Daniel Boone (OP) should consider a bypass. EDIT: Or just put a match in front of the stove end of his OAS, and see which way it draws. Or compare startup with the OAS connected and disconnected--to see if it makes startup easier. If that's the case, he can always add a bypass. Just a heads-up.

I also believe my assertion is valid in all cases, but won't muddy this thread discussing it here. Please feel free to start another thread if you want to talk about it more.

Personally, I think an OAS is a great idea, for increased efficiency and humidity.

HTH, and good luck Boone! If you do it, please post back here and tell us what you discover!
 
I would stop short of saying we are in agreement, but I will make consessions. AFAIK, most all basements are in a pressure deficit situation and opening the cold stove door should start the OAK flowing, purging all warm air from it, tipping the balance scale. There is the possibility that a very cold outside flue could out-downdraft the OAK and tip the balance scale the wrong way. If/once the air in the OAK is flowing in the right direction and warm air has been purged, it should not negate any flue length.

Should there be a persistent flue reversal that cannot be turned around by sending a safe source of hot air up the flue with a propane torch or heat gun, there is a real danger of the OAK reversing and hot flue gasses entering it should a fire be attempted to reverse the flue. I would propose a draw collar be installed on the flue to mitigate such risk.

I'm not sure having a bypass that shuts off OAK air and diverts to room air would be of any benefit. If the flue reversed persistently, the bypass could send hot flue gasses into the room.
 
I should point out that I have a positive shutoff on my OAK and when I am about to start the cold stove, I can feel the OAK pushing on the air wash of the door the moment I open th OAK. If I were to feel cold air come down the flue instead, I would never light the stove in the hopes the fire would reverse it. I would reverse the flue first by sending safe heat up the flue and by opening a nearby window to negate the pressure deficit.

This is easy for me to say since my class A runs up the inside of my house and so never reverses.
 
Thanks for an interesting discussion LL. The more I think about the OAS's effect on draft, the more interesting it is. I won't hijack poor Mr. Boone's thread further, but will give it more thought, and may start a new thread so I can bother everyone. :)

Good luck DBoone! I hope you post back to this thread, and tell us what you discover.
 
Hey LLigetfa,

You inspired me to study on chimneys, and I've realized that I'm wrong and you're right!

I appreciate your input, and politely challenging my thinking. Thanks.

To the esteemed Mr. Boone, I now believe that the effect of an OAS with it's source higher than the stove is minimal at best, and vanishes once the stove pulls air through it. I hope that's right. :)

Anyway, rock on with the 4" pipe, and good luck.
 
Wow you guys really went to town on this subject. I haven't posted on here in a while and forgot that you only get a notification email on the first response. I thought after the first response nobody cared about my post anymore (sniffle, sniffle). That is until I jumped on the post today and saw all your responses!!! I will need to read your post at least another time because some of it went over my head. I hope the OAK/OAS does not get too complicated. A reversal of outside air to exhaust gases sounds a little scary. I didn't really think about that, but if that is a risk I am sure I at least want to use metal duct work all the way out. Is the general consensus that an OAK would be beneficial to the operation of the stove?
 
I don't know how my graphical masterpiece got so small. It was much bigger when I drew it in MS Paint. I guess you all get the idea however.
 
Almost 2 years later and I still have not done this OAK install!! I still remember & appreciate this thread and the contributions you guys made to it. This time I am seriously going to do it. Hopefully in the next couple weeks. I even made a trip to Home Depot to buy some parts. I would like to fire up this thread again with a few more questions if you don't mind.

1. Is standard silver HVAC/clothes dryer type tubing sufficient & the best choice for this? I picked up a bunch of stuff today at Home Depot. I got both flex pipe and straight solid pipe. Not quite sure which I will use. Might use both.

2. The NC30 has about a 3" air intake pipe on the bottom. I am going to connect to this and then quickly put on a 3"x4" reducer to increase the pipe size to 4" for the majority of the run.

3. What does "positive" shutoff mean? Is this basically a valve or flapper in the OAK pipe that can allow you to open it fully or shut it off fully? Any idea if they sell these at Home Depot.

4. Some discussion was brought up regarding the storage room temps. This storage room gets very cold. When it gets real cold outside it can get down to 40-45 in there. There will be about 8' or half the distance of the OAK run through this room. Does this increase or decrease the need for insulation around the pipe?

5. Sounds like the by-pass idea, was eventually not recommended or possibly not needed?

6. I am trying to wrap my arms around the potential reversal issue. I find this possibility deeply concerning. I am not intending to build this system for such a possibility with such extreme temps. Once the fire is established and drawing in the right direction could you possibly see it switching back the other way? Wouldn't the "positive shutoff" come in handy to control this while you are initially starting up?

Any other final tips or words of wisdom would be appreciated.

Thank you!!
 
One other question in addition to the above. What does the kit from Englander for $72 do that I can't do on my own by buying parts at the hardware store?
 
From what I have learned about this particular stove, the outdoor air kit does not supply all combustion air to the stove so even if you had an ideal pipe setup, there would still be room air being burned and the chance of that smell in the basement. Some stoves do actually get 100% of their combustion air from the OAK intake like my old hearthstone and the current BK.

The NC30 has those holes behind the legs that feed the doghouse air, the larger primary intake, and then the sqaure secondary intake. Each a separate hole.
 
How about setting a cheap battery-powered smoke alarm by the air inlet to the stove...that would alert you to a draft reversal?
 
One other question in addition to the above. What does the kit from Englander for $72 do that I can't do on my own by buying parts at the hardware store?

You can see the kit here
http://www.englanderstoves.com/store/30-NC_Parts.html#278
I didn't buy it for exactly the same reasons. 6 feet of tube is not enough for me, so if I have to buy parts to make it work.... I am using 3" pipe off the stove to a reducer/expander to standard 4" dryer vent using a through the wall dryer vent kit that will be installed through my basement sill plate.
 
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