Outside Air vs Room Air

[Karl Hungus]

A blower door test won't tell you what you need to know. You can have leaks, but until you have leaks AND an appliance putting a suction in your home you won't necessarily have flow through the leaks.

A blower door test tells you how big your leaks are.
If you've got lots of leaks then why bother with the OAK?
Alot of new homes are not as air tight as they should be. And a Truely air tight house needs a HRV or ERV because of air quality issues.
They exchange the "dirty" inside air with fresh outside air after taking the heat and or humidity out of the inside air and put it in the incoming outside air so you are not bringing in cold dry outside air and exhausting warm inside air. A heat exchanger.

 

[Karl Hungus]

My home is very air-tight (final blower door test 0.8 ACH50). I have an OAK for the small stove in the lower level. If I go to light the stove when the clothes dryer or range hood is running, the stove backdrafts, forcing me to scramble to open a window, then run upstairs to kill the exhausting device. I get smoke out past the door seal and around some small cracks or openings around the air control mechanisms, despite the OAK direct connection. Once I get the stove fired up well, with hot flue gas running up the chimney, that draft through the OAK duct and up turns the stove into a minor exhausting device, able to compete with the dryer or hood, so those can go back on.

The house also has an HRV (heat recovery ventilator, basically an air-to-air heat exchanger) to provide fresh air for the house, something really required for a very tight house. Operating an exhausting device, like dryer or hood, makes the HRV run out of balance, as those devices compete with the fan exhausting interior air out through the HRV. I perhaps could run the stove without an OAK, as long as there was nothing to compete with it for air. I suspect I'd have trouble with backdrafting as soon as I ran some other device.

My advice for a new house built to be very tight: add the OAK, and pick a stove that lets you connect the duct directly to the stove. I rejected one stove after the mfg confirmed that there was no direct connection, and that the OAK duct would just dump raw outside air "into the vicinity of the stove," as in all night long after I let the stove burn out. Uh-uh, not in my tight, superinsulated house.

Dick, that's a very good air exchange rate. Our house isn;t finished and tested yet but I'd be happy to be where your at.

We are using an ERV, we don't have a hood over stove but do have a elec. dryer.
We are going to be using a masonry heater. It runs only for 2 or so hours instead of all the time like a woodstove does so I'm going to try it and see how it goes. I can add a OAK if I need to.

Dick, how well insulated is your house just for curiosity?
Our house has 8" sip panel walls Using neopor for the foam with 1" of foam over the outside of the SIP panels for a R35 for the walls, R60 attic, R25 basement walls and floor, fiberglas triple pane windows with a U of .19.
Our est heat load was 20Kbtus/hr for a 2800 sq ft house.

 

[Highbeam]

A blower door test tells you how big your leaks are.
If you've got lots of leaks then why bother with the OAK?
Alot of new homes are not as air tight as they should be. And a Truely air tight house needs a HRV or ERV because of air quality issues.
They exchange the "dirty" inside air with fresh outside air after taking the heat and or humidity out of the inside air and put it in the incoming outside air so you are not bringing in cold dry outside air and exhausting warm inside air. A heat exchanger.

You answered your own question. Regardless of how leaky your house is, sucking out conditioned interior air through the stove will cause that air to be replaced by dry, cold, outdoor air at a higher rate. This is the problem which will be remedied by the OAK connection to your stove.

You don't seem to understand that leaks in the envelope don't mean air exchanges. They are just holes until there is a pressure differential that will cause a flow of air in or out. Perhaps you are just saying that if your house is swiss cheese with leaks then the house is a lost cause and will be leaking conditioned air by natural convection so much that the difference made by the OAK is minimal. That's not the case. If it were, then you wouldn't be able to heat the place. Every cubic inch of air not sucked out of the room by the stove is a cubic inch less of infiltrated cold air that you must heat and condition.

 

[DBoon]

Very timely thread for me as I am finalizing plans for a old house renovation. The renovation will result in an airtight home - in the vicinity of what Dick Russell has (0.8 ACH50). The stove will be a Woodstock Absolute Steel (chosen for its efficiency, size, and most importantly, sealed combustion air intake) and will use an OAK. An ERV will be installed in the house. The bathroom, dryer vent and range hood exhaust will use backdraft prevention dampers to ensure no leaks in these supplemental exhaust locations.

OK, those are the details (you'll all ask for them, so might as well provide them up front )

Question 1: it was mentioned earlier that the OAK air intake should be below the firebox level. However, this would lead me to have an OAK air intake only 1 foot above ground level - a problem in our snowy climate. Can this be reasonably raised a couple more feet given that the chimney height is 23' and that there will be a backdraft damper on the OAK intake?

Question 2: Given that the OAK will be on the north side of the house (the predominant wind direction in the wintertime), is there some type of baffle I should install over the air intake to reduce the impact of air turbulence on the intake? The OAK will be next to the chimney not shielded from the wind, so it could be expected that the wind will create extra turbulence as it hits the chimney/house corner where the OAK intake will be.

Thanks for any thoughts and comments.

 

[Highbeam]

Very timely thread for me as I am finalizing plans for a old house renovation. The renovation will result in an airtight home - in the vicinity of what Dick Russell has (0.8 ACH50). The stove will be a Woodstock Absolute Steel (chosen for its efficiency, size, and most importantly, sealed combustion air intake) and will use an OAK. An ERV will be installed in the house. The bathroom, dryer vent and range hood exhaust will use backdraft prevention dampers to ensure no leaks in these supplemental exhaust locations.

OK, those are the details (you'll all ask for them, so might as well provide them up front )

Question 1: it was mentioned earlier that the OAK air intake should be below the firebox level. However, this would lead me to have an OAK air intake only 1 foot above ground level - a problem in our snowy climate. Can this be reasonably raised a couple more feet given that the chimney height is 23' and that there will be a backdraft damper on the OAK intake?

Question 2: Given that the OAK will be on the north side of the house (the predominant wind direction in the wintertime), is there some type of baffle I should install over the air intake to reduce the impact of air turbulence on the intake? The OAK will be next to the chimney not shielded from the wind, so it could be expected that the wind will create extra turbulence as it hits the chimney/house corner where the OAK intake will be.

Thanks for any thoughts and comments.

BK was pretty clear that they expect the intake to be below the stove. Call WS and see if they have a concern with your concept. I have never seen anyone put a backdraft damper in the OAK line. It would have to be flipped backwards of normal to prevent air from being expelled. In any case, it will be a restriction and could cause problems.

Intake on the windward side, no problem. Consider your stove supercharged. People worry too much about the chances of wind effecting operation of an outside intake.

 

[Karl Hungus]

You answered your own question. Regardless of how leaky your house is, sucking out conditioned interior air through the stove will cause that air to be replaced by dry, cold, outdoor air at a higher rate. This is the problem which will be remedied by the OAK connection to your stove.

You don't seem to understand that leaks in the envelope don't mean air exchanges. They are just holes until there is a pressure differential that will cause a flow of air in or out. Perhaps you are just saying that if your house is swiss cheese with leaks then the house is a lost cause and will be leaking conditioned air by natural convection so much that the difference made by the OAK is minimal. That's not the case. If it were, then you wouldn't be able to heat the place. Every cubic inch of air not sucked out of the room by the stove is a cubic inch less of infiltrated cold air that you must heat and condition.

If the leak is insignificant compared to the output why worry about it?

Dry air at 70F is 0.075 lbs/cf, so one pound of wood burned over an hour will take in 81.3 cf of air, or 1.36 cfm.
Net Heating Value of 1 lb of dry wood = 8030 BTU.
So if your house needs 80,000 btu's/hr to keep it warm you'd need to burn 10lbs/hr.
This equals 13.6 cfm of air for combustion.
Rough numbers to make a point.

If the leak is pulling in 13.6 cfm of air in and your woodstove is pumping out enough heat to keep 16,000 cubic feet over 70 degrees why worry?
13.6 cfm is .00085 of 16,000 cubic feet, insignificant.

The all homes have enough air leakage potential to feed the stove.

Have a buddy that heats exclusively with wood. He occasionally opens a window to cool things down if its too warm inside. Anyone who does that does not need a OAK.

If your using a wood stove and your house is over 70 all the time you don't need a OAK.
Its not like most woodstoves have a problem heating a typical home. If anything they usually provide more heat than req'd.

In fact an argument could be made a OAK in most homes with a woodstove hurts the indoor air quality. Better off to have a leak bringing in fresh air.

You don't want a absolutely air tight house. It would kill indoor air quality. You need to have air turnover. Whether that turn over is controlled via ERV/HRV or uncontrolled via air leaks.

 

[madison]

okiesmoker,
Using the dryer vent as an OAK... sharing an outside appliance air source is dicey, especially if it isn't in close proximity.

The intent of the dryer vent was not to share, sorry for the confusion.

I was trying to insinuate that a dryer vent is a source of replacement air and the OAK would be redundant. IF OAK's were designed to be sealed to the air intake of the stove then an OAK (IMHO) maybe of some value. I do not know of any that are designed as such. I've read quite a few threads on OAK's mentioning ..." cold air blowing out from under the stove"... and would never consider one personally. Again, this is only my opinion. Any penetration from the home will be a source of outside air, leaking, windows, doors, attic penetrations, dryer vents.

 

[Highbeam]

. IF OAK's were designed to be sealed to the air intake of the stove then an OAK (IMHO) maybe of some value. I do not know of any that are designed as such.

Ah, you just have to look for them. My BK takes 100% of its combustion air through the OAK nipple. My last stove was a hearthstone heritage that also took 100% of its air through the OAK nipple. My Englander NC30 fails at this and only one of the 4 combustion air inlets can be fed sealed outside air.

 

[DickRussell]

....Dick, how well insulated is your house just for curiosity?
Our house has 8" sip panel walls Using neopor for the foam with 1" of foam over the outside of the SIP panels for a R35 for the walls, R60 attic, R25 basement walls and floor, fiberglas triple pane windows with a U of .19.
Our est heat load was 20Kbtus/hr for a 2800 sq ft house.

Mine has similar numbers. The exterior walls are double framed (2x6 outer, 2x4 inner), with a 12" cavity full of dense packed cellulose, so wall R is perhaps 43 center of cavity, or upper 30s whole-wall. Lower level is set into a hill, so full foundation uphill, fully framed downhill, stepped foundation on the sides. Concrete walls are insulated to R20. Under the slab is R20. Attic floor is loose blow, R60. Windows a a mix of fixed glass and casements, U around 0.17 according to the mfg. Door glass is only double pane, though. Total conditioned space is about 4,000 sqft. Heat loss model says design maximum is 22 K BTU/hr, although best (but somewhat crude) operating data suggest that's more like 19K. With heat loss that low, the walls are never really in steady state, and that damps the delta T considerably. I would think that your house would see a lower heat loss, unless you are all on one floor.

 

[Van]

Wow, this thread has had quite a response.
Here's my two bits.

If you have bathroom ceiling vent fans you may have adequate air supply (positive pressure) for your stove as long nobody uses them LOL.

Of course outside air is much colder than inside air, and this has little affect on performance outside of starting a fire in a cold stove.
The three key factors for achieving a good fire are: 1. oxygen, 2. fuel, 3 heat.
Increase oxygen and heat and your wood fuel will burst into flames!

Cold air is denser and therefore holds (a bit) more oxygen than warm air, but this is actually negligible in wood stoves as verses motor engines.
Therefore the important factor for your wood stove performance is heat.
This is seen in newer stoves, which channel combustion air through the firebox so as to superheat it increasing combustibility, especially the secondary burn. Once a stove achieves operating temperatures, combustion air is sufficiently hot whether sourced from outside or inside.

On the other hand, a cold firebox coupled with cold combustion air spells disaster for starting a fire.
Is this not evident when starting a fire in a cold stove, which often requires the stove door to stay open/cracked until the fire and firebox temperature rises?

Okie, given your situation, I would not worry about about providing outside air.
Your challenge is to provide sufficient positive air pressure in your home (via a cracked window as worse case scenario).

On a personal note.
I installed my stove with both outside and inside air supply,and I still need to crack the stove door when starting wit a cold firebox.

 

[Coyoterun]

You answered your own question. Regardless of how leaky your house is, sucking out conditioned interior air through the stove will cause that air to be replaced by dry, cold, outdoor air at a higher rate. This is the problem which will be remedied by the OAK connection to your stove.

You don't seem to understand that leaks in the envelope don't mean air exchanges. They are just holes until there is a pressure differential that will cause a flow of air in or out. Perhaps you are just saying that if your house is swiss cheese with leaks then the house is a lost cause and will be leaking conditioned air by natural convection so much that the difference made by the OAK is minimal. That's not the case. If it were, then you wouldn't be able to heat the place. Every cubic inch of air not sucked out of the room by the stove is a cubic inch less of infiltrated cold air that you must heat and condition.

I was under the impression that stack effect will result in pressure differentials from the temperature differentials. So there is going to be air flow through the holes anytime there is a significant temperature differential, regardless of whether that heat comes from a wood stove or from a high efficiency natural gas furnace with its own sealed OAK and exhaust.

Does a medium size wood stove using 20-40 cfm really make much difference compared to stack effect? I suppose two story house vs one story and basement stove vs ground level also make a difference. Single story ranch on a slab doesn't have much stack effect compared to two story farmhouse over a basement.

 

[Karl Hungus]

On a personal note.
I installed my stove with both outside and inside air supply,and I still need to crack the stove door when starting wit a cold firebox.

I have a leaky '70's vintage trilevel. In certain weather conditions I have to open a door to get a draft going up the chimney.
Maybe 1 time out of 20.
This is with a cold firebox.

I think the need for a OAK is vastly overblown. The amount of air that a woodstove needs to run is so insignificant compared to existing leaks that I don't see any benefit.

 

[OhioBurner©]

I think the need is greater for a pellet stove which pumps more air. But my house falls into the swiss cheese category of doesn't really make a noticeable difference. I did put an OAK on my pellet stove and have run it both ways. Even still with the OAK I can feel the wind blowing inside on windy days and the air inside I've seen down to 15% RH. I think I've said it many times but I can't wait to get out of this house. Someday.

 

[okiesmoker]

Definitely lots of good responses and information. Thank all. I've read a few more threads and several articles about it. At this point, I'm not convinced I absolutely need it, but I think time will tell. Right now, my stove seems to breathe well, it heats well and I haven't had any issues with the stove "leaking" into the room, even with a couple of our "vents" to the outside in use.

 

[madison]

If the dryer vent has a flapper, which many do, it will be sucked closed by any in-home air requirement. Sure the flapper can be removed, and a screen attached, but that will likely get clogged with dryer lint.

My PE Summit Classic has a dedicated OAK sealed to the air intake. Came from the factory with a 4" knock-out right in the back of the pedestal base.

Are you sure that the OAK is really sealed? - if it is , that is new on PE's.

Highbeam - thank you for updating my incomplete knowledge on sealed OAK intakes.

 

[Adahn]

What is the typical way to duct an OAK for a stove placed in front of a fireplace? Masonry holesaw through FP floor?

 

[Burn Time]

if you have a gas or electric cloths dryer you already have an OAK

Real good point there about the clothes dryer vent acting as an OAK when not drying clothes. Bath and kitchen exhausts probably act the same despite the little backflow dampers. I installed an OAK to my large fireplace in my superinsulated house 26 years ago and never have had a problem with backdrafting. Two years ago when preparing to install a Hearthstone soapstone stove, I asked the dealer/installer about an OAK. He firmly said it was not necessary. He was correct. But I have a 35 foot flue for the stove. The draft was so fierce I had to install a stack damper to reduce wood usage. So where was the stove getting all the air for its phenomenal draft? I tried to defeat the stove draft by running four bathroom exhaust fans and the kitchen exhaust fan simultaneously. That did not faze the stove. My house has six inch walls with blown in blanket insulation, tight windows and no drafts. But here is another important point: I have an air and heat exchanger system that draws air from the moist rooms (baths and kitchen) and draws air from outside. The warm inside air passes through a heat exchanger where it warms the incoming outside air before the outside air discharges to the furnace air return, then the inside air is exhausted outside. It keeps the air fresh all winter and regulates the indoor humidity. No doubt the air and heat exchanger functions as an OAK. So I would say you need to examine not only the nominal tightness of your house but also the stealth sources of draft air.

 

[Attaboy]

This is my experience anyways, our home here is high efficiency, insulation values are R22 walls and R50 ceiling, windows are very high efficiency, the windows/house framing contours are well sealed to prevent unwanted air infiltration. I am running a 1999 free standing PE Spectrum Classic in the downstairs family room which basically heats the entire house with the occasional help from electric baseboards in very cold weather below -25° ( perhaps 5 -12 days per winter) the PE's heat channels itself up the central staircase ( like a chimney ) that leads to the open concept living room, dining room, kitchen and main house entrance areas, I have no OAK and to be honest I can count on the fingers of one hand the times I have had a serious problem in the last 30 years because of negative in house pressure ( the crack of a window for a few minutes fixes the negative pressure problem really quickly ), I do however have two bathrooms each equipped with a 4 inch fan air exhaust which also lets some air in when the fans are not running, also have a 4 inch exhaust on the clothes dryer which as well lets some air in and of course the kitchen exhaust fan which has something like 4'' x 12'' opening I believe and must not forget the attic trap door opening which must leak some air I am sure, all these do exhaust and all of them let some air in and very rarely can these all be operating at the same time, moreover just about all homes let some natural air in via cracks, crannies etc. even if it is limited and controlled during the building process, bottom line is I am getting plenty of combustion air from the combination of all of these orifices combined together.


In very cold areas as where we live in temps can go down to the -20° to -33° range, you need a really good OAK installation to ensure no condensation forms or you will eventually get water puddling. Many professional installers here are very leery of installing OAK's in many homes unless absolutely necessary to avoid numerous related problems.

Basically if you have no combustion air supply problem to begin with, why then have and add a uncontrollable OAK if absolutely not required. Do not do it strictly based on John or Joe's say or a neigbours say or whoever else's opinion and just on plain old hear say, your needs may not be there needs, all are different.

 

[electrathon]

I see a lot of the posts opposed to an OAK use the argument that "my house is leaky, I just rely on those leaks to supply the stove". The puzzling part of this argument is that we are trying to heat our houses. A common complaint heard is that people have a hard time getting heat to the far side of the house. So when you draw in cold air through leaks in the far side of the house, draw the cold leaked air across your living space to the stove, and up the chimney. A simple direct connected OAK will make the far side of your house warmer. Combustion issues are a very positive benefit, but as many people have commented they have leaky houses and it is not necessary for combustion. It does help heat the far side of your house though.

 

[Bill in the U.P.]

It seemed like such a good idea until it was tested.
http://www.woodheat.org/outdoor-air-supplies.html

 

[Karl Hungus]

I see a lot of the posts opposed to an OAK use the argument that "my house is leaky, I just rely on those leaks to supply the stove". The puzzling part of this argument is that we are trying to heat our houses. A common complaint heard is that people have a hard time getting heat to the far side of the house. So when you draw in cold air through leaks in the far side of the house, draw the cold leaked air across your living space to the stove, and up the chimney. A simple direct connected OAK will make the far side of your house warmer. Combustion issues are a very positive benefit, but as many people have commented they have leaky houses and it is not necessary for combustion. It does help heat the far side of your house though.

1st, all houses leak air. The amount of air the wood stove uses is insignificant compared to how much the house leaks.
2nd, If the far side of your house is not warm its not because of an insignificant amount of air drawn in from a wood stove.
Lack of insulation, massive air leaks and or the warm air from the stove is not being moved around are the causes.
3rd you assume that the air leaks are in the far side of the house.

Why are the leaks not closest to the wood stove instead of farthest away from it?

The leaks will change in amount of leakage due to pressure. If the wind is blowing from the east it will leak in different spots than if the wind is from the north. Read the article from the post above mine. It explains it rather well.

 

[Attaboy]

I simple direct connected OAK will make the far side of your house warmer. Combustion issues are a very The puzzling part of this argument is that we are trying to heat our houses. A common complaint heard is that people have a hard time getting heat to the far side of the house. So when you draw in cold air through leaks in the far side of the house, draw the cold leaked air across your living positive benefit, but as many people have commented they have leaky houses and it is not necessary for combustion. It does help heat t
he far side of your house though.

That is not necessarily so or true and I would love to read a verifiable study that proves this, haven't found one yet.

If you have cold rooms or areas it is simply because your stove or insert, central heating system and/or air circulation are not sufficiently powerful to circulate heat to those areas, in fact you have many home owners who do not have a wood stove, insert or fireplace that are equipped with central heating systems which push air through ducts through out the house who complain of cold rooms or inversely overly warm rooms or areas in the summer ( if equipped with central air ) simply because the duct work and air returns is poorly designed or insufficient for proper circulation.

In certain cold weather countries they have found that the solution to this is not to have one overly large wood stove to heat the house from a single point but rather to have 2 small wood stoves strategically placed to completely heat the house efficiently, this resolves the cold areas problem.

 

[electrathon]

That is not necessarily so or true and I would love to read a verifiable study that proves this, haven't found one yet.

I don't think it is rocket science. You pull out about 20 CFM of heated air out the chimney. If you have an OAK you are not pulling in cold air, you are using outside air. Without an OAK you are pulling in about 20 CFM of 20 degree air into your living area through leaks in your windows, doors, vents and etc. The arguments against an OAK are similar to the arguments against weatherization (caulking, weather striping, etc.).

Every point I am making refers to a direct connect stove. In this day and age it is hard to understand why a manufacture would not have direct connect fittings. My furnace requires outside air. Same reasons as mentioned why a stove needs it. Without outside air it will rely on leaks in the house drawing in cold air. Smart people figured out that a fire works best with a source of air.

 

[Coyoterun]

I don't think it is rocket science. You pull out about 20 CFM of heated air out the chimney. If you have an OAK you are not pulling in cold air, you are using outside air. Without an OAK you are pulling in about 20 CFM of 20 degree air into your living area through leaks in your windows, doors, vents and etc. The arguments against an OAK are similar to the arguments against weatherization (caulking, weather striping, etc.).

My main takeaway from the studies linked above is that an average sized house needs about 70-100 CFM of outside air exchange just for air quality purposes. So the 15-20 CFM exhausted through the chimney is only a small portion of the minimum amount that needs to be leaving the house anyway.
The impression I got is that sealing a house to less than 100 CFM of exchange is not beneficial. My house, being a 100 year old farmhouse, will be leaking far more than that and so any weatherization I can do is beneficial and will not impact the stove function.

 

[electrathon]

My main takeaway from the studies linked above is that an average sized house needs about 70-100 CFM of outside air exchange just for air quality purposes. So the 15-20 CFM exhausted through the chimney is only a small portion of the minimum amount that needs to be leaving the house anyway.
The impression I got is that sealing a house to less than 100 CFM of exchange is not beneficial. My house, being a 100 year old farmhouse, will be leaking far more than that and so any weatherization I can do is beneficial and will not impact the stove function.

Your house is a perfect example of the issue. Your house is old and leaky. For sake of discussion lets say you have 200 CFM of leakage, twice the desirable amount. Also remember that this number moves all the time, but lets keep it simple at 200. You build a fire, now you have 220. You are drawing into the heated envelope of the house even more coldness. Some from close to the stove, some far away, but all air drawn in from the suction created by the stove is going to be cold, outside air that you now need to heat to inside the house comfort. If a window or door could be sealed with a 20 CFM improvement on air migration anyone who understands home heating would be all over the idea. But for some reason keeping cold air out of the living area is po-pooed my wood stove people.

Reading through my above paragraph it popped into my head. This may be where the misconception comes from: Those of you who think that cold air is not drawn into your house by the chimney suction. Do you believe that the added suction of the chimney does not add to the added air infiltration into your house?