Reasons for an OAK.

Tonyray said:

[Reposting this from someone here who I think gave the best explanation for using an OAK

In an average home with normal air sealing, the effect on the stove's performance itself, in terms of how well it burns, will likely not be noticeable. But that's not the reason for an OAK unless you have a very tightly sealed home. For any situation where an OAK is added, the primary benefit is a reduction in the amount and rate of air exchanged from the inside to the outside of the home.

Your combustion blower is actively pumping air from inside to outside at a continuous, relatively high rate whenever it's on. Without an OAK, the air used for combustion is air you already paid to heat, drawn from inside the home. Because your house cannot sustain a vacuum (you and the other people in the house needing to breathe, for example) that air will be replaced by colder, often drier air pulled in from the outside through all the tiny little cracks in your home. You will then need to heat that air, and the needlessly expensive cycle continues. You would not, for example, drill a two-inch wide hole in one wall of your living room in January, then drill a three-inch wide hole in the opposite wall, and put a powerful fan in that hole, and turn it on all day, every day. Yet that is what is happenening when you don't use an OAK. Your stove is powerful enough to overcome that situation, but it works much harder to do so by burning more pellets than you would otherwise need to burn. You also feel more cold drafts near the floor, caused by the outside air being pulled toward the stove.

With an OAK, that combustion air is simply pulled in through the OAK, used for combustion, and exhausted back outside. None of your heated, nicely humidifed air is needed. You spend less money, and remain more comfortable. That's a pretty good deal for just installing a small metal tube.

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Also agree being the best explanation around, so much that I took my kit that was sitting around and installed it.
Immediately saw a difference, the burn was much better....

When I bought my stove Harman P68 seller told me to not install oak because it's too cold here and here due to the formation of ice in the oak. The temperature is usually colder than -30 degrees here .I think the reasoning makes sense.
I did a test my stove in the basement and my house is 2300sqft 3 floors and have a window next to the stove I opened the window a little and in this way the heat stays in the basement .When the window is closed I have a better flow of heat in the house on 3 floors the cold air near the ground is sucked into the stove going and create air circulation in the house

A friend of mine installed a new stove without an OAK. I brought him to my house and showed him what was happening with a simple experiment. My stove was off and I pulled the air intake tube off the fitting through my wall. Holding his hand there, He felt no air movement. Fired up the stove and did the same thing with his hand. There was a breeze coming in through the hole now. He was shocked at the volume of air coming into the house. The following weekend, we installed an OAK for his stove.

PWash said:

If your exhaust gas is the same temperature with OAK as it is without an OAK, then there is no net heat benefit. Cold combustion air with an OAK would result in less heat flux out of the stove than using warm combustion air drawn from the house. That means no net difference.

If there is a concern about maintaining inside air humidity, then an OAK would have a real benefit since the heat of vaporization of water is quite high. If I am reading this psychometric chart correctly, 70F air with 70%RH has 50% more enthalpy than 70F air with 20%RH (30 vs 20 BTU/lb dry air, respectively).

Both sides of the discussion have some truth.

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I don't think most people will understand what you are saying without some college level Thermodynamics, but it's absolutely right.

I have read elsewhere that there has never been any study done that shows the outside air kit will make more net heat in a average home.

ChandlerR said:

A friend of mine installed a new stove without an OAK. I brought him to my house and showed him what was happening with a simple experiment. My stove was off and I pulled the air intake tube off the fitting through my wall. Holding his hand there, He felt no air movement. Fired up the stove and did the same thing with his hand. There was a breeze coming in through the hole now. He was shocked at the volume of air coming into the house. The following weekend, we installed an OAK for his stove.

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Very good example ChandlerR,,,for my first OAK night I must say,,,I noticed that my exhaust flue was about 30F higher than before and the firebox also....

PWash said:

If your exhaust gas is the same temperature with OAK as it is without an OAK, then there is no net heat benefit.

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This statement would be true if the stove were exhausting into the house but since our stoves aren't made by Dr. Kevorkian, they exhaust outside.

PWash said:

Cold combustion air with an OAK would result in less heat flux out of the stove than using warm combustion air drawn from the house. That means no net difference.

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Yes it does because now the stove must maintain a higher output temperature (more BTU's) to heat the cold air being drawn into the house by the combustion blower.

Your statement also assumes that the heat exchanger is more than 100% efficient. I.E. the air is drawn out of the house, passes through the stove, gives up all it's heat to the heat exchanger and right back into the house before going outside.

If this were the case I.E. greater than 100% heat exchanger efficiency, then the net result is that you still have a fan sucking heated air out of the house that needs to be replaced and heated.

PWash said:

If there is a concern about maintaining inside air humidity, then an OAK would have a real benefit since the heat of vaporization of water is quite high. If I am reading this psychometric chart correctly, 70F air with 70%RH has 50% more enthalpy than 70F air with 20%RH (30 vs 20 BTU/lb dry air, respectively).

Both sides of the discussion have some truth.

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I don't think that's relevant unless you're trying to replaced lost humidity by evaporating water across the pellet stove?

I think the most simple way to think of it is to turn our pellet stove into an electric heater. In order to maintain a given temperature in the house, will this heater have to produce more BTU's over a given period of time if we cut a hole in the wall and put 40 CFM blower in hole that blows air outside?

Safety concerns are also a reason for an OAK. If a home goes into a negative pressure compared to the outdoors it can effect the ability for an appliance to draft. Such as a water heater, furnace or boiler etc. Basically air can come down a chimney and shield or block a water heater etc from drafting. I have seen it many times. Here is an online explaination may not be the best but it will suffice.

http://www.rheem.com/docs/FetchDocument.aspx?ID=d053fecb-6012-47a0-88be-f33f0d073b6b

I have heard it explained that outdoor combustion air doesn't make the appliance more efficient but makes the home more efficient. It is not as important on a pellet stove but parts like ignitors, flame sensors etc fare better with outdoor air than indoor air. Cleaners and chemicals have an adverse effect on them.

Golfeur said:

When I bought my stove Harman P68 seller told me to not install oak because it's too cold here and here due to the formation of ice in the oak. The temperature is usually colder than -30 degrees here .I think the reasoning makes sense.

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The ice forms on the outside of the OAK not on the inside. The condensation problem is easily avoided by wraping the oak with insulation.

I am surprised that nobody has mentioned the issue of competition for air. Every time an exhaust fan (bathroom or kitchen) is turned on or if some other appliance such as a clothes dryer or boiler turns on, the pressure in the house drops. That causes the pellet stove to burn richer increasing soot and creosote production. If you have a very leaky house this won't be noticed. You will just have more air turnover.

pageyjim said:

Safety concerns are also a reason for an OAK. If a home goes into a negative pressure compared to the outdoors it can effect the ability for an appliance to draft. Such as a water heater, furnace or boiler etc. Basically air can come down a chimney and shield or block a water heater etc from drafting. I have seen it many times. Here is an online explaination may not be the best but it will suffice.

.

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Yeah but your pellet stove's net affect on negative pressure in the house pales in comparison to a clothes dryer, range hood or bathroom exhaust fan.

checkthisout said:

Yeah but your pellet stove's net affect on negative pressure in the house pales in comparison to a clothes dryer, range hood or bathroom exhaust fan.

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It can be a cumulative effect and with a big stove on high it uses a lot of air also.

pageyjim said:

It can be a cumulative effect and with a big stove on high it uses a lot of air also.

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Agreed.

checkthisout said:

Yeah but your pellet stove's net affect on negative pressure in the house pales in comparison to a clothes dryer, range hood or bathroom exhaust fan.

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Think about it, how many times have you walked into someones basement and the door slams shut? It usually means there is a negative pressure down there usually caused by furnace and water heater.

Just an example. A 40K btu pellet stove should have 2000 cubic feet of space for combustion air. Maybe it is in a living room with an open floor plan next to a kitchen. Then add an oven, stove top and a range hood and you can more than double that figure. I only see advantages to installing an OAK.

chickenman said:

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With regards to an OAK; there is no situation that does not benefit from an OAK. If the system is designed right in the first place it will also save you a fortune in reduced fuel consumption over time.

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Agreed.

ewlsey said:

I don't think most people will understand what you are saying without some college level Thermodynamics, but it's absolutely right.

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Well, I thought maybe someone would understand it, oh well.

OK, that's it for me on this round. Believe whatever you think you read, thought you learned, heard your friend tell you, whatever. Several have tried to help, and we reached one or two. The rest can just wear socks for the draft and pay more. I'm done with this one.

Wilbur Feral said:

As the guy who wrote the original post quoted, I did so largely to combat this type of "logic", which I see over and over on the forum. Neither I nor anyone else can make it much simpler (many have tried), and these things are not matters of opinion. They are just a matter of physics and building science.

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That's the problem, your making it too simple. Like I said its much more complex then the reasons you state. You have applied physics and science to only half of the equation.

Wilbur Feral said:

As far as whether something is "drastically" changed, that's also not a matter of opinion except to the extent you get to define the word drastically. You can calculate the accelerated air exchanges rather easily if you know the CFM rating of the combustion blower. No one said the stove was "working twice as hard", nor that NO cold air is being introduced into the home.

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Yes they did say the stove was working twice as hard, and others have said their pellet consumption has gone down by up to half and you will save a fortune, just for a few examples(not necessarily on this thread). Its that kind of "drastic" differences that I deny. I am certain than OAK will typically make small improvements, don't get me wrong, I'm just playing a little devils advocate here. And their are plenty of articles out there on the net that claim no scientific advantage has been proven by OAK, I could dig up some of those but the last few I read I don't think were all that scientific really. Heck the amount of air most woodstoves consume is a lot less than what many often say is minimum air turn over for homes anyhow.

For one thing, all that hot air you already paid to heat and now blowing it out the stack isn't just all 'wasted' like everyone repeats over and over. That might describe your bathroom vent or something like that though, but in the case of a stove your feeding it into a device designed to heat. It should be slightly warming your combustion creating an ever so slightly warmer stove, and then it passes through a device called a heat exchanger that delivers most of said heat back to the home. Similarly, using an OAK, all that cold air that would be going into your home is still going into your stove, slightly lowering its temperature, and again passing through a heat exchanger to transfer most of that colder air into your home. Yeah heat exchangers are not 100% efficient or else this would be all a moot point (20-25% won't transfer back into the home). Most of these differences are going to be so small to even measure accurately though, without lab conditions and equipment.

My home is extremely drafty and poorly insulated. For me the difference of the oak is inconsequential, but is required for me so I have since implemented an OAK. I've had 3 wood stoves burning before (at the same time I mean) plus clothes drier, bathroom vents, whatever and no issues with air supply.

Some of the articles out there debunking the OAKs also bring up points about OAKs allowing backdrafting more easily in cases where the OAK inlet is in lower pressure like if put on leeward side of house or simply from wind changing direction. I'd say thats a bit of a stretch but I suppose a possibility.

checkthisout said:

I think the most simple way to think of it is to turn our pellet stove into an electric heater. In order to maintain a given temperature in the house, will this heater have to produce more BTU's over a given period of time if we cut a hole in the wall and put 40 CFM blower in hole that blows air outside?

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This kind of reasoning is nothing like whats going on here and is ridiculous.

ewlsey said:

I don't think most people will understand what you are saying without some college level Thermodynamics, but it's absolutely right.

I have read elsewhere that there has never been any study done that shows the outside air kit will make more net heat in a average home.

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I'll point out I have no college level thermodynamics either (my degree is electronics engineering). That is one good way to put it though, 'net heat'. Witht he oak it can make it seem a little warmer, especially if the drafts were being pulled from across your couch or chair where you could feel them more. But even in that situation the net temperature might still be the same, just you have colder air by the draft and warmer air by the stove without OAK. With OAK your stove might not be quite as hot but around where your drafts were might not be quite as cold either, so net is roughly equal.

I don't need science or physics . I can read English, and when the stove manual ,states that there stoves have to use an OAK CONNECTION. So simple do it or do not buy these stoves .We try to help other stove owners ,and your know it all attitude ,is not helping.

I don't know that much about pellet stoves vs wood stoves, but I've not seen a wood stove that requires an outside air connection except in a mobile home. Maybe pellet stoves are different.

chickenman said:

We have done the independent scientific testing when we get our stoves certified. THe improvement in efficiency averages out at 10%. THat is pretty good considering that pellet stoves dont have a huge output to begin with. Plus the sensory effect of reducing drafts is considerable.

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You started out good. 10% is a hard number that we can understand. I have no idea how this is calculated. I have no idea how this could possibly be consistent for every type of installation given that every home is unique.

Then you get off track and use words like "considerable". That's not scientific.

I can see newbies dropping bags of pellets and running into the night screaming "this is not why I signed up!" Do you want draft or no draft, that is the question.