To OAK or not to Oak

[Harvey Schneider]

I guess not, I have a digital instrument that measures outside and a few other manual ones that measure inside.
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Here is a very brief explanation of relative humidity. It basically is a measure of how much of the air's capacity to hold moisture has been filled. That capacity changes with temperature.
An example below.

The air has the same amount of moisture in it before and after it is heated. If the air outside at 10 degrees, for example, has 70% relative humidity, when heated, it's capacity to hold moisture increases. Because of that, it's relative humidity drops (to, perhaps 30%). It still contains the same amount of moisture, it's just that it could hold more. It will take that more from wherever it can find it; from the furniture, from house plants or from you.

 

[mikkeeh]

Wow::: I posted early in this thread, with what I thought was a logical argument for OAKS. I see there are folks that don't accept logic. Some folks are looking for scientific studies. Its been scientifically proven that smoking is hazardous to your health, and cost the taxpayers billions in healthcare....but people still do it???? Drinking and driving kill people and ruins lives....people still do it. There are folks that will not change their minds....and will not use logic...... And "common sense" isn't very common these days. But everyone has a choice. I choose to use and OAK. You do whatever floats your boat.

 

[St_Earl]

Alright, let's solve this once and for all...

(broken image removed)

 

[Madcodger]

I ran the numbers and for my situation it looks like $150 to $200 per heating season. That is about 25% savings for an OAK instalation.. Does that assume no air infiltration in the heated space? It seems the numbers are to good to be true. Any engineering types out there to give an opinion on the calculation? If the numbers are good, it is definitely worth the effort.

The existing air infiltration is essentially not part of the equation. All an OAK does is take the active pumping of internal air to the outside (through your stove) out of the equation, leaving you with the same air exchange (in and out) you had without your stove running. In other words, it closes that loop and separates your stove's combustion cycle from the existing air exchange of your home without the stove. That's its whole purpose for existing - to leave you with the same rate of air exchange rather than an accelerated rate.

As for the numbers being "too good to be true", I cannot help you. If you want to spend the time checking the calculator's accuracy, a number of physics texts can likely supply the various formulas. I'm going to trust the folks that built that calculator on this one, as their math appears solid from my quick review. Let other geeks, engineers, and data obsessed chime in... I have done all I can do.

 

[DamienBricka]

The downside of OAK is pulling moisture from the outside through your stove

The winter outside air is dry so you are not pulling humidity in the stove or the house

 

[Harvey Schneider]

The winter outside air is dry so you are not pulling humidity in the stove or the house

Not really accurate. Winter air can be saturated with moisture. It is just that saturated cold air holds a lot less moisture than warm air. The air only becomes dry when you heat it and its capacity to hold moisture increases. Defiant was incorrect, but so are you.
Don't feel that I am picking on you. Few people understand the concept of relative humidity and I just don't want misconceptions to become the accepted belief. It's the Engineer in me.
Read my post above.

 

[DamienBricka]

Not really accurate. Winter air can be saturated with moisture. It is just that saturated cold air holds a lot less moisture than warm air. The air only becomes dry when you heat it and its capacity to hold moisture increases. Defiant was incorrect, but so are you.
Don't feel that I am picking on you. Few people understand the concept of relative humidity and I just don't want misconceptions to become the accepted belief. It's the Engineer in me.
Read my post above.

I agree with you I guess I did not explain myself correctly.

 

[bondo]

Ok I am new to pellet stoves so I have a right to ask stupid questions. When you say a combustion fan pulls 80 cfm would that be with the damper wide open? For example my damper is only open about 1/2" so am I pulling in 80 cfm of air or 20 for example?

 

[Mt Bob]

Blowers are generally measured "free air",in their housing,no inlet or outlet restrictions.A stove has channels,bends,turns,damper adjustment,so realisticly the blower rating is not what is going through the stove.

 

[Harvey Schneider]

Ok I am new to pellet stoves so I have a right to ask stupid questions. When you say a combustion fan pulls 80 cfm would that be with the damper wide open? For example my damper is only open about 1/2" so am I pulling in 80 cfm of air or 20 for example?

With the damper partially closed the blower will move less air than when it is fully open. The relationship is not linear and half closed does not mean half as much air.
There is no direct way to relate the position of the damper to the cfm moved by the blower. The usual way to find out the amount of air moved is to measure the velocity of the air in a duct of known cross section. If the linear feet per second is know and the cross section is known the volume is the product of the two.

 

[tfdchief]

Sorry if I missed something because I did not read the entire thread. I installed my first wood stove in 1982. At the same time, I installed an 8 in make up air duct from the outside to the front of the wood stove insert, not directly connected, just a register grate in the floor in front of the insert. Then in 2011 I installed a new stove in the kitchen. Again, I did not direct connect an outside air but tapped into the 8 in outside air I already had and put a 4 in air opening below the new wood stove in the kitchen. With that set up, I have never had any problem with negative air. Even with exhaust blowers in the house going like bathrooms and kitchen blowers. Wood stoves use air from the house and need make up air. Lots of ways to do it, This just worked for me.

 

[bondo]

With the damper partially closed the blower will move less air than when it is fully open. The relationship is not linear and half closed does not mean half as much air.
There is no direct way to relate the position of the damper to the cfm moved by the blower. The usual way to find out the amount of air moved is to measure the velocity of the air in a duct of known cross section. If the linear feet per second is know and the cross section is known the volume is the product of the two.

Basically what I am saying is this. Earlier in this thread someone said the combustion fan moves 80cfm of air. What I am saying is if it where taking air from only a 1/2" gap instead of the whole 2" then isn't it taking in less air? So with that the equation made earlier about the volume of air sucked out of the house per hour would be different. Am I correct or way off? I am thinking of it like a bath exhaust fan for example. If it is 10" square but I block off 8" of it I isn't moving as much air out of the house. Just like the combustion fan wouldn't be taking as much air out of the house.

 

[Pellet-King]

OAK This!

 

[Harvey Schneider]

Basically what I am saying is this. Earlier in this thread someone said the combustion fan moves 80cfm of air. What I am saying is if it where taking air from only a 1/2" gap instead of the whole 2" then isn't it taking in less air? So with that the equation made earlier about the volume of air sucked out of the house per hour would be different. Am I correct or way off? I am thinking of it like a bath exhaust fan for example. If it is 10" square but I block off 8" of it I isn't moving as much air out of the house. Just like the combustion fan wouldn't be taking as much air out of the house.

You are correct, the blower will move less air when the damper restricts it. What I was trying to point out is that it is not a direct correlation ie. 1/4 the opening does not mean 1/4 the air moved. How much less air is moved is hard to say.
Even without a damper, the path through the stove is reducing the volume of air the blower will move compared to the label plate number.

 

[bondo]

You are correct, the blower will move less air when the damper restricts it. What I was trying to point out is that it is not a direct correlation ie. 1/4 the opening does not mean 1/4 the air moved. How much less air is moved is hard to say.
Even without a damper, the path through the stove is reducing the volume of air the blower will move compared to the label plate number.

Thank you for helping me on that. I am just curious if it is worth an oak or not. If I have a 2" hole and only have the damper out a 1/2" that in the grand scheme of things isn't really that much volume of air moved. Even though I have decent insulation and new windows there is still some leakage around windows, doors, walls, ceilings, etc.... I mean was house was built in 1900. If I put a hole in my wall that will add additional leakage around the pipe, not a lot but some. Then if the stove is off it will add even more. Add to that I have to put my oak to the north or west. I would cap it but wonder if wind could still affect me. I am not arguing for or against an oak I am just thinking out loud trying to decide. Everyone seems to be either you must do it or it is dumb. I am just really trying to think it out.

 

[Harvey Schneider]

Thank you for helping me on that. I am just curious if it is worth an oak or not. If I have a 2" hole and only have the damper out a 1/2" that in the grand scheme of things isn't really that much volume of air moved. Even though I have decent insulation and new windows there is still some leakage around windows, doors, walls, ceilings, etc.... I mean was house was built in 1900. If I put a hole in my wall that will add additional leakage around the pipe, not a lot but some. Then if the stove is off it will add even more. Add to that I have to put my oak to the north or west. I would cap it but wonder if wind could still affect me. I am not arguing for or against an oak I am just thinking out loud trying to decide. Everyone seems to be either you must do it or it is dumb. I am just really trying to think it out.

Another valid reason for an OAK is to prevent a backflow of smoke when there is a power failure.
If you have a rise in your vent, this might not be an issue. Power failure and/or a strong wind against the side of the house that has the vent can result in air traveling backwards through the stove and combustion products being released in the house.
Somebody else on the forum commented that they had to shut down their stove due to strong winds driving air back through their stove. That wasn't even concurrent with a power failure. Wind pressure was enough to overcome the combustion blower.
Both my stoves have a significant rise in the vent and I don't believe that backflow would be an issue. For the price of an OAK I got peace of mind and eliminated the forced heat loss. Seems like a simple decision to me. It's just the right thing to do.
You can probably tell from this that I am a very conservative Engineer. If something can go wrong, it should be prevented.

 

[bondo]

Another valid reason for an OAK is to prevent a backflow of smoke when there is a power failure.
If you have a rise in your vent, this might not be an issue. Power failure and/or a strong wind against the side of the house that has the vent can result in air traveling backwards through the stove and combustion products being released in the house.
Somebody else on the forum commented that they had to shut down their stove due to strong winds driving air back through their stove. That wasn't even concurrent with a power failure. Wind pressure was enough to overcome the combustion blower.
Both my stoves have a significant rise in the vent and I don't believe that backflow would be an issue. For the price of an OAK I got peace of mind and eliminated the forced heat loss. Seems like a simple decision to me. It's just the right thing to do.
You can probably tell from this that I am a very conservative Engineer. If something can go wrong, it should be prevented.

I have a 5 foot rise and tested it by unplugging it. No smoke entered the house. Then the breaker popped a few days later after it got overloaded and again no smoke. I feel pretty good on that front. I could see if the wind was just right though it could give me a problem.

 

[Polar Bear]

Besides the long established fact that cooler, denser air makes for more complete combustion:

(broken link removed to http://chimneysweeponline.com/hooa.htm)

Wood stoves should have'em too. Wish I had the guts to bore the hole in the back of my fireplace.

Some do. My father has an outside air kit for his woodstove. Burns amazing while only needing a QUICK clean in the spring.

Although it is the only one I've seen with one. Unsure why that is.

 

[jlupi]

My indoor humidity runs a consistent 30+%, you should be familiar with CT weather, last week during all that rain outside relative humidity was 98+%.

the reason it is 30 is its RELATIVE HUMIDITY as in relative to temperature. the humidity drops because you heated the air (it has the same amount of moisture in it). If anything breathing cooking showering etc adds moisture to the air.

 

[Mt Bob]

Just to throw another iron in the fire,if you live where you get fairly constant wind from a certain direction,and you put an OAK on the lee side of the house,you can drasticly reduce the stoves efficiency,lower output.OAK may not be for everyone.

 

[bondo]

Call me stubborn but I haven't seen anything yet to justify a hole in my house. I have the oak it came with the stove. I have holesaws and every other tool needed. I even have a spot marked. It would take only a few minutes to do it but the question is still is it worth it. The equations don't tell me anything. 80 cfm through a straw or through firehouse is much different. Yes there is a point to be made about recycling air that has been already heated. However a new hole through a house is going to cause additional cold air a spot to come through. How much condensation does it cause? Hot air and cold air cause condensation. If it is snowing or raining and I am sucking that air in does that cause additional problems? I did a scientific test. I held a piece of paper to my intake it had just enough suction to hold it there. I then went to my bathroom and held it to that exhaust fan. It sucked the paper in from about 2" away. The bathroom fan which is 70cfm is sucking a lot more then the stove is. I am not for or against an oak. I just have not seen anything that tells me it is a must or must not have yet.

 

[Luvmesomesamples]

Does anyone know if my 2012 Harman accentra insert has separate intakes for the combustion and distribution air? I am considering adding an oak but don't want by distribution air to be cold, but wouldn't mind my combustion air to me. Thanks!

 

[SwineFlue]

Call me stubborn but I haven't seen anything yet to justify a hole in my house. I have the oak it came with the stove. I have holesaws and every other tool needed. I even have a spot marked. It would take only a few minutes to do it but the question is still is it worth it. The equations don't tell me anything. 80 cfm through a straw or through firehouse is much different. Yes there is a point to be made about recycling air that has been already heated. However a new hole through a house is going to cause additional cold air a spot to come through. How much condensation does it cause? Hot air and cold air cause condensation. If it is snowing or raining and I am sucking that air in does that cause additional problems? I did a scientific test. I held a piece of paper to my intake it had just enough suction to hold it there. I then went to my bathroom and held it to that exhaust fan. It sucked the paper in from about 2" away. The bathroom fan which is 70cfm is sucking a lot more then the stove is. I am not for or against an oak. I just have not seen anything that tells me it is a must or must not have yet.

While there are definitely benefits to an OAK, the 80cfm intake number is pure fiction. In reality it's closer to 8. The 80 came from the rating of an exhaust blower, not an air intake. Most of what comes out the exhaust is the combustion gases (ie the burnt pellets); the air pulled in is far less.

That's why your bathroom fan is pulling more air than your stove.

 

[iceguy4]

Well I just spent the last hour reading this entire thread...and indeed real good stuff. REAL GOOD STUFF some exceptional posts about the attributes of an OAK.
I have quoted some posts that I feel MUST tilt you toward an OAK....if you will keep an open mind. the safety factor alone .

I could see if the wind was just right though it could give me a problem

This poster said 2 times his chimney rise compensated for his fan....then says this...are you willing to take the chance??

Somebody else on the forum commented that they had to shut down their stove due to strong winds driving air back through their stove. That wasn't even concurrent with a power failure. Wind pressure was enough to overcome the combustion blower.

I read this thread...his problem was attributed to no OAK (stove is not affected when OAK and exhaust in close proximity)

Another valid reason for an OAK is to prevent a backflow of smoke when there is a power failure.

Bingo

I understand your position completely however is it your contention that with an OAK installed vs the same situation with no OAK that a substantial volume of already heated air is conserved to the point that the continual loss of air/heat through convection is marginalized so much that the OAK is compensating enough so that the normal heat/air loss is is surpassed and makes the modification justifiable or is it insignificant unless you have a really tight home? That is where I am going with this.

Can anyone help me with this?

what I learned from this thread...relative humidity . having an OAK yes yes ...because its a no brainer.

 

[iceguy4]

So there, knock yourself out. Calculate for yourself. And thank the people at Lincoln Electric. I think I may go take up welding or something

excellent post. the calculator puts me @ $216/yr VERY believable.