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beermann · Sep 14, 2017

Hey everyone.

I'm nearly ready to pull the trigger on my installation of my wood insert into my old masonry fireplace.

Since I'll be drawing air from inside the house to feed the wood insert I was wondering...should I entertain the concept of making a small outside air hole through the back of the fireplace or even the ash dump. nothing large. I'd be referring to professionals to do the work and make it so I can cover/close it when not in use. Insulated cover.

I know I can just crack a window but the closest opening window is in the kitchen.

Thoughts?

Simonkenton · Sep 14, 2017

I would go ahead and fire it up and see how it goes. If you have an old fireplace, you have an old house. Probably not airtight.
I doubt you will notice the small amount of draft that the stove causes.

beermann · Sep 14, 2017

Simonkenton said:
I would go ahead and fire it up and see how it goes. If you have an old fireplace, you have an old house. Probably not airtight.
I doubt you will notice the small amount of draft that the stove causes.

Ok I'll use it for the winter before I make the jump on modifying anything. Is this kind of modification something people do or is it not a good thought?

It is not air tight which is why I got the insert with a blower. Last winter was a little pricey to run central heating on natural gas...ontario electricity and drafty house caused it to kick on every 10-15 minutes costing a fortune and using the old masonry fireplace wasn't appealing as I was told it would draw too much air and make my kids rooms down the hall colder.

The only limit to my free wood is my free time and physical readiness to keep chopping (until I have an electric splitter)

Simonkenton · Sep 14, 2017

Yes, lots of people use an outside air intake for their wood stoves. If the house is super air tight is could be necessary.
I have a pretty big stove, an Oslo 500, and I had all kinds of plans for an outside air intake. But, once I ran the stove several times, I realized that it was not needed.
For comparison, the fireplace pictured at left is in my house. I had to install a 6 inch by 14 inch, plus another 5x5 inch, outside air intake for this Bad Boy.

Remember that wood has to be really dry, needs to dry for 2 or 3 years. What kind of wood do you have up there in Ontario?
I have spent a lot of time in the southern part of your beautiful province, down on the border in Quetico Park.
Drove up there 9 times from Georgia to go on canoe camping trips. What a gorgeous place!

WoodyIsGoody · Sep 14, 2017

beermann said:
Ok I'll use it for the winter before I make the jump on modifying anything. Is this kind of modification something people do or is it not a good thought?

A house would need very tight construction before I would use one. Then they can be a very good thing but only if properly designed. I bet your gas furnace doesn't have an outside air intake either. Old houses don't need them and many modern houses don't either.

Ashful · Sep 14, 2017

Guys, an Outside air kit (OAK) is always better than none, as it reduces pressure drop in your house, thus reducing cold air infiltration. Though not necessary for the purpose of providing adequate draft for your stove, an OAK is always better than no OAK.

I would have them on both of my stoves, in this big old leaky house, if I didn't need to tunnel thru 20" thick rubble walls to get outside.

Simonkenton · Sep 14, 2017

So what kind of wood do you have up there in Ontario? I remember seeing, in the Quetico, lots of [of course] maple trees.

WoodyIsGoody · Sep 14, 2017

Ashful said:
Guys, an Outside air kit (OAK) is always better than none, as it reduces pressure drop in your house, thus reducing cold air infiltration. Though not necessary for the purpose of providing adequate draft for your stove, an OAK is always better than no OAK.

I would have them on both of my stoves, in this big old leaky house, if I didn't need to tunnel thru 20" thick rubble walls to get outside.

We're going to have to agree to disagree on that! Especially with modern efficient stoves like Blaze King catalytics that are designed to consume less air. If it's not beneficial, it's not better!

beermann · Sep 14, 2017

WoodyIsGoody said:
A house would need very tight construction before I would use one. Then they can be a very good thing but only if properly designed. I bet your gas furnace doesn't have an outside air intake either. Old houses don't need them and many modern houses don't either.

The heater does have one. The house is insulated fairly well just gets cold at -40 outside.

beermann · Sep 14, 2017

Simonkenton said:
Yes, lots of people use an outside air intake for their wood stoves. If the house is super air tight is could be necessary.
I have a pretty big stove, an Oslo 500, and I had all kinds of plans for an outside air intake. But, once I ran the stove several times, I realized that it was not needed.
For comparison, the fireplace pictured at left is in my house. I had to install a 6 inch by 14 inch, plus another 5x5 inch, outside air intake for this Bad Boy.

Remember that wood has to be really dry, needs to dry for 2 or 3 years. What kind of wood do you have up there in Ontario?
I have spent a lot of time in the southern part of your beautiful province, down on the border in Quetico Park.
Drove up there 9 times from Georgia to go on canoe camping trips. What a gorgeous place!

I have a big mix of wood that's been drying this summer outdoors in sunlight and windy spot but still under some tree branches for cover.

Cedar, pine, ash, maple. Just a big stack of mixed free firewood.

BKVP · Sep 15, 2017

So as many followers here know, I have a King in my 1895 Sears Craftsman house. The mail slot is any of the four sides of the front door! For the fist ten years of burning my King, it drove me nuts laying on the floor, in front of the stove, watching the NASCAR races...because a draft would come across the floor right into me!

So I added an OAK to my King and the very day I did so, the draft vaporized! I also noticed a very slight drop in fuel consumption and the air leak around the front door all but disappeared.

Take what you want from this real world application, but I think an OAK is a great idea. Before I installed the OAK, my grandson said "why do you burn the the air you already heated?"

Hogwildz · Sep 15, 2017

I'm good without an OAK. I think of any minor leaks as fresh air replacement in the house.

beermann · Sep 15, 2017

BKVP said:
So as many followers here know, I have a King in my 1895 Sears Craftsman house. The mail slot is any of the four sides of the front door! For the fist ten years of burning my King, it drove me nuts laying on the floor, in front of the stove, watching the NASCAR races...because a draft would come across the floor right into me!

So I added an OAK to my King and the very day I did so, the draft vaporized! I also noticed a very slight drop in fuel consumption and the air leak around the front door all but disappeared.

Take what you want from this real world application, but I think an OAK is a great idea. Before I installed the OAK, my grandson said "why do you burn the the air you already heated?"

My concern is exactly what you mentioned. So I think my next step is to just run it this winter and If I have any cold airways like that I'll just make an OAK from the rear of the fireplace to behind the insert or from the ash dump to under the insert. Will take some thought and proceed if it's too drafty

beermann · Sep 15, 2017

Hogwildz said:
I'm good without an OAK. I think of any minor leaks as fresh air replacement in the house.

Me too. Just concerned about making a room too cold. Sounds like I may be overthinking it.

Simonkenton · Sep 15, 2017

I am with hogwildz. I look at any air that comes in for the wood stove as fresh air for the house. I can't feel a draft any where.

To each, his own.

WoodyIsGoody · Sep 15, 2017

BKVP said:
For the fist ten years of burning my King, it drove me nuts laying on the floor, in front of the stove, watching the NASCAR races...

That drives me nuts too! But it has nothing to do with drafts, it's the way they just drive around in a big circle, every turn is a left hander! Boring. You want a real motorsport, try Moto GP! Real racing with lots of braking, tight turns, sweepers, chicanes and fast straights. BTW, you should hire a company to seal up all those "mail slots" around your front door if you don't have the time to do it yourself. More better.

Before I installed the OAK, my grandson said "why do you burn the the air you already heated?"

Smart kid. But he probably doesn't know your Blaze King already uses some of the heat it produces to pre-heat the combustion air before it's burned. Because it's not good to introduce air below freezing to the firebox. The hotter the air introduced, the easier it is to maintain an active combustor on low burn. Introducing colder air (rather than room air) just means it needs to be heated up inside the firebox before it can burn rather than outside the firebox.

WoodyIsGoody · Sep 15, 2017

Simonkenton said:
I am with hogwildz. I look at any air that comes in for the wood stove as fresh air for the house.

Agreed. Most homes have a bit of cold air infiltration, and that happens without a woodstove burning too, and the cold air naturally settles near the floor. Without a woodstove or forced air furnace running, the cold air just sits there. But fire up a woodstove and within an hour or two the stove's air intake naturally "vacuums up" this cold layer near the floor since the air intake is generally down low where all the cold air settles. A CAI lacks this ability to "vacuum" the cold air off the floor (and air infiltrates regardless of a CAI or not).

Hogwildz · Sep 15, 2017

You're going to have some "cold" air at the floor whether you have an OAK or not. A natural convection loop is going to pull cooler air across the floor to the stove while the heated air is going to rise.

Ashful · Sep 15, 2017

WoodyIsGoody said:
We're going to have to agree to disagree on that! Especially with modern efficient stoves like Blaze King catalytics that are designed to consume less air. If it's not beneficial, it's not better!

As long as stoves run on air, and that air must come from outside, there will always be some theoretical benefit to the OAK. Not saying it's necessary, obviously most get by without them, just saying it's always a theoretical advantage.

There are potential downsides to OAKs, though. Other than the obvious installation hassle, there are sometimes issues with venturi effect, if they're on the leeward side of the house on a windy day.

WoodyIsGoody · Sep 16, 2017

Ashful said:
As long as stoves run on air, and that air must come from outside, there will always be some theoretical benefit to the OAK.

When you believe there "must" be a theoretical advantage to OAK without analyzing the thermodynamics involved, you are skipping an important step. Namely, analyzing and identifying that supposed thermodynamic advantage. When you do that, you will see it evaporate right before your eyes.

Let's take a stove equipped with an OAK and a special diverter valve for test purposes (it can run on outside air or room air at the flip of a lever). It's operating on room air at a steady state (meaning a constant flue temp and rate of flow of exhaust). Let's say the stove is on low burn and flue gas temperature is 275 degrees. It's 10 degrees outside and 70 degrees inside. You can feed it 10 degree air or 70 degree air from the room. The air from the outside air intake needs to be heated 60 more degrees than the air from the room to maintain that same 275 degree air temperature and keep your cat active (or, in the case of secondary burn, to keep the secondary burn active). Where does that extra energy come from? The same place it comes from if you use 70 degree room air, from your wood. There is no free lunch. The only way around this is to say the 10 degree air will simply drop the flue temperature by 60 degrees (compared to using room air) and thus increase efficiency. But now your flue temperature is only 215 degrees which implies you have lost cat activity or secondary burn. The stove's burn rate must be increased to prevent this from happening and yet all the extra energy from increasing the burn rate to maintain a clean burn is going into heating up that outside air and not to heating your room. You can't have your cake and eat it too.

There is no gain in efficiency.

There are potential downsides to OAKs, though. Other than the obvious installation hassle, there are sometimes issues with venturi effect, if they're on the leeward side of the house on a windy day.

That issue can be solved by using multiple intakes connected together to average pressures on multiple aspects of the structure. But OAK's are not typically supplied with this capability - it has to be provided separately. And it still doesn't fix the fact that outside air is not more efficient to begin with.

cd36 · Sep 16, 2017

I have an outside air kit installed on my new construction tightly sealed house (full vapour barrier behind drywall). If I run the stove fan I can notice more resistance when opening outside doors and hear the rush of air, so the house is tight.

I've never ran without an OAK I guess I should try, but I feel it necessary in my install. Pulling cold air into the house from anywhere you don't want it to come in will lead to moisture condensing in the walls and mold issues eventually.

Also all air that is burnt in the stove will need to be heated one way or another. Either you pull cold air in elsewhere in the house and the wood stove has to heat it up, potentially resulting in a cold room, or it pulls it right into the wood stove to be heated without you ever feeling the cold air first, to me the second option is best.

The only issue I do have with my OAK is frost. If it's below probably - 35c outside I can get frosting on the side of the fireplace (should mention I have a RSF focus zc fireplace). I asked rsf about that and they said to make the flexible duct going to the fireplace longer to give it a chance to warm up first, I did that with not much benefit. I don't want to make it to long though since it will make it tougher to pull air in.

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WoodyIsGoody · Sep 16, 2017

cd36 said:
I have an outside air kit installed on my new construction tightly sealed house (full vapour barrier behind drywall). If I run the stove fan I can notice more resistance when opening outside doors and hear the rush of air, so the house is tight.

A properly designed and installed outside air intake is sometimes necessary if the construction is very tight. My place is tight enough to notice one fan slightly depressurizing the house but not tight enough to need an outside air intake.

I've never ran without an OAK I guess I should try, but I feel it necessary in my install. Pulling cold air into the house from anywhere you don't want it to come in will lead to moisture condensing in the walls and mold issues eventually.

You have it exactly backwards. When air migrates from a colder to a warmer place the dew point is rising so it can't condense and cause mold issues. That is the ideal direction for air to migrate in the winter which is one (of many) reasons I prefer not to have an outside air intake if I can avoid it. By making the stove and flue a closed system (separate from room pressure) you are preventing the stove from slightly depressurizing the house. If there are no exhaust fans turned on then the house has the same pressure as the outside. But only until the wind blows on the house. Then you have a high pressure area outside the windward side and a low pressure on the lee side. This causes warm room air to travel through the wall on the leeward side. As it travels through the wall it cools. If it cools past the dew point, any humidity in that cooling room air condenses inside your wall. This causes mold to grow in your wall. But a flue connected to the room air can slightly depressure the house which can prevent room air from migrating through the walls to the outside. In the summer, when it's cooler inside than outside, it's preferable for air to migrate from the cool indoors to the warmer outdoors. The stove is not run during these conditions so the lack of a cold air intake is not a disadvantage. Besides, there is unlikely to be enough of a temperature differential in the summer to cause a problem.

cd36 · Sep 19, 2017

Yup you're right I had a brain fart on that one, don't know what I was thinking. The cool dry air isn't going to condense as it moves through your wall. Depending how it is coming into the house though it can cause the warm moist air in the house to condense. This all depends though on how it is getting into your house.

And still pulling cold air into other rooms of your house would, I think, be less desirable to pulling cold air into the fireplace. That air has to be heated one way or another, but if it goes right into the fireplace I don't feel it, and if it comes in through a room on the other side of the house, it makes for a cold room.

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beermann · Sep 20, 2017

WoodyIsGoody said:
A properly designed and installed outside air intake is sometimes necessary if the construction is very tight. My place is tight enough to notice one fan slightly depressurizing the house but not tight enough to need an outside air intake.

You have it exactly backwards. When air migrates from a colder to a warmer place the dew point is rising so it can't condense and cause mold issues. That is the ideal direction for air to migrate in the winter which is one (of many) reasons I prefer not to have an outside air intake if I can avoid it. By making the stove and flue a closed system (separate from room pressure) you are preventing the stove from slightly depressurizing the house. If there are no exhaust fans turned on then the house has the same pressure as the outside. But only until the wind blows on the house. Then you have a high pressure area outside the windward side and a low pressure on the lee side. This causes warm room air to travel through the wall on the leeward side. As it travels through the wall it cools. If it cools past the dew point, any humidity in that cooling room air condenses inside your wall. This causes mold to grow in your wall. But a flue connected to the room air can slightly depressure the house which can prevent room air from migrating through the walls to the outside. In the summer, when it's cooler inside than outside, it's preferable for air to migrate from the cool indoors to the warmer outdoors. The stove is not run during these conditions so the lack of a cold air intake is not a disadvantage. Besides, there is unlikely to be enough of a temperature differential in the summer to cause a problem.

So does this mean if I have mild mold issues around my original 60+ year old windows that the fireplace may actually resolve this issue. That's cool.

WoodyIsGoody · Sep 20, 2017

beermann said:
So does this mean if I have mild mold issues around my original 60+ year old windows that the fireplace may actually resolve this issue. That's cool.

I would replace 60 year old windows. You're going to like having insulated glass!

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