Fireplace insert: Insulate the exterior, or not?

[pdqdl]

I am re-habilitating my fireplace insert, a Quadrafire 4100. It's been a great heat supplement to the house, but has really sucked since the stovepipe up the chimney went bad. Yeah, I know. I should have fixed it a long time ago. Still, apart from the fact that it was hell to get started, it burned clean and added heat to the house. I also need to replace the firebrick and backer board & blanket on the inside.

Rignt now I have it pulled out, with plans to install new, stainless steel double-wall stovepipe to the top of the chimney. Something like Menards' 6" pipe.

I also intend to modify the stove to take the makeup air into the firebox from the holes I intend to punch through the fireplace walls. I really don't like the fact that I am burning wood with warm "inside" air, instead of taking it from the outside.

Now that you got the picture, here's the question: This fireplace insert is basically a steel box inside of a steel box. The heater blower circulates warm air into the house from the space between the two boxes. The outside shell, however, is sitting in a damned cold fireplace with cold air coming in from above, and theoretically, from the holes I intend to make in the rear. This means that a significant amount of heat will be shed to the outside, when I'd rather keep it inside the house.

I wish to insulate the outside of the fireplace insert with some inch thick kaowool ceramic fiber blanket, something like this stuff:
https://brickwoodovens.com/products...ilns-ovens-pizza-ovens-furnaces-forges-stoves. I haven't decided whether or not to attempt attaching it physically with metal screws and galvanized roofing washers, or perhaps some other scheme that hasn't yet occurred to me.

I figure all the heat I can keep in the house will be an improvement, especially since that insert lets tons of cold air into the house when isn't burning. Your comments and advice are welcome!

 

[Bill G]

Pictures are needed.

 

[pdqdl]

Really? It's the back side of metal box, inside of a conventional brick fireplace. I guess I could, but I didn't think that mattered.

At this point, I don't even have the stove out of the fireplace all the way. Getting the flexpipe detatched has been extra difficult. I had to heat the dinky 1/4' sheet metal screws cherry red before I could get them out. Then I had to let it cool off before proceeding, now I'm completely cold to the thought of going any further tonight.

I'll put up some pics when I get it out all the way. OK?

 

[Del_]

I see no need for photos for this theoretical discussion.

Is the mass of the fireplace in the living space? If it's not, your idea sounds sound.
If it is in the living space, have you considered sealing the area between the double walled stainless chimney and the walls of the chimney it's in? I'm assuming there is an existing masonry chimney. This is what I've done with mine. My chimney is double wall Metalbestos chimney and I've used unfaced fiberglass insulation stuffed tightly between the liner and the masonry chimney for about 8 feet.

There is very little evidence that an outside air supply is of any benefit and I have never used on.

There is very good insulation for flex pipe and it would be significantly less expensive.

In my first house(1978 to 1993) I had a 7 inch heavy duty stainless steel liner fabricated(22ft long) and rested this liner on a 1/4" steel plate through which it passed. I then poured vermiculite insulation from the top and fill the space between the liner and the chimney. I used a RiteWay Model 37(which if still own) and it used a 7" chimney. It was way cheaper than 22ft of double wall chimney plus it fit well in the 12 x 12 flue tile. Worked great.

The Outdoor Air Myth Exposed​

https://www.woodheat.org/the-outdoor-air-myth-exposed.html

 

[pdqdl]

I see no need for photos for this theoretical discussion.

Is the mass of the fireplace in the living space? If it's not, your idea sounds sound.

No. The fireplace has a brick mass protruding only slighty from the wall. The bulk of it is outside the house.

If it is in the living space, have you considered sealing the area between the double walled stainless chimney and the walls of the chimney it's in? I'm assuming there is an existing masonry chimney. This is what I've done with mine. My chimney is double wall Metalbestos chimney and I've used unfaced fiberglass insulation stuffed tightly between the liner and the masonry chimney for about 8 feet.

8 feet from the bottom going up, or 8 feet from the top going down? That sounds like a good idea too, but wouldn't do much for the hearth area lower than the "damper" level in my fireplace. This is where I intend to bring in outside air.

There is very little evidence that an outside air supply is of any benefit and I have never used on.

That is a mistaken notion. Makeup air is a well established concept, and it is strictly an engineered concept for large HVAC systems. In large systems, they balance the need for makeup air to combust the fossil fuel with the environmental requirement to exchange breathing air inside the structure. In my particular case, until my fire builds enough heat, I cannot get smoke to go up the chimney until I open a window or door into the basement area with the stove. Chokingly thick smoke literally pours out the vents of the insert and it extinguishes the fire. Open the window first, cold air pours in, and smoke rises up the exhaust. My house is tight.

There is very good insulation for flex pipe and it would be significantly less expensive.

Link?

In my first house(1978 to 1993) I had a 7 inch heavy duty stainless steel liner fabricated(22ft long) and rested this liner on a 1/4" steel plate through which it passed. I then poured vermiculite insulation from the top and fill the space between the liner and the chimney. I used a RiteWay Model 37(which if still own) and it used a 7" chimney. It was way cheaper than 22ft of double wall chimney plus it fit well in the 12 x 12 flue tile. Worked great.

I like that idea. I have not researched the single wall stainless stovepipe idea yet. I know I am quite displeased with how long the non-stainless pipe lasted that I put in originally. My existing chimney has a stainless liner and is 12" square. It wouldn't be much expense to fill with vermiculite. It wouldn't meet building codes, however.

The Outdoor Air Myth Exposed​

https://www.woodheat.org/the-outdoor-air-myth-exposed.html

I have strong objections to that web page. The first premise is just mistaken, the second is quite true, and the third premise is not disproven anywhere on that web page.

My house, however, is tight enough to prevent proper draft, at least until it is hot enough to have a stronger draft pulling air into the house. I should have made a video of smoke pouring out the leaks in the stove until the window was opened.

The assertions about passive makeup air are certainly true for any whole house solution, but don't at all fit most woodburner situations. It also states erroneously that somehow direct to combustion air source is inherently more dangerous, yet makes not one salient point that proves its argument.​

Given that the air-exchange value of the home is already established for my home, prior to installing the wood burner, it is not a valid argument to claim that increasing the air exchange greater than its original design is somehow ok.​

​

The proper concept behind providing outside air is that when the flue draft sucks air out of the building, it is sucking COLD air into the building. This will be air sucked inside from every leak in every window, door, attic stairway, and every other way that can be found for air to come in the building. Then you must consider that the air that comes in might be 0°F, which will knock all the charm out of the bedroom furthest away from the wood burning heat causing the air leak. I've seen it, felt it, and can confirm that the need for makeup air is real. At least for my house.

By providing outside makeup air from a source close to the fire, there is no need to chill other parts of the house. My insert has features that are adapted to using outside air. When you consider that the fireplace insert is actually hanging outside the building somewhat like a window air conditioner, it makes perfect sense that it should draw outside air to blow up the chimney.

 

[Bill G]

Really? It's the back side of metal box, inside of a conventional brick fireplace. I guess I could, but I didn't think that mattered.

At this point, I don't even have the stove out of the fireplace all the way. Getting the flexpipe detatched has been extra difficult. I had to heat the dinky 1/4' sheet metal screws cherry red before I could get them out. Then I had to let it cool off before proceeding, now I'm completely cold to the thought of going any further tonight.

I'll put up some pics when I get it out all the way. OK?

Pictures are needed.
A picture is worth a thousand words
You need to remember the entire Montana Resident ordeal with his stove burning the walls and asking if welding blankets would help.
His picture told the story without saying a word.

 

[Del_]

Eight feet from the bottom. I figure that ought to prevent most air movement.

My understanding is that a house needs more air exchange than a woodstove needs to operate. You are right though about makeup air making rooms far from the woodstove colder if the house is leaky. If the house is tight it seems there should be minimal cold air intrusion is the distance rooms.

I'd have to google flexible chimney liner insulation for sources. I've never used flex liner or insulation.

I've never used direct connected outside air and don't plan to.

I do have a problem in the beginning and end of the heating season sometimes when the stove may be cooled off for a day or so at a time. I can get a down draft making starting the stove a smoky situation. I remedy this by using a piece of cardboard cut to fit my side door opening with a round hole in it's center to accept a hair dryer heated end. I load the stove starting it with the top down method, then I blow hot air into the cold stove resulting in warm air going up the chimney. This starts enough draft make smokeless start ups easy. I only do this a dozen or so times per heating season.

I've been 100% wood heat since 1979 and use to heat domestic hot water too before I moved to a two story house with no basement. I'd love to do DHW now but there are logistical problems.

I believe Woodheat.org is pretty much stop on.

I usually don't read Mr Bills postings and when I do, I hardly every respond as he's just a waste of time.

 

[pdqdl]

You need to remember the entire Montana Resident ordeal with his stove burning the walls and asking if welding blankets would help.
His picture told the story without saying a word.

Eh. The problem with that situation was that he didn't like the answers he got. We might get there too, knowing my nature and many of the folks here. Pictures won't change anyone's tendency to argue.

I'll be happy to put up some pics, but I can't get it out of the hearth. The damned flexible 6" exhaust pipe is stuck in the outlet, and I can't wiggle it even a little bit.

I got a vicious Ingersol-Rand air chisel that will make short work of it tomorrow. I can come in from the bottom, inside the stove and just push it out. But that is at work right now.

 

[pdqdl]

I do have a problem in the beginning and end of the heating season sometimes when the stove may be cooled off for a day or so at a time. I can get a down draft making starting the stove a smoky situation. I remedy this by using a piece of cardboard cut to fit my side door opening with a round hole in it's center to accept a hair dryer heated end. I load the stove starting it with the top down method, then I blow hot air into the cold stove resulting in warm air going up the chimney. This starts enough draft make smokeless start ups easy. I only do this a dozen or so times per heating season.

I've got that situation in spades, but I have that all figured out. Here is the problem: Most folks think that heat rises, and I start a fire in this here cold-ass stove, and the smoke will go up the flue. Right?

Not necessarily. That outside air is pouring down the top of your chimney at very cold temperatures. The column of air inside that chimney is heavier than the warm basement air trying to get out. So your natural home ventilation, which generally rises through the house and excapes out the attic vents is carrying air up inside the house, and that cold column of air inside the stove is pushing down and out. Until you put enough heat into that stove's chimney to force air up, it's gonna smoke inside the house. A match and some kindling won't be enough to do that.

If you think that isn't right, open your stove door, and light a match at the opening on a cold day. Watch which way the smoke blows before you have any fire ignited.

Now imagine a different scenario. The chimney is cold, the stove is cold, the hearth is cold, but there is no draw from the house to the combustion chamber of the stove. That air flow is blocked off by the plate covering the "inside air" opening and the well-sealed door where the wood gets loaded. The fire will be running on very cold (heavy) air coming in from outside the hearth. In that case, just a modest shot of hot air will rise up the chimney, just like it always does from a campfire running on outside air.

 

[pdqdl]

BTW: my draft is so bad on that stove that I have considered adding a draft inducer fan to the top until it starts drawing naturally. Installing a new stovepipe that isn't rusted out and falling apart at the bottom seems like a better idea, though.

First things first, right?

 

[Del_]

Once we get into the heating season my stove is seldom all the way cold and I have zero problems with back drafting. Any time the house is warmer than is it outside I have no back drafting.

If I do, and like you mentioned, I can tell as soon as I open the stove door.

Out comes the hair dryer and wood stove door blocking plate. This is draft inducer.

I think a motorized draft inducer at the top is getting really compacted and not a good solution.

If it's at the top of your chimney.....it's going to get covered in creosote.

How would you get the draft inducer out of the way once the chimney is warm enough to operate without it?

I've used the hair dryer method for about 35 years now.

I'm in a much warmer climate than you are. zip code 30011

 

[Bill G]

Eh. The problem with that situation was that he didn't like the answers he got. We might get there too, knowing my nature and many of the folks here. Pictures won't change anyone's tendency to argue.

I'll be happy to put up some pics, but I can't get it out of the hearth. The damned flexible 6" exhaust pipe is stuck in the outlet, and I can't wiggle it even a little bit.

I got a vicious Ingersol-Rand air chisel that will make short work of it tomorrow. I can come in from the bottom, inside the stove and just push it out. But that is at work right now.

Tough to insulate something you cannot remove to insulate.

 

[pdqdl]

So true! I'll get it out, but I need to get something stronger that just my hands.

The goal here isn't just to insulate the outside of the box. I need to modify the rear to accept outside air, I need to install a couple of holes in the rear of the hearth to accomodate makeup air, and I need to get it out of the way so that I can connect new chimney pipe up to the top of the roof.

The insulation is just an afterthought. Do or not do? If so, how best accomplished?

 

[pdqdl]

If it's at the top of your chimney.....it's going to get covered in creosote.

How would you get the draft inducer out of the way once the chimney is warm enough to operate without it?

I've used the hair dryer method for about 35 years now.

That creosote problem is why I didn't do it many years ago.
I like the hair dryer idea with the door seal. That never occurred to me.

What did occur to me was the roofing torch blowing about 50,000 BTU up the stove before I tried starting the fire. That works pretty well too! Somehow, I never left the torch at my house, and keeping the 20lb propane tank in the basement didn't seem like a good idea, either. But it definitely got the draft going up the chimney.

 

[StihlsawuserMS361]

I see no need for photos for this theoretical discussion.

Is the mass of the fireplace in the living space? If it's not, your idea sounds sound.
If it is in the living space, have you considered sealing the area between the double walled stainless chimney and the walls of the chimney it's in? I'm assuming there is an existing masonry chimney. This is what I've done with mine. My chimney is double wall Metalbestos chimney and I've used unfaced fiberglass insulation stuffed tightly between the liner and the masonry chimney for about 8 feet.

There is very little evidence that an outside air supply is of any benefit and I have never used on.

There is very good insulation for flex pipe and it would be significantly less expensive.

In my first house(1978 to 1993) I had a 7 inch heavy duty stainless steel liner fabricated(22ft long) and rested this liner on a 1/4" steel plate through which it passed. I then poured vermiculite insulation from the top and fill the space between the liner and the chimney. I used a RiteWay Model 37(which if still own) and it used a 7" chimney. It was way cheaper than 22ft of double wall chimney plus it fit well in the 12 x 12 flue tile. Worked great.

The Outdoor Air Myth Exposed​

https://www.woodheat.org/the-outdoor-air-myth-exposed.html

This is in response, maybe off topic, to Del's link regarding "Outdoor Air Myth"
I have a self-built '85 raised ranch home. I used to get leeward ceiling "darkness" for lack of a better term when using my wood stove. A buddy of mine clued me in on houses being too tight for wood stoves etc. Fabricated a manifold of outside air. Problem went away.
YMMV

 

[pdqdl]

I think the outside-air consideration isn't quite so important to the makeup air consideration as it is resolving the pressure difference between the chimney stack pressure and the basement air pressure.

Here are my observations:

1. Hot air rises, and this causes air exchange through the roof.
2. Cold air settles in the basement, where most wood stoves and furnaces are located.
3. In the absence of a hot fire, cold air is pouring into the basement through the chimney as make-up air for the hot air rising through the roof of the home. We see this every time we try to light a fire on a cold day in a cold stove.
4. If the draft of the fire in the wood stove is less than optimal, it becomes a nightmare of trying to get a fire burning to produce enough heat to overcome the naturally induced draft coming down the open hole in the ceiling.
5. Assuming that the wood burner is isolated from the negative air pressure of the home's innate air circulation (at the low point of the wood burner), putting the wood stove on outside air isolates it from the house circulation.
6. When the wood burner has only cold air to draw from to send up the equally cold air above, even a match will make enough heat to draw air..

These assumptions are all based upon the concept that the wood burner can be isolated from the differential pressure between the house basement and the outside. My insert is completely sealed from any air exchange with the firebox except through the air dampers and the door. Once you open the door, even with an excellent supply of outside air, it will draw cold air into the room.

After I set the stove up for an outside air supply for combustion, at least I will be able to get a fire started more easily. I cannot express how vile an experience it is to be forced to open the front door and the basement window to get a fire started in the basement.

 

[Del_]

I think the outside-air consideration isn't quite so important to the makeup air consideration as it is resolving the pressure difference between the chimney stack pressure and the basement air pressure.

Here are my observations:

1. Hot air rises, and this causes air exchange through the roof.
2. Cold air settles in the basement, where most wood stoves and furnaces are located.
3. In the absence of a hot fire, cold air is pouring into the basement through the chimney as make-up air for the hot air rising through the roof of the home. We see this every time we try to light a fire on a cold day in a cold stove.
4. If the draft of the fire in the wood stove is less than optimal, it becomes a nightmare of trying to get a fire burning to produce enough heat to overcome the naturally induced draft coming down the open hole in the ceiling.
5. Assuming that the wood burner is isolated from the negative air pressure of the home's innate air circulation (at the low point of the wood burner), putting the wood stove on outside air isolates it from the house circulation.
6. When the wood burner has only cold air to draw from to send up the equally cold air above, even a match will make enough heat to draw air..

These assumptions are all based upon the concept that the wood burner can be isolated from the differential pressure between the house basement and the outside. My insert is completely sealed from any air exchange with the firebox except through the air dampers and the door. Once you open the door, even with an excellent supply of outside air, it will draw cold air into the room.

After I set the stove up for an outside air supply for combustion, at least I will be able to get a fire started more easily. I cannot express how vile an experience it is to be forced to open the front door and the basement window to get a fire started in the basement.

I remember you saying your house was tight but houses less tight sometimes have a 'house chimney effect', meaning hot air is leaking from the living space into the attic space via air leaks near ceilings.

 

[Del_]

This is in response, maybe off topic, to Del's link regarding "Outdoor Air Myth"
I have a self-built '85 raised ranch home. I used to get leeward ceiling "darkness" for lack of a better term when using my wood stove. A buddy of mine clued me in on houses being too tight for wood stoves etc. Fabricated a manifold of outside air. Problem went away.
YMMV

I hear you. Note that www.woodheat.org says "sometimes outdoor air supplies are needed". Note that there are hear recovering house air exchange devices on the market.

If your house it 'too tight' for a wood stove they you may not be getting enough air exchange for a healthy house.

Here's some interesting info on home air exchange rates Vs. wood stove air need from the site:

"The average air consumption of a modern wood heater is in the range of 10 - 25 cfm, which is very small compared to the natural leakage rate of houses. Building scientists say that the air in a house must be exchanged at least every three hours, or one-third of an air change per hour, to control moisture from cooking and washing and to manage odors. One third of an air change in a 1500 square foot house is 4000 cubic feet, or 66 cfm. Note that this is the absolute minimum air change for healthy living and that most houses older than 20 years have natural leakage rates far higher than this in winter. So the air consumption of a wood stove is a tiny part of a much larger exchange of air between the house and outdoors."

Are you sure your house doesn't suffer from 'house chimney effect'?

 

[StihlsawuserMS361]

I'm learning quite a bit here, thanks to all.
If my math is correct,@ 25cfm, x 60 minutes in an hour equals 1500 cfh x 24 hrs equals 36,000 cf per day. Seems high?
I've helped a few put in pellet stoves. All had outside air intakes.
Am I confused?

 

[pdqdl]

Are you sure your house doesn't suffer from 'house chimney effect'?

If I am understanding the term correctly, that would be a negative house chimney effect at my house. And there I thought I was describing the problem pretty clearly already.

Yes, there is absolutely a pressure differential between the floors, with higher pressure at the base and lower pressure at the top of the house. It doesn't help that the top of the brick chimney adds about 8-9 feet above the upper floor to make the stack height taller than the attic. So far as I am aware, nothing is going to make that "chimney effect" go away, either. Sometimes we position a squirrel cage blower at the base of the stairs to force the cold air downstairs up to the warmer areas. This is particularly helpful in the summertime.

The lower floor is a half-basement, with no apparent insulation between the concrete foundation and the paneling covering it. It's always kinda chilly down there, but that is where the fireplace is, so I have no options for another location.