Fireplace insert: Insulate the exterior, or not?

[pdqdl]

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.

There won't be any appreciable leakage into the attic. There are no cracks whatsoever in the drywall, about 9 inches of blown cellulose insulation and a tight-fitting attic stairway. The attic fan louver will be leaking a bit of air, but not enough, I think, to cause any significant air movement. I'd speculate that the sewer vent stack is completely uninsulated as it passes into the attic, but I have never found a cold-air dump area on the lower floor. Quite frankly, I don't know where it is located; I think it's hidden inside a wall.

We only spend about $100 per month on natural gas in the winter time, so it is pretty well insulated. I've put in double pane windows everywhere, and insulated the heck out of them with spray foam between the aluminum window frames and the wood framing on the house. This house is a split level, with a double garage on the north half, and the "basement" on the south half is holding the fireplace insert.

 

[pdqdl]

An update: I am still defeated by the flexible pipe attached to the top/rear of the stove. I took my heavy-duty air chisel home from work, and it didn't even wiggle. It hits so hard that the stove coupler pressed into the stove started to come out and the air chisel was happy to blow holes in the heavy duty flex-pipe. I gave up. Tomorrow I'll get in there and chop it off with my battery operated disk grinder. I'll be able to extract the 2" remaining piece out when I can pull the stove out of the hearth.

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 am very interested in how long that single-wall pipe lasted. I cannot justify $50 to 100+ per foot for the double-walled chimney pipes. I figure if the flexible pipe is good enough to go up past the damper, it is good enough to go all the way to the top. I really like your idea about insulating the whole column with vermiculite, too.

How long did that last? After 22 years, the "stainless" flexible pipe that couples my wood burning insert out past the damper is still quite sturdy, although thoroughly rusted and stiff. I'd happily re-use it, but it looks like I will destroy it getting the stove out of the hearth, first. I figure if a single-wall flexible pipe is good enough for the first 4 feet, it ought to be good enough for the last 20 feet.

Anybody have objections to this plan?

 

[Del_]

An update: I am still defeated by the flexible pipe attached to the top/rear of the stove. I took my heavy-duty air chisel home from work, and it didn't even wiggle. It hits so hard that the stove coupler pressed into the stove started to come out and the air chisel was happy to blow holes in the heavy duty flex-pipe. I gave up. Tomorrow I'll get in there and chop it off with my battery operated disk grinder. I'll be able to extract the 2" remaining piece out when I can pull the stove out of the hearth.



I am very interested in how long that single-wall pipe lasted. I cannot justify $50 to 100+ per foot for the double-walled chimney pipes. I figure if the flexible pipe is good enough to go up past the damper, it is good enough to go all the way to the top. I really like your idea about insulating the whole column with vermiculite, too.

I lived in that house for 14 years moving from there in 1993. No problem at all during that time.

I'm sure it would last a very long time. I believe it was 20 gauge stainless but don't remember for sure. It was custom fabricated by a local heating and air company.

I doubt the owners are heating with wood today but they might be.

 

[gggGary]

We still need pics.
I've been a member of woodheat.org and a moderator of John's yahoo group for over a decade.
That adds exactly zero to my credentials but I'm not uninformed on the subject. Been heating with wood on systems I built and installed for over 30 years. Been involved in home building/construction industry for basically my whole life.

OK Yeah pics make a difference. the old 1000 words thingy.
Any design, materials used MUST be compliant with local/national codes.
Two strong reasons
No matter how much you think on it, gotcha's beyond a layman's knowledge lurk in DIY solutions.
If killing your loved one's isn't enough, don't give your insurance an excuse to tear up a check and walk away.

Comments, suggestions (so far without pics) I tend towards gentle suggestions but this IS pdqdl
You are fortunate, your stove has installation instructions, FOLLOW THEM, no exceptions.
https://downloads.hearthnhome.com/installmanuals/250_7201.pdf
Any homebrew chimney materials, insulation scheme is a non starter. DON'T do it.
Your install/system is far from ideal. Operation may require extra steps when starting a fire, have limitations, RE how far you can restrict airflow to extend burn times. Change the design, installation or learn to deal with it.
Is the masonry chimney, firebox you have completely code compliant? Materials, air spaces, clearance to combustibles?
Simple things first; do not underestimate the draft establishing power of a single sheet of newspaper lit and held high in the firebox. I've never had this fail.
Do not over or undersize your Flue, stick with 6" for the hydrofire 4100, use only code approved flue materials do NOT insulate between the flue and the masonry.
Creating a closed shelf to block air movement into the space is code and acceptable.
Be extremely careful with any combustion air supply vents, piping, You have stated that a reverse draft is possible, likely in your install. Think about the consequences of one occurring during operation of the stove.
We still would like to see pics.
I'll gladly debate fine points, but not code or manufacturers installation guideline compliance.
Thoughtfully, gggGary

 

[dboyd351]

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.

Definitely not as drastic as the 50,000 BTU roofing torch, but this what i use to get a draft going in my woodstove chimney. I build the fire, then stick this up the entrance to the chimney for about 30 seconds, then immediately light the fire. Works for me.
Also, your chimney is tall enough, isn't it?
when I first installed my woodstove, the guy that helped me do it had done many up in New York before moving down here. He told me how high he thought the chimney pipe needed to be, and we initially went with that, even though it was less high than the woodstove manufacturer recommended.
I had major problems with downdraft getting a fire started with that chimney and I ended up adding another 6 feet of pipe, 3 feet at a time and have never had a problem since. My experienced installer buddiy swore that wasn't the issue but firsthand experience and the stove manufacturer's instructions gave a much different answer.
I realize you have likely checked the chimney height, just passing on my experience.

 

[sean donato]

2 years ago, we had to replace/ fix our masonry chimney. We got several quotes for different procedures, and ultimately decided to go with I started stainless pipe. Our masonry chimney needed quite a bit of block work, so it didn't make financial sense to do a stainless liner since the chimney had to come down anyway. If the block would have been good, I would have went this route, it's a good bit cheaper then the selkirk double wall you mentioned, and will work every bit as good. I gave up on any non stainless stove pipe years ago as well. All it does is rot out at some point. Buy once cry once.
I wouldn't hesitate to insulate the heat exchanger portion of the furnace either. I can't see where letting heat intended for In the house go through the stone and outside is good. It's just wasted btu.
I would wait to see If you need a air make up pipe if you're using any form of insualted pipe, it will draft better then the masonary chimney. (Even if you had an old pipe jammed up there, the insulation makes a huge difference.)
And usually heats up very quickly. We had similar issues with our furnace, I'd crack the window open next to the furnace till the masonry chimney heated up. Don't have to with the insulated stainless chimney.

 

[gggGary]

I gave up on any non stainless stove pipe years ago as well. All it does is rot out at some point. Buy once cry once.

Also true of manufactured flue pipe, don't cheap out, pipe with non stainless outer wall exists, avoid it like the plague.
Fun fact; stainless steel is corrosion RESISTANT not corrosion PROOF, never use metal chimney brushes on SS liners. scratches start/harbor corrosion.

 

[pdqdl]

Any homebrew chimney materials, insulation scheme is a non starter. DON'T do it.

No homebrew plans pending. Insulating a firebox with fireproof materials, I don't think is a problem.

Your install/system is far from ideal.

How so? This is a fireplace insert in a fireplace. This is exactly what it was built for.

Operation may require extra steps when starting a fire, have limitations, RE how far you can restrict airflow to extend burn times. Change the design, installation or learn to deal with it.

No one has mentioned restricting air flow for extended burn times. I'm not sure where that comment comes from.

Is the masonry chimney, firebox you have completely code compliant? Materials, air spaces, clearance to combustibles?

Yes, it is. Furthermore, it has an excellent 12" x 12" stainless steel liner up the chimney.

Simple things first; do not underestimate the draft establishing power of a single sheet of newspaper lit and held high in the firebox. I've never had this fail.

I have. Time after time, after time. A single sheet of newsprint won't even make the draft in this firebox nervous... currently. Try to keep in mind that the 6" flue pipe has degraded significantly, and is far too short to create a proper draft. That is why I am fixing it!

Do not over or undersize your Flue, stick with 6" for the hydrofire 4100, use only code approved flue materials do NOT insulate between the flue and the masonry.

I am mystified by that statement. At this point, I will be planning to install a flexible stainless 6" flue pipe, appropriate for that burner. ALL the instructions for those pipes that I have found encourage, but do not require, an insulation layer to be added to the outside of the pipe.

Creating a closed shelf to block air movement into the space is code and acceptable.

Part of my plan!

Be extremely careful with any combustion air supply vents, piping, You have stated that a reverse draft is possible, likely in your install. Think about the consequences of one occurring during operation of the stove.

I'm not sure what is meant by "reverse draft". If you mean smoke coming out of the fireplace insert every time I try to start it, then you should know that is an "every time I start it" condition. The cause of this is known, however. My rusted-out flue pipe inside the brick chimney is the known problem. I am currently fixing that shortcoming.

We still would like to see pics.
I'll gladly debate fine points, but not code or manufacturers installation guideline compliance.
Thoughtfully, gggGary

So far, I have been unable to extract the flexible flue pipe installed 22 years ago. It is in very sturdy condition, and I have poor access to it. I'll be cutting it off tonight with an angle grinder, after which it should be easy to remove the stub from the collar on the stove.

I am glad to hear that you approve of sealing off the shelf behind where the damper used to be. Got any suggestions for the best method? At this point, I am considering either a fireproof backer board, or possibly a masonry solution. It will almost certainly involve some of the fireproof wool blanket to make a flexible closure around the flue.

 

[pdqdl]

Also, your chimney is tall enough, isn't it?

Yes and no. My draft problem is currently caused by the severely corroded old stove pipe I installed 22 years ago. This is why I am rehabilitating the installation, 'cause it's current condition sucks. That being said, I don't see any reason not to do a much better job than my original installation. Hence, this thread.

The brick chimney, however, is in excellent condition, minus the steel damper I took out many years ago.

 

[pdqdl]

Also true of manufactured flue pipe, don't cheap out, pipe with non stainless outer wall exists, avoid it like the plague.
Fun fact; stainless steel is corrosion RESISTANT not corrosion PROOF, never use metal chimney brushes on SS liners. scratches start/harbor corrosion.

Yeah, I did that non-stainless the first time. Worked great for a long time. Pretty damned poor for the last couple of seasons, though.

An explanation as to why I let the current flue pipe get so bad:
When I first put that insert in, I found it extremely heavy, with a front panel that slid vertically into position behind the front of the firebox as a cosmetic seal to the hearth. Unfortunately, the mantle on the fireplace included a protruding row of brick that required sliding that front panel in prior to pushing the firebox inside the hearth. In fact, iIt was such a tight fit that I had to grind out the bottom bricks to make clearance for the two upper corners.

Here is the tricky part of the installation: the front panel prevented any access to the height adjusting legs for the insert. THEN, when I pushed the firebox into the hearth, it dropped down from the front lip at the rear of the hearth extension. I have been quite fearful of the difficulty of dragging that insert back out, anticipating that the adjustable legs would catch on the higher brick lip at the front of the firebox, making it impossible to pull the insert out far enough to lift the faceplate vertically off the insert.

Yeah, this is where pictures would help. I'll get that tonight.​

My recent attempts worked out much better than I could have hoped, however. When I installed the insert, I didn't realize how much of the front would disassemble and lighten the load. Taking it out a couple days ago, I took off the front door, the heavy top plate, the heater blower housing, and it's associated cast iron grill. With over a hundred pound taken off, I just heaved it a few times and was able to pull it out by myself and slide up the front faceplate. I remain stalled on the removal because of the flexpipe flue that is resisting my extraction attempts.

Here is the damned flexpipe connection that won't come apart:
Side view: There is barely room for my hand to slip between the insert and the hearth to get this picture.

When I took this pic, I discovered the screw in the rear, which had evaded my probing fingers. That has since been removed. (Propane torch to cherry red the area, then a small vice-grip to turn it out) Notice the chisel mark at the front of the flexpipe. This is where my air chisel attempted to push out the pipe. Nope! That ain't gonna work.

View from the front:

As you can see, there isn't much room for a burly arm to get in and force out that flexpipe. That height gap is about 3" tall.

 

[gggGary]

Good reply, pic, and MORE info.
Less than ideal ~ basement, chimney, firebox is not completely inside the tall house.
Somethings we just have to live with, accommodate for.
on current flex liner removal; 12" blade in a sawzall, whack it off.
the flue support shelf; made of steel plate, bolt or brackets/bolt to masonry?
install steel blocks or pour masonry so floor of firebox where insert feet go is flush with front opening, insert now slides in/out no lifting.
Consider letting insert free stand, stick out onto hearth, allow open airflow around behind up and out of firebox.
Install 6" liner all the way to the top of the masonry chimney. It sounds like it just goes up partway now? I'll guess that would let a newspaper inducer work.
An electric heat gun should be a good/handy to use draft inducer.
intake air; since backdrafts are a known issue on your install, any intakes should be fire resistant. That is some of John's point, back drafts can/have sent fire back through intake pipe systems.
For consideration Other appliances are also back draft inducers/dangers; gas water heaters, stove hoods, bath vents, dryers, attic fans all can create negative pressure or also backdraft in tight/tall houses.
Read up Florida bungalow syndrome for reasoning/problems with very low fire rates/ overly closed dampers.

 

[gggGary]

PS a couple pics from 10' away, lets see the install!

 

[gggGary]

A 12x12 SS square? liner is HUGE a 6" round flue all the way up will improve operation.
I would want the liner to terminate on the new shelf, with a separate piece of flue to connect from the insert to the shelf.
It appears your height/space is quite limited hence the difficulties. A common problem.

 

[pdqdl]

Good reply, pic, and MORE info.
Less than ideal ~ basement, chimney, firebox is not completely inside the tall house.

Nope! The brick chimney is pretty much outside the walls of the house. Obviously from the outside.

I also see that the top of the chimney isn't quite as tall as the roof peak. Is ok, as that fireplace always drafted nicely with a fire going. Like the insert, however, not so good when starting cold!

Somethings we just have to live with, accommodate for.
on current flex liner removal; 12" blade in a sawzall, whack it off.

Hmmm... Hadn't considered it. I have both, will try both.

the flue support shelf; made of steel plate, bolt or brackets/bolt to masonry?

Not sure what you mean. Please explain.

install steel blocks or pour masonry so floor of firebox where insert feet go is flush with front opening, insert now slides in/out no lifting.

An excellent suggestion. I wish I had thought of that putting it in the first time. One thing I cannot do is raise the insert any higher than previously. The faceplate is tucked right up against protruding bricks at the top.

Consider letting insert free stand, stick out onto hearth, allow open airflow around behind up and out of firebox.
Install 6" liner all the way to the top of the masonry chimney. It sounds like it just goes up partway now? I'll guess that would let a newspaper inducer work.

6" liner to top is what I had originally, but not recently. I even sealed the top of the chimney so that no air was flowing down the outside of the 6" liner. Originally, it worked fine, but rust had its way with my cheap install.

An electric heat gun should be a good/handy to use draft inducer.
intake air; since backdrafts are a known issue on your install, any intakes should be fire resistant. That is some of John's point, back drafts can/have sent fire back through intake pipe systems.

You guys are more familiar with many different intake systems than I am envisioning. This will only be a few holes poked into the back wall of the fireplace to allow outside air into the open ports on the back side of the insert. I'm currently re-thinking that plan anyway. 25 cfm seems to be the accepted flow rate into this kind of stove. That's about the equivalent of a low speed computer fan; not too much in the big scheme of air flow inside the house. I am beginning to think that sealing up the chimney real tight to cold air flow might be better for the total heat exchange than making the backside of the insert frigidly cold with outside air.

For consideration Other appliances are also back draft inducers/dangers; gas water heaters, stove hoods, bath vents, dryers, attic fans all can create negative pressure or also backdraft in tight/tall houses.
Read up Florida bungalow syndrome for reasoning/problems with very low fire rates/ overly closed dampers.

I have no backdraft issues on this contraption, once the fire is well started. Obviously, turning on the attic fan is a smoky disaster, though. Both my house furnace and water heater use outside air and induced flue pipe venting, so that never affects the woodburner.

I love my Ruud on-demand water heater, too. I even have a water circulation pump connected to a flow detection switch, with a fancy timer. It will stay on for any time that I dial in, then remain off until cycled on again. I also have a thermostat on the circulation line, to prevent activation of the circulation system when it isn't needed. This is the problem with these on-demand water heaters! It can seem to take forever for the hot water to arrive. My circulation pump cuts that down to about 5-10 seconds for every faucet in the house except for the downstairs bathroom sink. Additional benefit: the circulation pump is switched on at 1/4 gpm, but the on-demand water heater doesn't come on until 1/2 gallon per minute. The low-flow faucet in the kitchen sink was tricky to get any hot water when initially put in. I fixed that pretty quickly! ​

 

[pdqdl]

PS a couple pics from 10' away, lets see the install!

Ok... I've been trying to hide my mess.

​

 

[dboyd351]

This is what I was referring to regarding chimney height
As initially installed the chimney only extended about as high as the 2 support poles attached to the chimney pipe. Didn't draw worth a crap and was several feet shorter that the stove manufacturer's recommendation based on top of the roofline and slope of the roof.
My "installer" friend was adamant the chimney height was not the problem and threatened to quit helping if I went to a taller chimney.
I ordered the extra pipe, installed it and my draft problem vanished.

 

[gggGary]

Ok... I've been trying to hide my mess.

​

We suspected as much, LOL but it's OK we won't think less of you, this IS the "worse before better" part of a SNAFU fix.
The physics of chimneys is fascinating.
The pressures/differentials involved are VERY small.
Like you say the air movement of a RUNNING stove is small, air flow during lighting, with lots of "bypass air" while the doors are open may be an order of magnitude greater.
House is not as tall as I was thinking, unless there's a second story that's out of the pic frame?
That chimney does seem to be on the "short side"
I added 8' to a similar set up in a previous house.
It can be revealing to go outside and watch smoke during a start up on a warmish no wind fall day
PS yes a plate/baffle top and bottom of the chimney is acceptable, desirable.
Yes it's far from ideal but you can prolly get around most of the issues with a lined/insulated firebox and full height 6" chimney insert/flue.
Plumbing vents should NOT be an issue unless there's a dried out floor trap(s) somewhere. This is A thing! But your nose usually reveals those.
It would be nice to avoid cutting drilling holes for intake air. Does the firebox have an ash pit/exterior door?

the flue support shelf; made of steel plate, bolt or brackets/bolt to masonry?

Not sure what you mean. Please explain.

It won't be easy because of your height restrictions but a plate that fits/closes off the masonry opening with a collar the flex liner sits on, then the plate bolted or held in place by other substantial means to the ceiling of the firebox. then a connector pipe from the fire box to the plate.
Some inserts have both a top and/or rear flue exit to make it easier to install in tight/height restricted spaces.

 

[pdqdl]

Read up Florida bungalow syndrome for reasoning/problems with very low fire rates/ overly closed dampers.

You may wish to save this link to the original analysis:
http://www.gulland.ca/florida_bungalow_syndrome.htm

 

[pdqdl]

We suspected as much, LOL but it's OK we won't think less of you, this IS the "worse before better" part of a SNAFU fix.
The physics of chimneys is fascinating.
The pressures/differentials involved are VERY small.
Like you say the air movement of a RUNNING stove is small, air flow during lighting, with lots of "bypass air" while the doors are open may be an order of magnitude greater.
House is not as tall as I was thinking, unless there's a second story that's out of the pic frame?

"Second story" is the main floor. First floor is the half-basement. Notice the ground level windows on the right side of the chimney.

That chimney does seem to be on the "short side"
I added 8' to a similar set up in a previous house.
It can be revealing to go outside and watch smoke during a start up on a warmish no wind fall day

Done that plenty. It still swirls around the house a bit; you can often smell wood smoke on the front porch with a southerly breeze. The draft inside the house is pretty much free of wind effects, though, providing I keep the cap on at the top.

PS yes a plate/baffle top and bottom of the chimney is acceptable, desirable.
Yes it's far from ideal but you can prolly get around most of the issues with a lined/insulated firebox and full height 6" chimney insert/flue.

Plumbing vents should NOT be an issue unless there's a dried out floor trap(s) somewhere. This is A thing! But your nose usually reveals those.
It would be nice to avoid cutting drilling holes for intake air. Does the firebox have an ash pit/exterior door?

No. In fact, the firebrick floor of the indoor hearth is several feet lower than the outdoor ground level. This means that any perforations to the chimney to provide outside air will have to be made just beneath the level of the damper on the inside. Hence, my newly formed reservations about pursuing any outside makeup air source.

Not sure what you mean. Please explain.

It won't be easy because of your height restrictions but a plate that fits/closes off the masonry opening with a collar the flex liner sits on, then the plate bolted or held in place by other substantial means to the ceiling of the firebox. then a connector pipe from the fire box to the plate.
Some inserts have both a top and/or rear flue exit to make it easier to install in tight/height restricted spaces.

No such features on my installation. I just ran that terribly stiff flexpipe out the damper opening and stacked the single wall (now rusted out) stove pipe up the chimney to the top with no support. It worked well, up until when it rusted out.

Supporting the new flue pipe is an adventure I have not addressed yet. I figured to build some sort of support when I closed off the large square former-damper opening. I tend to like things done with masonry, and I'm imagining sealing it up with some bricks and masonry close to an opening big enough for the flue pipe. An unresolved problem, at present. I still cannot get a good look at it, 'cause the insert is stuck on the rusty but strong flexpipe from 22 years ago.

I figure that problem can be solved once I see how stiff the new flex pipe to the top will be. I don't wish to build anything that will make it too tough to service the flue in the future, nor to clean out when necessary. Here's a joke for you: I've never had to clean the chimney or the rusted out flue pipe. These hasn't ever been any creosote buildup in all that time.

 

[sean donato]

Also true of manufactured flue pipe, don't cheap out, pipe with non stainless outer wall exists, avoid it like the plague.
Fun fact; stainless steel is corrosion RESISTANT not corrosion PROOF, never use metal chimney brushes on SS liners. scratches start/harbor corrosion.

I'm only currently aware of one mfg that offers a stainless inner and galvanized outer covering. Which I would never reccomend to anyone. One it looks like crap and two it's going to rust the entire system out eventually.
corrosion resistance is only as good as the person taking care of the pipe. Regular brushes and even mild steel fasteners will allow stainless to rust. Following the (every) mfgs recommendations you will never have a worry about rust/ corrosion build up with any quality stainless pipe.