Maybe bottom block off plates aren't always a good thing ...

Do you have a bottom block off plate on your chimney, and do you have problems with downdraft?

  • I do have a block off plate, no downdraft problems

    Votes: 5 62.5%
  • I do have a block off plate, and I do have downdraft problems

    Votes: 1 12.5%
  • I do not have a block off plate, no downdraft problems

    Votes: 2 25.0%
  • I do not have a block off plate, and I do have downdraft problems

    Votes: 0 0.0%
  • what's a block off plate?

    Votes: 0 0.0%

  • Total voters
    8
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iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
Hello everyone,

There was the first bit of chill in the air this morning and I looked at my woodpile as I went out to work and realized that it won't be long before I'm feeding it into my stove. It also reminded me that there was a topic that I meant to put out on the forum over the summer and never got around to it ... so here it goes.

Before I jump in, please understand that my goal here is simply to share my experience with attempting a block off plate on the bottom of my chimney. I'm sure some will see my comments as being controversial - that's ok, but I'm not trying to start a fight ... just a reasonable discussion on pros/cons of block off plates

I've had my Jotul F55 for two full seasons now, and its a great stove. My local dealer did the install, and by all accounts I would say they did a pretty good job. My stove is in the basement, attached to a 6" SS flex liner that goes up on old masonry terra cotta lined chimney that runs up the side of the house. From stove to the top of the stack it is about 30 feet. My stove also has an outside air kit (code in my area).

I never had an issue with poor draft in the first year and half of using my stove (on a daily basis in the winter), but I'm a tinkerer, so I like to try things. I read a lot on the forum about the good things a bottom block off plate can do improve draft, so I decided to give it a try. In January, I stuffed a bunch of Roxul up the chimney surrounding the liner and then crafted a simple block off plate and fastened it to the seal up the opening around the bottom end of the liner.

The next day, I went to light up my stove and it was impossible to light. I use the little fire starter squares (the Rutland ones) and normally they light up great and I have a roaring fire in half an hour. This time, I couldn't get the darn thing to stay lit. I decided to put some crumpled up newspaper in the stove and try again. Even that didn't want to light and when it finally did I was sorry - smoke started billowing out of the stove. I closed the door of the stove, but even that wasn't enough - smoke started coming out of the bottom of the stove where the air intake is! The smoke detectors went off and I had to open my windows in the dead of winter to air out the house (good thing we also have a gas furnace). I realized at this point that my stove had somehow developed a serious downdraft condition.

I have heard some people blame weather conditions for downdraft problems, which I don't doubt could happen, but I don't believe that was my issue. My living room fireplace had no draft issues at this same time, and it was a pretty typical stretch of winter for us with temperatures just a little below freezing... and all the neighbors had smoke going up from their chimneys

I tried again a few times over the next few days with the same result. I could actually feel the cold air coming down my chimney when I put my hand up to the baffle at the top of my stove. I tried a few tricks to get the draft moving upwards including putting a hair dryer up to the baffle and burning alcohol jelly to pre heat the stove. The alcohol jelly did work - but it took at least 45 min to generate enough heat to get the draft reversed.

I decided to speak to the guys at my local stove shop and see what they could recommend. They found it strange that I was having such a serious downdraft problem since it wasn't common on their installs. Then I admitted I had added the block off plate and they immediately said that was my problem. Given that my chimney ran outside the insulated envelope of the house, it got too cold when the stove wasn't being used, allowing for ideal downdraft conditions. They said in those situations, it is best to not have a bottom block off plate as it allows just enough heat from the house to get at the chimney to prevent most downdraft problems. They recommended that I remove my block off plate and see if that solved the problem.

I had been using my stove in this condition for almost a month and was frustrated with having to spend so much time preheating it every time (I usually burned one load of wood a day). I decided that I would give their suggestion a try and removed the block off plate and all the Roxul. The next day, the difference was apparent right away. I could no longer feel the cold air rushing down my chimney and I was able to get the fire going without any preheating or smoke coming back into the house. So I can say without reservation that my downdraft issues were primarily caused by the addition of the bottom block off plate.

I realize that without the block off plate and Roxul I am losing some heat from the house up the chimney, but in my case its a trade off I am willing to make as I don't want to risk creating downdraft conditions. Certainly my less than ideal chimney setup (outside the thermal envelope of the house) doesn't help either, but I have never once had a downdraft condition without the block off plate. I suppose my final conclusion would be that block off plates may work well for some, but consideration needs to be given to the individual setup and the potential problems they can create too.

Just for fun, I started a poll about block off plates and whether or not people have any problems with down draft.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Is your liner insulated like it should be? Regardless I see this as an example that shows that blockoff plates work by keeping the heated air out of an outside chimney where almost all of that heat will be lost.
 
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NoGoodAtScreenNames

Feeling the Heat
Sep 16, 2015
313
Massachusetts
Does a basement install with an oak mean that there is less temperature difference to help the stack effect than when the stove is in a probably warmer part of the house. I don't know? Far from an expert here...
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Does a basement install with an oak mean that there is less temperature difference to help the stack effect than when the stove is in a probably warmer part of the house. I don't know? Far from an expert here...
Yeah I suppose it could but I have never seen that cause an issue.
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
SS flex liner - so no, not insulated top to bottom. There is of course a block off plate on the top and Roxul up there too, so the top couple feet are insulated.

I agree some heat is lost out of the chimney, but like I said, I am ok with that compared to dealing with vicious downdraft. Someone who is concerned with keeping every bit of heat in the house may not be I suppose.
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
SS flex liner - so no, not insulated top to bottom.
ss flex liners can and should be insulated. Having it insulated will mean that first whatever heat that gets into the stove and up the stack will be in there longer and help with draft. It will also mean that preheating will be very easy.

Also the fact that the liner is not insulated means it is not installed per its ul listing which means it is not code compliant. And unless the chimney has the required clearance to combustibles it could be a fire hazard.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Oh I just noticed you are from canada. I honestly do not know the canadian codes so disregard the code comments I made they may not apply.
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
Also the fact that the liner is not insulated means it is not installed per its ul listing which means it is not code compliant. And unless the chimney has the required clearance to combustibles it could be a fire hazard.
Well I do recall looking at the install instructions for the SS liner and there was no mention of insulation being required, just that the clay liner it was being installed in was not damaged. It was also inspected by the city building inspector and a separate WETT inspection and everything was fully compliant and to code (this was required for my insurance). The clay liner is 10"x10" and the SS liner inside of it is 6" round, so there is definitely some air space in there.
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
ss flex liners can and should be insulated. Having it insulated will mean that first whatever heat that gets into the stove and up the stack will be in there longer and help with draft. It will also mean that preheating will be very easy.
Perhaps this is true - if the liner is insulated top to bottom then it may not get quite as cold and it may preheat sooner, but its not a change I am prepared to spend money on right now. I am guessing a majority of people have single wall SS liners like I do, but I could be wrong (maybe somebody should start a poll).
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Well I do recall looking at the install instructions for the SS liner and there was no mention of insulation being required, just that the clay liner it was being installed in was not damaged. It was also inspected by the city building inspector and a separate WETT inspection and everything was fully compliant and to code (this was required for my insurance). The clay liner is 10"x10" and the SS liner inside of it is 6" round, so there is definitely some air space in there.
Like I said I realized you were in Canada after posting that and I do not know Canadian code. But in general it is more stringent than us code. What liner was used?

But regardless of code issues The insulation on the liner would help your situation allot. I am not telling you you need to tear it out and insulate it but you were asking about what could help with your issues and I gave you an answer. Do with it what you will
 
Last edited:

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
I am guessing a majority of people have single wall SS liners
Again single wall ss liners can and should be insulated. And the performance gains will be more evident in an exterior chimney like yours.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
What I meant was is it possible they are getting their make up air from the outside of the liner which the plate is stopping and there isn't enough natural stack effect from the oak to offset the loss?
My original answer still applies And I am sure the top plate was sealed before so they were not getting makeup air in that way
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
My original answer still applies And I am sure the top plate was sealed before so they were not getting makeup air in that way
Yes, top plate was well sealed of course, I saw that happen.

I suppose I would consider a full top to bottom insulation of the chimney if it could be done relatively cheaply and easily, but I am not keen to spend time and money on something that I can't easily reverse given my previous problems.

I can't recall the liner brand off the top of my head, but I think I still have the paperwork around somewhere ... Jotul specifies in their manual the specs of liner that are acceptable and the liner did meet the spec.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
I can't easily reverse given my previous problems
Why would you want to reverse insulating the liner there is absolutely no down side to it.

I can't recall the liner brand off the top of my head
The only reason I ask is because of what you said about only requiring insulation if the clay is damaged. Which is not part of any listing here or Canada. And if it is in their instruction I would get ahold of them and ask where they came up with that requirement. I am not questioning that you saw it there but if I am going to ask them about it I need to see it myself.
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
Why would you want to reverse insulating the liner there is absolutely no down side to it.
That's what everyone said about installing a block off plate too :)
 

iluvjazznjava

Member
Oct 26, 2014
168
British Columbia
The only reason I ask is because of what you said about only requiring insulation if the clay is damaged. Which is not part of any listing here or Canada. And if it is in their instruction I would get ahold of them and ask where they came up with that requirement. I am not questioning that you saw it there but if I am going to ask them about it I need to see it myself.
No - the requirement was that the clay liner not be damaged and for it to be clean before the SS liner was dropped in. I would have to go back and check, but I don't remember the liner install instructions mentioning insulation at all.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
That's what everyone said about installing a block off plate too
And I still don't see keeping air you have already paid to heat in your house as a downside. But regardless there are no such possible downsides to insulating your liner. The only possible down side to an insulated liner is if you have an internal chimney with an un insulated liner that masonry structure will heat up and give off a little of that heat to the house. But that does not apply to you.

o - the requirement was that the clay liner not be damaged and for it to be clean before the SS liner was dropped in. I would have to go back and check, but I don't remember the liner install instructions mentioning insulation at all.
Well that makes even less sense many times you line a chimney because the existing liners are damaged. Are they saying that their liner cant be used to repair a chimney with damaged liners?
 

jscs.moore

Feeling the Heat
Sep 9, 2015
291
Eastern PA
ss flex liners can and should be insulated. Having it insulated will mean that first whatever heat that gets into the stove and up the stack will be in there longer and help with draft. It will also mean that preheating will be very easy.

Also the fact that the liner is not insulated means it is not installed per its ul listing which means it is not code compliant. And unless the chimney has the required clearance to combustibles it could be a fire hazard.
Hope a block plate doesn't cause any draft issues. I have a 28ft exterior chimney with an insulated liner (Olympia pre-insulated) and have always had a very strong draft, no issues at all. I have read many, many posts on the benefits of a block off plate for exterior chimneys and just had the dealer install a BOP about a month ago. My insert is in my living room (not in the basement) so I hope I don't experience any draft issues this season because that definitely wasn't an issue for me last year. My issues was trying to keep the insert cranking out enough heat when it got really cold (20s and below). I know part of the issue was the wood, but I really believe the primary issue was losing heat to 28ft of masonry?
 

Highbeam

Minister of Fire
Dec 28, 2006
17,472
Mt. Rainier Foothills, WA
As I understand it you don't have to insulate the liner unless your masonry chimney has failed and is no longer safe. As in you don't have to insulate the liner. Of course, I would agree that it is a superior job to insulate the liner but not a requirement unless your masonry is bad.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
As I understand it you don't have to insulate the liner unless your masonry chimney has failed and is no longer safe. As in you don't have to insulate the liner.
Not true at all here there is absolutely no code or requirement of the ul listing that says anything about condition or even existence of a clay liner. It is all about clearances from the exterior of the masonry structure and installing per the ul listing.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
Hope a block plate doesn't cause any draft issues. I have a 28ft exterior chimney with an insulated liner (Olympia pre-insulated) and have always had a very strong draft, no issues at all. I have read many, many posts on the benefits of a block off plate for exterior chimneys and just had the dealer install a BOP about a month ago. My insert is in my living room (not in the basement) so I hope I don't experience any draft issues this season because that definitely wasn't an issue for me last year. My issues was trying to keep the insert cranking out enough heat when it got really cold (20s and below). I know part of the issue was the wood, but I really believe the primary issue was losing heat to 28ft of masonry?
I have been in the industry a long time and have installed hundreds of block off plates and have never heard this complaint before. I am pretty sure you will be fine
 

Highbeam

Minister of Fire
Dec 28, 2006
17,472
Mt. Rainier Foothills, WA
Not true at all here there is absolutely no code or requirement of the ul listing that says anything about condition or even existence of a clay liner. It is all about clearances from the exterior of the masonry structure and installing per the ul listing.
That's the point, the liner is not required unless your masonry chimney no longer meets requirements for a masonry chimney. The masonry chimney is the chimney and your liner is just a conduit. I, same as the original poster, had a masonry chimney with a non-insulated liner installed some time ago. The liner is optional for a reason.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,893
central pa
That's the point, the liner is not required unless your masonry chimney no longer meets requirements for a masonry chimney. The masonry chimney is the chimney and your liner is just a conduit. I, same as the original poster, had a masonry chimney with a non-insulated liner installed some time ago. The liner is optional for a reason.
Or the liner is required if the flue is oversized.

But if you install a ul listed liner you need to install it to the listings requirements. I have never heard of any liner being tested for use with wood with out insulation. Therefore you need insulation on the liner to maintain that ul listing which is required by code. Just because you had one installed uninsulated does not mean it meets code requirements.

Can you show me in the code books where it says anything about the condition of the clay liners?
 

Highbeam

Minister of Fire
Dec 28, 2006
17,472
Mt. Rainier Foothills, WA
I have never heard of any liner being tested for use with wood with out insulation. Therefore you need insulation on the liner to maintain that ul listing which is required by code.
Okay, you are making a great leap there. I see what you did. Because you have never heard of a liner that doesn't require insulation to maintain the UL listing you "assume" that insulation is required for all liners to meet code. The code you reference is just that all UL listing requirements be met. Ah, that's a mistake. There is no code that requires an insulated liner for a fully functional and legal masonry chimney.

There is a parallel thread going on as well where you are the only one making this insulation requirement claim. Perhaps what you really mean to say is that insulation isn't required unless your particular liner brand requires it in that liner's manual.

https://chimneysweeponline.com/ffwrapwhen.htm
 
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