BTU loss to chimney masonry

[SculptureOfSound]

Hello!

I'm curious if anyone has some ballpark figures of BTU loss of an insert (or stove inserted into a fireplace) to the surrounding masonry.

I realize there are a lot of variables but i'm just trying to get very rough ballpark figures. I'm more concerned with an external chimney as that's what I have and also heat loss to the masonry on an internal chimney is not nearly as "lost".

So, if your insert is cranking along and putting out say, 30,000btu an hour, and temp is somewhere between 0f - 32f, roughly how much is lost to masonry. Let's also assume a block off plate IS installed. Are we talking like 1k btu an hour, or more like 10k?

 

[Simonkenton]

My fireplace pictured at left is an interior fireplace. That is, the back wall of the fireplace is the interior wall of the bathroom.
And if you run the fireplace for five hours, that back wall gets up to 105 degrees and stays there! For about 24 hours.
Massive, steady heat running into the bathroom, also, out the side walls.

So, you are correct, with an external fireplace, which most of them are, you are going to get massive heat loss to the outside.
I don't know how to measure it but it is a lot. Exterior masonry fireplaces are fatally flawed because, even if you are not running a fire, or a wood stove in there, that 10 degree outside cold is constantly running through the masonry, and having to be heated up by your house interior.

 

[shoot-straight]

i have an exterior brick fireplace. I took a thermal camera to see if I could see any hot or cold spots on mine both inside and out. I didnt see what I expected to see, which was not much.

That said wholeheartedly agree with all thats been said. Even thought of wrapping my and insulating my lower chimney because of it. Oh, if only I could have built my house again knowing what I know now.

 

[Pertzbro]

I have external chimney with insert. Original red brick chimney from 1922. 25 feet high.

I went out at -20degrees F to test after running all day with one of these:




I could not notice any significant difference between by siding and the brick. There was one spot that was 5-10 degrees warmer than anywhere else. I do not have a block off plate.

 

[SculptureOfSound]

I don't have an insert yet but with the fireplace doors closed the bricks in the firebox get mighty cold. Must be only 40 degrees or so. That's a significant constant infiltration of cold...just not sure what it even roughly translates to in btus.

For now I've Roxul'd the firebox and that makes a big difference. Now I'm just trying to decide if I pick my insert based on leaving enough room to still insulate. That would mean a smaller insert with less heat output, but also less heat loss.

 

[Pertzbro]

For now I've Roxul'd the firebox and that makes a big difference. Now I'm just trying to decide if I pick my insert based on leaving enough room to still insulate. That would mean a smaller insert with less heat output, but also less heat loss.

I would not downsize the insert because you have insulation. You'll want the biggest unit you can fit even if that means taking out the insulation. Things like size of logs, amount of logs will have a direct correlation between heat output and burn times. It can be frustrating trying to stuff a firebox full of wood that is too large or odd shaped that you cannot get the full potential of the firebox. Bigger is quite literally better for an insert.

 

[shoot-straight]

I have a freestander in mine now- it has a high lintel specifically for a freestander. I have considered trying an insert to see if it heats better.

 

[mellow]

Concrete board can be used or Micore and that will not take up much room. It will help keep the heat IN.