BTU losses to the hearth and chimney

Karl, I'll bump this to the top for you.

I have never run an insert but it seems reasonable there would be a benefit to adding the rock wool but I don't know how much.

You will get some heat loss with an insert to the fireplace because of the back of the insert blocked off to the room. You shouldn't get any more than a freestanding stove up the chimney, though, if you have a full reline and the throat of the chimney is blocked off and insulated. A blower on an insert will greatly limit the heat loss from the back by efficiently circulating a lot of the heat out into the room before it's lost through the second wall of the convection chamber.

I have several inserts in the family. I also have the Heatalator FP, but bolted the damper closed, and cut straight up though the sloped rear panel of the Heatalator so I could use 6" rigid liner all the way up.

I would guess that one is losing 30% minimum. a major portion of the radiated heat from the sides, back, top, and bottom is leaving the living area.

For that reason I added a small marble hearth extension, and installed a stove in the forward portion of the hearth, partly into the FP opening. I also added about 40 bricks, stacked 6" or so from the back of the FP. They get pretty warm, and help keep a bit more heat. I should probably add a bit of fiberglass insulation behind them.

I don't think I'd do another insert unless I was living a lot further South.

ATB,
Mike

What about wrapping the side and top of the insert with rock wool? I think this has been brought up before but people were worried about how this would effect the stove?

There are some peolpe that have done the insulating thing. I'm am concerned about the affect that insulating could have on the stove. I think it would violate the UL cert by not following the install guide.

I also know a few folks that run stoves on an extended hearth, then clean the pipe and set them back in the most of the way FP opening like an incert all Summer.

Todd said:

What about wrapping the side and top of the insert with rock wool? I think this has been brought up before but people were worried about how this would effect the stove?

Click to expand...

That would be a very bad idea. If the insert is unable to radiate and lose heat it could be severely damaged. The best thing to do is to install an insulated block off plate and use your insert's blower.

Somewhat related, insulate your liner to prevent it from losing heat and cooling your flue temps.

cmonSTART said:

Todd said:

What about wrapping the side and top of the insert with rock wool? I think this has been brought up before but people were worried about how this would effect the stove?

Click to expand...

That would be a very bad idea. If the insert is unable to radiate and lose heat it could be severely damaged. The best thing to do is to install an insulated block off plate and use your insert's blower.

Somewhat related, insulate your liner to prevent it from losing heat and cooling your flue temps.

Click to expand...

How is this a bad idea? I'm just curious since my PE is wrapped with rock wool behind the surround. I'm trying to heat the room, not my chimney. Laws of thermodynamics would say excess heat retained by insulation would go into the room. Curious to hear other opinions through.

BrotherBart said:

With the blower running on low or above I don't think you are losing a thing. What would be lost to the masonry is being moved out into the room.

That's my story and I am sticking with it. Don't believe it? Go outside with an infrared thermo and shoot the chimney right behind the stove when it is burning. I don't have an insert anymore but have that 30-NC buried to the hilt in the fireplace and the back of the fireplace firebox has never measured more than 210 degrees. With no insert shell around it and the blower not running. And the chimney bricks outside are the same temp as the pile of bricks stacked next to it that I am gonna use for a sidewalk.

Click to expand...

I'm thinking the same thing, I always run my blower on high and from doing calculations on heat-loss if it cost me 30% more to run an insert I would be burning a heck of a lot more wood than I am now. I do have a blockoff plate and rock wool as previously stated though.

There are too many variables here to make an exact determination, but it could be said that, in general, installation of an insert into a exterior masonry fireplace can cause some substantial heat losses to the outside.

Many factors are at play, some have been mentioned. Keep in mind that most exterior masonry fireplaces lose heat to the outside even when nothing is burning in there! That is, the rear wall and other parts can, depending on the exact construction method and outdoor temp, lose much more heat than an insulated wall.

Let me address the blower part of the deal. There are various parts to this. Obviously, a blower will move some of the heat out of the air chamber which otherwise may have soaked through and then been lost to the chimney structure. However, there is another factor - when you blow air around the outside of a firebox, it cools the firebox and slightly quenches the fire. It also acts to hold heat into the stove - which then end up going up the chimney. Either or both of these factors can come into play. Wood stoves are generally not engineered to perfection in this manner - that is, the designers often make a nice stove and then convert it to an insert. I don't think many of them actually test their inserts into cold fireplaces - although when I visited Travis Industries, they had a cold test chamber (they were using it only for their built-in FPX).

It's a total guess, but I would still put that 15-30% number out there for a exterior fireplace in a very cold climate (New England). This figure would decrease as the outside temp rises. Keep in mind some of the same would pertain to a stove on the hearth or in the fireplace.....so the comparison is against a freestanding stove with exposed stove pipe and insulated chimney inside the home.

On the other hand - if the fireplace is interior, the entire setup might end up being more efficient than the norm, as the masonry soaks up and later releases much of the heat from the insert and flue.

IMHO,

Convective portion of the BTU heat output would be minimally lost with an insert with a fan running. But would argue that the radiant heat loss would be significant comparing an insert embedded in a fireplace vs the same free standing model .