# Making an old fireplace insert more efficient



## Daniel Holzman (Mar 10, 2009)

I have a recently constructed Jotul fireplace insert. As you noted, this insert comes with a secondary combustion system, which is the horizontal pipe near the top of the firebox with holes in it. The purpose of this pipe is to allow secondary combustion of the gases which come off the wood, which in fact increases efficiency and reduces particulate emission. As to how Jotul came up with their design, I assume there was a lot of engineering that went into the system, not something I would care to guess at. The engineering would include the size of the pipe, the size of the holes, the placement of the holes, and the connection to the air inlet system.

The actual efficiency of a stove or fireplace insert is driven by thermodynamics, which sets a MAXIMUM efficiency of a stove at (T1 - T2)/T1, where T1 is the absolute temperature of the gas within the chamber, and T2 is the exhaust temperature. As you can see from the equation, the hotter you burn, and the cooler you exhaust, the higher the efficiency.

There is a minimum outlet temperature you must maintain when burning wood, else you get creosote. Most stoves set the outlet temperatures somewhere around 500 degrees F (use temperature 957 degrees Rankine for efficiency computations). The maximum burn temperature is not much greater than 1200 degrees F (1670 R). This limits efficiency to around 45 percent, however actual efficiency is lower since there is a considerable amount of water created during burning, the heat of fusion of which is lost since wood stoves are not condensing furnaces.

The most important ways to maximize efficiency of a wood stove are :

1. maximize burn temperature by controlling the air/fuel ratio

2. Minimize generation of water vapor by burning dry wood

3. Use secondary air to burn the volatile gases. Failure to burn these gases increases pollution, and wastes energy by reducing burn temperature.

4. control heat loss out of the firebox by insulating the firebox. This helps to maximize burn temperature.


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## roger erickson (Oct 10, 2012)

*colony hearth*

Hello,
my name is roger, and I am a new member, I read your thread on colony hearth fireplace inserts of 2010, hope you are still reading this stuff, looks to me like you and I have the same model, I think my model number is or was C-200 something like that, anyhow, the biggest thing I am interested in and hope you may be of help, if getting a replacement for the opening, as there is no window at all, I have made inquiries into finding some sort of welder to do the job, but that seems kind of futile, have you had anyluck with finding a way to get a window, or did you ever find a replacement for that part of it, let me know, thanks. Anything of interest about old earth stone northwest will be appreciated, 
Roger d erickson
silvercreek, wa


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