I had two options when it came to deciding if I wanted to go with a coil in my firebox, or on the outside of the stove, and I think it may be interesting to people to see why I chose the latter...going with an outside coil rather then the inside.
As someone else said, it is easier and gives you more square inches in which to capture the radiant heat. As is, in order to protect my walls from excess radiant heat I had to put up a tin heat shield, so my thoughts were, if I am going to put something up to protect my walls, why not install a device to help capture that radiant and heat and put it to good use? As is, I cannot hold my hand on my current heat shield without severly burning my hand...I believe there is plenty of radiant heat available to do what I want.
For me, that good use is plumbing my pre-heated water to my main heater...my propane boiler. Because I have radiant floor heat in a concrete slab, and a state of the art heating system, I only need to run water that is between 75-100 degrees in temperature. This water temperature is controlled by my computer system that monitors the heat I am losing in the floor, the heat returning from my heating zones and the temperature outside. It constantly changes according to how warm or cold it is outside. If the temperature outside goes down, the water must be warmer in order to make up for a concrete slab that is losing more heat; if the temperature goes up, the water flowing through my concrete slab goes down. In this way I have very even heat within my home and my boiler is only heating water a slight amount, thus saving btu's. It is because of this rather low water temperature that I don't want super hot water, I only want warm water, but lots more of it. An external coil will enable me to have that.
So what I want to do is, install a radiant shield with copper coils applied to the front of it, pump the water through a circulator pump hooked in conjunction with my boiler, and routed through a flow control valve, and supply my main propane boiler loop with hot water. After that my boiler system will take over and meter the hot water to my floors as needed.
In this way, I should have greater control over the heat emanating from my stove. My greatest fear is actually getting the water too hot and flashing to steam, this is another reason I chose to go with outside coils because I can more easily adjust the distance between the stove and the coil allowing me to fine tune a heat range.
Will this collected heat be enough to heat my 3200 square foot home? I severely doubt it, but by capturing wasted btu's in the form of errant radiant heat, I should be able to contain it, and pump it to where it is better used (my back bedrooms) and thus give me greater living comfort and spend less on propane.