I may be confused, (actually I usually am!) but I think there are some misconceptions about the Amtrol heat exchanger unit and how it is hooked to the boiler. Yanche was explaining how the controls should work, with one minor modification (the capability to run the circulator continuously) that should stay as it is. One word of caution, if you allow the Harman to produce hot water without limit you will have to install a tempering valve to protect your family
The attached sketch is out of the Amtrol manual (available at: http://www.amtrol.com/boilermate.htm
) and includes my suggested hook up based on three assumptions:
1. you are only running the boiler to heat water and in the event that the house is not warm enough.
2.our suggestions for air circulation in the other thread are going to cause the boiler to run even less than before
3. the control scheme for the circulator on the Amtrol will modified to run all the time as long as the Harman is fired up (perhaps a manual override switch)
A side benefit for keeping the water flowing through the coil all year will be a nice dry and non corroded stove during the summer months.
Also beatle, you said your expansion tank was 43 gallons, I think you might have expansion tank and your domestic supply tank in the Amtrol confused (see the drawing for it's probably current location on the boiler loop). You may have to increase the size on the boiler to account for the Harman, but I tend to think you won't have to.
Just my two sense based on my planning for installation of an Amtrol with my coal boiler. As I said before, nothing like a good science project to stir up the thought process!