Grandgracing, your post has got me to thinking, the reason your stove boils with the fan flap open all the way is it's getting natural draft with the blower off.(I'm sure your already aware of this, but it gives me an idea) I'm trying to figure out a way to hook up 2 thermostats, say for example, 1 thermostat runs a flap to allow natural draft, closes at 170 with maybe a adjustable screw so the fire always gets a small amount of air. The 2nd thermostat would control the draft fan, off at 150. It seems to me you will use less wood with a natural draft (instead of blowing half the heat up the chimney) But if the water temp continues to drop the fan will kick in.
I'm just trying to think how to accomplish this without overcomplicating it, also it would be nice to be able to adjust airflow amount for the fan and for the natural draft, independent of each other.
Fletchers setup could work if 1 thermostat controlled the flap he made, a second stat to control the fan. 1st stat opens flap all the time unless temp gets over 170, then shuts. 2nd stat shuts fan off at150? The only problem is this wouldn't allow you to adjust airflow independent of each other.
Also adding chimney length may help natural draft.
Maybe Fletcher has some thoughts on this idea?
Did you follow that? Sometimes I ramble a little