Oh wow. I have a good answer for that
... On warm days, I will cut back the primary air so the fire will burn cooler. Then to maintain draft, I give her extra secondary air (over the fire air). This extra air doesn't contributed to much if any combustion. Instead it just gets heated and goes up the chimney. I've been able to cut heat output to hardly nothing on days that its 65 degrees and then at night just turn up the primary for heat again. This only applies to anthracite I dunno about other fuels.