room blower fan

when the stove gets up to temp the blower fan comes on high speed and will not go to low speed my stove is a 25-PDV built in 11-92 with the old blower control with dials

The blower is a variable speed blower but this problem is either a bad blower or bad control panel. The easiest way to test is to put a rheostat and an ac cord on the blower and see if you cab vary the speed by turning down the voltage. If that works then the control panel is bad. Otherwise the blower is bad.