does not work in auto.. its a utica propane boiler-/hydronic system..

i was goin to swap out the thermostat from upstair (which works) for the inoperative one, to see if thats the problem, but my buddy said he does not think thats the problem... he thinks it mite be the 'aquastat'.. if it is this, how diff. is it to change myself, or do i need the propane co. to do it..thanks