"very old gas furnace" has anybody checked the heat exchanger?? and depending on the fittings and the rate at which the valve opens and closes...see to many veriables. it might be time for a new one, some antiques are hard to find parts for. but if the valve fits and has the same operating specs, you might be alright. maybe he should call a specialist to look the system over for leaks and other near future problems?? what has me concerned most is "very old"
