I have a customer that is smelling gas once and awhile. I did not smell it.. when furnace runs apparently he doesn't smell it. I checked all fittings in gas lines. My question is once gas valve turns off during operation and I put leak detector on the orifice going into burner and it bubbles. Maybe the gas valve is passing gas. Is it supposed to bubble with the left over gas in the manifold?
No, as soon as the gas valve closes any gas in the manifold would be burned off by the burner, at the end of a call for heat.
It sounds like the gas valve is bad and leaking gas.
Sometime gas valve passes a little gas through the manifold,u will find more such cases when furnace is not running maybe in summer.To check turn off heat at thermostat and then use leak detector at the air intake or in that area ,sometime u can sniff too.it may take some time to Icp up the gas bcoz it is little quantity.
I remember the odd fireplace gas valves doing this from brand new. The manufacturer said it was still within spec, to a certain amount. :-\
Hook up Manometer to inlet of gas valve... Shut off drop to furnace.. Observe manometer. Should not show any drop in pressure.