Gas valve change question


Old 03-13-08, 05:06 PM
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Gas valve change question

Swapped out an older Honeywell gas valve SV9500M2674 for a new SV9501M2528.

The furnace is a Tempstar (numg125) condenser type. Has a vacuum hose that runs to 1) switch on heat exchanger 2) fire box 3) gas valve, from the exhaust/condenser fan.

The new gas valve did not have a port for the vacuum hose pre-installed. The physical configuration is the same for both valves so I took out the plastic filler nut on the new valve and used the old brass fitting in what I assume to be the vacuum port. The furnace fires fine. If I remove the vacuum hose from the fan and try to re-fire the burners it fails to start like it is suppose to.....the as shipped Honeywell manual does not cover the use of this port so I am just a tad uncomfortable with my installation. Is this a proper use of this port on the new valve?
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Old 03-14-08, 07:24 PM
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You have what is called a dual-port or "differential pressure switch" (typical of direct vent units), which compares "and substracts" two negative pressure signals, one being the mild vac at the burner box, and the other being the deeper vac at the inducer blower.

I believe one could just do away with the connection to the gas valve, hence the lack of instructions with the new gas valve (not a problem to keep the connection, as you've done, but likely an exercise in futility). The installation manual of your unit only talks about the blower and burner box ports, there is no mentioning of the gas valve port. YES, I'm aware there's a tee by the burner box from which hoses go to the gas valve port, the burner box and the pressure switch.

Please note that when I say that the hose connection to the gas valve is likely inconsequential, I do not mean to say unhook and leave it hanging (the burner will immediately stop if that is done)...but I believe the tube coming from the switch could just connect to the burner box port and end of story. Try and let us know...

BTW, your unit's M/N is muGm (not nuMg)
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