Oil Fired Boiler and Gas Water Heater Venting in Same Flue

[Nick Ostrowski]

At my inspection today, I saw an oil-fired boiler and gas water heater venting into the same flue. The flue was relined in 2008 for the oil furnace and has the flexible steel liner. The gas water heater was just replaced in late 2011 and it appears the house has had this combination running for some time. The water heater flue pipe enters the chimney above the furnace flue pipe.

Is this setup allowable?

[Nick Ostrowski]

Anybody? Bueller???

[Bob Harper]

Oil and gas can be common vented if:
The common vent is sized properly
the vent works without spillage of flue gases
both units have primary safety controls

[Nick Ostrowski]

Thanks Bob.

[Ken Amelin]

Oil and gas can be common vented if:
The common vent is sized properly
the vent works without spillage of flue gases
both units have primary safety controls

Bob,

Can you expound a bit. What primary safety controls should we be looking for on both the oil fired boiler and the gas fired water heater?

[Bob Harper]

Ha! I was wondering when someone was going to ask that question. The sad part is, the code does not spell it out. What it means is some type of safety control that if the chimney is not venting properly, the safety control will shut off the burner. This is often in the form of a spill switch wired to the fuel control. Some AHJ's will count the cad cell on an oil burner as sufficient even though it performs flame proving but not vent proving. You could say the same about the safety pilot on the WH.

On drafthood equipped water heaters, some contractors who are trained by NCI will remove the draft hood and replace it with a bullhead tee with a double acting barometric damper and spill switch to a thermocouple interrupter so the pilot drops out upon fault. I've also wired a separate 24volt switch from this tee to the oil burner's primary control plus one on the oil appliance's barometric damper.

BTW, if you common vent a CAT I fan assisted furnace with a draft hood equipped water heater and the chimney becomes blocked, the furnace will happily continue to fire and vent out the draft hood into the home without tripping any of the three safety controls on the furnace: pressure switch, spill switch and high limit switch.

HTH