Combustion air damper

Icarus

Condition...~85k cast iron hot water nat gas. boiler in a basement oversized boiler room that is heated only by lost heat in piping and the boiler itself. The boiler is seldom used except on the coldest days (Pac. NW, pretty mild climate) as the client uses mostly wood stove.

The boiler is equipped with a motorized stack damper above the draft hood to keep cold air from running through the boiler while idle. Combustion air is provided to the room from a 10”x10” hole in the rim joist directly into the room. My question is this. Is there any advantage to installing a duct from the air inlet to direct air directly to the mouth of the boiler, and secondly is there some merit in adding a furnace controlled stack damper to shut the air off when the burner is not calling for heat, reducing cold air infiltration to the boiler room (and the house).

Finally, can I accomplish this simply by adding a barometric damper to the intake duct, such that it would only open then the room experiences negative pressure. (The boiler room is quite large, several hundred square feet, and the house is ~100+ years old so I don’t believe the boiler would ever be starved for air.

Any advice appreciated. Thanks

Icarus

EdTheHeaterMan

Your proposal has merit, however, it may be unnecessary. Calculate the total BTU capacity of all appliances in the basement, including clothes dryer if any. Then calculate the total volume of the space the equipment is in. You may be describing an unconfined space that needs no additional vent for combustion air.

But since you have a 10x10 existing opening, you can duct it to the boiler area and you can operate a motorized damper with an interlock switch to prevent the burner from operating if the damper is closed.

The barometric type idea is also doable however the damper should probably at least 16" round and have very little counterbalance weight to close it. That said the wind from outside could easily blow it open.

The electric motor damper is easily controlled, The barometric not so much. You would operate the combustion air damper in the same control design as the existing exhaust vent damper. Assuming the existing vent damper is properly wired and controlled with a fail-safe that does not allow the burner to operate if the damper fails to open.

Icarus

Thanks for the advice. As I said, I think the room is plenty big enough that no outside air would be required. (There were two Nat gas 50 gal water heaters in the room as well, but they are being decommissioned) I’m sure the vent was put in to satisfy the inspector. I like your thinking regarding the barometric vent. I think I will do a 6” duct, with a motorized damper to reduce infiltration and allow the boiler to draw extra room air if needed. I need to check to see if the stack damper is actually connected to the gas valve, fail safe. If not, we will abandon the vent. (This whole boiler will be replaced with a heat pump boiler in the next few years, so the few % savings from the damper is not worth replacing it.

Icarus