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Inside or outside intake vent?

Bruce Davis | Posted in General Questions on

New construction with ERV.  Gigantic utility room has two 97% DV gas furnaces & a 97% gas tankless water heater.  The two furnaces get their intake air from outside.  The water heater gets it’s air from inside the room.  I questioned it and the plumber said the room was so big, it was fine.

I’ve got two opinions.  One says while there is sufficient air to prevent any backdrafting, I’ll have negative pressure drawing air in though envelope leaks.

The other opinion is the negative pressure will be so minimal (maybe a bit while showering), that’s it’s nothing to worry about.  Adding the outside intake would also add another penetration to the envelope.  And, I have an ERV.

The plumbers will be there tomorrow anyway for something else, should I have them change the intake? Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bruce,
    Code mandates apply here. The International Fuel Gas Code requires one of the following options for equipment installed in a mechanical room: either (a) a ceiling duct to provide outside combustion air with a minimum of one square inch of free area for each 4,000 Btu/h input of gas-burning appliances in the room, or (b) a wall duct to provide outside combustion air with a minimum of one square inch of free area for each 2,000 Btu/h input of gas-burning appliances in the room, or (c) a fan that provides 1 cfm of outdoor air for each 2,400 BTU/h of gas-burning appliances in the room.

    By the way, the presence or absence of an ERV has nothing to do with this question. Your question concerns the provision of combustion air to an atmospherically vented appliance installed in a mechanical room. This isn't about ventilation air for a house.

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