Thread: Combustion Air - Request for interpretation

Inspection today - a Navien on demand water heater was installed for the entire house. NR-240 running on natural gas with a maximum input of 199,000 BTU/HR. The wall mouted unit was installed without a direct vent. Combustion air is pulled in from a small utility room of about 60-72 sf and 8 foot ceiling. One end of the room is next to the garage. The other end, nearest the WH, opens into the living area of the lower level of the split-foyer home. With a volume of 480-576 CF but both doors are normally closed, I concerned with the adequacy of the combustion air supply. The installation manual implies that there should be a 14x14 vent opening into the main lower level room. I'm a little concerned about the combustion air but I thought I would ask some plumbing gurus for input on the subject. I attached a picture of the installation manual section on combustion air venting and a picture of the mounted unit with the front cover off.

This house was built a year ago and had a lot of cheap jack-leg trades working on the house. It would have cost maybe $50 to install the WH as a direct vent appliance.

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So why not recommend a fan in a can and be done with it. First instinct is usually right.Why get in too deep, are you an hvac pro? No , so leave it to them.

I would definitely say to put a grill in the door to the house. Given the age, it's hopefully relatively tight and the garage door should be weatherstripped. The basic rule for furnaces and water heaters is 1 sq inch interior make up air per 1000 btu and you maybe have 16 making it past the closed door.

The label you are pointing to says that it needs 199 sq in. MINIMUM, round that off to a 10" x 20" opening for net free opening.

Put louvers, metal or wood, or even a screen in that opening and the opening suddenly needs to get larger, much larger, and if the louvers are wood then the opening needs to become HUGE (wood louvers are deemed to provide 25% net free area, so a 800 sq in. (put a 40" x 20" opening in the door?) opening would provide the required minimum of 200 sq in. net free area.

Thanks guys. You confirmed my suspicion.

Originally Posted by Ben Tressel

I would definitely say to put a grill in the door to the house. Given the age, it's hopefully relatively tight and the garage door should be weatherstripped. The basic rule for furnaces and water heaters is 1 sq inch interior make up air per 1000 btu and you maybe have 16 making it past the closed door.

Ben, Are you saying to put a vent in the garage to house door and bring in combustion air from the garage?

I would recommend this unit be tested with combustion analysis, clock the gas meter because I'm not confident a negative pressure gas valve can supply 200CFM through a 1/2" steel pipe at any length. I'd also soap bubble test every joint in that pvc venting followed by a combustion analyzer. I would recommend MUA from outdoors. I would use a combustible gas sniffer in the exhaust vent to see if there is unburned fuel.