Ceiling vent grate near water heater firestop.

[Killer95Stang]

Quick question... The picture I attached shows the ceiling of a small 29"w X 29"d water heater closet that has access from the exterior of the house. The vent exited directly through the center with a metal firestop. It also had two of these grates right next to the vent that were probably there to allow hot air to escape up into the attic. I now plan on installing a tankless in the same closet venting through the same center hole (replacing existing with proper vent material), but wonder if I need these grates up in the ceiling? Does it pretty much make the firestop useless being so close to the vent? The whole closet will be covered with proper 5/8" drywall, before I put the new water heater in. So what do you think? Does the extra heat of a tankless require that I keep the venting grates up there to keep the heat from collecting in the ceiling?

[jadnashua]

Any appliance with a burner needs combustion air. If this is in an area where things can freeze, it becomes tough in an unheated compartment. Providing adequate air may mean in between firing, things can get cold enough to freeze. Some tankless systems have antifreeze programs that will turn them on to prevent them freezing up. But, this may not protect the inlet and outlet pipes. A tank has a large volume of heated water, and will turn on to keep it at the desired temp. A tankless has essentially no water heated in side it unless there is a call for heat, or it activates its antifreeze program. The thing may work, but it will be firing fairly frequently if the area is cold, even when you aren't calling for any hot water.

For maintenance, normally, you'd want more space around the thing.

[Killer95Stang]

I think I need to explain my situation a little better.

- I live in sunny socal, where we didn't see temps below 40 degrees this year. Freezing is not an issue.
- Standard 40 gallon gas fired tanked water heater has been in this closet for 50 years ( since house was built).
- I'm replacing tanked heater with Rheem direct fired condensing tankless unit.
- tankless draws combustion air through a sealed concentric vent pipe. Unit also exhausts through the center of the same vent pipe.
- closet will be fully insulated before the drywall goes up.
- door to closet is 20" shorter than the ceiling, so a pocket of hot air will be held as the air rises up.

The more I think about it, the less I'm worried about it. I just wanted to know if I should remove the grate from the ceiling.

[hj]

EVERY gas water heater NEEDS one vent near the floor AND one in or near the ceiling. Their purposes are to admit combustion air and VENT any gas leaks or combustion fumes.

[Killer95Stang]

EVERY gas water heater NEEDS one vent near the floor AND one in or near the ceiling. Their purposes are to admit combustion air and VENT any gas leaks or combustion fumes.

Thanks hj,

I'll leave the vent up in the ceiling when I drywall and make sure it doesn't get plugged up when I blow in insulation recover the areas where I removed the ceiling. I can probably form a sheetmetal sleeve to keep the insulation away from the vent opening.

Currently the door to the closet has openings covered with perferated metal at the top and bottom which will allow outside air to be drawn in. With the Tankless being a sealed unit with all combustion air being drawn in from the intake roof vent, I don't think I would starve the unit without vents in door and attic. The tankless has no openings anywhere on the case. I just don't want the radiant heat from the unit itself to be an issue..

[Dana]

EVERY gas water heater NEEDS one vent near the floor AND one in or near the ceiling. Their purposes are to admit combustion air and VENT any gas leaks or combustion fumes.

How does a, "... direct fired condensing tankless unit." need combustion air to be supplied from the surrounding space?

Venting the closet to the conditioned space via the door would provide enough dilution to prevent explosive mixtures in the closet for anything but the most egregious of gas leaks, and make it more likely that minor leaks would be detected.

Random open hole leaks to the exterior just adds to the uncontrolled ventilation/infiltration, making for a less efficient building envelope from a heating & cooling point of view.