A local solar installer is telling me that I shouldn't put my battery box (24 two-volt cells) in my utility room since that room also has a propane tankless hot water heater. I had thought that the 2" vertical hydrogen vent from the box to the outside would be sufficient to avoid any danger of explosions. Should I take his advice and find somewhere other than the utility room for one of these two utilities? What do most people do?

Thanks,
...Terry

If your HW heater is direct-vent/sealed combustion with no combustion-air exchange with the room air it should be fine. If yours isn't, some models can be retrofitted for sealed combustion with kits available from the manufacturer. (Most Takagi HW heaters that use room air for combustion are retrofittable.)

That makes sense, Dana, and it is a direct-vent heater with an external air supply. But it isn't documented as 'sealed combustion', and the owner's manual says 'Do not store or use gasoline or other flammable vapors and liquids in the vicinity of this or any other appliance'. It's a Bradford White 150, by the way.

I suppose one option is to pay more by buying gel batteries -- the extra cost would be a trade-off with the cost of building a battery closet in the utility room with an outdoor air supply, and I'd get the no-maintenance benefits of the gel batteries. Anyone out there have good experiences with gel batteries?
...Terry

Get AGM batteries if you want no maintenance. In my experience, much better than gel.

Another option with flooded batteries is using Hydrocaps, a power vent at the top of the box/cabinet controlled by the charge controller, and fresh-air inlet at the bottom of the box/cabinet. This combo would minimize risk in a room with a direct-vent gas heater, no?

You guys seem to only be seeing half the potential problem, yes, you should be concerned with the H given off by the batteries, and the H is lighter than air, so an unobstructed vertical route out of the battery box would provide venting there, however, Propane is heavier than air. While your appliances may never leak any gas, you have to provide a "just in case" route for propane to be vented. The gasses from either source have the potential for explosive ignition by either source although the batteries alone have little potential, is is in the maintenance and disconnection for maintenance that the ignition spark is possible.

I would recommend that if you have to have them in the same "room", build a sealed box around the battery area, with a fresh air in AND out vent, then vent the room at a low point and relatively high point for the water heater.