Obviously I'm getting carried away here, ...but there you go. I'm me!
A couple of years ago I installed a Precision Temp ShowerMate
propane water heater in the boat. This replaced my wall-mounted "illegal" (according to ABYC) Paloma point-of-source unit. What makes the ShowerMate acceptable to ABYC is interconnects that won't allow propane to flow unless there's ignition, water is flowing, and the forced draft exhaust is functioning.
The unit itself is about the same size as the 110V, 5 gallon water heater I used to have in my lazarette and is installed in the same spot. The old unit finally heard my plea for relief from the 110V load and sprung a leak by chewing itself apart from the inside.
The ShowerMate wasn't difficult to install, but required some troubleshooting to get it to work properly. This resulted in adjustment to the diameter of the propane lines which had gotten too long for their own good. Another adjustment, which is the real subject of this blog entry, was to reduce the resistance in the exhaust.
Skipping some of the details, suffice it to say that I had originally run the 2 inch stainless steel spiral wound exhaust duct to the old thru-hull for my genset exhaust. That unit had gone kaput a few years ago and I just ripped it out, leaving the hull valve in place. As you might know, the opening in a hull valve even for a 2 inch diameter hose is substantially less than 2 inches in diameter, so it constituted a serious restriction in the water heater exhaust.
To solve this problem, I replaced the thru-hull with the water box pictured above. The short pipe coming into the bottom from the left attaches to the thru-hull fitting and the long pipe coming into the bottom from the right attaches to the exhaust duct. The idea here is to allow free exhaust flow, but restrict incoming water to the water box itself. When water does come in through the thru-hull, it has to fill up the entire box before it can flow into the exhaust duct. Nifty, huh?
Again, this design was executed for me by Hjorth Consulting in Ballard, WA. It was expertly made out of 316 stainless steel. Here's a pic of the installation: