Installing a Water Heater

When replacing the pressure water system on our project boat, we thought it would be fun to install a water heater. But where to put it? Like most early ’70s boats, our Norlin 34 lacks interior volume compared to modern boats. The need for the heater to be mounted below the engine’s heat exchanger (to prevent problems with coolant circulation) further complicated matters.

The only logical place was under the starboard settee, where the water heater would help offset the weight of the water tank under the port settee; the problem, once again, was space. I needed to find a heater that was less than 13 inches in diameter and less than 24 inches long. Many water heaters are as fat as they are tall, I thought this might be a deal-breaker.

A Web search turned up Indel Marine’s Isotemp Slim line of heater; these are skinnier versions of the company’s Basic range. At 21in long and 11 3/8in wide, the Slim 15 would fit under my settee with room to spare for plumbing connections. At 4 gallons, I reasoned its capacity would be adequate for its primary uses—hosing off the kids with the deck shower after swims in the chilly New England water, and washing-up after meals. The heater itself is skinned with 316 stainless steel and comes with a mixer valve to control water temperature, a pressure-relief valve and a drain.

Coming up with a secure mount for 23 pounds of heater containing 33 pounds of water was the next problem. I briefly considered glassing semicircular plywood mounts to the hull sides to cradle the heater, but soon found an easier solution. I strengthened the front and sides of the settee base with some lengths of 1in x 2in oak, and suspended the heater from two 1/2in x 2in aluminum bars that I bolted to the oak bearers. This is plenty strong enough, and keeps the heater clear of the hull.

I spent some time contemplating the best way to route the coolant-bearing hoses from the freshwater-cooled Yanmar 2GM engine to the water heater. The further the tank is from the engine, the more heat is lost though the hoses. Finally, I broke out the hole saw. I bought the requisite NPT fittings and 5/8in ID water heater hose online, and, after soliciting some advice from the Yanmar mavens on the forums, unbolted the two blank plugs in the engine cooling system and installed the hoses. I had to run them along the hull bottom, which is not ideal; better to route them where there is no chance they’ll be abraded or pinched, which can lead to total engine coolant loss. I plan to install shutoff valves in both hoses, close to the heat exchanger, to guard against this.

I’d replaced all the boat plumbing with Whale semi-rigid tubing, which had adapters to fit the NPT fittings on the heater. Connecting up the freshwater plumbing to the heater was easy enough; I put a check valve in the supply line to make sure that hot water wouldn’t find its way into the cold water line, and also fitted a shutoff valve so that I could bypass the heater if necessary. (The heater also has a 115V heater coil, but I have not yet hooked it up to the AC distribution panel.)

Then I switched on the water pump and filled the tank, opening and closing all three hot water outlets to bleed the system. Once I was sure there were no leaks in the boat plumbing, I fired up the engine and topped up the coolant reservoir to make up for the fluid forced through the hoses. Within 10 minutes I had hot water; yet another project ticked off my lengthy list.

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