Sharing a painful lesson I've learned over the past two months with my Universal Diesel 16 HP engine on my O'Day 30. I bought a new Force 10 water heater at the beginning of the season, with the engine coolant loop, and installed it the way the old one had gone in. All season, I've been having problems with the engine badly overheating after about 30-45 minutes of operation. Not being a mechanic, I did some basic fixes like changing the impeller, found the entire exhaust line was crystallized so replaced it and added a water cooler unit on the exhaust. Also replaced exhaust hoses, etc. When these didn't help the problem I cleaned out the heat exchanger unit on the engine and also the exhaust manifold. Still overheated. Nearing the end of a two week cruise on the coast of Maine, and getting frustrated with the engine, I disconnected the water heater loop. Voila. The engine worked perfectly. When I put the water heater back in the loop, the engine overheated. A call to Force 10 in British Columbia solved my problem. On the last page of their installation booklet in the smallest type size is a note, an important note, to install a separate loop directly from the exhaust manifold to the heat exchanger, in addition to the water heater loop. This is because the water heater has a high temperature shut off valve. My engine was heating up, the water heater was closing down the anti-freeze flow, and the engine overheated. Conclusion: Read every word in those instruction books no matter how much you think you know what you're doing.