I recently purchased an '87 Freedom 30 and have been sailing her with great pleasure for a few months now.
One bug I’ve noticed is that the hot water output hose would come undone from the tank seemingly at random. I paid a bit more attention to it, and realized that it was happening when the hot water tank would heat during engine run. With the tap closed, the water pressure would build and have nowhere to go - the hose pops off with a subtle bang. Since I realized this I’ve started dumping water pressure before I start the engine to good effect; I’ve stopped the hose from coming off any further.
My question is how do other boats deal with this? Do I need some form of pressure relief valve? Do I already have one that may or may not be working?
Thanks!
I had this exact problem, and it was completely infuriating. The first sail of every season, the hose from the water heater would heat up, get soft, and the pressure would cause it to pop off the fitting and dump my entire water tank into the bilge. I would put it back on, then re-tighten the hose clamps after the hose material had warmed up a bit, which would solve the problem for the season. However, after the system was winterized and recommissioned it would happen all over again the next spring! Maddening.
Anyway, after 3 spring shakedown cruises without running water, I solved it. The problem was that the brass fitting on my water heater outlet port was not a proper hose nipple—it was really just a tube with some grooves cut into it. I ordered a proper fitting in stainless that actually has hose barbs on it, and that did the trick. Tightening the hose clamps forms a proper seal that doesn’t pop off when the fitting heats up. I believe the threads are 3/8 NPT.
Another possible solution I didn’t end up implementing was to order a short length of high temperature silicone tubing, to replace the first few feet of hot water line from the water heater. However, the new hose barb worked so well that I decided I didn’t need to do anything else.
So the solution is “make it so that it can’t come off” 
That makes sense. Installed presently is a PVC barbed fitting with some standard braided tube. I was considering finding a way to fit a household pressure relief valve, but I think replacing the fitting is a better first step.
Yeah Josh, exactly. The hoses in your water system should have enough stretch to accommodate the increased pressure from heated water. Also, a relief valve would work, but you’d be losing fresh water every time you run the engine.