hot water system expansion tank

[Trojan]

The expansion tank on a home unit is to prevent what they call (water hammer). It's a loud banging noise when the water is turned off. Some times the pipes even shake. Some homes have them some don't. The only reason an expansion tank is used on a boat or a well water system is to smooth out the water pressure and to keep the water pump from cycling on and off in short cycles making your pump wear out premature. Short cycle as in a waterlogged system. There should never be enough hot water expansion to trip the pop-off unless the water tank heat controls are bad or the pop-off is bad. Some home water systems are operated at 180 PSI. and use the same pop-off. I believe there are only 2 (home) hot water tank pop-offs sold. A 3/4 and a 1 inch npt.. Mohn sells a Faucet that requires a minimum 16o psi water pressure and won't work on a boat. Don't ask me how I know. They retail for $286. bucks and its not stated in the literature. Change your pop-off and go from there. A cheap fix. The expansion tanks are supposed to be inserted in-line on the water pump side of your water system before the system check valve that separates the boat water and the shore water. Your pressure gauge should show what the system pressure is running at.

BILL

[Trojan]

The expansion tank on a home unit is to prevent what they call (water hammer). It's a loud banging noise when the water is turned off. Some times the pipes even shake. Some homes have them some don't. The only reason an expansion tank is used on a boat or a well water system is to smooth out the water pressure and to keep the water pump from cycling on and off in short cycles making your pump wear out premature. Short cycle as in a waterlogged system. There should never be enough hot water expansion to trip the pop-off unless the water tank heat controls are bad or the pop-off is bad. Some home water systems are operated at 180 PSI. and use the same pop-off. I believe there are only 2 (home) hot water tank pop-offs sold. A 3/4 and a 1 inch npt.. Mohn sells a Faucet that requires a minimum 16o psi water pressure and won't work on a boat. Don't ask me how I know. They retail for $286. bucks and its not stated in the literature. Change your pop-off and go from there. A cheap fix. The expansion tanks are supposed to be inserted in-line on the water pump side of your water system before the system check valve that separates the boat water and the shore water. Your pressure gauge should show what the system pressure is running at. There should also be a preasure regulator at the input of the shore water connection.

BILL

[GaryNW]

The 31 EC does not have a shore connection and I am certainly not going to add one. My marina water pressure is high (but not anything like 160); I assume the fire department had something to do with that. I can't attach any kind of nozzle to my white FDA-type hose- it would burst it. Even open-ended all of the fittings leak which is why I only use it to fill the water tank.
The water is hot- probably 180 on both the element and engine loop. I have not gotten around to finding and adjusting any thermostat. I see a check valve at the cold water going into the tank which I assume is to keep hot water out of the cold side.
On the engine loop- I was reading a sea trial recently on a high end trawler. The author noted that the water was too hot underway and that nothing could be done because you are stuck with engine temp. Seems to me this could be corrected with a 3/4" thermostat housing, a 140 degree thermostat and some plumbing. Just deny water flow to the tank once the temp hits 140.
Gary

[krush]

For engine loop heating, I'd just get a valve or a pump with a valve and tap it in to the thermostat on the hot water heating. When the temp gets to your setpoint (130 or 140), it closes the valve from the engine or shuts off the pump.

The beauty of using a pump is that you can suck lots of waste heat from the engine after shut down. You would need to put a switch in line so the pump/valve does not open when the engine isn't hot.

[kwerges]

Perfect timing for this thread. While at the boat this weekend my wife turned on the hot water heater. After a couple of hours, I noticed the water pump running intermittently (I always use water from my water tank and refill, as opposed to leaving a pressurized hose attached to the boat). I checked the bilge and found hot water in the bilge. Apparently my hot water pressure valve let off some pressure. We later noticed that if we leave the hot water heater on for more than about 30 minutes, it will release some hot water. From this thread, it sounds like my release valve might be weak...