I'll qualify this by stating that my background is plumbing
design for over 15 years. I'll further qualify this by saying I'm not real familiar with the boating
systems but from what I've read above the systems are very similar with the addition of the heat exchanger
which is dependent on the temperature of the engine water
So first question is what is the temperature of the coolant water
from the engine
to the HWH? Since I don't see any type of automatic bypass valve at the heat exchanger
to bypass the water heated by the engine, pressure above the rated blow off valve (yellow tagged) could be exceeded (normally set at around 180F) if the engine was run for an extended time and the water temperature from the engine was excessive to the point it raised the water temperature at the HWH above it's safety
temperature. The electric
element in the HWH has it's own t'stat to limit it's heating
temperature normally set at or about 120F.
Anyway back to the pressure relief or Blow off valve.
Back in the good old days Hot Water Tanks
were blowing up killing people left and right. The fix was the addition of the pressure relief valve in the tank which on pressure reached above 60% of the tanks
(pressure vessel)rated max pressure a devise shall relieve the pressure in the tank. The National Plumbing
Code goes on to say that this relieved pressure shall "spill" over the floor. The reason for this is so if there is an issue with over pressurization (water in the bilge) it will be seen and the problem fixed. The other issue most people don't realize is that these inexpensive valves are suppose to be replaced every 2 or 3 years. The springs normally fail (lower pressure to open)and the valves start to leak. These valves have also been known to fail closed in highly calcified water conditions where calcium build up and seals
them shut. That's why they are suppose to be exercised once a year. There is a tab on the top you lift
which opens the valve. Problem is probably 90% of the time after doing this they leak so I just replace them every 3 years.
Under no circumstances should you pipe this safety
valve back into the system.
The Accumulator Tank
mentioned above is used for reducing the run time of the water pump and that only. It has nothing to do with water temperature but does effect the water pressure. It allows the water pump to build up water pressure in the system and shorten the run time of the pump. Typically adjustable and set between 35 to 65lbs on house systems. It has a rubber bladder in it and doesn't like hot water. It is installed as close as possible to the pump. I'm not sure what the boat pressure systems are but would like to know if someone does.
The temperature at the "tap" can depend on run lengths and initial temperature at the HWH tank. I'm not a fan of the whole house system mixing valves since I like my kitchen sink water temp potentially higher than my shower
temp. The temperature drop after a whole system balancing valve limits this to whatever it's set at and can drop the temp at the kitchen sink by quite a bit depending on the pipe run.
So can someone enlighten me to the typical engine water temperature at the heat exchanger in the HWH and the typical pressure at the kitchen sink? This would go a long way in determining the set points of devices.