In days gone by the use of a hot water expansion tank was of little concern, because smaller
boats tended to have only a 4 gal or so
heater and be plumbed with a mile or so of vinyl hose, and the hose would generally expand enough to accommodate what expansion there was.
Today, with larger heaters and tubing that's more rigid we'd like a better method.
The problem with only having a hot water expansion tank is that when only cold water is used you have no accumulator effect, so you either need a pump designed for use without an accumulator tank, or install both an accumulator tank and an expansion tank., (so many tanks, so much space, so much tubing and fittings,) it's no wonder so many boats are
plumbing nightmares.
The recommended water
heater inlet check valves are another unnecessary appurtenance.
With the low flow/low pressure/low demand
boat systems, (as compared to your home/city water,) the typical brass/bronze "flap valves" installed are quite prone to sediment build-up on the flap, one little bit of debris and they don't seal anyway, now their worthless. Answer?
1, Install an accumulator tank downstream of the pump.
2, Remove the heater inlet check valve.
Now you've got a system that gives you the benefit of an accumulator on both hot and cold sides, (pumps models be da**ed, your whole system works better with an accumulator tank, and I might add, a
remote adjustable pressure switch like a Square D, the 20>40psi model).
And the expansion of the hot water simply backs-up into the inlet tubing, the accumulator tank handles it all in stride.
Loss of hot water is almost immeasurable, 2>3 ft back from the water heater you'll never see the difference.
Now you have a simple system, no hassle, no extra tank, no additional tubing/fittings, and can set the cut-in/cut-out the way you want it.