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Fitting Accumulator and Expansion tanks to water system

I have decided to install an accumulator and also an expansion tank in our boat's water system, hoping to gain two benefits:
1. To stop the irritating "kick" the pump makes every few minutes, which I assume is due to the water pressure falling downstream of the pump, causing it to cut in briefly to re-pressurise the system (we have to switch the pump off at night so we can get to sleep). There are no leaks in the system, so I am guessing that the pressure drop is due to water being forced the wrong way back through the pump.
2. Extend the life of the calorifier by reducing the pressure stresses on it and preventing warm water finding its way back into the cold water feed and coming out of the cold tap.
My question is about where to place the non-return valve(s). I realise I need one to isolate the expansion tank and calorifier from the pump and cold water side, but am wondering if it would make sense to fit another one between the water pump and the accumulator? I'm concerned the pressure from the accumulator will continue to force water back through the pump as well as pressurising the system as its supposed to.
Please confirm if I am right in thinking the accumulator needs pressurising to somewhere bewteen the pump's cut in and cut out pressures and the expansion tank needs pressurising to something a bit less than the pressure relief valve setting on the calorifier?
Many thanks for your advice.

Asked by: Colin Wilks  | 5.42pm, Tuesday 15 January

WW says:

Generally, the non-return valve is in place just before the cold water entry into the calorifier: it usually means it is easier to get at as well!
Most systems do not include a second NRV, and they can reduce flow somewhat, so fitting two might not be the best possible plan. Some pumps already have a NRV fitted (so once primed, they do not have to self-prime again).
You are right in assuming that backflow through some pumps does cause a pressure drop (though not very much!): also, as the calorifier cools down as hot water is drawn off (or by natural) this reduces the volume of the hot water, which then decreases the pressure inside the system, so requiring a "top-up" from the pump. This is often the major cause of pump running.
The difference in volume between water at 80C and 20C is quite substantial and that can cause quite a few runs of the pump during the night as the pressure decreases. If the calorifier is cold, you will often find the pump stops kicking in- at least as often.
You are spot on with the pressure setting, though often it is a case of trial and error- a car-type piston pump works well and should give you the pressure you need.
Hope that helps!

Mark Langley  | 6.36PM, Tuesday 15 January

On closer reading, you say that the pump is kicking in every few minutes- it might as you suggest that the pump is failing- but also the pressure relief valve (PRV) on the calorifier might be leaking off (especially if it goes straight into the engine bilge where you might not notice it) so might be worth checking as well.
The NRV, by the way, is important to prevent contamination of the cold (drinking) water side, by warm water flowing back.

Mark Langley  | 6.39PM, Tuesday 15 January

Fitting a filter on the input side of the pump will help prevent any debris getting into the pump. If you do not have one this may be the cause of the pump valves not holding the pressure, and causing the pump to kick in.

Rupert Smedley  | 6.46PM, Tuesday 15 January

Readers say:

Thanks very much for that. There is no filter fitted before the pump and I had planned to add this whilst doing the other work. It sounds like the pump may well be failing, but I shall install in the normal way (without a second NRV) and see what happens before replacing the water pump.

Colin Wilks  | 10.07AM, Wednesday 16 January

We fitted nrv's on the non-pressure side of the pump (ie between the tank inlet and the pump) on all our hire boats to reduce the regular cut-in's. Didn't reduce flow rate and solved the problem of run-back through the pump due to wear or debris in the pump diaphram.

Cliff Eden  | 5.58PM, Monday 21 January

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