Expansion / Accumulator tank sizing and choice

[Tintin]

My new (to me) boat has a calorifer but I cannot see any expansion tank or accumulator tank, which may explain why the water pump is cracked and broken and leaking.

It is a 34' Beneteau First 325, with sink and WC basin bpth with Hot and Cold , and it has a small shower.

What size expansion tank does it need on the hot side do you reckon?

What size accumlator tank do you reckon?

Also, from what I have seen in pictures a regular domestic zilmet type expansion tank will suffice for both purposes - or are there special marine versions?

 

[Dill et Bound]

Mine has a blow off valve on top..
You sure your system has not frozen at some time???

 

[Hoolie]

Depends on the size of your calorifier. I found a website - address now lost - that will calculate expansion tank sizes. I have a 2 litre for a 15 litre calorifier. You need a separate accumulator for the cold system as there's usually a non-return valve in the feed to the calorifier. A 1 litre accumulator is usually enough.

 

[vyv_cox]

Whatever caused your pump to crack, it certainly was not the absence of an accumulator. Many boats, including mine, are not fitted with accumulators from new and their pumps last perfectly well. The benefit of fitting one is that the flow of water from the taps is smoother, without causing the pump to kick in as soon as the tap is opened. In some installations the accumulator is used to take the expansion of hot water in the calorifier but this is often not necessary. It is normal practice to place it close to the water pump in the feed to cold and hot water but if there is a NRV at the accumulator to prevent hot water from flowing back into the cold it may be necessary to fit one in the hot water circuit.

If you are changing the broken pump anyway you might consider one of the new types that lhas variable flow, in which case an accumulator might not be needed anyway. The accumulator I fitted to my boat is a 1 litre one but on a motorhome with a similar water system it appeared far smaller than that.

 

[David2452]

As Vyv says, lack of an accumulator is an unlikely cause of your pump damage.
If you really want to get technical and precise then use the following formula:

V= eC
l-p1
p2

V = The total volume or nominal size of the expansion vessel. It is not the acceptance volume.
C = The total volume of water in the system (Bars).
P1= The fill pressure of the system in Bars absolute (atmospheric or gauge pressure plus 1 Bar).
P2 = The setting of the expansion/pressure relief valve in Bars Absolute (atmospheric or gauge pressure plus 1 Bar).
E = The expansion factor that relates to the maximum system requirements.

I would use that for large installs but fur for most applications of 20 to 35l found aboard small vessels a 2l one is plenty and will also allow a quick hand wash at night without striking up the pump, for potable water systems a white colour (or plastic marine type) is the proper designation, the red ones are for closed loop heating systems.