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Water System

Just changed the hot water tank old one was 75lts new one 55lts now the pump over runs for a long time what size accumulator do I need to fit I have 15.25mts narrow boat

Asked by: Patrick Moran  | 7.58pm, Friday 16 October

WW says:

A basic 2 litre accumulator would be fine for this- although, within reason, larger ones are OK as well!
What you might consider doing is, as well as having an accumulator immediately after the water pump/pressure switch in the cold line, is to fit a second accumulator on the hot water pipework after the calorifier (hot water tank). This will enable the expansion of the water as it is heated to be allowed for, which reduces the stress on the metal calorifier cylinder. The PRV (pressure relief valve) on the calorifier will vent water as it is heated up, but this only takes into account more extreme expansion. An accumulator fitted on the hot water size (which must be in the hot water pipework leading from the cylinder) will reduce the strain on the system and can prolong the life of the hot water tank.
The pressure accumulator in the cold water side is really there to prevent the pump rattling on-and-off, as well as smoothing out the flow. If you have a largeer accululator, the pump, once it cuts in, will run for a long period of time. However, it is possible to alter the air pressure above the diaphragm to change how much balancing force it applies- if you have changed your cylinder, this might be worth checking (often you can use a car-type footpump with gauge).
An accumulator fitted after the calorifier is there to protect it (and missed out by most boatbuilders!) and this could potentially be larger, though even here a 2 litre unit would suffice.

Mark Langley  | 11.21AM, Monday 19 October

I should add that the non-return valve of the calorifier prevents the accumulator in the cold side from allowing for pressure changes. As the water heats from say 15 to 70 degrees celcius, the water expands considerably, placing additional pressure on the hot water pipework, but more importantly on the metalwork and joints of the hot water tank itself- which can lead to premature failure. It is the lack of an accumulator on the hot water side that is probably a key reason that a calorifier would fail on a boat (the other being major frost damage)

Mark Langley  | 11.23AM, Monday 19 October

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