Are two accumulators better than one?

[Graham_Wright]

Connecting up the pipework for my fresh water system, I have realised that an accumulator immediately after the pressure pump should perform its primary job of preventing short cycling as well as taking up the expansion fed back from the calorifier.

I can see that, if the primary accumulator is fully charged due to absorbing the cold water pressure, its secondary function of absorbing the calorifier expansion is compromised.

But I cannot see how a second accumulator could be isolated from the cold water supply. It would seem that, via the circuit through the calorifier, it would merely parallel up the first.

I should add that there is also a calorifier pressure release valve for safety.

Any comments please to prove or disprove the logic?

 

[Chalker]

Quite right, the pressure will simply build to the same level throughout the system. The added pressure from calorifier expansion should be small compared with the pump.
Small black tubular accumulators are simply a container.
Larger accumulators have a bladder inside that can be partially inflated by a bcycle pump.
I arranged that my calorifier pressure relief vented into the cockpit so that I could see if the pressure got too high.

 

[vyv_cox]

I don't believe that a correctly set up accumulator would be 'fully charged' in absorbing the water pressure. Internal air pressure should be sufficient that some part of the internal volume is available to absorb further thermal expansion.

A second accumulator is required when a non-return valve is fitted to the calorifier inlet to prevent hot water from expanding back into the cold water supply to the taps. In this case thermal expansion cannot go back to the primary acumulator and a second is required downstream of the calorifier.

 

[Graham_Wright]

A second accumulator is required when a non-return valve is fitted to the calorifier inlet to prevent hot water from expanding back into the cold water supply to the taps. In this case thermal expansion cannot go back to the primary acumulator and a second is required downstream of the calorifier.

That of course is true (and I eventually worked it out for myself!) BUT, it does not prevent the pump also charging the "downstream" accumulator forwards through the non return valve.

Perhaps a higher pressure in the latter would ensure some remaining capacity.

I do not like the idea of continuously losing expansion water to the bilge;- it just doesn't seem right.

 

[vyv_cox]

I have posted my simple method of achieving the optimum pressure in an accumulator several times, but in case you have not seen it before here it is again. I have never considered it for a twin-accumulator installation but I'm sure you could modify it to suit.

Ensure your water tank contains plenty. Let the air out of the accumulator. Turn on a tap and note that the pump is running constantly. Begin to pump air into the accumulator, noting the increasingly intermittent running of the water pump. Once the stopped period of the water pump decreases as you continue to add air, you have gone too far. Stop pumping, bleed a little air out of the valve.

I assume that having done this with the first accumulator with the cold tap you would repeat for the second using the hot one.

Just to add - if the calorifier relief valve is constantly lifting it could either be rated at a lower pressure than the water pump pressure switch, or it could be calcified and needs cleaning.

 

[Graham_Wright]

I have posted my simple method of achieving the optimum pressure in an accumulator several times, but in case you have not seen it before here it is again. I have never considered it for a twin-accumulator installation but I'm sure you could modify it to suit.

Ensure your water tank contains plenty. Let the air out of the accumulator. Turn on a tap and note that the pump is running constantly. Begin to pump air into the accumulator, noting the increasingly intermittent running of the water pump. Once the stopped period of the water pump decreases as you continue to add air, you have gone too far. Stop pumping, bleed a little air out of the valve.

I assume that having done this with the first accumulator with the cold tap you would repeat for the second using the hot one.

Just to add - if the calorifier relief valve is constantly lifting it could either be rated at a lower pressure than the water pump pressure switch, or it could be calcified and needs cleaning.

Mine are both diaphragm accumulators (not that that affects the advice).

Perhaps a higher pressure in the latter would ensure some remaining capacity.

- what do you think?


Thanks.

 

[Tony Cross]

I have posted my simple method of achieving the optimum pressure in an accumulator several times, but in case you have not seen it before here it is again. I have never considered it for a twin-accumulator installation but I'm sure you could modify it to suit.

Ensure your water tank contains plenty. Let the air out of the accumulator. Turn on a tap and note that the pump is running constantly. Begin to pump air into the accumulator, noting the increasingly intermittent running of the water pump. Once the stopped period of the water pump decreases as you continue to add air, you have gone too far. Stop pumping, bleed a little air out of the valve.

I assume that having done this with the first accumulator with the cold tap you would repeat for the second using the hot one.

Just to add - if the calorifier relief valve is constantly lifting it could either be rated at a lower pressure than the water pump pressure switch, or it could be calcified and needs cleaning.

We used to have this problem on our old calorifier. The pressure release valve was marked as 3bar, which was barely above the shut-off pressure of the pump. We changed the old (17-years old) tank last year for a new stainless steel one, it has a pressure release valve rated at 7bar. No water in the bilge since.

 

[Graham_Wright]

We used to have this problem on our old calorifier. The pressure release valve was marked as 3bar, which was barely above the shut-off pressure of the pump. We changed the old (17-years old) tank last year for a new stainless steel one, it has a pressure release valve rated at 7bar. No water in the bilge since.

Thanks - I think that should work. Need to check the calorifier recommended maximum safe pressure - probably a bit higher than the safety valve.

 

[Graham_Wright]

Thanks - I think that should work. Need to check the calorifier recommended maximum safe pressure - probably a bit higher than the safety valve.

Maybe a better solution would be to use separate pumps for hot and cold water. That should solve all the problems.