Accumulator Sizing ?

[Cancunia]

My 25 year old boat has no accumulator so I'm thinking of fitting one to save undue stress on the pump but I'm not sure what size is needed. I've searched on here and see things like 'the bigger the better' but how big is big?  There are loads of 0.75L accumulators for sale including from brands like Seaflo, is that big enough for a small 36 foot boat with a sink and a calorifier tank?

Can I use a central heating expansion vessel? 

Thanks. 

[Jen-in-Wellies]

7 minutes ago, Cancunia said:

My 25 year old boat has no accumulator so I'm thinking of fitting one to save undue stress on the pump but I'm not sure what size is needed. I've searched on here and see things like 'the bigger the better' but how big is big?  There are loads of 0.75L accumulators for sale including from brands like Seaflo, is that big enough for a small 36 foot boat with a sink and a calorifier tank?

Can I use a central heating expansion vessel? 

Thanks. 

Don't use a central heating expansion tank. They have no anti corrosion coating inside, so will rust through in no time. Proper expansion tanks, or accumulators are coated inside to protect them. It makes them more expensive, but that is for a reason.

Central heating systems have corrosion inhibitor additives in them, so can get away with uncoated steel.

[jonesthenuke]

Get one designed for potable water, some of the ones for central heating are not suitable. Also check that the connection is stainless. I bought one which had a plated mild steel threaded connection and after about 6 years it failed with a pinhole corroded through the thread.

[MtB]

12 minutes ago, Cancunia said:

My 25 year old boat has no accumulator so I'm thinking of fitting one to save undue stress on the pump but I'm not sure what size is needed. I've searched on here and see things like 'the bigger the better' but how big is big?  There are loads of 0.75L accumulators for sale including from brands like Seaflo, is that big enough for a small 36 foot boat with a sink and a calorifier tank?

Can I use a central heating expansion vessel? 

Thanks. 

 

It's all a matter of taste. 

 

The bigger the accumulator, the longer the on/off/on cycle will be. I find a 2 litre accumulator about right. 

 

 

I've just bought one of these, coincidentally. 

 

https://www.bes.co.uk/catalogsearch/result/index/?q=25745

 

And no you need one for "potable water" not central heating. 

[Cancunia]

Thanks for the quick replies. I did wonder if 0.75L would be big enough and was thinking about 2L. I'm fairly sure that I have enough space for a 2L tank but don't want to take up space without good reason. 

Any ideas why the smaller tanks have 2 water connections and the larger tanks have only 1? 

[MtB]

2 minutes ago, Cancunia said:

Any ideas why the smaller tanks have 2 water connections and the larger tanks have only 1? 

 

Errr.... they don't normally!

 

Can you post some examples perhaps? 

 

 

Maybe you are confusing the Schrader valve on them for a water connection...

[Cancunia]

1 minute ago, MtB said:

 

Errr.... they don't normally!

 

Can you post some examples perhaps? 

 

 

[MtB]

7 minutes ago, Cancunia said:

 

Oh I see. That is just to save you installing a tee fitting in the pipe! 

 

But hang on, thats a pump not an accumulator! 

 

 

[IanM]

1 minute ago, MtB said:

 

Oh I see. That is just to save you installing a tee fitting in the pipe! 

 

I think that's a combined accumulator and pump.  Or that's what it reads like.

[Cancunia]

Based on the replies & the pricing, I'm now aiming for a 2L tank. Probably Zilmet (potable) as I've used them before with no problems. Points taken about not using a CH vessel, shame as they are a bit cheaper (for a reason). 

 

Thanks everyone.

[Canal321]

I fitted a cheap 0.75 L as shown, works fine 👍.

[Cancunia]

10 hours ago, Canal321 said:

I fitted a cheap 0.75 L as shown, works fine 👍.

No picture / link?

[Bee]

12 hours ago, Canal321 said:

I fitted a cheap 0.75 L as shown, works fine 👍.

And that, sir, is a bargain.

[MtB]

12 hours ago, Canal321 said:

I fitted a cheap 0.75 L as shown, works fine 👍.

 

Except the picture shown is a 2 litre vessel. 

 

 

[Cancunia]

I get no picture on the post from @Canal321?

 

[Canal321]

22 hours ago, Cancunia said:

One of these.

[Cancunia]

1 minute ago, Canal321 said:

One of these.

Ah, the one that I posted! Yes, I was looking at them and then decided that a few pounds more would get me a 2L version. Did you have any problems with the pipe connections? I saw some comments (maybe on Amazon) about the connectors leaking when using jubilee clips.

[Canal321]

I didn't use the supplied connections. I fitted 1/2 inch to 15mm compression tap connectors.

[David Mack]

1 hour ago, Canal321 said:

I didn't use the supplied connections. I fitted 1/2 inch to 15mm compression tap connectors.

Did you find 1/2 inch NPT to 15mm compression fittings, or did you just use 1/2 inch BSP? The thread pitch is the same (for 1/2 and 3/4 inch but not other sizes), but the thread form is different.

[Canal321]

6 hours ago, David Mack said:

Did you find 1/2 inch NPT to 15mm compression fittings, or did you just use 1/2 inch BSP? The thread pitch is the same (for 1/2 and 3/4 inch but not other sizes), but the thread form is different.

Hi, yes I looked into all that, I just used 1/2 inch bsp, they screwed on fine and tap connectors seal on the fiber washer anyway. All is fine. 

[jonesthenuke]

On 09/04/2025 at 11:21, MtB said:

 

It's all a matter of taste. 

 

The bigger the accumulator, the longer the on/off/on cycle will be. I find a 2 litre accumulator about right. 

 

 

I've just bought one of these, coincidentally. 

 

https://www.bes.co.uk/catalogsearch/result/index/?q=25745

 

And no you need one for "potable water" not central heating. 

 

This looks to have the  plated mild steel connection like one the one that I fitted. It lasted less than five years, failing with a pinhole corroded through the very thin threaded connection. I would avoid and look for one with a stainless threaded section.

[Cancunia]

I've ordered a 2L accumulator, now for what may seem daft questions about installation.... 

Should I fill it with water before connecting to the pipework?

My 'Whale' pump documentation has a cut out pressure, but not a cut in pressure. I'm guessing that they can't be the same so can I assume the cut in is lower and if so, by how much?

 

Thanks.

Edited Monday at 11:54 by Cancunia

[Tony Brooks]

No, don't fill it with water. If you set the pressure to about half the cut-out pressure, it will be near enough.

[MtB]

8 hours ago, Tony Brooks said:

No, don't fill it with water. If you set the pressure to about half the cut-out pressure, it will be near enough.

 

Determining the cut-out pressure being the hard bit! Unless the OP has a pressure gauge and a tee fitting to connect it into the water pipe. 

[Tony Brooks]

Just now, MtB said:

 

Determining the cut-out pressure being the hard bit! Unless the OP has a pressure gauge and a tee fitting to connect it into the water pipe. 

 

True, but it is not vital to be absolutely accurate, so using the Whale documentation will be near enough - unless someone has been tampering with the pressure switch.