Accumulator pressure

[wandering snail]

Just wondering if the pressure should be set to the cut-in pressure of the pump or the cut-out pressure?

[Boater Sam]

Neither.

I usually set mine to half the pump cut out pressure. There has been much discussion in the past but this works for me.

[Machpoint005]

ie halfway between cut-in and cut-out?

It strikes me that the best advice would be "suck it and see". The OP won't do any damage to her system!

[WotEver]

Jabsco recommend 2 or 3 psi below the cut-in pressure. I’m sure a bit higher won’t hurt though. 

 

https://www.jabscoshop.com/files/Accumulator and Expansion Tank Instructions ZPWL4 doc595.pdf

 

At what pressure should I set the accumulator tank?

The pressure in the tank is adjusted by means of the air valve in the closed end.
The tank is supplied with an internal pressure of about 2.8 bar (35 psi). Reduce this pressure to 2 - 3 psi below the cut-in pressure of the pump (refer to pump instructions for details). This pressure adjustment is best done before installation of the tank, using a car-type pressure gauge to determine when the required pressure is reached

[BEngo]

Theoretically, to get the maximum possible volume into/out of an accumulator in the main supply pipe the pressure should be set at the pump cut-in pressure.  A couple of psi below allows something for real world physics.

An expansion vessel on the hot water side of a calorifier or other stored water heater should be set to the cut off pressure of the pump.

N

[Machpoint005]

1 hour ago, WotEver said:

The tank is supplied with an internal pressure of about 2.8 bar (35 psi). Reduce this pressure to 2 - 3 psi below the cut-in pressure of the pump (refer to pump instructions for details).

 

It is you, or the Jabsco instruction manual, that confuses the issue by mixing units? Why should the user have to work out what 2 or 3 psi below 2.8 bar means? They might not have a pressure gauge that shows both systems!

[Tony Brooks]

I think that typically half the cut out pressure is roughly the cut in pressure but easier to measure with a tyre gauge.

[GUMPY]

I have always set them 3-5 psi below the pressure rating of the pump. Mine is set around 20psi for a 25psi pump never looked at cut in or out pressures, I get about 15 litres or so out of the system before the pump cuts in, mind you it takes nearly 2 minutes until it switches off ...........vessel is 25litres.

[WotEver]

2 hours ago, Machpoint005 said:

 

It is you, or the Jabsco instruction manual, that confuses the issue by mixing units?

It’s Jabsco. I simply cut ‘n pasted from that pdf. 

 

If you prefer, set the pressure to 0.2 bar below the cut-in pressure. It’s nearasdammit the same. 

2 hours ago, Machpoint005 said:

Why should the user have to work out what 2 or 3 psi below 2.8 bar means?

They don’t. They just have to know what 2 or 3 psi below their pump cut-in pressure is. 

Or 0.2 bar below... either one works. 

Edited March 12, 2019 by WotEver

[Sea Dog]

34 minutes ago, WotEver said:

It’s Jabsco. I simply cut ‘n pasted from that pdf. 

 

If you prefer, set the pressure to 0.2 bar below the cut-in pressure. It’s nearasdammit the same. 

They don’t. They just have to know what 2 or 3 psi below their pump cut-in pressure is. 

Or 0.2 bar below... either one works. 

Plus, anyone who struggles to make a simple percentage-based approximation out of those figures will probably do themselves a mischief if allowed to use an air pump without adult supervision.