Water pump cycling on and off

[DannyB]

When I open a tap on board I can hear the water pump cycling on and off. With the tap open, there used to be almost an equal time of on and off, I've become aware that the off time is almost non existent. Now there is a small red tank immediately after the pump which has a tyre valve on top of it, could it be that this tank needs pumping up? Does anyone have any idea what pressure should be in this tank ? I ran my tank dry a couple of weeks ago, and I've only noticed this pump behaviour since then.

 

[ghostlymoron]

The little red tank is an accumulator which provides a pressure reservoir so that the pump doesn't have to start every time you draw a cupful of water. There's two types, one has an internal membrane and rarely need recharging, the other doesn't and need recharging more often. Sounds like pumping it up a bit would solve your problem.

 

[ghostlymoron]

2 - 3 psi below the cut in pressure of the pump (refer to pump instructions for details).

 

[VicS]

When I open a tap on board I can hear the water pump cycling on and off. With the tap open, there used to be almost an equal time of on and off, I've become aware that the off time is almost non existent. Now there is a small red tank immediately after the pump which has a tyre valve on top of it, could it be that this tank needs pumping up? Does anyone have any idea what pressure should be in this tank ? I ran my tank dry a couple of weeks ago, and I've only noticed this pump behaviour since then.

What GM says

You may also find this pdf useful/interesting

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

 

[johnalison]

I had a little red tank which only lasted a couple of years before the diaphragm went, and the replacement didn't do much better. I would suggest getting a new, perhaps Jabsco, accumulator tank, preferably the diaphragm type. There is or was a Jabsco tank with no diaphragm which needs occasional bleeding with air, but this is a bit of a hassle if it is hard to access. My system normally operates without cycling when one or both taps are fully open.

 

[VicS]

What GM says

But since the pump is now running for longer to pump the same volume of water it suggests that the pump performance has deteriorated. A little bit of debris trapped in one of its internal valves could cause this. It should not happen though if you have a strainer in the pump suction plumbing.
OR
Maybe the strainer is partly blocked by debris as result of running the tank empty.

Start with easy to do checks, Accumulator pressure and strainer, work towards the more complex tearing down of the pump.

 

[johnalison]

Start with easy to do checks, Accumulator pressure and strainer, work towards the more complex tearing down of the pump.

Certainly, but it's not unduly hard to get into a pump if you really have to, even for a non-engineer like me. If you undo the four screws at the base it comes apart easily and in my experience doesn't leak when put back together. You just need to note which way round it comes off. It is not necessary to take the pressure switch off as well.

 

[AndrewB]

A couple of other wrinkles in addition to the good advice above.

Many boats have 25 psi pumps with a cut-in pressure of 15 psi. That means, as Ghostlymoron indicates, the accumulator pressure needs to be set at 12 psi. You can use a tyre pump. But gauges designed for measuring tyre pressure don't read at all accurately at such low pressures - you'll need to buy a pressure tester that is good for this. They are cheap enough from Halfords.

2. Although it may say 15/25 psi (or whatever) on the pump, the setting on pumps is very easily disturbed so that the actual cut in/cut out pressures are very different, in which case you won't be able to set your accumulator properly. Although there is usually a simple adustment screw on the pump, unfortunately it is not that simple to test what the actual pump pressure is, so making adjustments is tricky. If you think water pressure at the taps has been unusually high recently, this may be the cause of your problem.

 

[vyv_cox]

My method for setting accumulator pressure needs no gauges and is pretty accurate. First, let all the air out of the Schrader valve. Put a bicycle pump on it and be ready to pump. Turn on a tap. The water pump will run continuously. Now begin to pump air into the accumulator. The water pump will begin to run intermittently, being off for longer periods until eventually as more air is introduced these periods will reduce again. Let a little air out until the water pump off period is at its maximum. The accumulator pressure is now exactly right.

I last did this more than 10 years ago and it has never needed any adjustment.