Pulsing pump

[GraemeM]

Nearly a month into our French trip, no major issues with our VW compass calypso, despite her 13 years of pootaling around the uk. The exhaust pipe snapped, but managed to get a very nice chap in a small, but very professional garage to fix that. We have a blog if anyone is interested http://gandscampervan.co.uk

 

One annoying thing is the kitchen mixer tap when turned to cold pulses unless running full bore. Its the pump is pulsing on and off. Same thing does not happen on hot. Any thoughts?

 

Thanks Graeme

[Derek Uzzell]

As your Calypso’s tap ‘pulses’ it’s (near) certain that the water-pump is the pressure-sensitive diaphragm type an example of which is referred to in this link

 

http://vandogtraveller.com/mounting-shurflo-water-pump/

 

Standard practice to reduce ‘pulsing’ is to fit an accumulator (examples here)

 

https://www.google.co.uk/?gws_rd=ssl#q=carravan+water+pump+accumulator

 

The reasons why pulsing occurs when your mixer tap is providing a restricted flow of cold water, but does not occur when on full-bore cold supply or on the tap’s hot-water setting are as follows:

 

The ‘part cold’ setting limits the water flow at the tap: some cold water flows from the tap and the pressure in the cold water hoses gradually drops until it causes the pump to kick in momentarily which produces a sudden spurt of water from the tap. This ‘pulsing’ will repeat indefinitely, but if the tap is fully opened, enough cold water will flow from the tap to cause the water pump to run continuously rather than intermittently and pulsing won’t (or is much less likely) to take place.

 

When the tap’s hot-water setting is selected the water in the hot-water boiler will act as a damper. Although the pump may still run intermittently if the tap is on ‘part-bore’, the pressure changes caused by the pump’s on-then-off-then-on-again action won’t find their way through the water in the boiler and into the hoses leading from the boiler to the taps.

 

'Pulsing’ is a pretty common complaint when a leisure-vehicle’s water system includes a pressure-sensitive diaphragm pump

 

http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Hot-water-pump-pressure-pulsing-/35409/

 

You could try adjusting your pump’s pressure switch (assuming it has one!) but adding an accumulator/surge-damper is likely to be the most successful approach.

 

More here

 

http://www.swift-owners-club.com/support/knocking-pumps.html

[GraemeM]

Thanks Derek that gives me some things to look at and ponder the solution.

Cheers

[Alanb]

I find it rather strange that we have not found this pulsing effect to be a serious problem. Perhaps the answer is in the plumbing. The pipework in our 10 year old PVC is reinforced plastic hose, which can expand slightly, and provides some cushioning.

 

Some more modern installations use rigid plastic pipework of the "Speedfit" or similar variety. Presumably these installations do not provide any cushioning.

 

For once I must disagree slightly with Derek, who usually makes flawless and very informative posts. It is not the water in the boiler that provides the cushioning, but the small amount of air that is trapped in the system.

Water cannot be compressed by any significant amount, hence its former use as a hydraulic fluid.

 

 

[Derek Uzzell]

I did not suggest that the boiler’s ‘damping’ effect was due to the water in the boiler being compressed. The boiler’s cold water inlet will be near the boiler’s base and the hot water outlet near the boiler’s top, so - when the boiler is full - there’s 10kg or so of water between the inlet and outlet and the weight of the water will produce a damping effect. Having said that, I can well accept that boilers do not completely fill with water and (as you say) a pocket of air is always present above the water and it’s this that does the cushioning.

 

My Hobby motorhome’s water system used reinforced plastic hose and a SHURflo pump, and cold-water pulsing was not a problem. My Rapido motorhome uses semi-rigid pipework and the same SHURflo pump as the Hobby’s and cold-water pulsing is also not a problem. Neither motorhome’s water system included an accumulator/surge-damper.

 

I suspect that all systems that involve a pressure-sensitive diaphragm water-pump and no accumulator/surge-damper will have a propensity to pulse when cold-water flow is low, but some seem to do this more than others.

 

Your statement that “...we have not found this pulsing effect to be a serious problem” implies that your system will pulse, but not enough to irritate. That’s probably the case witjh my Rapido (and probably would have been so with the Hobby). If I deliberately sought to make the taps pulse I expect I could, but in day-to-day use it’s not an issue.