universal shower pump starts with delay

[mafj]

I have fitted a universal shower pump too boost hot water from gravity system to a fancy kitchen faucet (min pressure 1bar).
The pump is RP80SU rated 2.4bar 10m of pipework away from the faucet, cold is mains 3.5bar.

I have tried initially positive feed pump, but the flow from the faucet was 1L/min band it took time before pump started (not mentioning pump was 1.3bar and the flow was 3L/min tops).

Now with RP80SU it is better as the hot water flow from the faucet picks up much quicker, and following that pump kicks in quickly.
However, this is rather erratic and the outcome often can be meager flow from the faucet for several seconds before the pump kicks in.


Question: What could be the problem.... or is it supposed to be this way?


a bit of admission: the RP80SU pump comes from ebay, and is 5yo or so. Could be that pressure vessel is dodgy?

 

[Madrab]

I'd be checking the pre-charge pressure on the expansion vessel first.

Another problem you have is that's it's 10M away from the outlet, it'll take a sec to sense the drop in pressure, then another sec to pick that up again. How far away from the cylinder is the pump?

2nd hand pumps are an unknown factor too, you don't know how much abuse it has taken.

 

[mafj]

The pump is 4m away from cyl - in 22mm in pipework including gravity loop in the loft (the top of the pump is bang on at 600mm below the bottom of the cold water tank)..

I'm reading up about the vessel pre-charge...

 

[mafj]

It looked the vessel was near flat. I pumped it - it was marked 18PSI, - I gave it my best shot... but I only have a leaky pump with approximate gage.
It feels definitely more reliable but I still can get it to 'deadlock' state with ten attempts.

 

[Madrab]

Make sure you turn off the pump, then shut off the HW cylinder and depressurised the downstream side of the pump before pre-charging (open a hot tap and leave it open)

 

[mafj]

I have not resolved the problem entirely but mitigated it somewhat (time for the pump to kick in).

I have done some pump/vessel characterization (made sure to depressurise water side before adjusting as advised).
The pressure when tap is closed is 2bar.
When tap is opened very slightly - dripping, the pump kicks in when pressure drops to 1bar... or when 100ml+ of fluid pours out.
(I think it is both but would like to know what triggers it exactly: pressure or flow or both).

When the tap is opened abruptly then it takes 100-140ml for the pump to kick in. It might take 3.2s (minimum) or much more depending on the pressure of the air in the vessel.
The issues seems to be that with tap opened abruptly pressure drops instantly to way below 1bar (measured 1m before kitchen tap). Low pressure means low flow through the resistant faucet (1.6L/m @ 2m head, 3L/m @ 1bar),
through which at least 100ml needs to squeeze.
To compare, I have connected shower (low pressure fittings) to the piping and the pump is up within 1.2s off opening (3m away from the pump in 15mm pipework).

Now, this is what I have been suspecting might be the cause of the issue with the kitchen faucet:
The pressure vessel does not deliver enough flow - some restriction inside the pump - so the pressure in the upstream piping drops when tap is opened.
With shower it works as even on gravity alone the flow is 3L/min.

The pressure vessel has nominal volume of 0.6L. When not pressurized at all the pump can fit up to 270ml into it, but when pressurized it might be as low as 100ml (probably at too high air pressure).
Pump overrun is 2.54s at moderately low air pressure and 180ml get squeezed into the vessel.

 

[mafj]

Is it possible to tweak the pipe electronics/swap pressure sensor? I'd like to make the pump to set off at 1.5 bar.

 

[Madrab]

A universal pump activates with a drop in pressure, hence the reason for having the pressure vessel. When the pump runs and you switch off the water, the pump overruns a little to charge the downstream side and the wet side of the vessel, the pump has a pressure switch that activates the pump when the downstream side drops in pressure and kick starts the pump.

You may find that if the pump is >5yrs old that it's not the best any more, switch may be less sensitive etc. I would presume there is a way to adjust the sensitivity but not sure what you would adjust with that model of pump.

 

[mafj]

I'd think that the pressure vessel would hold the high pressure and flow for a bit though... However with 100ml it would not be for more than 1s at 6l/min

 

[Madrab]

The switch should be sensitive enough to register the smallest drop of its set pressure when the outlet is opened and start the pump, if it isn't then it may need looked at. May need a strip down and clean and the switch response checked, there's only one seal kit for it so not sure how serviceable it may though if it isn't starting properly.

 

[mafj]

Tweaking the pressure vessel was the right direction.
After getting somewhat adequate non-leaking car foot pump with manometer I realized the pressure I applied beforehand was way below of what I thought it was.

Note the salamander datasheet states 14psi for that model - RP80SU. The pressure vessel itself carries 18psi sticker.
I also asked salamander support and they advised going to 22psi after learning about my situation (and getting my pump's S/N). They confirmed no service kits for these pumps.

I probably got it slightly higher that 22psi (no way to tell really).
Now pump responds below 1.5s for kitchen and around 1s shower.

It was a journey that set off at delay of 5s+ and by this time my expectations got me to the point that 1.5s makes me happy.

Thanks Madrab