Negative head pump very slow to kick in

[DesRocks]

Hi, I'm a general builder who has fitted many bathrooms over the years but I'm having problems with a negative head pump issue.
There's an ensuite shower room in a loft conversion, so the shower head and basin taps are higher than the cold water storage tank , therefore a negative head situation. The toilet is separate on the mains.
I fitted a 1.8 bar showermate Stuart and Turner negative head pump, this was a few years ago, and for a while it was ok and it's always eventually started, but there have always been occasional times when the pump takes up to 30 seconds to kick in when an outlet is opened.
The client, a friend of mine, has always just lived with the problem but now they are selling the house so I'm trying to find a solution.
I've tried changing the pump, changing the pressure vessels, ensuring there are no blockages and also installed automatic air vents and tried fitting non return valves on the inlet/supply side of the pump, but nothing seems to fix the problem.
The pump is situated on the floor next to the hot water cylinder, on the floor below the loft conversion, it has dedicated supplies from the cylinder and the cold water storage tank.
The cold water storage tank is in the eaves of the loft conversion at the same level at the shower room floor, and the shower head is about 1.8m above the floor level.
One thing that might be a problem is that the pipework had to drop below the eaves level, and under the door opening, then up into the wall of the shower cubicle, so there is a low point in the run which I guess could cause air lock issues, hence the air vents, but once the pump kicks in there isn't any air coming out of the taps/shower head.
I would really appreciate any advice from the plumbers on here if they have any suggestions that may solve the issue?
Thank you, Des.

 

[Johntheo5]

Does water start flowing immediately you open a hot or cold tap, even if the pump doesn't cut in, you should get ~ 0.25 to 0.5L of water from the little pressure vessel on the pump?, don't think that any NRVs should be installed on the pump suction side. Are you sure that the pump takes 30 secs to kick in or is it kicking in and not delivering any water for 30 secs or so?.
"DO NOT install a non-return valve, or devices which contain non-return valves, in the suction (inlet) pipework to the pump. The pump must be free to vent to the supply tank at all times."

Are you sure they make a negative head Showermate? "Outlet head must be below bottom of cold water tank and be able to pass a minimum of 1 litre/min gravity flow" and
"Symptoms
Pump will not start
Insufficient gravity flow
Check flow rate minimum of 0.8 l/min required on full hot and cold"

They do make a Showermate Universal (negative head) Pump though.

 

[DesRocks]

Hi John,

Thanks for your reply.
Sorry, yes the pump is a showermate universal and it's being used in a negative head situation. I've attached an image of the pump info.
When I open a tap on the basin in the loft level ensuite, a tiny amount of water comes out, maybe 0.05 to 0.1L max, and the pump doesn't kick in, then 20 - 30 seconds later it kicks in fine. Once it's running it's works well, and there's no trapped air escaping etc
I fitted the non-return valves as a last resort to try something different, but I'll remove them tomorrow. It was still doing the same thing prior to me fitting the NRVs.
The pump is sat on the floor next to the hot water cylinder on the next level down from the loft, so about 2.5 metres below the CWS tank in the loft eaves, and the shower head is about 1.8m above the bottom of the CWS tank.
When you say - "Outlet head must be below bottom of cold water tank and be able to pass a minimum of 1 litre/min gravity flow" doesn't that refer to a positive head scenario, as the outlet head is 1.8m above the bottom of the CWS tank?
Any advice would be greatly appreciated

Thanks for your help

Des

 

[Johntheo5]

Yes my comments were mainly about the positive head pump. Is there anywhere close to the pump discharge where you can tee in a pressure gauge, the pre charge pressure for this pump is 1.5bar but don't know what pressure the switch closes at, if you can install one you can see what it fall to when you open a tap and then see how (if) much further it must fall before the pump starts.

 

[DesRocks]

Ok thanks, I'll try that