Water pressure mystery!

[bobnotthe builder]

We had a kitchen renovation and I noticed the flow of the kitchen tap was very low. I understand that flow and pressure aren't the same thing but we the kitchen tap flow is 3L per minute so pressure is a likely issue. The plumber measured the pressure at 0.7 bars - measured at the point about 60cm above the stopcock where the water diverts into a water softener. 20cm below that diversion the mains branches off to supply the outside garden tap so we have mains water for the garden.

I asked Thames Water for a pressure check and their man told me the pressure in the street is 1.6 bars and that the garden tap pressure is 1.2 bars. (The street tap is 8m in an almost straight line (apart from where it comes up through the floor) to the stopcock and the garden tap pipe is 4m further on but does a couple of bends.

So the mains water goes from 1.6 in the street to 1.2 from the garden tap. Where the mains water diverts above the stopcock is at least 1.2 bars but 20cm above it, where it diverts to the softener it's 0.7 bars!

Can anyone explain what they think might be going on? I'm not 100% sure about the plumber but the TW guy seemed to know what he was doing - he just could only measure the outside tap and the street pressure - couldn't start taking things apart etc. Thanks for any suggestions/advice in advance.

 

[Madrab]

What kind of pipe comes into the house? Is it blue/black plastic or lead or some other pipe? Losing 0.4 bar from the street to the house is quite significant. So the mains pipe from the toby into the house may be a restriction or it has lots of bends for some reason.

Losing 0.5 bar wouldn't usually be down to 200mm of pipe unless its quite restricted somehow, so it may be down inconsistences in the gauges. I'd get the outside tap checked again and confirm @ 1.2bar, then use the same gauge for the internal test just to confirm.

 

[bobnotthe builder]

I’m not sure what comes into the house but where it comes up through the floor it’s copper. The junction to the garden tap is copper but smaller diameter.

(I didn’t get what you meant by ‘toby’ - road?)

Even so the 0.7 reading right above the junction to the garden is weird.

In your experience could the change in pipe diameter cause a drop of 0.4 bar?

I think the drop to 0.7 is likely to be a problem with the meter as he showed me a photo of the gauge attached to the pipe and the reading was 0.7.

When the TW guy took the street meter reading he showed it to me on a dial attached to the meter and then backed this up by showing on a digital one.

 

[Gasguru]

Unfortunately some of those readings aren't helpful as they are not defined as either static pressures (with no water flowing) or dynamic (with water flowing out the taps at a particular flowrate).

You can assume the pressure in the pipe in the street is 1.6 bar (measured with no water flow) and that should be sufficient pressure for a reasonable flowrate even with a 1/4 turn kitchen tap.
I'd guess that TW also measured the pressure at the garden tap (noting 1.2 bar) at a particular flowrate - they didn't tell you what that flowrate was but lets assume something like 8 litres/min to prove their minimum flowrate & pressure can be achieved.

Your plumber couldn't solve a simple issue so I'd be inclined to ignore his readings.

Given kitchen fitters often leave open pipe ends for debris to fall in and use reels of PTFE tape on fittings then I'd be checking the tap isolating valves/strainers/hoses etc for blockage (and read the tap specs just in case it requires some insane high pressure).

 

[bobnotthe builder]

The kitchen tap is a 4 in 1 (including boiling water) and the minimum pressure is 1.5. Thing is the plumber measured pressure as 0.7 before it even gets to the tap. I expected flow rate to be affected by the 4 in 1 but no way can I expect anything decent if water entering it is only at 0.7bars. 1.6 at the street would be fine if the pressure wasn’t being affected by whatever it is! I think I need another plumber!

 

[Gasguru]

Get yourself a 4 bar pressure gauge and a brass reducing bush to screw it into the 3/4 BSP thread washing machine hose. Most gauges are an 1/8 or 1/4 BSP. Then you can monitor the street pressure (under no flow) at different times..

 

[Moss5]

The static pressure (i.e. NO TAPS RUNNING) will be the same throughout the system, regardless of pipe diameter. The tap spec demands 1.5bar min pressure. Expect poor flow rates at inlet pressure below 1,5bar.

 

[foxhole]

Friend had a boiler replaced and suffered low pressure for a week before finding plumber had not opened a valve fully in his loft , pressure was restored .