Pressure and flow required for unvented system

[jenningsp]

I am planning to have a 250L unvented system installed. From another thread have come to the conclusion that most of the main unvented systems have similar performance. Having read various other threads it is clear that pressure and flow are important. My situation is as follows:-
1) existing cold water supply pipes are 15mm
2) pressure at first tap in the chain is approx 4 bar
3) flow measured at 30 litres per minute based on 3 open taps
4) unit will be approx 5 metres above tap where pressure was measured
5) shower mixers are Bristan Java thermostatic
6) currently two showers are run from a pumped (at 1 bar I think) gravity supply and give acceptable performance
7) however bath fill from the unpumped 3 metre head is lousy (Marflow Tangent taps which look great, supposedly operate at low pressure, but I suspect are only suitable for mains pressure)

I am planning to add a further ensuite shower hence the need to change system - unlikely that all three be be used at the same time.

I can see that these figures are marginal but is anyone prepared to share experience about the likely end result. In terms of making it better would a 22mm direct supply to the unit make a significant difference.

The builder will choose the plumber - hence wanting to double check as much a possible before installation starts.

 

[chrishutt]

The 4 Bar pressure is presumably static (no flow) pressure. It would be interesting to know what the dynamic pressure is at this point, say with 10 l/m, 20 l/m and 30 l/m flow. This would tell us what pressure drop the existing pipeline causes.

You ought anyway to consider how easily or cheaply the existing 15mm supply might be upgraded to 22mm. What size and material is the existing pipe from the mains, both in your front garden and from the main in the street? Ask your local water company if you're not sure.

Shower mixers may have been set up for low pressures and require some modification to work at higher pressures. For example some have optional flow restrictors. Check the original installation instructions if you can access them.

The unvented cylinder location needs to allow for the run to outside of a discharge pipe (22mm or possibly bigger) from the safety valves. Also insist that the unvented cylinder is installed and signed-off by someone with the "unvented ticket", who might possibly have a clue what they're doing.

 

[ChrisR]

If you get 30 l/min at peak usage times through 15mm, you're doing pretty well.
You might find no real problems with your current supply, but if you run two showers or baths expect to see the flow reduce.
It would certainly be better supplied by a larger pipe.

 

[jenningsp]

I have measured pressures again as follows:-

Static 4 bar - as before

1 tap open - delivering approx 14 litres / min - 2 bar

2 taps open - delivering approx 20 litres in total - 1.6 bar

3 taps open - delivering approx 30 litres in total - .8 bar (position 1 metre after water meter).

Have contacted water company (Severn Trent) about increasing size of water meter from 15mm to 22mm - same as output from stop cock - supply from road is I think slightly larger black pipe. They were not helpful - could (or would) not give cost or leadtime.

In terms of increasing cold water pipes from 15mm to 22mm in the house the options are:-

1) Very easy to do - tee off 22mm pipe from first floor (before kitchen and utility taps) to unvented cylinder giving approx 2.6 metres in 22mm

2) possible to do without major disruption - tee off 22mm immediately after the water meter and take a new route (via integral garage internal wall) directly up to unvented cylinder giving approx 12 metres in 22mm.

Just to confirm the work will be done by a properly qualified installer and subject to BC inspection. However, any further advice or comment gratefully received - its best to be as forwarned as possible! Thanks.

 

[chrishutt]

Taking your figure for 20 l/min flow, the pressure drop from the main to the pressure measuring point (1 metre after water meter) is 4 - 1.6 = 2.4 bar. 15mm (12.5mm bore) pipe with a flow of 20 l/m will give a pressure drop of around 0.5 metres per metre run, or 0.05 bar/metre.

If you have 15mm pipe from the main this pressure drop would be right for a run of about 2.4/0.05 = 48 metres. However some of this resistance will be due to valves, fittings and bends so the actual distance will be less. How far is it to the main in the street?

If the above were correct and you increase the supply pipe to 22mm (19mm bore) you would have a pressure drop of only 0.06 metres per metre run, so for 48 metres run you would have a total pressure drop of 0.06 x 48 = 2.9 metres, so the pressure at the same measuring point would be 4 - 0.29 = 3.7 bar for a flow rate of 20 l/m. All figures are approximate - errors will be jumped on by ChrisR.

The above applies to the pipe from the main to the pressure measuring point. There will be additional pressure losses beyond that point to the various outlets, but it's clear that the major part of the pressure drop occurs upstream of the pressure measuring point. So it's not worth worrying about downstream pipework until the upstream problem is resolved.