Improving the flow rate

14 Feb 2015
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United Kingdom
I have a combi boiler and am expanding into the loft with a second bathroom.

I'm going to get a hot water tank into the system to ensure I have good amount and pressure of hot water. But, I'm unsure of the best way to improve the cold water flow rate or pressure.

Incoming I seem to get a good flow rate and pressure. The pressure at the combi boiler is really low though and on the 1st floor quite poor.

Is there much I can do other than replace the pipe work, starting with the incoming pipe to the combi..?
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When you say "incoming you seem to get good flow rate and pressure" where is this measured from and what are the figures?

From that point to the combi, what is in between with regards to pipe sizes, lengths & turns?
I've measured at the main line off a point that normally goes to the washing machine, this is a couple of turns and about 6 foot of pipework to the combi. It's 2.5 bar. I haven't measured in the 1st floor bathroom, but from a normal tap I know it's way down, at least half that probably even worse.

The incoming pipe is 15mm and I'd expect very old and poor, being a pre-1890 home. Replacing that incoming pipe to 25mm I think will improve the whole house. I'm not sure if replacing any internal pipework will help, but it looks like it's in fairly good condition.
To give you a starting point you want to the check, static pressure at that point, dynamic pressure at that point and flow rate (L/Min).
6' of pipework with a couple of elbows should not diminish the delivery to the combi by any great amount but if the flow is crap to start with then the combi will struggle.
You might have good pressure but flow, if the mains is 15mm, may not be ideal especially for house with multiple outlets. Are you sure the bathroom is supplied from the mains or do you have a cold water cistern located somewhere at the top of the house? If it's mains I wouldn't normally expect to see a 50% drop in delivery in what equates to say 3M in height unless there was a very long pipe run with lots of bends.
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Without more information its irrelevant speculating.

Knowing the boiler make and model would help. The will only give about 8-14 li/min regardless of make.

An unvented cylinder will give whatever the mains pressure and flow will allow but dynamic flow rates are what is required. ideally about 22 li/min @ 1.0 bar for a good shower.

You are only allowed to pump up to 12 li/min from the mains. Stuart Turner have a very good pump for that but its about £300 I expect.


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