I'm told we have a 20mm "poly" water feed to the house, it goes-in to a 15mm stopcock and then 15mm around the house. The pressure and flow rate at the upstairs cold tap (mains fed, not gravity!) is very good. As part of a move to a mains-pressure hot water system it's been suggested that the stopcock and pipe to the cylinder should be changed to 22mm to "improve the flow rate" - how can this be? I would have thought that the limiting factor was the pipe run from the main to the stopcock and if this has a bore of somewhere near 15mm it doesn't matter what I do in the house. Am I wrong? Is there any benefit in using larger pipes downstream of the stopcock?
:-) Thanks ;-) Being resourceful (SWMBO says "tight") I was thinking more of a widget to adapt a tyre pressure gauge ('seems like it should work) or some other ingenious way of adapting something I might already have. Dave S
Remember that this is really only part of the story. The static pressure will be constant wherever you measure it (allowing for height of course). The point in whether or not pipes need to be changed will be more an issue of the dynaimic pressure and the flow rate. Flow rate can be measured very cheaply with a stop watch and a bucket. In practice it is flow rate and flow distribution that is more important.
It all depends of the relative lengths of the pipes. 20m of 15mm internal bore will have a greater opposition to flow that 10m of same. If you had
10m of 15mm internal bore outside & 10m of 22mm inside, the opposition to flow would be less.
If your current internal plumbing is in 15mm then you can get a feel for this by opening the stopcock fully & doing a timed fill of a 2 gallon bucket from the closest tap to the stopcock, then repeat for the furthest tap from the stopcock. If it's far slower then think about changing, otherwise not.
As to the pressure thing, is it really pressure you are interested in or flow, ie. can I get a healthy shower and can I fill the bath in a reasonable time. You should be able to get a feel for this by filling the bath with cold only or rigging up a temp shower head with cold feed only.
Btw, I wouldn't plumb my hot flow on a mains pressure system in 22mm as it would delay the arrival of hot.
Only flow rate is really important, provided the pressure is enough for a good shower. However, there is a reasonable correlation between pressure and flow rate, as it is the pressure that makes the water move. There are plenty of exceptions, though.
Provided you are happy with the flow rate provided by the cold mains, and realise that opening a hot tap might mean that the same flow is shared between the taps (it might not, of course), then you can use 15mm for the hot water, too.
There is some benefit to using 22mm within the house even when 15mm is used outside. You must not think of the system as a "weakest link" effect. Many people do and find the system odd. It is actually more akin to an electrical network with resistors. The 22mm has less resistance than 15mm, so having a system half 22mm and half 15mm is better than a system all 15mm. It doesn't matter which order the pipework is in.
However, in a hot water system, there is a disadvantage to using 22mm in that there is more cold water sitting in the pipes to clear, which means a longer wait for hot water and less efficiency when all the hot water in the pipe sits around cooling off. This disadvantage doesn't apply to cold water mains, though.
The ideal is to use 25mm+ MDPE incoming main and then 22mm throughout for cold, preferably from a manifold system to reduce side effects. The hot is then run through 15mm, provided flow rate is good, or 22mm if required to reduce pressure losses leading to slow flow rates (very unlikely to be required on an instantaneous combi, due to low achievable flow rates).
Why would you get water in the gauge? Just get a plastic 2litre Coke bottle, attach a screw-in Schraeder valve from an old bicyle tyre to a hole drilled near the top, attach the bottle to the tap with a bit of rubber hose and a couple of hose clips. Pressurise the air in the bottle by turning the tap on, check the pressure at the valve.
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