1st fix for possible HW cylinder upgrade

18 Apr 2007
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United Kingdom
I'm renovating a property which after works are complete will be 5 beds, 4 bathrooms.
All 4 bathrooms will have rain head showers and one will also have a bath.
Incoming CW supply is in 25mm MDPE to internal stopcock and then 15mm to CW outlets. Pressure is 3.5bar & flow rate just over 20l p/m.

Boiler is 32kW system boiler in basement with vented HW cylinder (also in basement) approx 5m pipe run away from boiler. Tanks in loft 2 floors up.

Pressure is currently good for showers etc in current config of 2 bathrooms (2 showers, 1 bath). I doubt though if the gravity fed HW will be sufficient with the addition of 3 bathrooms.

To upgrade I'm looking at unvented cylinder options but need to understand what the pipe size requirements are for the 1st fix on the outlet side of the cylinder. Will 15mm be sufficient from the cylinder with current pressure to ALL outlets or should I run dedicated for bath/showers in 15/22mm?
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No, definitely not. I would also recommend you perform a dynamic test on the cold mains to ensure you have enough pressure and flow to supply that number of outlets, especially if they could be used at the same time.

Always try to feed an unvented with uninterrupted 22mm from the mains to the combi valve and 22mm from the cylinder to the branches to each outlet. I'd also recommend you take the cold water supply to the showers from the balanced side of the combi valve in 22mm to the branch to each outlet.
Won't the 22mm from the cylinder to the 1st floor where it splits off to the 3 bathrooms mean a longer wait for hot water?
Assuming the feed from stopcock to cylinder is in 22mm then with a flow rate of 20l p/m and 3.5 pressure mean the outlets on 1st floor can all be in 15mm (3 showers, 3 basins, 3 toilets & 1 bath).
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Won't the 22mm from the cylinder to the 1st floor where it splits off to the 3 bathrooms mean a longer wait for hot water?

Not significantly so over 15mm no, it not about pipe diameter rather than distance the hot water has to travel.

If you are worried about time for the hot water to reach the outlets that may be a fair distance away then adequate lagging of pipework is a must and a secondary HW re-circulating setup may be advised. The 22mm supply is all about suffucient flow to adequately feed multiple outlets without increased noise/wear or drop in performace, it's not about the time for the hot water to reach the outlets.

Did you take dynamic pressure/flow readings, or are your readings static?

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