Hot water cylinder - 15mm cold return?

13 Oct 2009
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United Kingdom
Hi experts!

I just wanted to see if anyone could provide some insight on what seems to me a slightly weird bit of plumbing on my hot water cylinder.

The cold return from the coil is reduced from a 1" fitting down to a 15mm pipe immediately on exiting the cylinder, only to be changed up to a 22mm pipe a short distance away.

Photo shows the 15mm length of pipe (one end has a 28mm to 15mm reducer which connects to the 28mm to 1" coupler that fits the cylinder, while the other end leads to a 22mm to 15mm reducer used in reverse to connect to 22mm pipe).

I was thinking of binning that and changing the run to be 22mm all round, is it worth it?

Given that the pipes for all the other connections are 22mm what (if any) negative effect would having this 15mm reduced flow have?

Any insight on why this might have been done like this?

Many thanks in advance!
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With a pumped system a short length of 15 mm is usually adequate to provide enough flow through the cylinder coil.

Yours has a lockshield valve for setting the flow as well although just that length of 15mm is probably adequate to cause sufficient restriction.

The valve should technically be set to give the required differential in accordance with the boiler requirements.

In practice its a compromise as the setting would be different between HW only and HW plus CH.

Having an auto bypass theoretically helps but practically upsets the differential.


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