The limiting factor may be the incoming cold size. On the house side after the main stop tap, if the pipe is 15mm then there is little point in increasing the bore anywhere else in the property.
It is usual except in very large or commercial properties to use 15mm pipes for feeds to outlets. If the rising main is 25mm or larger and the property has more than two bathrooms then using 22mm between the stoptap and the cylinder (where the supply feeds both cylinder and cold water outlets) is a good idea.
Having a larger bore after the cylinder just means a longer wait before the hot water reaches the outlet.
We have influence over the size of the water main because we're getting a new connection from the water meter to a new stopcock. The new stopcock will only be about 2.5m from the pressurised tank and I am also in an area that has really good water pressure.
Would it make sense specifying a 25mm blue mains pipe rather than a 20mm so I can use 22mm from the new stop cock to the cylinder?
Regardless of the input to the cylinder, all cold feeds to the rest of the house can be in 15mm including the bath.
Hot would drop down from the 22 leaving the cylinder to 15 at the first branch after it leaves the cylinder and then also be 15mm for everything.
Another question. With the distances I've got, is it worth installing a pumped hot water circuit with a return? I'm concerned that the run off fo rthe kitchen, which is the furtherest point from the cylinder is going to waste a lot of water waiting for it to get hot.
How would that that work with the pressurised cylinder?
I agree waiting for hot water to arrive at a tap is frustrating but the disadvantages of a circulating system are so great they are rarely used in a domestic environment. Even if the pipes are heavily lagged, the energy wastage is significant.
Eastworth wrote:I didn't understand the "hot and cold supplies share the same pipe" and the "cold side of the hot water circuit"
The "cold side of the hot water circuit" is the pipework leading from the mains stop tap to the hot water cylinder.
If for example you fill a bath running both hot and cold taps together, the water to BOTH individual taps comes originally from the mains through the stop tap. Wherever both supplies are combined in one pipe its bore should be 22mm.
If you wanted to avoid using a larger bore, you could tee immediately after the stop tap and take one 15mm feed uninterrupted to the cylinder and a second 15mm supply and branch off to each cold tap/outlet.