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I'm renovating a cottage. We're geting a pressurised unvented hot water tank run from an airsource heatpump fitted by an installer, but I want to do the rest of the plumbing myself.

We're planning three en-suites, each with shower, basin and toilet. One will have a bath as well. Planning to run all the showers off the pressurised hot and mains cold.

The cottage is 10m end to end and 5 m wide.

The ensuites are at the extreme ends and the hot water cylinder is downstairs in the centre of the building.

The longest run will be 6m from the tank to one ensuite and then a further 5m on to the kitchen.
The next longest will be 6m and then a further 3m to the ensuite that has the bath.

My question is what size pipes should I use? 15mm or 22mm. I was planning to use plastic piping.

For the hot supply I will have 22mm from the tank for the first 600mm of so, but after that, do I reduce it to 15mm for the whole house? Or should I do the first limbs in 22 and then drop down to 15?

For cold, should I be running 15mm or 22mm from the stopcock?

All suggestions gratefully received.

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.

Thanks Plumbob.

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.

Have I got all that that right?

Eastworth wrote: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.

If you are replacing the incoming main, consider up sizing to 32mm. This is the maximum permissible size for a standard free connection.

Eastworth wrote:Regardless of the input to the cylinder, all cold feeds to the rest of the house can be in 15mm including the bath.

Use 22mm pipe anywhere the hot* and cold supplies share the same pipe. (*By hot, I mean the cold side of the hot water circuit.)

Use 15mm everywhere for cold feeds to outlets including the bath (except as above).

Eastworth wrote: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.

Seems reasonable.

That's brilliant, thanks.

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.

Thanks again. Once last question and it's me being a bit dim.

On one of my posts, I'd asked:

"Regardless of the input to the cylinder, all cold feeds to the rest of the house can be in 15mm including the bath."

and you replied:

Use 22mm pipe anywhere the hot* and cold supplies share the same pipe. (*By hot, I mean the cold side of the hot water circuit.)

Use 15mm everywhere for cold feeds to outlets including the bath (except as above).

I didn't understand the "hot and cold supplies share the same pipe" and the "cold side of the hot water circuit"

Could you please explain for me?

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.

Is that any clearer? :-)

Much clearer, thanks :)