combi DHW supply/flow pipe size

Have a dedicated pipe from the stoptap to the combi with nothing teed off except the cold to the showers just before the combi. Also have the shower hot direct from the combi, and again a dedicated supply. 22mm would be better for the reasons you stated, however 15mm should be fine if a dedicated cold feed.

On my combi the tail at the bottom of the boiler for taking cold water is 15mm, and so I guess is the pipework within the boiler. In this case I can't see any benefit it having 22mm pipe from the main, or am I missing something?

flow rate is not limited exactly by pipe bore alone, its a function of pressure, length and bore.

So if you want flow rates to be high, use large bore wherever you can.

The downside is the space it tales up, the heat that may be locked inside it and lost, and the cost.

I tend to think in terms of 22mm almost everywhere, these days.

"Anita Palley" wrote

Yes

Phil

Yep! You could have 25 litres/min and 3.5 bar at the stoptap and with lengthy 15mm pipe with elbows have 20 litres/min at the end and much reduced pressure at the end of the pipe.

Also hot water takes longer to come through a 22mm pipe - the volume of the pipe is proportional to the square of the radius (or diameter), the flow rate to some lesser power. This means it's brill once the hot water has arrived, as the flow is larger, but there's a longer period flushing out the cold.

Fair point.

Considering all opinions so far - how about 22mm dedicated cold supply from stopcock to boiler and then 15mm from boiler to shower/taps. This will minimise pressure drop on the cold side and give shortest waiting time for the hot water.

Fair point.

Considering all opinions so far - how about 22mm dedicated cold supply from stopcock to boiler and then 15mm from boiler to shower/taps. This will minimise pressure drop on the cold side and give shortest waiting time for the hot water

Sounds good. Have the shower hot back to the combi and cold top the shower taken off the combi just before the cold inlet. Have a shower mixer with pressure equalisation valve inside. Or fit one at the combi on the two shower lines.

Use a thermostatic shower designed for combi and multipoints, i.e. high pressure cold and widely varying pressure hot and very fast reaction to temp changes. They work extremely well.

Pressure equalisation valves on a combi where the hot water pressure changes significantly as the load changes don't work at all well with a shower (unless perhaps you are the only person in the house). You need a fast acting thermostatic valve anyway, so just get the right type in the first place.

That would be my choice if starting from scratch, except that I would not bother with a 'dedicated' supply if doing it in 22mm. I would try to make sure the boiler's cold supply is upstream of most/all other usage points.

You will find these integrated equalisation valves.

You need primarily a valve with a pressure equalisation valve inside. The thermostatic part is very secondary.

Which is near impossible in most cases.

=)

well it is impossible in my case, but I know what's important now, thanks for all your help, much appreciated

And this comes from a self styled expert? The pressure stays the same, prat. The flow rate alters.

Pleese eff off as you a total and utter idiot.

Not much point if the combi can only heat it at 14 l/min. Draw water faster and it will just be cooler.