38kw combi boiler supplying a shower. 22mm or 15mm pipework?

[Ahearsum]

38kw combi boiler supplying a shower. 22mm or 15mm pipework?

I am refurbishing a 1st floor bathroom and installing a shower to replace a bath and have a 22mm/15mm pipe quandary. There is currently a newish Worcester Greenstar 38CDi Classic combination boiler and my question is should I redo historic mixed diameter pipework. The boiler has 15mm mains-in and DHW out connections. At some point in the past there was a cold water tank in the attic and presumably a hot water tank somewhere. There is a half meter of 15mm pipe coming from the DHW boiler output which becomes 22mm supplying the bathroom bath and sink 3m away. The 15mm mains water pipework comes up from the ground floor into the boiler cupboard and also is connected to an historic 22mm pipe to supply the bathroom. This looks an easy route that the previous installer took.
I was considering replacing all the 22mm pipework with 15mm as that seems to be the general internet consensus from what I can gather but I want to make sure I am not going to throttle the supply to my shower as the aim is to have a GREAT shower. Any advice gratefully received.

 

[Aquarius]

15mm, and the 22mm should have been removed.

 

[Chuck]

I was considering replacing all the 22mm pipework with 15mm as that seems to be the general internet consensus from what I can gather but I want to make sure I am not going to throttle the supply to my shower as the aim is to have a GREAT shower. Any advice gratefully received.

Not many people would describe a combi-supplied shower as 'GREAT'. They're okay, particularly if you use a well-designed spray but 'economical' is the first adjective that comes to my mind.

Not much point in using 22mm from a combi, it just means you waste more water and time waiting for the hot water to arrive. The boiler power is the limiting factor.

 

[Ahearsum]

15mm, and the 22mm should have been removed.

Thanks. Apart from the benefit of quicker to arrive hot water through a 15mm pipe run (although at 3m it is a short run) are there any downsides to leaving the 22mm there that you can think of?
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Not many people would describe a combi-supplied shower as 'GREAT'. They're okay, particularly if you use a well-designed spray but 'economical' is the first adjective that comes to my mind.

Would there not be some difference in the power of a shower with boiler of a higher wattage (assuming a high enough mains pressure)? 38kw boiler is quite powerful no?

Not much point in using 22mm from a combi, it just means you waste more water and time waiting for the hot water to arrive. The boiler power is the limiting factor.

I find flow a little confusing. In my mind, bigger pipes mean a higher flow rate which means more water coming out of my shower head (assuming, again, a high enough pressure and a boiler powerful enough to heat that increased flow). Is that wrong thinking?
Would the amount of time and water wasted with a 22mm pipe be that significant over a short run such as 3m?

 

[Ben-gee]

If you’ve only got 15mm coming into Combi, not much point having 22mm going out.
But try it both ways and see if there’s any difference as you seem quite resistant to advice.
Report back your findings as it could be useful to others.

 

[Aquarius]

Thanks. Apart from the benefit of quicker to arrive hot water through a 15mm pipe run (although at 3m it is a short run) are there any downsides to leaving the 22mm there that you can think of?

If you leave the 22mm in you may lose some heat, but probably negligible.

Chuck has explained about taking longer to heat up, so although you will get more flow through the 22mm you will also be trying to get the boiler to heat up a bigger section if you had 22mm coming out of it - if this makes any more sense? Bigger flow rates were required for gravity fed systems as it wasn’t pumped like your mains is.

 

[Ahearsum]

you seem quite resistant to advice.

That's a little unfair I think. I'm very grateful for the advice but always want to understand why so will ask questions. Its for clarification reasons, not to pooh pooh someone's help.