Pipe size length bends junctions to Combi - affect on water flow rate?

[Andy VC]

Hello! Advice required, please.

Trying to understand implications of pipe size, length of run, and number of junctions etc. en route between the incoming water feed to the Combi-boiler. For various reasons it is difficult to get from stopcock to boiler without taking indirect route back across room under kitchen floor, directly upwards to ceiling, through wall into utility, back along 2 walls and into boiler, around 12+ metres (with various bends and junctions). On the way, (whether a good idea or not), it would seem simpler to branch off to all the various places requiring cold feed (as shown in diagram).

The question is - if flow rate is 16 litres / minute as it enters house, if using 22 mm copper pipe around to boiler, and it's also 22 mm for the initial branches off en route (before the branches drop to 15 mm at sinks, appliances, etc.), would flow rate still be at or near 16 litres / minute arriving at boiler?
Diagram attached.

(If flow rate arriving at boiler is going to be impacted, it may be possible [but more disruptive] to take a more direct route to boiler as shown by 'dashed pipe' - would this be better? or unnecessary?)

Any advice/comments welcome.

(As you have probably gathered, I am not a plumber or boiler installer! but would like to understand this as I have had conflicting information and don't want to have a Combi-boiler installed that is not going to receive enough water flow rate to work to its potential. I am looking at something like 40W Worcester Greenstar 8000 Combi, which wants around 16 l/min incoming flow rate. Not included in diagram, but for information, will also be about 15 rads, with 22mm Flow/Returns, branching to rads on 15mm)

(I am aware many would advise not using a Combi for house with more than one bathroom etc., and go for boiler with hot water tank etc., but for various reasons I would like Combi if possible.)

Thank you for any help.

 

[Timmy D]

You’ve got 16 lpm entering house, you’ll lose some to pipework but either way, a 40kw boiler is way oversized.

If it’s sending out 50c or higher hot water, you’ll use some of that flow rate as cold to reduce the temp. So now the boiler has even less.

With that flow rate, you’d get away with a 28kw boiler. You’ll also only be able to run one shower and will likely notice flow drop when someone uses a tap in the property.

 

[Andy VC]

You’ve got 16 lpm entering house, you’ll lose some to pipework but either way, a 40kw boiler is way oversized.

If it’s sending out 50c or higher hot water, you’ll use some of that flow rate as cold to reduce the temp. So now the boiler has even less.

With that flow rate, you’d get away with a 28kw boiler. You’ll also only be able to run one shower and will likely notice flow drop when someone uses a tap in the property.

Thanks for your reply.

So, by "losing some", does that mean, it should still be near 16 lpm arriving to boiler given the pipe scenario shown (or in your experience maybe 15, 14, 13 or less?)

(Note, I got between 16 and 18 lpm when testing using bucket and timer at kitchen tap).

Would taking the pipework more direct to the boiler (dashed path in diagram) (and branch other cold feeds after first arriving at boiler) make any real difference to flow rate arriving at boiler? (or am I just missing the point and that is not the main issue/problem!?)



And even if near 16 lpm incoming to boiler, in practice, even though quoted Worcester figures say stuff like "Hot water flow rate 40°c ∆T (litres per minute) 14.3 l/min", that is not achievable in reality at hot taps / bath / shower etc.?

In reality, when the boiler manufacturer sites quote outgoing flow rates (e.g. 14.3 /m 40°c ∆T), is it a misunderstanding to assume (like I have) that this means if incoming to boiler is better than 14.3, then I can achieve 14.3 at a shower!? (because e.g. the boiler will be set to send out 50°c water [at an even lower flow rate], which is then mixed with the cold at the shower to get a comfortable temp.?)

But wouldn't a 28kw combi work even less well for getting hot water to taps/bath/shower, and the flow rate would be even less? (I am obviously not fully understanding).

Thank you, your input is much appreciated!

 

[Ben-gee]

Your diagram indicates, bathroom and en-suite and future wetroom.
Three such rooms run from a combi will not be a sensible set-up.

 

[Andy VC]

Your diagram indicates, bathroom and en-suite and future wetroom.
Three such rooms run from a combi will not be a sensible set-up.

Yes, acknowledged (may only run one shower off Combi, and use electric shower for other(s), etc.).

But what about how pipe run, junctions and diameter affects water flow rate from inlet at stopcock through to Combi? Would just using a 22mm pipe, even though it branches off in various places en route, mean the flow rate (of e.g 16 lpm on entry) would still be around 16 lpm at entry to boiler? - I cannot find much information to get to grips with that. (I note there are clear tools / guidance to calculate Gas pressure loss along pipe run from meter to boiler in a gas pipe system, but nothing similar for water flow that I can see.)

 

[Ben-gee]

It’ll be fine at the boiler the long route, but you’ll have the problem of using simultaneous outlets- but this will happen whatever route you take. It won’t make hardly any difference so use the route you can do most easily.

 

[Andy VC]

It’ll be fine at the boiler the long route, but you’ll have the problem of using simultaneous outlets- but this will happen whatever route you take. It won’t make hardly any difference so use the route you can do most easily.

Thanks for the advice. Appreciated.

So, just so I've got it!

1. I don't need to worry about the long route with multiple pipe branches off on way to boiler with regard to impact on incoming flow rate - flow rate to boiler will not be much affected - is that right?

2. My main concern should actually be how well the Combi can cope serving multiple outlets at potentially the same time (even if it is managing its maximum suggested output flow rate of 14.3 l/m 40°c ∆T) ?

 

[Ben-gee]

It’s not just the outlets that the combi is supplying, it’s also the cold outlets as everything is supplied from the same incoming main.
The longer route to boiler won’t matter if none of the other outlets are being used as you’ll only lose a small percentage in frictional losses.

 

[Andy VC]

It’s not just the outlets that the combi is supplying, it’s also the cold outlets as everything is supplied from the same incoming main.
The longer route to boiler won’t matter if none of the other outlets are being used as you’ll only lose a small percentage in frictional losses.

Right. Understood re longer route being okay, when other cold outlets are unused. But when those other cold outlets are used (sinks, dishwasher, washing machine, toilet...) there IS going to be a flow drop into the boiler. Hmmm.

Would I be correct (or not!) to think that if the incoming supply went (mostly) to the boiler first, then that would be advantageous for the boiler in terms of less flow rate drop (first in the queue, sort of thing!) (with the majority of branches off to the other outlets being incorporated into pipework routed AFTER the boiler)?


And I've now been wondering about the incoming mains pipe size. Any thoughts on the relationship of cold feed pipe diameters, between the various internal cold feed copper pipes themselves, and between them and the incoming mains pipe (from what I can see, it looks about 22mm [3/4 inch ?], but I've not measured it, and can't at moment - it's black, poly pipe of some sort)? Does this incoming pipe size dictate what copper pipe size is sensible to take on from stopcock to boiler, and elsewhere? Would using a fatter pipe for the stopcock-to-boiler stretch of pipework [even if fatter than the incoming mains pipe!?], compared to the rest of the copper piping, be beneficial, make no difference, or be detrimental (in maintaining highest flow rate possible to boiler) ?

So many questions! The more I think about this, the more complicated it seems.

 

[John.g]

Problem might be with the poly pipe internal diameter, if you can measure the OD then ID can be determined, a pressure gauge reading, even static, taken anywhere would be a help as well, also estimated distance from mains stopcock to your house.