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.

 

[Timmy D]

Whatever way it’s piped, opening another outlet will reduce the available flow to the boiler/other outlets.

Nothing you do to the piping will make a worthwhile difference.

If you want more flow rate, you need to get more from the street.

 

[Andy VC]

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.

Bit of a delay, but now double checked, and found marking on black poly pipe - looks like "TELCON BS 1972 ...CLASS D". Old Alkathene pipe?! Must be the 1/2 inch OD, which from what I have found is just 14.1 mm ID.

A water authority guy was looking at stop tap at water meter a couple of weeks back and he did a pressure test at an outside tap for me and it was just about 3 bar. Distance from mains stopcock to internal stopcock, in kitchen at the far side of the house, is about 16 metres.

I assume the problem is with the poly pipe internal diameter then ??

 

[Andy VC]

Just to clarify what I have now found with regards to drop in water flow as other cold outlets are used, as a number of you have made clear is an issue (thanks to all for steering me in the right direction):
Flow at, for example, utility sink cold, by self, 18 lpm.
Also turn on bath cold, flow at utility sink drops immediately to 12 lpm.
Turn on 2 more taps, flow now down to 10 lpm.
Turn on 2 more (utility plus all other sinks/bath), flow down to 8 lpm at utility sink (and bare dribble out of upstairs bathroom sink).

[PS. if wondering why I seem to be a bit unsure about current pipework setup, or surprised that water flow drops so much with just a couple of cold outlets running, it's because we haven't actually managed to move in to this house yet as there have been a few unexpected issues that need sorting before we can, including sorting out various water damage issues (!), and installing new boiler, replacing radiators etc. and now perhaps sorting incoming pipe from water meter!? Therefore I am just really discovering how the water setup is working, or NOT working very well (compared to what I have experienced before)]

 

[Andy VC]

If you want more flow rate, you need to get more from the street.

Looks like that may be the only answer.

About 16 meters away; inside the house the current 1/2 inch pipe runs under the suspended timber floor, then outside there is 6 meters or so to external stopcock at water meter (under garden, and then less than meter of path, which looks like it was previously dug up, probably to put in the water meter at some point.)

Is that a big/expensive job?

 

[John.g]

I have similar pipework in a 1971 built hose but its a measured 21mm OD X 11.5mm ID, its 1/2" (imperial) so 1/2" ID, nominally. If yours is 1/2" ID then it can't be 14.2mm ID? except you measured this.
I have a 21M run of this and I get measured static/dynamic pressures of 3.6/0.95 bar, measured on a outside tap at a bath (3/4") tap flow of 14.4LPM. Theoretically, re pipe loss calcs, the pipe friction loss should only be 1.16 bar, ie static/dynamic of 3.6/2.44 bar but of course, as in all cases, I don't know if the pressure at the mains stopcock is a dynamic 3.6M, it may well fall as well, depending on the mains supply pipe diameter.

Anyway using the pipe loss calculations, and if you accept that the pipe ID is probably 11.5mm or thereabouts then a flowrate of 16LPM will result in a pipe loss of 1.07 bar and theoretically leave you with a pressure of 1.93 bar, (3.0-1.07) at the end of the 16M run.
You have no control over the mains pressure but of course you can renew the 16M run in 3/4" or 1" ID, ~ 28mm or 34mm OD, the equivalent pipe losses are then negligible at 0.09bar&0.02bar and are still only 0.3bar/0.08bar at a flow rate of of 30LPM. (at the end of the 16M run).

Friction Loss Calculator - National Pump & Energy

Calculate your efficiency losses due to friction in your system pipeline with NPE's easy-to-use Friction Loss Calculator.

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[ShaunCorbs]

Even with a new supply the water might drop eg won’t stay at a solid 16lpm as the water services in the area might not be upto all capacity etc

Tbh new 25mm water service and install an accumulator

 

[Andy VC]

Even with a new supply the water might drop eg won’t stay at a solid 16lpm as the water services in the area might not be upto all capacity etc

Tbh new 25mm water service and install an accumulator

Replace supply pipe with 25mm (ID or OD?) ? AND put in accumulator (had not thought of that) ?

 

[Andy VC]

Note, I'm having problems replying to some people's posts ...

 

[John.g]

GB pipework is metric (unlike ours which is still imperial) so 25mm is probably ~ 19mm ID or(3/4"), you can look up a table to give you the relevant numbers but probably not a huge difference in installation cost if going for a larger diameter.
There are two types of accumulator, one builds up a accumulated vol when pressure rises due to reduction in demand, the other is a pumped type.
Suggest buying a pressure gauge that you can stick on the end of a outside tap and monitor the pressure over various usages/times before making any decisions.

 

[Andy VC]

Many thanks for everyone's advice and comments. I think I have a better understanding of the issues now, even though I am not sure of the best way to move forward to improve the situation!

Thanks for now.