On 24/07/2019 21:37, Roger Hayter wrote:
Roger Hayter wrote:

John Rumm wrote:

On 24/07/2019 09:03, RJH wrote:
On 22/07/2019 14:29, RJH wrote:
On 22/07/2019 14:08, Robin wrote:
On 22/07/2019 11:21, wrote:
On Monday, 22 July 2019 10:45:04 UTC+1, RJH wrote:
I'm about to relocate a couple of radiators, and plan to loop some
10mm
plastic pipe behind the skirting boards into the 15mm main run.
Is there a calculation/formula that determines what losses are at
play?
Or if the resistance to flow might become a problem?

8mm carries 1500W, 15mm-6kW, 22mm-13kW. The first limiting factor is
the velocity of the water in the pipes. If it exceeds 1.5m/s which
is the maximum velocity specified in BS 5449 you will start to get
noise problems.

The second limiting factor is the pump head and pipe length/resistance.

See
http://www.users.waitrose.com/~robin...ter/pump/pipes
izing.html



Those figures seems much lower than


https://www.heatweb.co.uk/w/index.ph...culations#Limi
ts_imposed_by_velocity_versus_common_pipe_sizes


and with the way many 30-odd kW domestic boilers are installed.

PS BS 5549 was replaced by 12828 - but with the same recommendation
about 1/5m/s


Thanks both - quite a bit to go on there!


A couple of Qs if I may:

The resistance increases quite a bit as the 15mm goes to 10mm. But only
for a short run (about 1m), where it goes into the rad via 15mm. This
implies the flow, and the rad outlet, is now at this reduced level. Is
this correct?

Flow resistance is proportional to both the pipe internal diameter, and
also the length of the pipe. So a short run of smaller diameter pipe
will have less effect than a longer one.

For the sake of future readers who may know that you are an expert, I
take it you are using "proportional" in the colloquial sense of "depends
on". As you know. it is actually proportional to quite a high power of
the internal diameter, the exact relation depending on whether there is
any turbulence.



Since I'm doing corrections, I suppose it's reciprocal resistance, flow
per unit pressure rather then pressure per unit flow, that goes up with
diameter!

Perhaps proportional is a poor choice of word since that normally
implies that you can swap in an equal sign and a constant and you have
yourself an equation. The influences on flow resistance are more complex
than simple variables than you rightly state. Perhaps flow resistance
would be better described as a function of pipe length and diameter and
other variables. :-)

The thrust of what I was getting at was, yes a reduction in diameter
will restrict flow, however a very short length reduction in diameter
will have much less effect than a longer run at the same smaller
diameter. So using a pipe insert that reduces a nominal 15mm^2 to 10mm^2
for 20mm will not be anything like as "damaging" as a 5m run of 10mm^2
micorbore pipe.

--
Cheers,

John.

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