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Old July 13th 07, 01:24 PM posted to uk.d-i-y
olo olo is offline
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Default combi DHW supply/flow pipe size

Hi folks,

need your advice on pipe size - what would you recommend (15mm or
22mm) as a supply (cold) to and flow (hot) from combi boiler to taps?
Boiler can produce 14 ltrs/min so one would think that 15mm pipe is
enough. However there are opinions that 22mm is preferred because
boiler will get priority in getting cold water.

thanks for any info,


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Old July 13th 07, 01:40 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size


"olo" wrote in message
oups.com...
Hi folks,

need your advice on pipe size - what would you recommend (15mm or
22mm) as a supply (cold) to and flow (hot) from combi boiler to taps?
Boiler can produce 14 ltrs/min so one would think that 15mm pipe is
enough. However there are opinions that 22mm is preferred because
boiler will get priority in getting cold water.


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.


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Old July 13th 07, 02:38 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size

On 13 Jul, 13:24, olo wrote:
Hi folks,

need your advice on pipe size - what would you recommend (15mm or
22mm) as a supply (cold) to and flow (hot) from combi boiler to taps?
Boiler can produce 14 ltrs/min so one would think that 15mm pipe is
enough. However there are opinions that 22mm is preferred because
boiler will get priority in getting cold water.

thanks for any info,


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?

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Old July 13th 07, 02:48 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size

Anita Palley wrote:
On 13 Jul, 13:24, olo wrote:
Hi folks,

need your advice on pipe size - what would you recommend (15mm or
22mm) as a supply (cold) to and flow (hot) from combi boiler to taps?
Boiler can produce 14 ltrs/min so one would think that 15mm pipe is
enough. However there are opinions that 22mm is preferred because
boiler will get priority in getting cold water.

thanks for any info,


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.
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Old July 13th 07, 03:05 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size


"Anita Palley" wrote


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?

Yes

15mm pipe from the stoptap to the boiler with associated elbows etc creates
a pressur drop.
Using 22mm right up to the 15mm boiler connection guarantees minimal losses
before the boiler.

Phil




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Old July 13th 07, 03:40 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size


"TheScullster" wrote in message
...

"Anita Palley" wrote


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?

Yes

15mm pipe from the stoptap to the boiler with associated elbows etc
creates a pressur drop.
Using 22mm right up to the 15mm boiler connection guarantees minimal
losses before the boiler.


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.

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Old July 13th 07, 04:10 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size


"The Natural Philosopher" wrote in message
...
Anita Palley wrote:
On 13 Jul, 13:24, olo wrote:
Hi folks,

need your advice on pipe size - what would you recommend (15mm or
22mm) as a supply (cold) to and flow (hot) from combi boiler to taps?
Boiler can produce 14 ltrs/min so one would think that 15mm pipe is
enough. However there are opinions that 22mm is preferred because
boiler will get priority in getting cold water.

thanks for any info,


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.



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.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)


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Old July 13th 07, 04:57 PM posted to uk.d-i-y
olo olo is offline
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Default combi DHW supply/flow pipe size

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.

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Old July 13th 07, 05:00 PM posted to uk.d-i-y
olo olo is offline
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Default combi DHW supply/flow pipe size

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

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Old July 13th 07, 05:13 PM posted to uk.d-i-y
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Default combi DHW supply/flow pipe size


"olo" wrote in message
oups.com...
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.


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.



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