Could someone explain why the flow rate of hot water from a combi boiler to
a tap open at a constant rate reduces as the water heats up?

"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi boiler
to a tap open at a constant rate reduces as the water heats up?



The tap washer was compressed - the heat swells it and reduces the flow.

On Fri, 05 Sep 2008 23:00:59 +0100, John wrote:

"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi boiler
to a tap open at a constant rate reduces as the water heats up?

The tap washer was compressed - the heat swells it and reduces the flow.

I'd guess it's the metal works of the tap rather than the washer itself,
but I've noticed the phenomenon too and agree it's thermal expansion of
some sort acting within the tap.

--
John Stumbles

The floggings will continue until morale improves

On Fri, 5 Sep 2008 22:24:23 +0100, "Gareth"
wrote:

Could someone explain why the flow rate of hot water from a combi boiler to
a tap open at a constant rate reduces as the water heats up?

Mine doesn't.

EricP" wrote in message
...
On Fri, 5 Sep 2008 22:24:23 +0100, "Gareth"
wrote:

Could someone explain why the flow rate of hot water from a combi boiler
to
a tap open at a constant rate reduces as the water heats up?

Mine doesn't.

Mine does.

On Sat, 6 Sep 2008 00:01:34 +0100, "R D S" wrote:

EricP" wrote in message
.. .
On Fri, 5 Sep 2008 22:24:23 +0100, "Gareth"
wrote:

Could someone explain why the flow rate of hot water from a combi boiler
to
a tap open at a constant rate reduces as the water heats up?

Mine doesn't.

Mine does.


Weird.

I just tried it in case I had missed something.

Still doesn't.

Gareth wrote:
Could someone explain why the flow rate of hot water from a combi boiler
to a tap open at a constant rate reduces as the water heats up?



Doesn't it happen regardless of type of boiler?

I am fairly sure that many taps do this - with consequent changes to the
sound the water makes as it flows through them - even when fed by
gravity. I know that I have often opened a bath tap at cold, left it
running at a modest rate and on returning found it barely dribbling bit
hot. Of course, the sound aspect is most noticeable on a tap that is
making quite a bit of noise, typically when not fully open, but some
always make a noise.

Agree with other posters - simply must be mainly thermal expansion of
tap components. I'd hazard a guess that ceramic disc taps would be less
likely to exhibit this behaviour. Wonder if changes in physical
characteristics of water as if warms up also have an effect?

--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
onset.
Although common it frequently goes undiagnosed.
www.thyromind.info www.thyroiduk.org www.altsupportthyroid.org

Rod wrote:

Agree with other posters - simply must be mainly thermal expansion of
tap components. I'd hazard a guess that ceramic disc taps would be less
likely to exhibit this behaviour. Wonder if changes in physical
characteristics of water as if warms up also have an effect?

It seems more noticeable on main pressure hot water systems - presumable
since the tape needs to be far less "open" to get a flow and hence any
reduction is more significant.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
Rod wrote:

Agree with other posters - simply must be mainly thermal expansion of
tap components. I'd hazard a guess that ceramic disc taps would be
less likely to exhibit this behaviour. Wonder if changes in physical
characteristics of water as if warms up also have an effect?

It seems more noticeable on main pressure hot water systems - presumable
since the tape needs to be far less "open" to get a flow and hence any
reduction is more significant.

And I thought I'd be the only silly bugger posting at this sort of time. :-)

Yes - that makes sense. And thinking about it, at least some experiences
of such taps have been with fairly high hot water pressure - whether
combi or not.

--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
onset.
Although common it frequently goes undiagnosed.
www.thyromind.info www.thyroiduk.org www.altsupportthyroid.org

"John" wrote in message
...

"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi boiler
to a tap open at a constant rate reduces as the water heats up?



The tap washer was compressed - the heat swells it and reduces the flow.


My fathers has a flow limiter and a temperature stabiliser on it.. when you
first turn it on it has some stored hot water and the flow is quite fast,
after a few seconds it slows down as it hits the maximum flow rate for the
heat output.
If your pipe work is just the right length it would just be getting hot at
the tap when this happens.

In article ,
Rod wrote:
I am fairly sure that many taps do this - with consequent changes to the
sound the water makes as it flows through them - even when fed by
gravity. I know that I have often opened a bath tap at cold, left it
running at a modest rate and on returning found it barely dribbling bit
hot.

If that's happening on a gravity system something's wrong. Possibly an
airlock somewhere.

--
*If tennis elbow is painful, imagine suffering with tennis balls *

Dave Plowman London SW
To e-mail, change noise into sound.

John wrote:
"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi
boiler to a tap open at a constant rate reduces as the water heats
up?
The tap washer was compressed - the heat swells it and reduces the
flow.

Ah, but what if the OP has ceramic taps?


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk

"Dave Plowman (News)" wrote in message
...
In article ,
Rod wrote:
I am fairly sure that many taps do this - with consequent changes to the
sound the water makes as it flows through them - even when fed by
gravity. I know that I have often opened a bath tap at cold, left it
running at a modest rate and on returning found it barely dribbling bit
hot.

If that's happening on a gravity system something's wrong. Possibly an
airlock somewhere.

--
*If tennis elbow is painful, imagine suffering with tennis balls *

Dave Plowman London SW
To e-mail, change noise into sound.


My combi has a flow limiter on the input side of the boiler - used to come
in with a 'clunk' but I think it has seized up now as I never hear it. As
thermal expansion is quite small, I thought that heat softening a tap washer
and making it swell to a pre-compressed state was quite a bit more likely.
As others have said, with mains pressure, the tap would not be open very far
so the effect would be more noticeable. Also - could suspended air have an
affect? My hot is quite milky in appearance until it stands a while.

"dennis@home" wrote in message
...


"John" wrote in message
...

"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi boiler
to a tap open at a constant rate reduces as the water heats up?



The tap washer was compressed - the heat swells it and reduces the flow.


My fathers has a flow limiter and a temperature stabiliser on it..

What make and model?

On Sat, 06 Sep 2008 08:12:33 +0100, dennis@home wrote:

"John" wrote in message
...

"Gareth" wrote in message
...
Could someone explain why the flow rate of hot water from a combi
boiler to a tap open at a constant rate reduces as the water heats up?



The tap washer was compressed - the heat swells it and reduces the
flow.


My fathers has a flow limiter and a temperature stabiliser on it.. when
you first turn it on it has some stored hot water and the flow is quite
fast, after a few seconds it slows down as it hits the maximum flow rate
for the heat output.
If your pipe work is just the right length it would just be getting hot
at the tap when this happens.

Mostly the effect is in the tap, but as you say there a few models where
some feature of the boiler comes into effect.

Those with a store of HW will last more than just enough to warm up the
tap.

Another model which has this feature is the Pott Puma combi which did this
the other way around, restricted the flow of HW until the boiler was
working steadily.





--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Sat, 06 Sep 2008 05:28:36 +0100, John Rumm wrote:

Rod wrote:

Agree with other posters - simply must be mainly thermal expansion of
tap components. I'd hazard a guess that ceramic disc taps would be less
likely to exhibit this behaviour. Wonder if changes in physical
characteristics of water as if warms up also have an effect?

It seems more noticeable on main pressure hot water systems - presumable
since the tape needs to be far less "open" to get a flow and hence any
reduction is more significant.

I'll second that observation.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Sat, 06 Sep 2008 10:35:02 +0100, John wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
Rod wrote:
I am fairly sure that many taps do this - with consequent changes to
the sound the water makes as it flows through them - even when fed by
gravity. I know that I have often opened a bath tap at cold, left it
running at a modest rate and on returning found it barely dribbling
bit hot.

If that's happening on a gravity system something's wrong. Possibly an
airlock somewhere.

--
*If tennis elbow is painful, imagine suffering with tennis balls *

Dave Plowman London SW
To e-mail, change noise into sound.


My combi has a flow limiter on the input side of the boiler - used to
come in with a 'clunk' but I think it has seized up now as I never hear
it. As thermal expansion is quite small, I thought that heat softening a
tap washer and making it swell to a pre-compressed state was quite a bit
more likely. As others have said, with mains pressure, the tap would not
be open very far so the effect would be more noticeable. Also - could
suspended air have an affect? My hot is quite milky in appearance until
it stands a while.

This is (IME) air coming out of solution, if possible try to turn the
boiler down on the HW setting and/or increase the flow for DHW. This
effect means the boiler is over heating the water and you may also get
lime scale build up of the water is hard.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html