We have a combi boiler and when you run the hot tap after a few minutes,
the hot water starts to flow quite well.
When you run the water for the bath, the taps has to be turned virtually
off to get really hot water out.
I do appreciate that the hot water is dependant upon the speed of which
the water passes through the boiler.
Here's the bit I am curious about. From the boiler carrying the hot
water is a 15mm copper pipe. This then feeds into the original 22mm
copper pipe to the bath which is approximately 8 feet away. Isa it
because of the 22mm pipe and the 3/4" tap that the tap has to be
virtually off before water gets really hot.
Comments most welcome.
I can't see the pipe size making any real difference in this situtation.
Also you did not mention what make/model of boiler it is. The power of
the boiler will dictate what flow rate of hot enough water you can get.
As Lee said, some people like to partially turn off the cold water inlet
valve on the boiler (usually just a quarter turn valve just under the
boiler - you will need a screwdriver to turn it probably). That way you
can limit the supply of cold water to a rate that the boiler can heat to
"hot enough". This will make running a bath a bit simpler since you can
turn the bath tap full on, and know you will get hot rather than tepid
On Sun, 09 May 2004 16:07:51 +0100, troubleinstore wrote:
The same flow from a bath tap on a basin tap will look
pathetic from the bath tap. Especially if the tap is of a traditional
style with a bell mouth spout.
Some boilers allow you set throttle the HW flow for the whole
installation, this feature may well be useful to you.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
I thank the person who informed me that I had ommitted the bioler
It is a Alpha 'OCEAN' 240XP.
I have had a look where the cold water supply goes into the boiler and
there is what appears to be a screw head on the mains water inlet valve.
Is this the thing I have to turn to regulate the flow into the boiler.
There are no other valves on the cold water inlet pipe.
I can't find anything in the installation and servicing instruction
booklet that refers to HW flow.
Yep, the bath taps are of the traditional style.
According to the SDA boiler database program that has a max power of
just over 23kW. Not sure if that is its input or output power however.
Since its efficency is low at 70% that could mean the actual power going
into heating the water could be as low as 16kW.
That translates to a feeble 6.5 litres/min at a 35 degree rise (or
better but ot great 9.5 l/m if the 23kW is output rather than input
power). That ought to be OK for a shower - but running a bath is going
to be on the slow side.
Sounds like that could be the one. You may find it is of the ball valve
type - when the screw head is inline with the pipe it is full on, and
rotated 90 degrees it is completly off. Try running a hot tap full on
and then slowly turning the screw a little and see if you can reduce the
flow rate. You should be able to find a point where the water is running
hot enough with the tap fully on. Remember that if you do this the
setting of the flow restriction will change in the summer as the ground
water gets warmer.
It is low enough they probably thought the less said about it the better ;-)
According to the Installation & Servicing Instruction booklet:-
The maximum output available for domestic hot water is 23.3 kW (79,500
Btu h), capable of providing 9.5 litres/min with a temperature rise of
35 deg C.
We didn't know anything about central heating boilers when we moved in
and so we were in the hands of the heating engineer. I must admit though
that looking at this so called proffessionals quality of work, if needed
again at my next house, I will do all the central heating and just have
someone to commission the boiler.
Knowing what I know now from reading various threads on this group, a
better boiler would have been installed.
Just a thought...
We have an Alpha Ocean 280E boiler. Have you got the hot water
thermostat control (I assume there is one as there is on ours) set
somehwere near maximum? This, I think, should set the maximum water
temperature irrespective of the flow rate. However, on ours wherever it
is set you can still get VERY hot water initially, before the
thermostsat kicks in, and if you turn the hot tap down. With it set to
max the bath tap can be on full and the water too hot to touch.
Yep, the temperature control is set at max on the boiler.
Turn hot tap on the bath fully on and you get lukewarm water, turn it
down so as you get less flow than the sink tap and you get really hot
water, out of both taps. That was why I was asking in the original post
if the 22mm original pipework that the 15mm hot water pipe from the
boiler fed into would make any difference.
On Mon, 10 May 2004 12:04:15 +0100, troubleinstore
I believe the 'flow' is generally set by biggest restriction in the
system. In you case this could be the 15mm pipe or more likely the
water heater itself. Once the water hits the 22mm it simply slows down
because of the increased volume of the pipe (but the same l/min).
My instant water heater is fed with (unrestricted) 15mm mains water
but also uses (mostly) 22mm to the bath as I was due to have a
cylinder there at one point. The only 'issue' for me is how much water
you have to run (waste?) before you get the hot through.
We get 'enough' rate to fill a bath with 'bearably' hot water but
wouldn't be able to run the sink or basin taps at the same time (2 of
3 of us (electric) shower anyway) ;-)
All the best ..
T i m
On Sun, 09 May 2004 21:44:36 +0100, troubleinstore wrote:
Don't use the 1/4 turn service isolator for controlling the flow rate.
Put a gate valve or stop valve that works well when partially open/closed.
Turning the the HW thermostat on the boiler to a high setting permits the
boiler to make scaldingly hot water, it will not enable it to make more
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
I'm trying to get my head round this...
The HW thermostat seems to control the max temp of the hot water but
does not alter the flow through the boiler. So with the thermostat on
the highest setting if I have 12.5 l/min running through the boiler I
will epxect it to be heated to 45C, assuming the cold is 10C. By turning
the HW thermostat down I can reduce this temperature. If I reduce the
flow of water through the boiler I can increase the maximum temperature
- if I can raise 12.5 l/min by 35C then I should be able to raise 10 l/m
in by more than 35C but will the thermostat limit the max temp of the water?
If I want a shower at say 42C through a thermostatic shower mixer valve
how does it work and where should the bolier HW thermostat be set? The
thermostatic shower mixer valve will take hot and cold water and mix it
to make it 42C. If, to do this, it has to reduce the hot water flow then
won't the boiler heat the hot water even hotter and so cause the
thermostatic shower mixer valve to reduce the hot flow even more etc
etc. If the hot water isn't at 42C to start with then my shower will
never be hot enough, but it would seem from the above that I should have
my HW thermostat set at the highest value, although I can see that this
isn't a good idew when it comes to non-thermostatically controlled taps
at washbasins and sinks.
I'm probably being dim here but I would be interested in the theory
behind all of this.
You would increase the temperature - but not the maximum. The boiler
should not produce water hotter than the maximum temperature set on its
thermostat regardless of how low the flow rate.
It could also do it by leaving the hot flow untouched and raising the
cold flow rate. Not sure if they all use the same technique - guess I
will find out shortly when I install a thermostatic shower mixer ;-)
If that were the case it would end up reducing the flow until the max
temperature as set on the boiler was reached. Thereafter a further
reduction in flow rate will have the effect of lowering the mix
I would set it a bit hotter than the maximum temperature you are likely
to want for a bath or shower. That way the mixer can achieve the
required mix and you don' run the risk of boiling the next person
washing their hands under the hot tap.
Not dim really - good set of questions in fact - I will be interested to
hear the responses who have showers of this type on a combi.
I would have thought that if the boiler is non modulating, it will cut
out once the temperature set on the thermostat is reached.
If this is the case it may be best to use the mixer with the flow to
set to maximum, so the mixer does not restrict the flow from the
Also thermostatic mixers have a maximum hot water operating
temperature, so it would be worth setting the thermostat on the boiler
some way below this.
On Sun, 09 May 2004 16:07:51 +0100, troubleinstore
You can get a bit of a temperature drop through uninsulated copper
piping over that distance.
In our house, we insulated the hot water pipe between the boiler and
the bath / shower. It made a real difference to the temperature.
If you are someone who likes to spend ages in the shower, then a hot
water tank is a pain. It's runs out of hot water. I also find that the
water pressure of a hot water tank tends to be quite low.
The only real advantage for a hot water tank is the ability to fit a
power shower. However, this empties the hot water fairly quickly.
If the hot water tank is in the loft below the header tank then the
pressure will be much better, but, like when I was living at home, the
hot water tank was in the bedroom and even filling a bath from the tap
was like watching paint dry.
On Mon, 17 May 2004 17:06:31 +0100, troubleinstore
My brother had a hot water tank and a power shower. The power shower
pump would pump the water from the tank at a very high pressure. It
was like having a shower under Niagra Falls.
However, even when the tank was very hot, it could only provide
sufficient hot water for around 10 minutes.
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