HPM
IN THE
MIX
“The fl ow rate from mixer showers varies
depending on the available head of water and
restrictions within the supply and valve”
Thermostatic mixers:
common questions
Thermostatic mixers remain very popular and are perfect for homes with a readily
available supply of hot water, whether preheated then stored in a cylinder or heated on
demand by a combi boiler. Steve Saunders, senior technical manager at Triton Showers,
highlights some of the most frequent questions raised during his training courses.
Q. I installed a thermostatic bath and
mixer shower supplied from a combi
boiler and I cannot achieve a hot enough
showering temperature. What could the
problem be?
Steve says: The two most common causes
of this symptom in a bath and mixer
shower (and on other outlets) are:
• The fl ow of water through the shower
is too strong, meaning the boiler
is unable to heat the water to the
required temperature. To establish if
this is the case, isolate the cold supply
and measure the hot water fl ow/
temperature through the mixing valve
when it is on its maximum setting.
If it is still under temperature, it will
be necessary to fi t some form of fl ow
limiting device on the inlet or outlet of
the shower
• A blockage has occurred in the hot
supply which is restricting fl ow. With
a combi boiler it is normal for the
cold fl ow through the mixing valve
to be signifi cantly greater than the
hot fl ow because there will always be
more restriction through the boiler
than through the cold pipework. To
determine whether or not there is
suffi cient hot fl ow available, turn the
mixing valve to the full hot position.
Next, check that the boiler has ignited
and remains so.
Q. I have fi tted a thermostatic mixer
shower to a combi boiler system only
to fi nd the shower temperature is
fl uctuating. I thought a thermostatic
mixing valve should remain at a constant
temperature?
Steve says: The most common reason
for this is when the combi boiler is not
fully modulating. This results in the
shower not maintaining a stable hot
water temperature. As hot water entering
the shower increases in temperature, a
thermostatic mechanism will adjust to
restrict hot water entering the valve. In
turn, fl ow through the boiler is reduced
and the temperature reaches a point
where the thermostat switches off the
burner. Water temperature will then
reduce before the thermostat resets, which
will result in the shower constantly cycling
between hot and cold.
Q. The fl ow from the mixer shower I
recently installed is less than I expected.
What is the usual performance of a
shower on a gravity system?
Steve says: The fl ow rate from mixer
showers varies depending on the available
head of water and restrictions within the
supply and valve. To ensure optimum
performance from gravity fed mixer
showers, the following guidance should be
followed:
• Use as few 90° elbows as possible and
use pulled bends
• Use full way lever or gate valves – do
not fi t restrictive stop taps or 1/4 turn
service valves
• Follow the manufacturer’s
recommendations regarding pipe size
• Ensure the mixing valve is specifi ed for
a low-pressure application
• Always fl ush out the pipes before
connecting to avoid debris blocking
inlet fi lters.
Q. How can I determine the pressure I
will get to a showerhead when a mixer
shower is connected to a gravity supply?
Steve says: The principle is that for
every one metre head of water, there
will be a pressure in the showerhead of
0.1 bar. This is a theoretical pressure
as restrictions within pipe fi ttings and
the shower mixing valve will reduce the
pressure slightly.
Providing only minimal restrictions are
present in the supplies and mixing valve,
Triton installers on a training course
which would have prompted some of the
questions answered by Steve Saunders
the additional measurement (height)
of water within the cistern will usually
compensate for pressure losses in the
supplies.
So, a shower requiring 1 bar minimum
operating pressure would need to be
connected to a cistern located ten metres
above the showerhead.
Q. The property I am working in has a
Thermal Store hot water system. How
many mixer showers can be fed from one
of these appliances?
Steve says: The output from a thermal
store unit depends on its size (stored
volume), heat transfer capability and the
water pressure entering the unit. Some
are only capable of delivering suffi cient
hot water to one mixer shower whereas
others can supply three or four showers
at the same time. Always check before
selecting any shower that the capacity of
the heating appliance is compatible with
the required usage.
Q. Can I use one twin ended pump to
supply water to three mixer showers?
Steve says: It is possible to supply a
number of mixer showers from one pump
providing it is suitably rated to deliver
the fl ow and pressure that will cater for
simultaneous shower use.
For example, three showers are
required to deliver 10l per minute at a
minimum delivery pressure of 1 bar when
operated individually. Therefore, a pump
capable of delivering 30l per minute at 1
bar running pressure is required.
If a number of showers are supplied by
one pump, supply pressures will fl uctuate
when showers are being operated
concurrently. Thermostatic mixing
showers should be fi tted to ensure a stable
showering temperature is maintained.
ENQUIRY NUMBER 112
34 Heating & Plumbing Monthly | OCTOBER 2018 | www.hpmmag.com
/www.hpmmag.com
