My in laws have a mixer shower that me and he fitted a few years ago. Their
system then was a gravity fed solid fuel system, with a copper cylinder and
a massive cold water header tank in the loft. The shower was fed from these
two tanks. They have now had the system changed to a gas combi boiler and
all is fed at mains pressure, the main reason for the change is they are
getting on a bit and the thought of having to go for the fuel at the bottom
of the garden started to look less appealing!
The problem is if anybody flushes a toilet, turns a cold tap on etc. when
the shower is in use it gets a bit hotter due to the 'now shared' cold water
pressure. It doesn't get hot enough to scald so isn't a major rproblem but
if it can be quite easily and quickly remedied the FIL would like to do it.
Due to him being nearly 70 and the bathroom looking really nice with flush
fitting shower valve and head, replacing it with a thermostitic mixer is NOT
Any ideas / thoughts most welcome.
Replacing it with a thermostitic mixer suitable for combi/multipoint
is probably the only thing which will give a satisfactory fix.
The combi installer should have warned you about this as it's entirely
predictable. (I wonder if they checked the mains pressure and flow
rate was suitable for a combi?)
Other things you might try...
Reduce the flow to the toilet cistern so it makes less impact on
the water pressure, at the expense of taking longer to fill. If
it's got an isolating valve, you could try half closing it. You
could try the same for other water consuming items.
If your mains water pressure is reasonably high, you could fit a
pressure reducing valve on the shower cold water supply, so it
doesn't see pressure fluctuations, providing the supply pressure
doesn't drop below the pressure reducing valve set pressure.
IANA plumber but did see something in the Screwfix catalogue that might
You can get inline thermostatic mixing valves, that you could connect
into the existing hot water supply going to the shower, plus a
connection to the cold water supply.
That will limit the hot water temperature available at the shower.
Whether you would be allowed to use one on this system, is a different
It might help.
What worries me a bit is that a sudden demand for water from - say - a
cistern will reduce *both* the cold and the hot flow. So even if you balance
the pressures, the reduced flow through the combi may result in a higher
outlet temperature - so the mixed temperature will *still* rise.
Opening the main stop cock a bit may reduce the effect as may closing
down the toilet cistern inlet but are unlikely to totally cure the problem.
Using a pressure balancing valve may work better than a pure (wax
capsule) thermostatic valve as it will respond more quickly to a step
change in pressure.
http://www.plumbingpages.com/featurepages/Greens.cfm for a case study
almost exactly matching your situation but it's 40 quid for the parts alone:
http://www.plumbworld.co.uk/344-0000 or a bit cheaper here:
http://www.bes.ltd.uk/products/110.asp (this price + vat & post).
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