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Touchy Taps

Question: I have a shower hose that attaches to the hot and cold taps in my
bath. When I want to use the shower I turn on the hot tap fully and then, very
slowly, turn on the cold tap in the belief that the water temperature will
adjust slowly until it is just how I want it.

However, the temperature instead turns dramatically from the scalding hot to
the icy cold even though the adjustment is slow. Why is this and why does the
temperature not change gradually?

Answer: The hot tap on your bath is fed from the cold water tank in the loft
via the hot water cylinder. The head on this (the height above tap level) in an
average modern house with an upstairs bathroom is about 4 metres, giving a
pressure at a closed tap of about 34 kilopascals.

The cold water, on the other hand, may be fed from the rising main—that
is, directly connected to the water supply—and have a pressure of about
310 kilopascals, which is obviously much higher.

When the hot tap is turned on for a shower, a full flow of water is obtained
only by opening the tap wide, because the pressure that is available to push the
flow through the nozzle is low. However, when the cold tap is opened slightly,
the high pressure in the cold main produces a relatively high flow, which
immediately swamps the mixture with the cold water.

The pressure behind the shower hose nozzle also rises because of the increase
in flow, and this back pressure reduces the flow of hot water, increasing the
cold effect even more.

Where fixed showers are plumbed into both hot and cold, the cold should be
connected directly to the storage tank in the loft rather than to the rising
main, to make the pressures of hot and cold water roughly equal.

This, of course, also makes the job of regulating the temperature much easier
and you are far less likely to be blasted by icy water or scalded by a hot
burst.

In most areas of Britain, it is in fact illegal to plumb mixing fixtures into
the high-pressure cold water main, but it is often done by lazy plumbers or
builders who are trying to save money by cutting corners.

Indeed, the practice of connecting a shower directly to the cold water main
carries a hidden danger. If the shower temperature has been regulated to a
comfortable level, but then someone draws water from a cold tap elsewhere in the
house, say to fill a kettle downstairs, then the cold pressure at the shower
head is suddenly and considerably reduced, and the person taking the shower may
be scalded.

If the plumbing is done correctly, this cannot happen. In this case the worst
thing that is likely to happen is that someone will draw hot water from the
downstairs hot tap, reducing the hot pressure at the shower, so the person
showering gets a sudden cold blast of water!

Tony Finn

Hull, East Yorkshire

Answer: Domestic water tap valves rely on water pressure to push them open.
It is the differences in this water pressure through the hot and cold taps that
causes the difficulties experienced by your questioner.

The problem of fine control of the mixed water temperature arises when the
high-pressure mains cold water meets the back pressure produced by a fine shower
rose and as a result attempts to overpower the low-pressure hot stream.

This causes the hot tap’s washer to slam shut like a non-return valve, with a
resulting loss of contribution from the hot side. Achieving a fine control with
crude domestic taps is frustrating at best, so the tap temperature see-saw game
begins for the poor person in the shower.

The cures for this problem are various: to get a broader shower rose with
more holes and thus lower back pressure (unfortunately, this means that
water-saving needle jets are out); to get a mains-pressure hot water service
(the drawback here is that this is very expensive); to put a pressure reducer on
the cold supply to the bathroom (this is difficult to install); or you could use
a trick that an old plumber once showed me for dealing with low-pressure
taps— take the tap washer out of the hot tap and slightly flatten it so
that it sticks in the tap spindle and is therefore less likely to drop down and
slam shut. The last option is the easiest to try and, at worst, you will only
have to get a new washer if it doesn’t help to solve the problem.

Nigel Holmes

Queensland Environmental Protection Agency

Rockhampton, Queensland

It used to be illegal to connect taps and the entire cold water supply
directly to the mains supply in Britain, partly because of the fear that
contaminated water might be drawn back into the mains if the pressure fell, thus contaminating other
households further down the line, and partly because many products in British
households had been designed to cope with the lower pressure that comes with
having a separate water tank. The mains supply would be too powerful for these
to operate correctly and they may have been damaged by the higher pressure.

Since the 1980s, however, the law has changed, and houses are now built with
plumbing designed to cope with the pressure from the cold water main. Hot water
is also provided from a cylinder that is designed to take the pressure of the
mains, so the hot and cold water pressures are balanced.

This has been the practice in much of Europe for some time. If you do live in
an older house, here’s a very simple solution to the problem:

Answer: To save money and also the risk of scalding, you can set the hot
water thermostat in your system at the temperature level at which you want your
shower, and use only the hot water for your shower.

In my opinion it is absolutely stupid to heat water to too high a temperature
and then cool it with cold water!

D. G. Jones

Mechanical engineer

Valbonne, France

This week’s question

Tongue torture? Why do fizzy drinks produce the sensation they do? I’ve heard
that they stimulate the pain receptors in the tongue.

Can it be true that when you drink a simple glass of sparkling water you are
really enjoying a form of low-intensity masochism?

Tom Starling

London