Pressurised
Hot Water Systems
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If like us you have the somewhat dubious pleasure of a pressurised
hot water system the you have scope for reducing both your water
and heating costs. Having suffered leaking tank, a failed expansion
tank and a failed seal which let large volumes of water at mains
pressure flood the house and bring down a ceiling I have a healthy
distrust of these systems since while you get similar pressure
in the cold water system hot water causes much more expansion and
contraction leading to a higher chance of failure than with a more
conventional hot water system which runs at a much lower pressure.
These systems are not for the faint hearted and should only be
modified if you are sure you know what you are doing. They are
also supposed to be checked annually by a competent person.
Principles Of Operation
These systems operate can operate at full mains water pressure
(depending on the pressure into the house). The hot water tank
is fed directly from the cold water supply via pressure reducing
valve and a non return valve. The pressure reducing valve is
normally set to 3 bar. To cope with the fact that as it heats
up water expand there is an expansion tank split in half by a
membrane. The lower half is directly connected to the hot water
tank and the upper half is pressurised by air at a pressure of
3bar. The tank also has a pressure relief and temperature relief
valves to stop an explosion if the pressure or temperature get
too high.
Reducing Water And Heat Use
The way to achieve savings is to adjust the pressure reducing valve
setting from 3 to 2 bar. This is slightly more difficult than
it sounds since you need to keep venting water after each small
adjustment to reduce the tank pressure due to the action of the
pressurised expansion tank. Having adjusted the pressure to 2
bar (most tanks have a pressure gauge fitted) I then reduced
the air pressure in the expansion tank to 2.5 bar by letting
some air out. The reason for doing this is to more nearly centralise
the membrane. Doing these adjustments reduces the flow rate from
the hot taps and hence tends to reduce water usage plus reducing
water heating costs as well.
Caution - These systems should only be modified by a competent
person.
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