|Having read about unvented hot
water systems it might seem somewhat of a contradiction that mains pressure
water can be achieved from a vented cylinder - but it can.
They say that the best ideas are the simplest.
Well, that could be argued about what's known as a 'Thermal Store'.
To the untrained eye a thermal store and an
unvented cylinder look very similar. They are typically cylindrical in shape
and the are both full of hot water. The difference is how they produce mains
pressure hot water.
We have covered the unvented cylinder in
the section above. So we shall now focus on the thermal store.
At the outset it must be said that one of
the most attractive properties of a thermal store is the fact that it is
vented. This is to say that the body (where the great volume of water is
stored) is at atmospheric pressure. It is therefore inherently safe as
dangerous pressure build up is impossible.
A thermal store is heated in exactly the
same way as any other cylinder (directly or indirectly). But unlike nearly
all others, the water in the cylinder is not destined to appear at
any taps or outlets - water is used solely to store a large quantity of thermal
energy (a little like the chemicals in a battery store electrical energy).
We therefore view the cylinder as a thermal battery or
The way a thermal store achieves mains
pressure hot water is by imparting the stored energy (from the hot water
stored at atmospheric pressure) into incoming cold water at mains pressure.
This is done by a process known as heat exchange and is achieved by
using.... (surprise surprise!)... a heat exchanger.
There are different types of heat exchange
device/method available with thermal stores, but essentially they all to the
same job - they take heat from the open vented source and transfer it to
mains pressure water as it passes through. In this way vented and unvented
can exist in proximity to each other but never mix.
Different heat exchange methods claim
different performance characteristics. Some are simple (immersed heat
exchangers for example), some may require complex control (plate heat
exchangers for example).
Whichever method is used they all share
they all share they same benefit - mains pressure hot water from a vented
cylinder that is inherently safe.
Thermal stores, apart from being inherently
safe, offer several other advantages that are beyond the capability of an
- Thermal stores run much hotter than a
normal or unvented cylinder (which must be set at the temperature you
want water to arrive at taps). So, litre for litre a thermal store holds
more energy. To prevent any risk of scalding, the temperature of the
mains pressure water leaving a thermal store is governed. The method by
which this is achieved varies according to the heat exchange method.
Many utilise a thermostatic mixing valve which prevents water from being
delivered above a set temperature.
- A thermal stores can accept multiple
heat sources simultaneously. This can also be true for vented and
unvented cylinders but more importantly thermal stores can accept heat from
'uncontrollable' heat sources such as wood burning stoves and Aga's that
cannot be turned up or down as required. They are also ideal for
appliances that operate by virtue of gravity (unpumped / thermosiphon)
- A thermal store can be configured at
the 'centre' of your domestic hot water system taking all sorts of heat
inputs (including solar, ground source and air recovery) then give back
energy not only for domestic hot water, but also central heating, under floor heating
etc. by taking feeds directly off the thermal store.
So, for example, a wood burner heats the store,
then hot water from the store is pumped off around radiators - the wood
burner now contributes to your central heating! Everything that
heats the store contributes towards its function. You may begin to
appreciate the versatility of this type of system.
- You do not need to notify Building
Control when you install a thermal store. G3 Regulations only apply to a
vessel containing more than 15 litres of hot water under pressure. The
body of a thermal store is vented and there is less that 15 litres of
hot water in the heat exchanger. Thermal Stores are not governed by these regulations.
- Because there is no inherent danger
with a thermal store they may be fitted by a competent person without
Thermal stores are particularly useful (and
in some situations essential) in conjunction with renewable energies fuels
and are available in various models including heat pump and solar. You might
like to take a look at some of the models: