What follows is a simple,
but detailed explanation of the different boiler types generally available.
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In a conventional boiler gas jets
play onto a cast iron heat exchanger through which
passes to be heated. If used to supply taps, hot water cannot be
provided on demand but must be stored, usually in a copper
cylinder. This type of boiler has relatively simple controls and
tends to be more reliable as less can go wrong. Energy consumption
can be high but the cost of this is moderated by the low
maintenance costs. They are versatile in that they can be used in
almost any type of property and can be pumped or gravity fed.
diagram to enlarge.
combination boiler heats water for the central heating in the same
way as a conventional boiler but can also provide the taps with
instant hot water. The main difference is that the hot water for
the taps is fed directly from the mains rather than from a hot
water storage cylinder. This can also be useful in a small
property where space for a cylinder is at a premium. The internal
workings are increasingly technical and can be perceived as
unreliable but are becoming more accepted. Other than the
lack of an airing cupboard one problem can be that in the event of
a breakdown the user can be without both heating and hot water whereas
on a conventional system an electric immersion element can usually
be inserted into the cylinder to provide hot water for the taps. With
a combination boiler, hot water and central heating
requirements are provided from the one unit. As well as providing
central heating water into the radiators, it provides all of the
domestic water for baths, sinks, and showers.
The big advantage with a combination boiler, or a
"combi" as it is usually called, is that it not only
delivers continuous hot water, but more importantly delivers it at
mains pressure too. So, with a combi system you can have a really
effective shower without needing an expensive "Power Shower" booster pump.
Combis are also considered to be amongst the easiest systems to
install, because they eliminate the need for both an expansion
tank in the loft and a hot water cylinder in an airing cupboard.
The combi does have limitations. Most standard combis take 40
seconds to heat water.
Combis provide maximum pressure through only one tap at a time. If
you have two taps running, the powerful flow rate is diminished in
one (or both) of the taps. So, if you want high temperature and a
high output flow rate from your combi, choose a high capacity or a
Combi's provide maximum pressure through only one tap at a
time. If you have two taps running, the powerful flow rate is
diminished in one (or both) of the taps.
By adding a
storage tank to a combi, the problems of flow rate reduction are
progressively overcome, depending upon the size of the tank. Now
you can use a couple of taps simultaneously, without an
unacceptable drop in performance.
The advantage of this system over a conventional boiler and
storage tank is that the hot water never runs out. Even after
running a bath, a combi storage boiler doesn't need time to
recover before you can use it again. So there's no need to plan
your hot water requirements, hot water is always there.
However, the flow rate from an average combi storage boiler does
not match the high flow rate from a modern system with an unvented
Like conventional boilers, a system boiler can provide central heating and hot water from a cylinder if required. The key difference with a system boiler is that all the major components are built in to the boiler.
For example the pump, normally installed remote from the boiler, in built in; so is the expansion vessel, which replaces the feed/expansion tank often installed in the loft. The safety valve, the automatic air vent and even the programmer are also
included. With these components built in installation time is reduced significantly, fewer materials are required, costs are reduced and servicing is simplified. The added bonus of a "dry loft" removes the worry of any leak or frost damage to
Typical System Boiler with Unvented Hot Water Installation
Click on diagram
to enlarge and use back button to return
can be fuelled by gas, oil or solid fuel. All require a lined,
flue to expel the potentially harmful gases. Solid fuel back
boilers can only provide hot water when the fire which heats the
house is lit. Gas and oil back boilers however can work
independently of the fire front so are able to provide hot water
all year round. An electric immersion heater can be used as with a
boiler uses modern technology to maximise the fuel efficiency of
or combination system.
The boiler is designed so that the cooler water returning from the
radiators is passed through a secondary heat exchanger to be warmed by
the hot flue gases which are normally expelled in to the air. The
warmer water is then sent back to the radiators.
It is known as a Condensing boiler because the water from these flue
gases 'condenses' in the secondary heat exchanger and drains away at
the bottom of the boiler.
Although condensing boilers will become increasingly popular as
dramatic savings can be made on fuel consumption they are at present
still in their infancy and may be more prone to breakdowns. They are
also more expensive to buy although grants can often be obtained to
assist with the extra cost.
Additional information about
Boilers with a storage tank (hot water cylinder) are better
capable of coping with the demands of multiple use and can deliver
water at a high temperature and at a high flow rate too. Unvented
tanks allow mains pressure water delivery, rather than relying on
gravity as traditional open vented systems do.
Unvented tanks remove the need for separate cold water storage in
They are most suited for family homes, and in properties where
roof space is limited.