A condensing
boiler makes better use of the heat that it generates from
burning fuels such as gas or oil. In a conventional boiler some
of this heat is wasted because the boiler releases very hot
waste gases from its flue.
A condensing
boiler uses some of the heat from these waste gases to heat
water returning from your central heating system, so it requires
less heat from the burner. This makes your condensing boiler
more efficient.
The efficiency
of a boiler is normally expressed as a percentage - some new
condensing boilers can be up to 92% efficient compared to new
non-condensing ones that are around 78% efficient and older
boilers that are only 55 to 65% efficient.
How does a condensing boiler reduce
carbon dioxide emissions?
Carbon dioxide
savings come from burning less fuel to meet your heating needs.
Imagine that
one unit of fuel potentially contains enough energy to heat your
home for an hour. Burning that fuel in a boiler that is 100%
efficient would heat your home for an hour.
Burning that
unit of fuel in a boiler that is 90% efficient would only give
you enough to heat your home for 54 minutes, and if it?s 60%
efficient you'd only get 34 minutes per unit.
So, the lower
the efficiency of your boiler the more units of fuel you need to
burn to keep your home at the right temperature.
The more fuel
you burn, the more carbon dioxide you emit.
How does a condensing boiler work?
Like
conventional boilers, a condensing boiler burns fuel to heat the
water in a metal heat exchanger.
A condensing
boiler uses an extra-large heat exchanger (or sometimes two) to
maximize heat transfer from the burner and recover useful heat
from the flue gases.
In condensing
mode the flue gases give up their ?latent heat? and exit the
flue in a visible plume of water vapour at 50-60?C - they?re
usually 120-180?C in a non-condensing boiler.
At the same
time water or ?condensate? is produced which must be drained
away.
Are they only efficient when in
condensing mode?
No, a
condensing boiler is always more efficient than a conventional
non-condensing one, due to its larger and more efficient heat
exchanger.
Every new
boiler has an efficiency figure called a Sedbuk value which
stands for Seasonal Efficiency of Domestic Boilers in the UK.
The rating
represents the average annual efficiency the boiler can achieve
in typical domestic conditions, making reasonable assumptions
about how it's used, the climate, controls, and other
influences.
This indicates
how efficient your boiler will be taking into account its
efficiency in condensing and non condensing mode.
Can I ensure it is always
condensing?
Not entirely,
but there are some things you can do to help by ensuring that
the water returning to the boiler is at the right temperature to
encourage condensing - around the mid 50s degrees centigrade.
It is
important to make sure that any heat-only boiler is not too big
for your central heating demand or it won?t operate efficiently.
Make sure that
all efficiency measures such as insulation and double-glazing
are in place, then size the boiler to meet the remaining space
heating demand (you can work this out yourself using the
calculators at the
Sedbuk or
Energy Savings Trust
websites, and fit good heating controls.
A basic room
thermostat and fixed boiler temperature isn?t flexible enough
because the set temperature doesn?t take account of how much
heat is actually needed. Best is an outdoor weather sensor
(known as 'weather compensation'), which enables the boiler to
run the central heating only as hot as is necessary, and to be
in condensing mode for most of the time it is operating.
You can
achieve a similar effect by turning your central heating output
temperature down as the weather gets warmer and up as it gets
colder.
Having larger
radiators will increase the efficiency of most heating systems
by a small amount, because the circulating water loses more heat
moving through your system and returns at the right temperature
for condensing.
Deliberately
choosing larger radiators than you need means that you can
operate the whole heating system at lower temperatures (again
encouraging condensing) but still get the right amount of
heating for your room.
But it will
often be neither cost-effective nor practical to change your
radiators just to achieve this small gain. If you?ve had
double-glazing installed since your radiators were fitted they
are likely oversized anyway.
To comply with
building regulations in England, Wales and Scotland, new or
replacement boilers should be rated as Sedbuk band A or B, that
is more than 86% efficient.
The Sedbuk
boiler efficiency database lists all of the boilers in
production, their efficiencies and estimated running costs in
examples of typical houses.
These
calculations were made when gas cost just 1.63 pence per kWh.
With current
gas prices of two to five times more these days, it?s clear that
the more gas you use, the more you?ll save now and in the future
by going for the most efficient boiler you can afford.
But there is a
little financial point in paying much extra for a Sedbuk band A
boiler of 91.5% versus 90%, unless doing as much as you can to
curb carbon dioxide emissions is vital to you.
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