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Glossary
of Stove Terms
Baffled by the terminology? Click on any of the underlined terms below for
a full explanation in plain English.
Anti-Corrosion
Inhibitor. All of the steel components
in a central heating system have a limited life including the
boiler and the pressed steel radiators it serves. It makes sense
to try and prolong the lives of these components for as long
as possible. Anti-corrosion fluids work by 'plating' the exposed
steel surfaces with a microscopically thin substance that inhibits
rust. Typically, it does other things as well – like driving
out dissolved oxygen and lubricating the impellor on the circulating
pump. And inhibitors are now available that include an anti-freeze
component to save you having to drain the system if you leave
a building unoccupied in cold weather.
Several brands of corrosion inhibitor
are available including one of the originals "Fernox"
stocked by most builders' merchants. To treat your system just
mix up the recommended doseage in a bucket and tip it into the
feed/expansion tank. The circulating pump will do the rest.
Chemical
Flue Cleaners. There are many magic
potions on the market – some better than others. At simplest
you can try sprinkling zinc powder on the fire as the Scandinavians
do. The chemical reactions vary from one substance to another
and in some instances literally produce a very slow, controlled
chimney-fire that reduces the soot to a harmless ash. Using chemical
flue cleaners with wet fuel is a waste of time since moisture
neutralises the effect. Also, note that chemical flue cleaners
must NOT be used with stainless steel chimney systems including
flexible metal liners since they may react adversely with the
metal and seriously shorten its life.
Chimneys
– PreTreatment Prior to Cleaning.
Creosotes and tars adhere to a chimney with great tenacity and
may be impossible to remove mechanically with a brush. For this
reason it is wise to pre-treat your chimney for a few days before
you call the chimney-sweep. During this period, bleed as much
warm, dry air as possible to the chimney. Smokeless fuel is particularly
effective if your stove can burn it, but if your stove is a 'pure
woodburner' burn only the driest fuel you can lay your hands
on. (Demolition timber is usually bone dry and well-suited to
the job.)
Chemical flue cleaners are worth trying
since they will often help to break up the tars into a loose
powder which is easy to remove with a brush. But be aware that
the effects of a chemical flue cleaner are likely to be be neutralised
by any moisture in your fuel so it's a waste of time using these
substances if your logs are unseasoned. Note that chemical flue
cleaners must NOT be used with stainless steel chimney systems
including flexible metal liners since they may react adversely
with the metal and seriously shorten its life.
Cleanburn.
Every stove has an optimum burning
temperature. When it runs too cool, combustion is incomplete
and smoke is produced. When it runs too hot, heat losses to the
chimney increase and the life of the appliance may be shortened.
The ideal therefore is to keep the stove operating somewhere
in the middle-zone. Cleanburn is a technology that aims to make
that zone as wide as possible, so that the stove burns efficiently
(and smokelessly) across the widest possible range of outputs.
Several techniques are used in support
of each other. Typically the firebox is lined with insulating
bricks so that the fire is kept as hot as possible. Glass door
panels may be double-glazed for the same reason. And incoming
air is often pre-heated by being passed through a duct before
it gets to the fire.
Keep in mind that not all implementations
are as good as they might be and variants described as "air-wash"
may not be the same thing at all. Also, "Cleanburn"
isn't infallible. If you persist in burning wet wood, or running
your stove in 'slumber' mode for extended periods – or if
you load your stove up with a high-output central-heating boiler
- you'll scarcely notice the difference betwen a "Cleanburn"
stove and a standard model.
Properly used though, it is a genuine
breakthrough and extremely impressive to behold. Less fuel is
used, door glasses remain clean and chimney fouling is drastically
reduced. The best bet is to specify Cleanburn, pay the modest
extra cost involved – and run your stove exactly as the
manufacturer instructs.
Convector
Stove. A convector stove is typically
a radiant stove housed within a sheet metal 'overcoat.' Because
of its design it will have slightly greater bulk than a radiant
stove for a given output. This type of stove heats the room using
a stream of warm air which is induced to convect between the
hot body of the stove and the outer casing. Because the surface
temperature of a convector stove remains much cooler than the
surface temperature of a radiant stove, it may be considered
a safer appliance. The penalty is that you don't feel warm with
a convector stove until the whole room has reached a comfortable
temperature since the appliance itself gives out little radiant
heat.
Domestic
Hot Water - Direct & Indirect Systems.
Click here for a full discussion and illustrations on
this topic.
Freestanding
Stove. A freestanding stove needs
no wall or hearth and can be installed in the centre of a room
on a suitable fireproof base, though in practice most freestanding
stoves in the UK are installed close to a wall and flue into
an existing chimney. Available in an enormous range of outputs
to heat anything from a caravan to a castle. Potentially the
most efficient configuration available since all surface areas
can emit heat freely into the room. When an existing open fire
is closed off to allow a freestanding stove to be installed it
is generally necessary to extend the hearth forward – this
raises obvious cost and aesthetic considerations.
Heat Loss. Heating engineers work out how much heat you
need by calculating the amount of heat LOST from a given space
under a particular set of circumstances. For instance, a sitting-room
that is heated to 21ºC. on a day when the temperature is
minus 1ºC. may have a heat loss of 3 kW. To arrive at this
figure you have to calculate how much heat is lost by conduction
(through walls & windows etc,) and how much is lost by ventilation
(as occurs when warm fuggy air is replaced by cold fresh air.)
The two figures are then added together. In the UK, heat loss
calculations are generally based on an external temperature of
minus 1ºC. (or minus 2ºC. in some parts of Scotland.
It isn't worth designing for freakish Arctic conditions that
only occur once in a blue moon – but if you are struck by
a cold snap at minus 1ºC. be prepared for the fact that
your heating system won't cope. In such circumstances you'll
need some auxiliary heating or some thermal underware –
or quite possibly both!
For a full discussion on this topic
click here.
Hybrid Stove. The term "hybrid" has been coined
by Backwoodsman and may not be recognised elsewhere! We use it
to describe stoves that incorporate the features of an open fire
& closed stove within a recessed configuration. Look no further
than the Dovre
2000 or Jotul
1 80 RH. What sets these appliances
apart is that they can be operated either as a highly efficient
closed stove – or as a much less efficient traditional open
fire. They may also be installed in such a way as to warm several
adjacent room by means of ducts set into a central brick chamber.
Hybrids aren't really compatible with water heating and if you
find a model that seems to offer this facility, avoid it!
Inset Stove. Designed to be recessed into an existing fireplace
or a pre-built brick chamber served by a chimney. They are generally
space-saving and therefore well suited to use in small rooms
or where it is impractical to extend the hearth forward for a
freestanding stove. Central-heating insets deliver most
of their heat to hot water and very little to the room they are
located in, so when this type is installed it is frequently necessary
to have a radiator in the room to 'top-up' the output from the
stove.
Insets designed to be accomodated within
a British Standard Opening (21" x 16", or 22"
x 18") are restricted in fire-box size and have a short
flue path. This arrangement is less than satisfactory for wood-burning
and such appliances ought really to be considered suitable for
use with smokeless fuel only.
Low Temperature
Thermostat. For an illustrated discussion
of this topic click here.
Multifuel
Stove. Virtually all stove manufacturer's
now offer a "multifuel" option within their range.
Some do a remarkably good job of bridging the gap between what's
best for woodburning versus what's best for solid fuel. Others
are little more than a botch-up and are best avoided at all costs.
Be aware that the compromises get more difficult as the appliances
gets bigger in size. It's one thing to design a reasonably efficient
4kW 'multifuel' stove to heat the spare bedroom – it's quite
another to produce a 14 kW central-heating stove that runs equally
well on anything that's thrown into it.
The main features to look out for (apart
from quality and durability of manufacture) are (a) grate design
and (b) air controls. There are now "two-way" grates
that operate rather like a venetian blind. The bars open and
riddle for use with solid fuel and can also be rotated and closed
off when wood is being burned. This arrangement only makes sense
if two sets of air-inlets are provided to serve the two configurations.
Since most 'multifuel' stoves are in
fact biased in their efficiency towards one fuel or another it
pays to know what type of fuel you'll be burning and choose your
stove accordingly.
Output Ratio. If you plan to install a central heating stove,
getting one with the correct output ratio is critical. For instance
a stove that delivers 3 kW to the room and 6 kW to hot water
may be exactly what you need, in which case you will be happy
with the result. And you will still be happy with a stove that
delivers 4kW to the room and 8 kW to hot water since the output
ratio remains at 1:2
But what happens if you need 1.5 kW
to the room and 7 kW to hot water? Now you're in trouble because
if you run either of the above stoves to suit your room, the
hot water output will slump to just 3 kW and if you drive the
stove hard enough to deliver anything close to 7kW of hot water
you run the risk of turning your sitting-room into a sauna bath!
The lesson to be learned here is –
never order a central-heating stove until you are in a position
to specify its required output with accuracy. If you are unable
do this yourself, get someone competent to do it for you. There
are no short-cuts, the heat-loss for every room in your house
must be calculated and taken into account.
What if the heat loss calculations
have been done, the correct stove has been specified and the
output balance still seems wrong? In this case, suspect the radiators
themselves. Each panel must be accurately matched to the room
it is required to heat and each should be fitted with a thermostat.
Any mistakes in this department can throw things out in an otherwise
well balanced system.
Radiant Stove.
Single skin stoves develop a high
surface temperature that radiates heat directly out into the
room. The closer you sit to them, the warmer they feel. Yet the
term is something of a misnomer since radiant stoves deliver
most of their heat in the form of a convective airflow
– which means that a correctly sized radiant stove should
succeed in getting all parts of a room comfortably warm within
a reasonable period of being lit. Typically more efficient than
an equivalent convector stove, but more of a hazard to young
children and to the elderly/infirm because of the high surface
temperature.
Slow Burn. You can run almost any closed stove at a low
output for several hours without attention. Your success in achieving
slow burns depends not just on the stove, but also on your fuel,
chimney draft and operating technique. However, slow burns are
not good practice because a fire that is severely 'throttled'
suffers from incomplete combustion and produces high levels of
tar and creosote. Properly run, a stove is so easy to light and
so quick to heat up that there is scarcely any argument for leaving
it running for long periods in your absence – any more than
you would leave a gas or electric appliance running.
Solid Fuel
Stove. Solid fuel is "air-greedy."
It needs to burned on a raised open grate so that air can pass
through the fire in a continuous stream. Anything that disrupts
the air flow will have an immediate impact on combustion efficiency.
Therefore ash and clinker must be removed at frequent intervals.
Solid fuel also needs a minimum bed
depth. If you spread the coals out too thin over a wide area
they will simply go out.
So the ideal solid fuel stove incorporates
a narrow, deep firebox together with an easily riddled grate
and a decent sized accessable ash-pan.
Because of the intense temperatures
found in a solid fuel stove the stove walls are invariably lined
with fire-brick rather than the sacrificial cast-iron panels
fitted in some woodstoves.
Solid fuel is harder to light than
wood and harder to keep "ticking over" at low output.
On the other hand it produces a much higher sustained output
to hot water than wood and is in every way the superior fuel
for central heating systems. Solid fuel stoves can be more compact
than their woodburning counterparts because the fuel has a lower
bulk density. It also burns with a much shorter flame eliminating
the need for a lengthy fluepath.
A point to bear in mind is that bituminous
or 'household' coal is a poor fuel for any closed appliance.
Unless it is burned at very high temperatures, the volume of
soot produced soon fouls up the fire-box and flueways. Therefore
it is best to stick to one of the recommended grades of smokeless
solid fuel.
When buying a solid fuel stove, look
for quality and weight of construction, adequate firebrick lining,
a grate that can be riddled easily and quickly – and decently
proportioned ash-pan that won't need to be emptied every couple
of hours.
And if you install a solid fuel stove,
make sure you never allow ash in the ashpan to build up to a
level that brings it into contact with the grate. If you do,
you'll regret it since nothing destroys grate-bars faster.
Stove Ratings
- British Thermal Units. (BThUs.)
Galloping metrication is putting paid to BThUs; most domestic
heating outputs are now expressed in kilowatts. In stove terms
a boiler needed for domestic hot water needs to have an output
of around 8,000 - 10,000 BThUs. Central heating boilers start
at around 18,000 and go up to a little over 45,000 BThUs. Above
this you really ought to be thinking about a kitchen-based boiler
optimised for the job, rather than a room-heating stove with
a central-heating capability. To convert BThUs to kilowatts,
divide by 3,412
Stove Ratings
- Kilowatts. Stove outputs are invariably
expressed in kilowatts and range from around 4kW for the smallest
models up to a whopping 20 kW for monsters intended for ball-rooms.
But how are the figures arrived at? Even to those in the business,
this is something of a mystery. Does a manufacturer run his appliance
flat out, measure the output and de-rate it by 20%? Or does he
take an average output based on a 4-hour refuelling cycle using
seasoned hardwood? Or does he just take a wild guess based on
the surface area of the stove? The truth is, no-one really
knows! Manufacturers have been known to swop from one method
of measurement to another simply to gain a competitive advantage.
All of which makes a mockery of calculating your heat requirement
to third decimal place accuracy.
Or does it? The fact is, if you do
your homework properly and specify a stove that doesn't appear
to match its published specification you have a strong case for
redress. Manufacturers know this and generally err on the safe
side. But in doing so they have to make a variety of assumptions
about your chimney draught, fuel and method of operation.
If you bought a stove rated at 6kW
and it appears only to deliver 3kW it's more than likely the
fault lies with you than with the manufacturer. Get your
chimney swept and checked, experiment with different fuels –
and if all else fails consult your stove supplier on alternative
operating techniques that might cure the problem.
Temperature
Gradient. For a full discusson on
this topic click here.
'U' Value. For a full discussion of this topic click here.
Woodburning
Stove. Classic woodburning stoves
like the Jøtul 602 don't contain a grate as such
at all. Instead, the floor of the stove is equipped with a shallow
'sump.' Before you commission the stove, you pour ash or sand
into the sump to a depth of around 1" (25mm.) Logs are then
burned on this soft bed which traps and holds the glowing embers
until they are burned to a fine ash rather like talcum powder.
The ash is in fact so light that most of it gets sucked up the
chimney and it is seldom necessary to de-ash the stove more often
than once a fortnight.
Because dry wood is a highly reactive
fuel, the amount of air allowed to reach the fire must be strictly
controlled. This can only be achieved by building a stove that
is effectively "air-tight" under normal operating conditions.
The principal features to look out for are (a) heavy duty constructional
materials throughout, (b) heavy duty and precise fitting doors
equipped with efficient seals, and (c) precision air-inlet valves.
Wood is a relatively bulky fuel, so
a good-sized fire-box is called for – particularly if coniferous
species are to be burned. Wood also produces a long flame, so
good design calls for a lengthy flue-path – often extended
by means of extra internal baffles.
These criteria are quite different
to those that apply for a stove optimised for use with anthracite
or other grades of solid fuel. Be careful therefore to choose
a stove that is at least biased towards the type of fuel you
will most burn.
Zone Control. City hotels and other big institutions save
a fortune by switching off heat in unoccupied bedrooms and in
other areas where it's not needed. You can take a leaf out of
their book and save too. At simplest, zone control means focussing
heat only where it's needed. A good example might be a central-heating
stove based in the sitting-room. During the day, the stove delivers
sufficient heat to keep the room warm, but at night time when
outside temperatures fall, the stove is burned up more brightly
and the circulating pump is turned on to heat radiators in the
hall, bathroom and bedrooms.
This arrangement can be highly effective
at driving down your fuel bills without incurring you in any
discomfort. And the bigger your house, the bigger the savings.
To be effective though, your installation has to be designed
with care taking into account all heat-losses and the output
characteristics of your stove. If your stove only heats the sitting-room
to a comfortable temperature when it's driven at full output
(as may be the case with some inset models) you will have to
run your circulating pump more or less continuously and turn
off all unwanted radiators by hand.
A point to consider with zone control
is that it needs to be handled with care in situations where
there are signs of dampness caused by condensation. If you produce
large volumes of warm humid air in one part of a house and allow
them to migrate into other areas that are cold and poorly insulated,
you may well end up with areas of damp, peeling wall-paper and
cupboards that turn your leather boots and belts green. To minimise
this problem, aim to extract moisture from the air wherever it's
produced. For instance an extractor fan over the cooker and in
the shower-room makes sense in every household – as does
diluting air-borne moisture by providing adequate fresh-air ventilation.
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