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.