Which Coal Should You Burn in a Central Heating Boiler?

When choosing coal for a central heating boiler, the first question is not “which has the most calories?” but “what size does my boiler burn?” Put large lumps in a stoker boiler and the feed mechanism jams; put nut-size coal in a hand-fed boiler and the small pieces fall through the grate, air cannot pass between them and the boiler burns inefficiently. This article covers the right coal for each boiler type, how to weigh calorific value and ash, and a buying plan for apartment and estate managements.

Stoker Boiler or Hand-Fed Boiler?

Stoker (automatic-feed) boilers carry coal from a hopper into the combustion chamber themselves, with an auger or similar mechanism, and adjust the feed to the heat demand. For this to run smoothly the coal must be small and evenly sized.

Hand-fed boilers are filled by hand through a door; the coal burns on a grate. Larger coal is used here so that air can pass between the pieces.

The air-pollution information note of the Manisa Provincial Directorate of Environment and Urbanisation makes the same distinction: coal used should be in the 18-150 mm range, that is, not smaller than the size known as “walnut”; coal in the 10-18 mm range known as “nut” may be used in automatic-feed (stoker) boilers. The same note recommends stoker boilers for centrally heated buildings.

Nut-Size Coal: Fuel for Stoker Boilers

Nut size is coal screened to 10-18 mm. Evenly sized pieces let the auger turn without jamming and send an even amount of fuel to the combustion chamber. Coal with a lot of dust and fines causes trouble in a stoker, so for nut-size coal the quality of screening matters as much as the calorific value. The product we sell for stoker boilers is nut-size central heating coal; its size and declared values are on the product page.

Walnut-Size Coal: For Hand-Fed Boilers and Stoves

Walnut size consists of larger pieces and lets air pass through the grate; it burns more evenly in hand-fed boilers and stoves. For hand-fed boilers, see the size and values on our walnut-size central heating coal page; the product's current stock status is shown there too. In boilers with a wide loading door, large-lump block coal is also used.

If you are not sure which size your boiler needs, check the manual or the boiler's label; the manufacturer states the suitable fuel and size there. Going outside the manual lowers efficiency and can damage the boiler.

Calorific Value and Ash: What to Look For

• Calorific value: with higher-calorie coal less fuel burns for the same heat and the boiler is filled less often. When comparing, note the basis of the figure; “dry basis” and “as-received basis” values differ for the same coal.
• Ash: high-ash coal leaves more ash, so the boiler and ash pan need cleaning more often. In large buildings that means more work in the boiler room.
• Sulphur and moisture: sulphur corrodes the chimney and boiler; with damp coal part of the heat goes into evaporating water.

All of these values are printed in the table on the bag. For a reading guide, see what the values on a coal bag mean.

For Apartment and Estate Managements

In apartment blocks and estates with a central boiler, buying coal is planned differently from buying for a single home:

• Coal store capacity: measure how much the boiler room can hold in advance and split the season's needs into several deliveries to match it.
• One type of coal: switching size and type mid-season upsets the boiler settings. Settle on a coal that suits the boiler and stay with the same product all season.
• Pre-season maintenance: the same note asks for chimneys and boilers in centrally heated buildings to be cleaned and serviced before the winter season starts. For a checklist, see chimney cleaning and stove maintenance.
• Written quote: for a board decision, ask for a written quote including VAT and delivery, and compare quotes for the same coal type and size.

For bulk coal purchases by apartment blocks, estates, schools and businesses, you can request a quote on our corporate supply page. Other kinds of coal are on our coal page.

Source: Manisa Governorship, Provincial Directorate of Environment and Urbanisation, “Information Note on Air Pollution” (in Turkish; coal sizes, stoker boiler recommendation, pre-season chimney and boiler maintenance).