How to Burn Coal in a Boiler or Stove: Lighting, Air, Overnight Loading

First, the right coal for your boiler

Half of all "the coal won't burn" complaints come down to a grade that does not suit the equipment. Anthracite is hard to ignite and needs a strong draught: a fan or a tall chimney. Long-flame coal DG (ДГ) lights easily and burns on natural draught, but in a fan-assisted boiler it burns out twice as fast. The short rule:

• Boiler with a fan or strong draught - anthracite AO 25-50 mm or AM 13-25 mm.
• Automatic auger-fed boiler - only anthracite AS 6-13 mm, free of slack.
• Stove, masonry heater, pot-belly stove, boiler without a fan - long-flame DG 50-100 mm.

More detail in our articles "Coal grades" and "Anthracite or long-flame coal".

How to light it

You cannot light coal with a match: it needs a bed of embers. The sequence is the same for a boiler and a stove:

1. Clean the grate. Remove the previous day's ash so that air can come through from below. A clogged grate is the number one reason coal "dies out".
2. Paper and kindling, firewood on top. Open the ash-pit door and the flue damper fully. Let the wood burn for 10-15 minutes until you have steady embers.
3. Keep the first layer of coal thin. 3-5 cm of DG or small anthracite on the embers. Anthracite is easiest to light via DG: a few shovels of long-flame coal under the anthracite replace half of the firewood.
4. Once the first layer is glowing (red, with no black patches, 15-20 minutes), add the main load.

Anthracite poured onto a weak fire simply smothers it: it contains few volatiles and only ignites from a glowing bed. If the load has not caught, do not add firewood on top - open up the embers in the centre and give it air.

How much to load and how often

The main load goes up to 2/3 of the firebox height, but no higher than the secondary air openings, if there are any. Level the layer so there is no "crater" in the centre. Guide figures for a 15-25 kW domestic boiler:

These are average figures: in a top-burning long-burn boiler a load of DG can last 12-20 hours, while in an old stove anthracite burns through faster. Add coal onto a still-active glow when the layer has burned down by about half, not when only a few embers are left in the firebox.

Air: primary, secondary and draught

Primary air comes from below through the grate and feeds the burning of the coal itself. Secondary air is supplied above the layer and burns off the gases. Anthracite, with 3-8% volatiles, hardly needs secondary air - primary air does all the work. With long-flame coal, which has over 35% volatiles, part of the heat goes up the chimney as smoke and soot if there is no secondary air.

Control the output with the ash-pit door or the draught regulator at the inlet, not with the flue damper at the outlet. A half-closed flue damper is the most common cause of carbon monoxide fumes: gases with nowhere to go come back into the room.

A blue flame above the coal is carbon monoxide (CO) burning. While it is there, never close the flue damper or the stove shutter. A carbon monoxide detector in a room with a boiler or stove costs a few hundred hryvnias and saves lives.

How to bank the boiler overnight

1. About an hour before bed, add a full load onto the glowing layer.
2. Let it burn up for 20-30 minutes with the ash-pit door open, so that the volatiles burn off rather than build up in the firebox.
3. Close the ash-pit door partly or set the draught regulator to a lower output. Leave the flue damper open.
4. With anthracite in manually fed boilers, some people cover the layer with fines on top to slow the burn. Do this only with the flue damper open and only if the boiler allows it.

In the morning, do not pour in fresh coal straight away: first stir the embers with a poker, give them air for 5-10 minutes, then add coal.

Cleaning: ash, clinker, heat exchanger

• Ash - daily, once it has risen to grate level. Ash in the ash pan reaching the grate blocks the air and overheats the grate bars.
• Clinker - fused lumps that do not pass through the grate. Remove it at every loading. If there is a lot of clinker, the cause is either high-ash coal (ash content above 15% in the test report) or too strong a blast, which melts the ash.
• Heat exchanger and flue passages - once every 1-2 weeks. A 1-2 mm layer of soot noticeably lowers boiler efficiency, and the same tonne gives less heat.
• Chimney - inspect before the season and sweep at least once a year.

8 mistakes that make coal burn badly

1. Anthracite on a weak fire or in a stove without draught - it smoulders and goes out.
2. DG in a fan boiler at full output - it burns out in a few hours.
3. A half-closed flue damper instead of control via the ash-pit door - fumes and condensation in the chimney.
4. A clogged grate and a full ash pan - the coal "suffocates".
5. Too thick a layer straight onto the kindling - it smothers the fire.
6. Stirring too often - fines fall through, channels form in the layer, and part of the coal ends up in the ash unburnt.
7. Wet coal from under the snow - part of the heat goes into evaporation, and the bags freeze together. Here is how to store coal to avoid this.
8. Low return water temperature (below 50-55 °C) - sulphurous condensation forms on the boiler walls, the boiler corrodes and soot builds up faster.

When the coal is to blame, not the boiler

If you do everything right but the coal is hard to light, leaves many heavy incombustible lumps and more than 20% ash by weight, the bag contains rock or slack. Ask the seller for the batch test report: ash content, moisture, volatile matter and calorific value. If there is no report or the figures do not match reality, change supplier. To test new coal risk-free, start with a trial bag.