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Fire, heat and cooking

Sweep and inspect a flue

Twice a year burning wood, per HETAS. What tar looks like at each stage, why the third stage cannot be swept out at all, and what to do in the first thirty seconds of a chimney fire.

A flue is a pipe carrying hot flammable gas through the middle of your house. Everything about chimney maintenance follows from that sentence.

In ordinary times this is a job you book. In a long emergency, with a stove running harder than it ever has on wood that is not as dry as it should be, it becomes a job you do yourself, more often than you would think, with tools you should already own.

How often

Twice a year is the baseline for wood, and the sensible pattern is once before the heating season and once part way through it.

In an emergency, assume more often. The frequency in that table assumes seasoned fuel, a well-designed installation, and a stove run properly hot. Take any of those away, which a crisis usually does, and the build-up accelerates sharply. If you are burning wood that is wetter than you would like, inspect monthly.

The best guide is what comes down: if a sweep produces a serious quantity of deposit, shorten the interval. If it produces almost nothing, you can lengthen it.

The three stages of creosote

Creosote is unburned wood gas that cooled and condensed on the flue wall instead of burning. Knowing which stage you have tells you what to do about it, and the third stage is the one that changes your plans.

Stage one: soot. Dry, black, dusty, brushes off easily. This is normal and is what regular sweeping removes. Nothing to worry about.

Stage two: flaky, crunchy tar. Hard black or brown flakes, shiny in places, like burnt cornflakes stuck to the liner. Still removable with a stiff brush and effort. This is a warning: it means the flue is running too cool, or the fuel is too wet, or the stove is being smouldered rather than burned.

Stage three: glazed creosote. A hard, shiny, tar-like glaze coating the liner, sometimes running like treacle. It looks almost like black gloss paint.

Glazed creosote is almost always the result of the same three things: wet wood, a stove damped down and left to smoulder overnight, and a flue running cold, usually because it is oversized or runs up an external wall.

Sweeping it yourself

Tools: a flue brush the correct size and shape for your liner, screw-together rods, a dust sheet, a bin bag, a torch, a mask, and a mirror if the flue has a bend.

Getting the brush size right matters. Too small and it polishes rather than cleans; too large and it will jam, and a jammed brush most of the way up a flue is a genuinely awkward problem. Match it to the internal dimensions, and use a round brush for a round liner and a square one for a masonry flue.

From below, which is usual for a stove:

  1. Let the stove go completely cold. Not warm. Cold, and ideally overnight.
  2. Sheet the floor and everything nearby. Then sheet more than you think. Soot travels.
  3. Remove the baffle or throat plate from inside the stove so the flue is open.
  4. Seal around the appliance opening with a sheet taped in place, with just enough gap to feed rods through, or the room fills with soot.
  5. Feed the brush up, adding rods as you go, and work it up and down in short strokes rather than one long push.
  6. Always turn rods clockwise, whichever direction you are moving. Rods unscrew anticlockwise, and a brush left at the top of a flue with the rods unscrewed from it is the classic self-inflicted disaster.
  7. Go all the way to the top. You will feel the resistance change and, on a straight flue, see daylight or feel the brush emerge.
  8. Withdraw slowly, then let the dust settle for ten or fifteen minutes before opening up.
  9. Clear the deposit from the stove and the base of the flue.
  10. Refit the baffle. A stove run without its baffle overheats the flue badly.

Inspecting while you are there

The sweep is the opportunity. Look for:

Check the draught before relighting. Hold a lit match or a smoking taper near the open stove door: the smoke should be drawn briskly up. If it hangs, or rolls into the room, do not light the fire until you know why. See carbon monoxide and ventilation.

A chimney fire

You will know. A chimney fire is loud, a roaring or rumbling like a jet, with dense smoke and often flames and sparks from the pot. The flue gets extremely hot very quickly.

In order:

  1. Ring 999 and ask for the fire service. First, before firefighting. Do not wait to see whether it settles.
  2. Close the stove down completely. Shut the door and close every air control. You are starving it of oxygen, which is the only one of the three fire requirements you can take away.
  3. If it is an open fire, gently damp the fire itself with a small amount of water or sand. A small amount. Do not throw a bucket of water up a hot flue.
  4. Get everyone out, and get pets out.
  5. Move furniture and anything flammable away from the chimney breast, if it is safe to do so on your way out.
  6. Check upstairs rooms and the loft for heat or smoke coming through the chimney breast, if you can do it safely. This is where the fire spreads into the structure.
  7. Do not go back in.

Preventing the build-up in the first place

Sweeping manages the symptom. These prevent it.