Smoking Fire Places

How a fireplace works

The up-draught or ‘pull’ in a flue results from a combination of the height of the flue and

the difference in temperature between the flue gases and the outside air. Very simply the

column of hot gases in a flue is lighter in weight than an equivalent column of cold air

outside, so the pressure inside a warm flue is less than the air pressure outside. It is this quite

small difference in pressure that creates up-draught. The warmer and taller a flue the better

the draught. 

Large voids or pockets, rough surfaces and sharp bends or steep angles in a flue are

resistances to the flow of flue gases and reduce the draught.

Wind at the top of a chimney may have a positive or negative effect on draught, depending

on the surroundings and the position of the chimney top in relation to the roof.

A Smokey Fireplace

Faulty chimney draught may show itself either in a sluggish fire and general poor

performance of the appliance, but may cause the actual escape of smoke and fumes into the

room.

When smoke or fumes actually escape into the room, the more usual reasons are:

A Insufficient draught to carry away all the smoke.

B No up-draught.

C Chimney terminates in high-pressure zone.

D Down-draught and difficult site conditions

Symptom A

Some smoke or fumes, but not all, escape into the room without any sign of being blown back

by the wind. Smokiness is constant and usually occurs regardless of weather conditions,

although the wind may have some effect in either worsening or improving matters. This

usually means insufficient draught.

Air-Starvation - Throat too large or badly formed - Fireplace opening too large - Partial Blockage -       Unsuitable size flue - Unsuitable Chimney Pot or Cowl - Air Leaks

Symptom B

No up-draught at all, but no sign of blowing back. Usually constant and unaffected by wind

conditions.

Cold Flue - usually if most of the stack or on the exterior of the house

Symptom C

Intermittent blowing back with the wind in a certain quarter. The degree of smokiness will

vary with wind strength.

Chimney in high pressure zone

Symptom D

With the wind in a certain quarter the draught stops or reverses and causes smoke emission.

The draught returns when the wind ceases or changes direction.

Windows or Vents in low pressure zone - Wind Currents from obstructions near pot