A Guide to Common Chimney Problems and Solutions

Modern chimneys that are well designed, constructed and maintained are unlikely to develop faults or perform undesirably. However, problems can occur and in the first of a series of articles, OFTEC’s Matthew Northcott looks at some of the most commonly experienced issues and remedies.

Part 1. Insufficient Draft

Part 2. No Updraft

Part 3. A Chimney Situated in an Unfavourable Position

Part 4. Down-Draught and Potential Solutions

 

Part 1 – Insufficient Draft

One of the four most documented reasons for chimney problems is insufficient draught. Symptoms include some smoke or fumes, but not all, escaping into the room without any sign of being blown back in by a down-draught. As a result, smokiness is consistent and usually occurs regardless of weather conditions, although wind may either improve or worsen matters.

Insufficient draught can be caused by a number of factors:

 

Air Starvation

Fireplace Opening too Large

Partial Blockage or Constriction of Flue

Flue Offset too Low, too Abrupt or too Long

Summary

 

Air Starvation

All solid fuel burning appliances need a flow of air into the room to support combustion. Insufficient flow causes reduced air speed through the fireplace opening which prevents all smoke carrying into the flue.

This is more likely in modern dwellings with high insulation levels or in well draught proofed rooms with a solid concrete floor.

Test – Open a door or window, preferably when there is no wind to complicate matters. If smoking into the room decreases, the issue is usually air starvation.

Remedy – Check ventilation into the room is in accordance with regional building regulations. It may be necessary to allow more air into the room whilst avoiding unpleasant draughts.

Firstly, check if it’s possible to reduce the volume of air flowing up the flue so the ‘demand’ for air is less. A large throat over the fire should be reduced to 110mm ± 10mm to provide a smooth, streamlined entry into the flue. The throat opening could also be made adjustable using an easily fitted throat restrictor, to suit the ‘pull’ of the flue when the chimney is warm, whilst allowing full opening for relighting the fire when the chimney is cold.

Fitting a louvered grille or ventilator over the door can also help by drawing air from an internal room which is warmer than outside air. Extra air inlets fixed into floorboards adjacent to the hearth, or in walls if the floor is solid, may also help.

In some cases, the most effective way of dealing with air starvation is to install a modern roomheater in place of the open fire. This drastically reduces the volume of air induced up the flue from the room.

 

Fireplace Opening too Large

Where the fireplace opening is too large in relation to the flue, smokiness is likely, particularly if there is a badly formed throat. The issue is very common with dog or basket grates standing in very large openings.

Test – All that can be done is to check the size of the flue and compare it with the opening. If the flue size is not equal to 15% of the fireplace opening, smokiness can often occur. However, there are many variable factors so it’s difficult to lay down a firm rule.

Remedy – Altering the size of the flue is usually expensive whilst reducing the size of the opening is often impractical. All that can be done with an inset fire is to consider replacing it with a closed roomheater.

However, if a customer wishes to retain a dog-grate or similar, smoking back can often be overcome by fixing a metal register plate across the underside of the top of the recess. This should be accompanied by a metal canopy over the fire with its constricted end projecting through the register plate as high as possible to connect to the flue gather.

Another option is to install a purpose-made free-standing fire with an integral throat restrictor. Such a fire can be placed in a large recess without smoke-back occurring, providing that the height of the recess is not greater than the outlet end of the canopy and the flue is properly constructed.

 

Partial Blockage or Constriction of Flue

This usually occurs at changes of direction in a flue (bends/offsets) and may be due to bad workmanship or foreign matter/ soot debris accumulation at the bend. A common cause is mortar that was dropped down the flue when it was built and now partially blocks the bend.

Test – By inspection using suitable camera equipment or chimney sweeps’ brushes. Either method should reveal any obstructions and sweeps’ brushes may dislodge them if a scraper is fitted to the end.

Remedy – In particularly bad cases, if the blockage can’t be removed by other means, the flue will have to be opened for clearing.

 

Flue Offset too Low, too Abrupt or too Long

Flues are frequently carried over, or ‘offset’, to one side of the chimney breast, either to make room for other flues or to by-pass the fireplace upstairs. Sometimes this offset is too low, or the bend too abrupt, or the traversing length too long. Sometimes, all three faults are found in one offset. The result is poor chimney draught and a smoky fire.

Test –The easiest way to check the route of a flue is often with a chimney sweeps’ rod or suitable camera equipment. This will indicate the height of the first bend and give some idea of the angle of traverse.

Remedy – There is usually no alternative to opening the front of the chimney breast, or back if it is on an outside wall, and rebuilding the offending part of the flue. It is such a major operation that all other possible causes of smokiness should be checked first.

 

Summary
Before diagnosing a problem, it is recommended that a discussion first takes place with the customer to determine any useful information about the possible cause(s). Useful questions to ask include: when was the problem first noticed; when was the fire or appliance was last serviced and the chimney last swept; and, has anything changed about the property which may affect air flow?

Information on further causes of air starvation, alongside additional reading, can be found in OFTEC’s Solid Fuel Heating Technical Book, available from OFTEC Direct.

In the next issue of fireplace.co.uk, Matthew Northcott of OFTEC will discuss the common causes of no up draft.

 

Part 2 – No Updraft

In the second of a series of articles looking at the most commonly experienced issues with chimneys, OFTEC’s Matthew Northcott looks at the problem of no up-draught. If no up-draught is the suspected issue, look out for smoke escaping into the room, but no sign of blowing back into the room. Symptoms will be constant and unaffected by weather conditions.

No up-draught is usually caused by two main factors:

 

Complete Blockage

A Cold Flue

Summary

 

Complete Blockage

If a flue is not regularly swept and kept clean, soot deposits can build up over time and completely stop any up-draught. The same thing can happen when pieces of a chimney pot, slate or brick fall down the flue.

It is important that the appliance is not used until the blockage is cleared.

Test – Inspecting the flue using suitable camera equipment or chimney sweep brushes should reveal any blockage. 

Remedy – It is preferable to use chimney sweep rods to remove the obstruction – or at least indicate the exact position where the flue should be opened to deal with the issue. Future regular sweeping of the flue is important for safety.

 

 

A Cold flue

An exposed chimney can rapidly cool the flue gases and draught is never as effective as it is in a well-insulated flue. Also, if the fire has been out for some time, or a flue damper or thermostat has been closed for several hours, the chimney draught may well be reduced sufficiently to cause smokiness/ fume emissions back into the property.

Uninsulated cast-iron or other single wall flue pipes should never be used as a chimney outside the heated envelope of the property. Severe chilling of the flue gases, particularly when an appliance is burning slowly, results in unsightly condensation but more seriously, reduces the flue draught so much that dangerous fumes may be emitted from the appliance because there is little or no flue draught to draw them safely up the flue.

Test – Carefully light a smoke match at the base of the flue. After a brief delay, the smoke should be carried up the flue.

Remedy – Give the draught an initial ‘boost’ by lighting a firelighter at the base of the flue to warm the column of air in the flue.

Domestic boiler dampers should always be left in the fully open position unless very windy conditions make it difficult to control the boiler by its combustion air control.

For thermostatically controlled appliances, an escape of fumes is most noticeable in the morning after the thermostat has been shut for several hours overnight. Giving the boiler a little more work to do by adding a towel rail or small radiator to the gravity system will slightly increase the flue gas temperature.

Any uninsulated outside flue pipe should be replaced by an insulated prefabricated chimney system. Find out more about the anatomy of your chimney in this article “A Guide to Understanding Chimney Height and Draft“.

 

Summary

Before diagnosing a problem, it is recommended that a discussion first takes place with the customer to determine any useful information about the possible cause(s). Useful questions to ask include: when was the problem first noticed; when was the fire or appliance last serviced and the chimney last swept; and, has anything changed about the property which may affect air flow?

Further information on investigating chimney problems can be found in OFTEC’s Solid Fuel Heating Technical Book, available from OFTEC Direct.

In the next issue of fireplace.co.uk, Matthew Northcott will discuss the issues around chimneys terminating in a high pressure zone and recommended solutions.

 

 

Part 3 – A Chimney Situated in an Unfavourable Position

In the third of a series of articles looking at the most commonly experienced issues with chimneys, OFTEC’s Matthew Northcott discusses how to identify and resolve the problem when a chimney is situated in an unfavourable position.

 

How Wind Effects Chimney Draught

Wind at the top of a chimney may have a positive or negative effect on draught, depending on the surroundings and position of the chimney top in relation to the roof and other nearby features such as trees or adjacent buildings.

If a chimney is located lower than a nearby structure, in many cases the roof, and on the windward side of that structure, the chimney is likely to be located in a region of high wind-pressure. There will be a low pressure region on the sheltered side of the building and this difference in pressure can result in air being sucked down the chimney and out to the lower pressure side.

This can affect the chimney performance and produce smoke emissions by either preventing up-draught or if the pressure is high enough, causing a down draught.

Symptoms to look out for are intermittent blowing back when the wind is in a certain quarter. The degree of smokiness will vary with wind strength and normally only occurs when there are openings such as doors or windows on the low pressure side, but not the high pressure side.

Test – Open a door or window on the windward wall of the building. This should equalise the pressure and restore up-draught.

Remedy – If possible, the chimney should be extended in height beyond the region of high-pressure. This can be done experimentally in the first place by trying various lengths of sheet metal pipes to determine the extra height needed.
In most cases, if the chimney is 0.6 to 1 metre higher than the ridge or other structure responsible for the pressure zone, the problem is eliminated.

Uninsulated flue pipes should not be used as permanent extensions as they will chill the flue gases and possibly cause further trouble.

Regional building regulations impose a definite limit on unsupported chimney height. If the permitted extension does not prove sufficient, then further investigation is required.

 

Summary

Where possible, it is always recommended that a discussion first takes place with the customer to find out any information that might be useful in identifying the possible cause(s) of the problem.

Useful questions to ask include: when was the problem first noticed; when was the fire or appliance last serviced and the chimney last swept; and, has anything changed about the property which may affect air flow?

Further information on investigating chimney problems can be found in OFTEC’s Solid Fuel Heating Technical Book, available from OFTEC Direct.

The final instalment of OFTEC’s articles on common chimney problems in the next issue of fireplace.co.uk, will focus on the issues of down-draught and difficult site conditions.

 

Part 4 – Down-Draught and Potential Solutions

In the final article of the series looking at the most commonly experienced issues with chimneys, OFTEC’s Matthew Northcott discusses the problem of down-draught and offers potential solutions.

Down-draught is often caused by two main factors.

The first of these is when openings into a room such as doors, windows or ventilators, are located in a low-pressure region. This problem mainly occurs with short chimneys, such as those on bungalows or on the top one or two storeys of blocks of flats, and also with inset open fires.

‘Suction’ in the room, although normally seen when a chimney terminates in a low-pressure zone, can sometimes cause smokiness regardless of the chimney positon.

In these cases, the flow of air around the property creates regions of high and low pressures. These can cause a down-draught if doors and windows are positioned so that suction in the room sufficiently exceeds pressure and therefore overcomes the natural ‘pull’ of the flue. In certain circumstances, this can be difficult to cure.

Test – Aromatic test smoke can be used in the room to show the movement of air. Usually the smoke will drift towards doors and windows in low-pressure regions, thereby indicating that air is being drawn out of the room.

Remedy – As with most other draught problems, some experimentation is usually necessary. The normal up-draught can be increased by:

replacing the existing fire with a closed room heater which dramatically reduces air flow and maintains a higher temperature in the flue gases fitting a draught-inducing pot to increase up-draught fitting a throat restrictor to reduce air flow and increase the temperature in the flue draught-proofing doors and windows etc. which are adjacent to the suction areas.
The second most common cause of down-draught is wind currents.

Downward striking wind currents often occur around chimney tops with higher structures such as trees and buildings nearby. Chimneys on the lee slope of a hill or in a valley may also suffer from this form of down-draught.

Test – Observe the position of the chimney top in relation to other higher structures or in respect of surrounding land contours. Note whether smoke has difficulty escaping from the chimney pot and is seen to blow downwards. The use of coloured smoke pellets may help to show this.

Remedy – The simplest remedy is to protect the top of the chimney with an anti down-draught chimney pot such as the ‘OH’ pot or the ‘Marcone/Vort-X’ pot.

Sometimes a cluster of chimney pots close together on the same stack may result in smoke and fumes from one flue passing down an adjacent one, particularly where fireplaces are near to each other in communicating rooms.

Raising the level of termination of one flue by fitting a tall pot is sometimes sufficient to break the siphon effect. If, however, adjacent. flues (often in old properties) are not in use, the problem is quickly resolved by capping the unused one. It is important this is done in such a way that the flue can be easily opened up if it is likely to be needed in the future.

Summary
Where possible, it is always recommended that a discussion first takes place with the customer to find out any information that might be useful in identifying the possible cause(s) of the problem.

Useful questions to ask include: when was the problem first noticed; when was the fire or appliance last serviced and the chimney last swept; and, has anything changed about the property which may affect air flow?

Further information on investigating chimney problems can be found in OFTEC’s Solid Fuel Heating Technical Book, available from OFTEC Direct.

 

Helpful Resources

OFTEC- www.oftec.co.uk – For help with oil fired and solid fuel appliances

NACE- www.nace.org.uk – Specialists in Chimney lining and flues

Document J Regulations- www.gov.uk – Statutory guide. Combustion appliances and fuel systems

 

With thanks to contributors

Matthew Northcott – OFTEC