Operating principles for chimney or flue systems

Remove combustion products to the outside and draw in air for combustion.

Principle of hot gases in chimney systems

Hot gases rise, creating a pressure difference called 'up draught' via convection.

1. Natural draught 2. Room sealed

Open flues are the simplest design of all flues, relying on heat rising to create the up draught.

They can be either natural draught or forced draught.

In appliances with open flues, air for combustion is taken from the room where the appliance is located.

Products of combustion are removed up the flue by natural draught.

Air for combustion must be supplied to the room and appliance from outside via a vent.

This ensures proper combustion and removal of combustion products.

Room sealed (balanced) flues

These appliances draw air for combustion directly from outside and discharge combustion products through the same flue assembly. The air is directed straight into the appliance, making them safer than open-flued appliances.

Types of room sealed flues

Natural draught and Fan assisted (forced)

Room sealed natural draught

Both the combustion air (in) and the products of combustion (out) are situated in the same position outside the building. The products of combustion are evacuated from the building through a duct that runs through the combustion air duct.

Room sealed fan assisted systems work similarly to natural draught flues but use a manufacturer's fan for positive and safe evacuation of products.

The fan ensures a positive and safe evacuation of all products, making the flue smaller and more versatile in placement compared to natural draught flues.

Fan assisted boiler versions

There are two versions: 1. Fan positioned on the combustion product’s outlet, creating negative pressure. 2. Fan positioned on the fresh air inlet, creating positive pressure.

Negative pressure in fan assisted boilers

Created when the fan is positioned on the combustion product’s outlet from the heat exchanger.

Positive pressure in fan assisted boilers

Created when the fan is positioned on the fresh air inlet, mixing gas and air to the burner.

Modern condensing boiler design

Uses the fan positioned on the fresh air inlet to create positive pressure.

The section of flue connecting the draught diverter to the appliance, creating the greatest up-draught. Must be at least 600 mm in length with no bends or changes of direction.

Allows pressure fluctuations in the flue system to be moderated. Functions include diverting down-draught, diluting flue gases, reducing the pull of the secondary flue, and preventing loss of flame stability.

Carries the products of combustion towards the outside. Should be straight, contain no 90° bends, have changes of direction at a minimum angle of 45°, and be twin-wall insulated.

The part where flue gases evacuate to the outside. Helps discharge gases to the atmosphere and prevents wind, rain, leaves, and birds from penetrating the flue.

The effect of layout on chimney/flue systems

The design and installation of chimney and flue systems affect air intake function and the safe dispersion of combustion products. Several factors need careful consideration.

Flue route and number of bends

All chimneys and flues should take the straightest route possible to minimize frictional resistance. Bends create resistance, slowing gas flow. A minimum of a 45-degree angle should be maintained where bends are necessary.

Effective height in chimney/flue systems

Effective height differs from physical height as it accounts for frictional resistance encountered en route. Manufacturers provide complex tables for calculating this.

Flue temperature needs to rise quickly and remain constant for efficiency.

For a flue to work efficiently, the temperature needs to rise quickly and remain constant.

Effects of a cool flue on gas movement.

When the flue is cool, the flue gases move slowly, creating condensation and potential flue gas spillage.

Time required for a chimney/flue to heat up fully.

It can take 5–10 minutes for the chimney/flue to heat up fully.

The position of the terminal is crucial for efficient evacuation of flue gas products.

A velocity of 6m/s is required for safe evacuation, and terminals need an opening twice the cross-sectional area of the flue.

Location requirements for a terminal

A terminal should be located where it will not be a nuisance and outside a pressure zone.

Minimum flue-height termination for a pitched roof with

At ridge, or 600 mm above, or at least 1.5 m measured horizontally to the roof line

Condition for raising the flue if within 1.5 m of a nearby structure

The flue must be raised to 600 mm above that point

Condition for raising the flue if within 2.5 m of a nearby structure

The flue must be raised to 1000 mm above that point

Layout and features of chimney and flue construction depend on the material they are constructed from.

Manufacturers provide full instructions on the layout.

Types of rigid chimneys

- Brick and masonry - Pre-cast blocks - Metallic flues

A type of chimney construction material.

Rigid-brick and masonry chimneys (class 1 flue) are designed for fireplaces or appliances.

These chimneys are specifically constructed to accommodate fireplaces or appliances, ensuring proper ventilation and safety.

Chimney dimensions prior to 1966

Chimneys installed before 1966 were 9" x 9" or 225mm x 225mm.

Chimney materials and dimensions after 1966

After 1966, chimneys were either 175mm round or 200mm² and made of clay, pumice, or concrete.

Reason for change in chimney size and materials after 1966

Larger chimneys were too big for gas appliances, slowing down flue gases. Newer materials prevent deterioration.

Pre-cast flue blocks are made from concrete and are an integral part of house construction.

Concrete pre-cast flue blocks are essential for building houses.

A common issue with pre-cast flue blocks is poor 'pull'.

Poor 'pull' can affect the efficiency of flue systems.

Not all gas appliances can be installed on pre-cast flue systems.

Compatibility of gas appliances with pre-cast flue systems must be verified.

Always check the manufacturer's instructions when dealing with pre-cast flue blocks.

Manufacturer's instructions provide crucial guidelines for installation.

Chimney and Flue Systems Flashcards

Student preview

60 questions

Show all answers

1.

FLASHCARD QUESTION

Front

Operating principles of chimney/flue systems.

Back

Chimney/flue systems operate by channeling exhaust gases from a fireplace, furnace, or boiler to the outside atmosphere, using principles of draft and pressure differences.

2.

FLASHCARD QUESTION

Front

Types of chimney/flue systems.

Back

Types of chimney/flue systems include masonry chimneys, metal chimneys, and prefabricated flue systems.

3.

FLASHCARD QUESTION

Front

Components within chimney/flue systems.

Back

Components of chimney/flue systems include the flue liner, chimney cap, damper, and smoke chamber.

4.

FLASHCARD QUESTION

Front

Effects of layout on chimney/flue systems.

Back

The layout of chimney/flue systems affects draft efficiency, heat retention, and safety, influencing how well the system removes exhaust gases.

5.

FLASHCARD QUESTION

Front

Layout and features of chimney and flue construction.

Back

Chimney and flue construction features include vertical alignment, proper insulation, and the use of appropriate materials to ensure safety and efficiency.

6.

FLASHCARD QUESTION

Front

Termination requirements for chimney/flue systems.

Back

Termination requirements for chimney/flue systems are detailed in building codes and standards, ensuring safe and effective exhaust of gases.

7.

FLASHCARD QUESTION

Front

Basic inspection and testing procedures for chimney/flue systems.

Back

Basic inspection and testing procedures include checking for blockages, inspecting the flue liner, and ensuring the damper operates correctly.

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