Boiler Gases: Where to Vent Them

Let the boiler breathe out

Nearly every type of heating system emits the results of combustion-waste gases, which produce corrosive fumes and condensation. It is, therefore, essential to have a flue to take away these gases, without harming the fabric or penetrating the home. A flue is also necessary for the correct combustion of heating appliances. Insulating both the flue and the appliance may also be necessary.

All fuels produce gases and water vapour when burnt. With solid-fuel open fires, these harmful by-products are greatly diluted with warm air which enters the flue above the fire and carries them away. Unfortunately, this also results in a great deal of wasted heat escaping up the chimney.

While modern gas and oil-fired boilers are able to make rather more efficient use of the fuel consumed, if they are to perform satisfactorily, without risk of damage from the effects of condensation, special attention must be paid to the flue.

A chimney not only provides a safe means of carrying away the products of combustion but also provides the ‘draw’ which pulls in air for efficient combustion of the fuel being burnt.

The effective draw of a chimney can be reduced by a number of factors. A ‘cold’ chimney on an outside wall will slow down the escape of flue gases. The number of bends should be as few as possible, since these also tend to hinder the flow of the flue gas. If the flue is too large, this may cause the chimney to smoke.

Design considerations

One effect of a badly designed chimney is to increase the amount of condensation (condensate) inside it.

Whenever a fuel is burnt in contact with air, a small amount of water vapour is produced with the products of combustion. With the modern high-efficiency appliance, the majority of the heat produced is converted into useful heat. The gases which pass out up the flue are, therefore, at a lower temperature than those from the traditional open fire.

When these low-temperature gases meet the cool walls of an uninsulated chimney, the water vapour which is held in suspension in the flue gas condenses on to the walls of the flue.

This condensation will then absorb other combustion by-products to form an acid solution which, over a period of time, will attack and damage the brickwork of the flue.

If allowed to continue, this attack can lead to instability of the chimney and bad staining of internal wall decorations. The highest concentration of vapour in a flue occurs when gas is being burnt and the lowest occurs when solid fuel is burnt.

Therefore, if condensation is to be avoided, the chimney serving a gas appliance must be well insulated.

To eliminate this problem a chimney must be:

• Insulated to reduce heat losses from the flue gas;

• Lined with a smooth and non-absorbent impervious material, so that condensate does not cause damage;

• Only of sufficient size to convey the full output of gases away from the appliance;

• As straight as possible, since bends tend to increase the possibility of condensation. If bends are necessary, the minimum angle of the flue to the horizontal should not be less than 45°.

The size of the cross-sectional area of a flue liner depends to a large extent on the type of fuel being burnt and the input rating of the appliance.

Whenever possible, the size of the flue recommended by the manufacturer of the appliance should be used.

Check the size of the flue on the appliance outlet so that you can arrive at the correct size of the flue lining.

As gas appliances tend to produce more condensate than other fuels, provision should be made to collect any which may occur.

The space between the tube and the flue brickwork is then filled, under pressure, with a liquid vermiculite composition. After about a day, to allow for setting, the tube is deflated and removed.

The resulting flue is then at the correct diameter and also thermally insulated in one operation. This method of lining, called ‘Insuflu’, is suitable for flues of up to 230mm diameter and up to 26m high.

10. November 2011 by admin
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