Central heating design considerations

Thermal comfort

Apart from the provision of Domestic Hot Water, the main reason that someone has central heating installed is to ensure that they and their family are provided with warmth at a level at which they can remain comfortable. This "Thermal Comfort" is a measure of a person's satisfaction with his or her surrounding or thermal environment.

Thermal comfort is achieved when a desirable heat balance, between the body and surroundings are met. These conditions will be:

• Air temperature at feet level, not greater than 3 degrees C below that at head level.
• Airflow past the body is horizontal and at a velocity of between 0.2m and 0.25m per second. A variable air velocity is preferable to a constant one.
• Room surface temperatures not above the air temperatures.
• Relative humidity of between 40% to 60%.
• Air temperatures between 16-22 degrees C, dependant upon the type of activity being carried out, age of occupants and the level and quality of clothing.

Building exposure

When a building is located in a position where it can be subjected to severe weather conditions such as on the top of a hill, by a riverside, at the coast, or in any extreme open location, allowance should be made for an increased heat input. Other factors include: -

• Temperature Extremes
• Site Location
• Humidity Conditions
• Level of Exposure
• Wind Chill Factors
• Frequency & Duration

Under these conditions a general rule is to add 10% to the heat losses, but this should be based on local conditions and increased even more if the location is particularly severe.

External design temperatures

The British standard BS 5449 states that the external design temperature should allow for all but the most extreme conditions and a figure of -1 degrees C is usually chosen. Many developers still rely on this minimum BS Standard of -1 degrees C irrespective of the anticipated external temperature extremes.

Temperature variations in throughout the UK over the last few winters have proved this minimum standard to be less than adequate especially during extreme weather conditions. A suggested fundamental pre-requisite to system design is the selection of local weather condition information, since absolute maxima or minima are unsuitable, as they would lead to uneconomic design temperatures.

Internal design temperatures and ventilation rates

The following internal design temperatures and air change rates are recommended for the design of full and part central heating systems and are based on providing the customer with acceptable comfort levels throughout. The minimum design temperature and air change rates required by BS 5449: 1990 are set out in the table below.

Allowances for intermittent heating or fast heat up periods

Where a heating system may not be used continuously, additional capacity will be required in both radiator and boiler outputs to enable room design temperatures to be achieved in a reasonable period of time.

Method 1

BS 5449 States that the requirement is met providing the system is run on continuous operation (i.e. one on & one off time control setting) to maintain the stated internal design temperatures. (No additions are made to the overall calculated heat loss)

This method would suit a group-heating scheme, or people who work from home. However this method allows smaller radiators to be installed and is often used to reduce tender and installation costs.

Unfortunately it does not provide adequate comfort conditions to those people who are on low incomes who cannot afford continuous heating nor does it provide adequate comfort conditions for those people who demand heat to coincide with their intermittent occupancy pattern.

Method 2

An alternative method to show compliance is detailed. BS 5449 states that under normal operation of the system (i.e. 2 on and 2 off time control settings) a 10 % - 30% addition is made to maintain the stated internal design temperatures.

This method gives acceptable results when a system is not used continuously, however, for modern well insulated new build properties a 10% addition will be insufficient. An alternative suggested by many energy consultants is a calculation of between 12 & 30 watts per Meter Square.

It's worth noting here that if you specify BS 5449 on it's own you could get either method, even though it could be inappropriate for the end user. When competing for a contract, it is imperative that the same criteria is used, designs based on method 2 will require larger radiators than those based on method 1.