This section gives guidance on specifying gas and oil-fired warm air heaters for space heating in new and existing buildings to meet relevant energy efficiency requirements in the Building Regulations. It includes guidance on measures, such as additional controls, that can be used to gain heating efficiency credits to improve the heat generator seasonal efficiency.
4.2 Scope of guidance
The guidance in this section covers the warm air heaters listed in Table 15. The guidance also covers indirect gas or oil-fired heat exchangers (as used in large ducted systems for office blocks, shopping and leisure complexes, etc.) to provide heating and fresh or conditioned air. Warm air central heating systems are not within the scope of this section but are covered in the relevant heat generator section and Section 10 Air distribution.
****Table 15 Warm air heaters and test methods****
4.3 Key terms
Heat generator seasonal efficiency of air heaters, since they operate under the same conditions at all times, is equivalent to their measured steady state thermal efficiency (net calorific value), which can be obtained from the heater manufacturer’s data and converted to efficiency (gross calorific value) using the conversion factors in SAP 2012 Table E4.
For indirect-fired heaters, data values for heat output and net heat input are measured using the efficiency test methods described in BS EN 1020, BS EN 621 or BS EN 13842 as appropriate.
For direct-fired heaters, the efficiency should be calculated using the method described in BS EN 525.
The calculation of the thermal efficiency (net) should:
• take account of the heater and the exhaust chimney within the building envelope
• exclude fans.
Effective heat generator seasonal efficiency is the heat generator seasonal efficiency with added heating efficiency credits (see below). It is the value entered into NCM tools such as SBEM to calculate the building carbon dioxide emission rate (BER).
4.4 Warm air heaters in new and existing buildings
Warm air systems in new and existing buildings should have:
a. an effective heat generator seasonal efficiency which is no worse than in Table 16
b. a controls package featuring, as a minimum, time control, space temperature control and, where appropriate for buildings with a floor area greater than 150 m2, zone control.
4.5 Heating efficiency credits for warm air heaters in new and existing buildings
Heating efficiency credits can be gained by adopting the additional measures listed in Table 17.
****Table 17 Heating efficiency credits for additional measures applicable to warm air heaters****
Destratification fans and air-induction schemes
It is recognised that destratification fans and air-induction schemes may improve the efficiency of a warm air system and significantly reduce the carbon emissions associated with the heating system. The benefits of these measures are already taken into account by the NCM so no heating efficiency credits can be gained by using them. Note that warm air systems with air induction schemes or destratification fans should not be confused with central heating systems that have air distribution.
Example: Using heating efficiency credits to exceed the minimum effective heat generator seasonal efficiency for a warm air heater
A proposed building has a gas-fired forced-convection warm air heater without a fan to assist transportation of combustion air or combustion products. When tested to BS EN 621:2009 the thermal efficiency (net calorific value) is found to be 91%, which meets the minimum effective heat generating efficiency recommended for this type of system in Table 16.
The minimum controls package specified in paragraph 4.4b comprises zone, space temperature and time controls. Table 18 shows how credits are awarded by adding optimum stop control, modulating burners and destratification fans.
****Table 18 Example to illustrate the allocation of heating efficiency credits to a warm air****
Effective heat generator seasonal efficiencynet thermal efficiencyheating efficiency credits
The effective heat generator seasonal efficiency is therefore 95%, exceeding the minimum standard by 4 percentage points. The value that should be entered into the accredited NCM tool to calculate the carbon dioxide emission rate is 95%.