13.1 Scope of guidance
This section provides guidance on the specification of micro-combined heat and power (micro-CHP) packages for dwellings to meet relevant energy efficiency requirements in the Building Regulations.
The guidance covers micro-CHP systems with an electrical output less than 5 kWe which are:
• capable of exporting electricity to the grid, and
• controlled in such a way as to avoid heat dumping.
13.2 2 Key terms
Heating plant emission rate (HPER) is the annual carbon dioxide emissions from fuel and power consumed by the heating plant, offset by the emissions saved as a result of any electricity generated by the heating plant, divided by the heat output over a year. It is measured in units of kg of carbon dioxide per kWh. To calculate HPER it is necessary to know the plant size ratio. Note: The HPER includes any auxiliary space and water heating that may be necessary, i.e. it represents the performance of all heating plant needed to provide space and water heating service to the building, assuming a standard demand pattern.
Plant size ratio (PSR) is defined as the nominal heat output of the heating plant divided by the design heat loss (the average heat loss of the building on a cold day with a temperature differential of 24.2°C). Note: For a given heat demand, the PSR determines the part-load condition for the heating plant.
13.3 3 Micro-CHP systems
a. For new systems, the HPER of the micro-CHP package (calculated as in sub-paragraph c. below) should be no greater than the carbon dioxide emission factor for the fuel divided by the minimum efficiency for a regular boiler using that fuel, at the PSR determined as in sub-paragraph b. below. The design heat loss of the dwelling should be calculated using the Energy Saving Trust’s Whole house boiler sizing method for houses and flats16.
b. The PSR for the micro-CHP system when operating in the intended dwelling should be calculated as defined in paragraph 13.2 above.
c. The HPER of the micro-CHP system should be calculated at the PSR determined in sub-paragraph b. above, using the methodology set out in DECC’s Annual Performance Method (APM)17 and the performance data for the micro-CHP package established by testing according to BSI PAS 6718.