Plant size ratio PSR
The plant size ratio (PSR) is the nominal heat output of a 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: The heating plant emission rate 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 hating service to the building, assuming a standard demand pattern. For a given heat demand, the plant size ratio determines the part-load condition for the heating plant.
The SAP Supporting Document:: "Calculation Method: CALCM:01. SAP Revised heat pump performance method Issue 1.2" published by BRE 5 September 2017 mentions Plant Size Ratio (PSR) thus:
"Heat pump annual performance calculations were first introduced in SAP 2005 via a SAP Appendix Q procedure. In SAP 2009 they were adopted and included in a newly expanded SAP appendix (Appendix N) which includes provision for heat-led micro-cogeneration. Heat pumps were included in the same appendix as cogeneration because both technologies have an annual performance that is highly dependent on the plant heating capacity divided by the design heat loss of a dwelling. This is known as the Plant Size Ratio (PSR)."
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