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)."
[edit] Related articles on Designing Buildings
- Accredited energy assessor.
- Air tightness.
- Are Energy Performance Certificates accurate?
- BS EN 15232 Energy performance of buildings: impact of building automation, controls and building management.
- Building energy models.
- Building heating systems.
- Building performance metrics.
- Building services compliance with the building regulations.
- Building services.
- Carbon ratings for buildings.
- Corrosion in heating and cooling systems.
- Display energy certificate.
- Domestic building services compliance guide.
- Emission rates.
- Energy certificates for buildings.
- Energy performance certificate EPC.
- Energy Performance of Buildings Directive.
- Energy related products regulations.
- Gross calorific value.
- Heat pump.
- Heat recovery.
- Heating.
- Heating plant emission rate.
- Heating system glandless circulator.
- Low carbon heating and cooling.
- Minimum energy efficiency standard (MEES).
- National Calculation Method.
- National Retrofit Strategy NRS.
- Non-domestic building services compliance guide.
- Overheating.
- Performance gap.
- Product Characteristics Database
- Radiant heating.
- SAP.
- Simplified Building Energy Model.
- Space heating system.
- The EPC consultation in the context of changes to the NCM.
- The Home Energy Model and Future Homes Standard assessment wrapper.
- Underfloor heating.
- U-value conventions in practice: Worked examples using BR 443.
Featured articles and news
Building Safety recap January, 2026
What we missed at the end of last year, and at the start of this...
National Apprenticeship Week 2026, 9-15 Feb
Shining a light on the positive impacts for businesses, their apprentices and the wider economy alike.
Applications and benefits of acoustic flooring
From commercial to retail.
From solid to sprung and ribbed to raised.
Strengthening industry collaboration in Hong Kong
Hong Kong Institute of Construction and The Chartered Institute of Building sign Memorandum of Understanding.
A detailed description fron the experts at Cornish Lime.
IHBC planning for growth with corporate plan development
Grow with the Institute by volunteering and CP25 consultation.
Connecting ambition and action for designers and specifiers.
Electrical skills gap deepens as apprenticeship starts fall despite surging demand says ECA.
Built environment bodies deepen joint action on EDI
B.E.Inclusive initiative agree next phase of joint equity, diversity and inclusion (EDI) action plan.
Recognising culture as key to sustainable economic growth
Creative UK Provocation paper: Culture as Growth Infrastructure.
Futurebuild and UK Construction Week London Unite
Creating the UK’s Built Environment Super Event and over 25 other key partnerships.
Welsh and Scottish 2026 elections
Manifestos for the built environment for upcoming same May day elections.
Advancing BIM education with a competency framework
“We don’t need people who can just draw in 3D. We need people who can think in data.”
Guidance notes to prepare for April ERA changes
From the Electrical Contractors' Association Employee Relations team.
Significant changes to be seen from the new ERA in 2026 and 2027, starting on 6 April 2026.
First aid in the modern workplace with St John Ambulance.
Solar panels, pitched roofs and risk of fire spread
60% increase in solar panel fires prompts tests and installation warnings.
Modernising heat networks with Heat interface unit
Why HIUs hold the key to efficiency upgrades.