How to Optimise SAP/SBEM CHP Heat Share with Heat Pumps


The ‘fraction of heat’ value for CHP is an important component in SAP/SBEM calculations. The larger the heat share from the CHP, the greater the carbon and cost savings. 

Combining electric heat pumps with CHP can significantly increase the share of low carbon heat, particularly when the project has a comparatively low electrical demand for the CHP. This is often the case in schools and ‘landlord’ areas within multi-residential developments.

As electricity consumers, heat pumps create additional electrical load for on-site CHP generation. Heat pumps can be activated during periods of reduced power consumption; for example, by topping-up low night-time electrical loads.

This combination of low carbon and renewable energy can offer a powerful solution and the concept is tried and tested in many Danish installations.

“Combining CHPs and Heat Pumps in district heating schemes is standard practice in Denmark” 

Beata Blachut
Technical Manager CHP / LoadTracker
SAV Systems

Beata Blachut

Energy, Cost & CO2 Emission Reduction

The Sankey diagram compares the energy cost and CO2 emissions from a single XRGI 15 LoadTracker CHP + ASHP combination with ‘traditional’ grid supplied electricity and heat from boilers.

Sankey Diagram

Energy Cost Saving*



CO2 Reduction*


*compared with conventional gas boiler & grid electricity


Case Study – The King’s School, Witney

Kings School Whitney

The Project

A school campus expansion project, providing additional space to accommodate both upper and lower school pupils at the same campus.

The Challenge

To achieve CO2 reduction through on-site energy generation, Satisfying ‘Part L’ and SBEM while minimising project costs and payback time.

The Solution

  • One 15 kWe/30 kWth LoadTracker CHP
  • Two Mitsubishi ‘Ecodan’ 4.8 kWe ASHP
  • Together supplying 83% of space heating & DHW demand.


  • Combining low carbon and renewables
  • Power generation at point of use
  • LoadTracker CHP with real-time automatic modulation to match site power demand
  • Minimal use of high CO2 grid electricity
  • Low noise levels of 49 dB(A)
  • Long service intervals (8,500 hours)
  • Simple control strategy to enable ASHP when required


  • Carbon footprint savings
  • Operational cost savings
  • RHI compliant ASHP installation

High and Low Temperature Circuits

CHP and Air Source Heat Pumps combine to supply the high and low temperature circuits in the school.

The Heat Pump is located on the lower temperature return from the underfloor circuit (35˚C), and provides 5˚C of pre-heating.

The CHP supplies a stable and controlled 80˚C to the DHW circuits and the FlowMaster pump and valve precisely mixes this high grade heat with the heat pump return to achieve the target underfloor flow temperature of 45˚C.

Balancing CHP Demands

Combining CHP and Heat Pump can be an ideal way to balance the loads in multi-residential developments, where the ‘landlord’ electrical demand is comparatively low and heat demand is high.

The Heat Pump provides a steady electric load for the CHP, increasing the available operating hours, maximising the heat share and reducing electricity export.

The low carbon site-generated electricity also reduces the carbon footprint of the ‘renewable’ heat pump!


Summary of Site Demand

Annual site electricity 71,789 kWh

Heat pump electricity 21,563 kWh

Electricity price 13.19 p/kWh

Annual site heat and DHW 246,445 kWh

Gas price 3.48 p/kWh


Carbon Footprint Savings

17 tonnes of CO2 emissions could be reduced by installing a CHP system relative to a conventional mains supply/gas boiler system.

Carbon footprint savings table

This is an equivalent to 17% reduction of CO2 emissions.

Cost Savings

The use of LoadTracker CHP would result in annual savings of £4,292 relative to a conventional mains supply/boiler system.

Cost savings table

This is equivalent to saving 22% on energy bills

CHP HP share

Mitsubishi Heat Pump

CHP unit

Thermal Store or Just a Buffer?

The arrangement and management of stored heat is critical when designing CHP systems.

In a basic system, a “buffer vessel” acts as an oversized header to avoid short-cycling of the CHP unit.

In a sophisticated, optimised CHP system, a real “Thermal Store” meets specific objectives and brings significant operational and economic benefits to the system.

Thermal Store Objectives

To provide a substantial displacement between the time of heat production and heat usage

To enable the CHP unit to meet peak heating loads greater than the CHP heat output

To allow heat demands lower than the minimum output of the CHP to be met

To optimise CHP operating time

Storage Tank

CHP Thermal Store Management

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