Module 83: Integrating centralised hybrid heat pumps with independent room units for energy-efficient concurrent heating and cooling

This module looks at how independent room units are successfully integrated with centralised hybrid heat pumps for energy efficient heating, cooling and hot water

This CPD will consider how modern variants of water-loop room systems are successfully integrated with centralised hybrid heat pumps to provide year-round energy-efficient heating, cooling and hot water, with reduced life-cycle cost and environmental emissions.

There are numerous global applications of four-pipe and two-pipe water distribution systems serving room terminal units – most typically, fan coil units (FCUs) comprising a filter, coil(s), possibly condensate drainage and a fan – that provide heating and cooling to rooms. In CIBSE TM43, a simple analysis of the CO2 emissions for a fan coil unit system installed in an office-type ‘example building’ compare favourably with most other HVAC systems. If used with a well-designed fresh-air air-handling unit (AHU), a fan coil unit system will comply with Building Regulations, and is surpassed only by more expensive and less flexible systems, such as chilled beams/ceilings.

Four-pipe systems have two independent water circuits – one with chilled water for room cooling, the other with hot water for heating. All terminal units in four-pipe systems are equipped with two independent coils, and can cool or heat according to space requirements. Four-pipe systems are extensively used in temperate climates such as the UK’s, particularly where there is no clearly defined seasonal operation. No summer/winter changeover is required, as cooling or heating can be produced at all times, and the control of the temperature of each room is independent of others.

In comparison, a single, two-pipe, water circuit can be used either for space heating or for cooling, but not both at the same time. A summer/winter ‘switch’ is used for a seasonal changeover so simultaneous cooling and heating in the same system is not possible. Successful operation of ‘changeover’ two-pipe systems in climates such as that of the UK is likely to be challenging and rarely applied.

Both four-pipe and two-pipe systems are typically served by either a chiller and boiler combination or a heat pump arrangement.

In commercial or institutional buildings, cooling and heating loads often coincide. Traditional systems – with separate chillers and boilers – do not allow the recovery or shifting of heat from one space to another. A hybrid heat pump allows this energy potential to be recovered and usefully applied.

Refrigerants with GWP below 2,500

A hybrid heat pump is a packaged heat pump equipped with a flexible and versatile heat-recovery system, which offers the options to deliver cooling only, heating only, or cooling and heating at the same time.

Each unit is equipped with three heat exchangers: the so-called ‘main heat exchanger’, where chilled water is produced; the heat recovery or ‘secondary heat exchanger’, where only hot water can be produced; and the condenser/evaporator, where heat rejection or heat absorption takes place. This last heat exchanger can be a finned coil, in the case of air-cooled units, or a refrigerant-towater heat exchanger, in the case of a water-cooled unit. In each operating mode, only two heat exchangers are activated.

When only chilled water is required, the unit will operate like a normal chiller – the heat will be removed from the main heat exchanger and rejected at the condenser (A1 mode in Figure 1).

Figure 1: Working principle of the single refrigerant circuit in a hybrid heat pump (E = main heat exchanger; C = compressor; V = expansion valve; R = secondary heat exchanger; S = condenser/evaporator)3

When chilled water and hot water are required at the same time, the unit will switch to heat-recovery mode – the heat removed at the main heat exchanger producing chilled water will be rejected to heat recovery, producing hot water (A2 mode in Figure 1).

If the chilled water requirements are satisfied, but there is still a demand for hot water, the unit will switch to heat-pump mode, using the third heat exchanger as the evaporator and rejecting the heat to the main heat exchanger, or to heat recovery, producing hot water (A3 mode in Figure 1).

The unit can change its operating mode, according to system requirements.

If the unit is equipped with two independent refrigerant circuits, each circuit can operate in A1, A2 or A3 mode independently from each other. The control logic of the unit will optimise the operation of each circuit to minimise the energy consumption.

Traditional systems, including chillers or traditional heat pumps have a typical energy efficiency ratio (EER – ratio of the cooling output to the total power input) of approximately 3 (air-cooled) and 5.2 (water-cooled), or heat pump coefficient of performance (COP – ratio of the heating output to the total power input) of approximately 3.2 (air-sourced) and 4.5 (water-sourced).

Hybrid heat pumps can deliver typical values for a total efficiency ratio (TER – ratio of sum of cooling plus heating outputs to total power input) in the range of 7 to 9.

The secondary heat exchanger of the hybrid heat pump can be connected to the domestic hot water primary circuit. Air-sourced units with semi-hermetic R134a screw compressors can operate with outdoor air temperature down to -10°C while maintaining economic hot water production up to 50°C.

In a four-pipe system (Figure 2), the main heat exchanger only provides chilled water to the circuit dedicated to space cooling, while the secondary heat exchanger supplies hot water to the circuit dedicated to space heating. Air-sourced R410A units with scroll compressors have been designed to produce hot water at 45°C during normal operation with an outdoor air temperature of -10°C in winter.

A make-up boiler is indicated in Figure 2, as it may be necessary, in some cases, to top up the heating capacity, or the hot water temperature.

Meeting the heating loads

Traditional flow water temperatures of 80°C are not available from heat pump technology, so the fan coil units must be designed to provide the appropriate heat output at flow water temperatures closer to 45°C.

Example application

In Milan, a 40-year-old, four-storey office building was refurbished and extended to create a further two floors. The project replaced the existing two-pipe heating and–cooling plant – comprising a liquid chiller and a boiler – with a hybrid heat pump capable of producing both chilled water and hot water, independently and simultaneously.

The L-shaped building had different exposures, so areas had very different heating and cooling loads, with concurrent opposite loads, particularly in mid seasons.

Cooling was required from external temperatures of -5°C (15% max load) to 35°C (100%), heating from -5°C external (100%) to 18°C (20%).

The old two-pipe system was replaced by a more versatile four-pipe system that supplied 263 FCUs and two AHUs from a hybrid heat pump (Figure 3) with a nominal cooling/heating capacity of 550 kW/396kW.

Figure 3: Hybrid R134a screw compressor heat pump – 550kW cooling capacity with 7°C chilled water at 35°C ambient, 396kW heating capacity with 45°C hot water at -5°C ambient

The actual measured performance of the system was compared with a model of a traditional system that used a liquid chiller for cooling and a condensing boiler for heating.

The quantity of heat available from the secondary heat exchanger is enough to provide the heating requirements of the building from external temperatures of 18°C down to around 6°C. For lower outdoor temperatures, the heating and cooling loads of the building will be covered by the hybrid heat pump operating one refrigerant circuit in ‘cooling and heat recovery’ mode (A2 operating mode in Figure 1) and the other circuit operating in ‘heating only’ mode (A3 operating mode in Figure 1). Because of the Milan climate, no additional boiler was needed for winter top-up, so no gas connection was required for the building.

From 35°C to 18°C external temperature, the unit operates as a chiller (A1); from 18°C to 6°C, it operates in ‘cooling and heat recovery’ mode (A2); from 18°C to -5°C, it provides cooling/heat recovery and acts as a heat pump (A2 and A3).

By undertaking a 15-year life-cycle cost (LCC) analysis of the two solutions, the capital cost of the hybrid heat pump solution is 28% higher, but it offers a payback period of 1.2 years, while the lifecycle cost is 23% lower.

A similar system comparison was performed by modelling the same building in the more extreme climatic conditions of Stuttgart, Germany. (For a comparison of outdoor air temperatures, see Figure 4.) Stuttgart has a significant number of hours with external temperatures lower than -5°C, even during daytime, with a recent minimum recorded temperature of -12°C. In these conditions, all-year-round use of a heat pump is not possible, and a condensing boiler was added to the model for operation whenoutdoor temperatures were below -5°C. The modelled behaviour of the system is similar to that of the real case in Milan, with the principal difference being that when the free heating is not available to cover the heating load of the building, the hybrid heat pump will operate as a heat pump (A3 operating mode) down to -5°C outdoor temperature.

Figure 4: Average minimum and maximum external temperatures for Milan, Stuttgart and London (based on monthly data) (Data source:

The capital cost of the new Stuttgart system is 54% more expensive than the traditional system, but provides a payback period of significantly less than two years, and LCC will be reduced by 25%, assuming 15-year operation. In Stuttgart, there are more hours where simultaneous heating and cooling are required, and where the heat recovery mode (A2) can be applied.

The heating energy available as free heating reduces the amount of primary energy needed to satisfy the heating loads of the building, thus resulting in the potential reduction of CO2 emissions, as shown in Figure 5.

Figure 5: Comparative CO2 emissions and energy use3


The use of a hybrid heat pump can provide a significant improvement to the environmental performance of a building with disparate loads, by reducing running costs and lifecycle costs, plus primary energy use, and so cutting environmental emissions. With appropriate systems, this can be successfully implemented across a wide range of climatic zones.

© Tim Dwyer, 2015.

Further reading:

For authoritative guidance on EU F-gas regulations see information sheets downloadable from Gluckman Consulting


  1. CIBSE TM43 Fan coil units, 2008.
  2. CIBSE TM53 Refurbishment of non-domestic building, 2013.
  3. Janes, M et al, Energy efficiency in the modern buildings: Energy saving through the application of hybrid heat pumps with simultaneous and opposite loads – a case history and a numerical simulation for a four-pipes system, Rhoss SpA, Italy, 2014.