This case study, provided by Neil Hudson of S&P Coils, is based on the use of heat pumps as the heat source and banks of radiant panels as the heat emitters to provide comfortable classroom conditions in schools designed to meet building bulletins 87 and 101.
Heat pumps have traditionally been associated with medium or low water temperature operation and hence have been deemed suitable only for low temperature emitters such as underfloor heating systems. The advent of high temperature heat pumps opens up the possibility of combining these with more effective emitters, one of which is radiant ceiling panels which provide increased flexibility due to their modularity and, as a result of their low thermal inertia, provide quick response and heat-up times.

Unlike conventional combinations of heat sources and emitters, the energy efficiency of the heat pump/radiant panel combination will increase at part load conditions.

Heating load calculations are usually based on worst case scenarios for space heat losses. While this worst case condition may apply for a few hours each year its use in sizing equipment invariably leads to oversized conventional equipment. One solution is to design the equipment against conditions which occur more commonly.

The combination of high temperature heat pumps and radiant panels can circumvent the above arguments allowing equipment to be sized conventionally against worst case conditions knowing that its efficiency will increase and control will not be compromised at part load, under which the equipment will operate for 99% of the heating season. As radiant panel outputs increase almost linearly with mean water temperature their outputs are directly linked to the control of the heat pump's water delivery temperature throughout the performance range. The excess in available panel area at part load, rather than being a hindrance now, becomes a positive benefit when coupled with the high temperature heat pump as it allows the heat pump to operate at lower water supply temperatures and hence at a higher level of energy efficiency.

Conversely, and beneficially, the relatively high supply water temperatures associated with the latest range of heat pumps allows economical numbers of radiant panels to be selected against the worst case design conditions.


The following analysis is aimed at demonstrating the annual carbon dioxide savings that would be expected compared to the use of a conventional boiler. While it is only strictly valid for the case study in question a good approximation of the available savings for other situations can be assessed by straight comparison with the design heat source requirements. Biomass boilers are not used in the comparison as the decision between biomass and competing technologies cannot be made on the basis of explicit emission savings; other less obvious factors need to be considered.

The heating system in question is intended to provide the heating requirements for a number of classrooms within a school. These could represent the entirety of a small school or could be an individual zone of a larger complex.

Conditions and equipment

Five standard classrooms are considered and the occupancy levels, heat losses and other design data are given in the table above.

The selection of panels in 3 m modules is given as an example only and actual panels sizes will vary as will the layout of the room and suspended ceiling. The selection of 19°C as a space temperature is based on the use of a radiant heating system; such systems can operate successfully at lower space temperatures as the increased temperatures of the room surfaces provide a higher 'operativeæ temperature than would be available for a warm air heating system. Space air temperatures would typically be 2°C to 3°C lower for a comfortable radiant heating system.

The heat source selected is a high temperature heat pump module consisting of three individual heat pumps. These are all connected to a common 400 litre capacity buffer tank and are controlled by a single weather compensated controller. Each of the individual heat pumps has its own circulating pump to ensure full flow through the heat exchanger, a secondary pumping system is employed on the other side of the buffer tank to supply the radiant panel runs.

Actual/seasonal system performance

At the design conditions given above the combination of heat pumps and radiant panels will provide the required heating capacity against the worst case conditions. These conditions require that the heat pumps generate hot water at the maximum temperature available i.e 65°C. At this temperature the coefficient of performance of the heat pumps (COP = useful heat output/rate of energy input) is low, registering a value of only around 2.1.

The COP for any heat pump is always maximised thermo-dynamically by minimising the temperature difference between the supply water into which heat is transferred and the ambient air from which heat is absorbed. This variation in COP is dampened when using AWHT0061 heat pumps because of the Enhanced Vapour Injection feature of the scroll compressors used. This technology allows for injection of high pressure/temperature vapour midway through the compression process and allows a single compressor to attain compression ratios which would normally be associated with multi stage compression.

The rate at which the heat pump absorbs electrical energy for its operation remains reasonably flat as the temperature of the ambient air increases, the rate at which the heat pump generates heating energy in the form of hot water, however, increases with rising ambient temperature hence the increase in the COP.

If the heat pump delivered its heat constantly at the worst case design condition then it would operate at a constant COP of around 2.1. As the ambient temperature increases, however, the heating load is reduced allowing the heat pump to supply water at a lower temperature and permitting operation at higher COP. These lower supply temperatures are sufficient for the radiant heating system to maintain the room at the design comfort temperature because of the reduced room heat losses.

In order to undertake a meaningful analysis of the overall energy requirements and applicable carbon emissions, it is necessary to quantify the performance parameters of the heating system across the range and duration of conditions that would be expected throughout the annual heating season. Before doing this. it is advisable to take an overview of the techniques used by the heat pump controller to match the supply water temperature to the instantaneous heat load.

The algorithm built into the heat pump's controller automatically varies the supply water temperature in response to a change in the ambient temperature. There are several heating curves incorporated in the controller and the selection of the correct one depends on the type of emitter being used.

Essentially, the heating system output is determined by the heating curve operating on the sensed ambient temperature. A room temperature sensor is supplied with the heat pump control system and its sphere of influence on the water delivery temperature can be increased from a default setting of 0 per cent should fine tuning of the system be required.

To assess the annual cost of heating in terms of carbon emissions, an accurate record of ambient temperatures and their duration throughout the heating season is required.

Ventilation loss

Heat loads are based upon the set of classrooms identified in the case study parameters and corrected for actual ventilation losses that would be expected. The worst case ventilation loss is based on a ventilation rate of 8l/s per person which is stipulated as being the design ventilation rate in building bulletin 101. This document states, however, that the minimum daily ventilation rate should be 5l/s per person so as to maintain the CO₂ level below the acceptable threshold. Based on full occupancy, a more realistic heat load is obtained by using 5l/s per person rather than the worst case 8l/s, particularly when taking into account the unrealistic assumption of full occupancy throughout the working day.

Carbon savings - the bottom line

Using current government figures for the CO₂ emission associated with the national grid of 0.562 kg CO₂/kWh the emissions associated with the heat pump supplying an annual 20674 kWh of heat will be 0.562 x 5441 = 3058kgCO₂. This can be compared with the alternative of gas fired boilers using the following assumptions for boiler seasonal efficiency and government emissions figures: Condensing boiler: maximum seasonal efficiency = 88 per cent, CO₂ emissions @ 0.206 kgCO₂/kWh, total emissions = 0.206/0.88 x 20674 = 4840 kgCO₂

Conventional boiler: maximum seasonal efficiency = 75 per cent, CO₂ emissions @ 0.206 kgCO₂/kWh, total emissions = 0.206/0.75 x 20674 = 5678 kgCO₂

Normalised emissions: air source heat pump = 1;Condensing boiler = 1.58; Conventional boiler = 1.86.

From the above, carbon savings of 37 per cent will be achieved compared with a modern, efficient condensing boiler. This figure is somewhat higher than rule of thumb savings quoted for air source heat pumps which indicate savings of between 20 per cent and 30 per cent. The increased savings are not only associated with the efficiency of the heat pumps being proposed but are also a result of the detailed analysis of the actual operating conditions. An analysis based on 24-hour operation would show a reduced saving but would be inappropriate for the situation being studied.

While boilers of all types show some variations in efficiency at different loads, these variations are small compared with the variations in heat pump COP. Heat pumps should be treated differently to boilers if an accurate assessment of the savings is made. Mean values of COP do not adequately quantify the potential savings.

neil.hudson@spcoils.co.uk