Air source heat pumps – how do they compare to condensing boilers?

Air Source Heat Pumps (ASHP) are generally considered to be a more energy efficient option to gas boilers. By extracting warmth from the surrounding air, the heat energy released can be many times the energy required to power the equipment. In some instances however, a gas-fired condensing boiler may be a better choice, as ASHP systems are, by their very nature, dependent on the ambient temperature and more efficient at certain times of the year.

How ASHP’s work

An ASHP system consists of a compressor and a carefully matched evaporator coil and heat exchanger. A refrigerant liquid, with a boiling point as low as 40°C, circulates within the system, evaporating when absorbing heat from the outside air. The resulting refrigerant gas is then compressed, adding more heat energy and raising its temperature to around 56°C. This heat is passed via the heat exchanger into water or air and used to provide space heating, for instance through low temperature convectors, or via under floor heating systems.

Calculating efficiency

The following formulae are designed to help installers make informed heating choices, analysing the relative efficiency by finding the break-even point, or point where both systems are as efficient as each other. The point can be calculated using either emissions or cost.

Gas Cost per Unit                             =                                Electricity Cost per Unit
Gas Condensing Boiler Efficiency      =                                Electric ASHP Efficiency

The formula can be re-arranged to find the break-even CoP for ASHP as follows:

Electric air sourced        =          Electricity cost per unit      x     Gas efficiency
Heat pump efficiency                     Gas cost per unit

Putting typical figures in this place gives:

Break-even cost EVC =

0.09£/kWhr (electricity) x 0.9 = 2.3 CoP for energy equality
0.035 £/kWhr (gas)

The same formula can be used for emissions analysis:

Gas emissions                                       =                Electricity emissions
Gas condensing boiler efficiency           =                 Electric ASHP efficiency

The formula can be re-arranged to find the break-even CoP as follows:

Electric air sourced heat pump efficiency =

CO2 emissions from electricity per unit x boiler efficiency
  CO2 emissions from gas per unit

Result:

Break-even CoP EVC =

0.432 kg CO2/kWhr electricity x 0.9 = 2.24 CoP for emissions
0.193 kg CO2/kWhr gas

The figures used in the equations can be adjusted as prices, efficiencies and emissions change with time and consequently used to calculate the viability of gas driven heat pumps or any other similar comparison.

These calculations show that when an air-sourced heat pump has a Coefficient of Performance (CoP) of less than 2.3, its CO2 emissions are greater than a gas-condensing boiler and vice versa; when the ambient temperature is higher and the CoP is more than 2.3 the ASHP is the more efficient system. When choosing heating equipment, the questions that need to be asked are: at what ambient temperature does a system become more or less efficient and what percentage of the heating season is above or below this figure?

When used externally to a building, Colt’s outdoor unit ASHP provides a break-even point of about -3°C with an overall break-even point of 2°C, a temperature that is infrequently met in the UK.

With the vast majority of the UK heating season above this temperature, Colt’s ASHP provides a more efficient option compared with a condensing gas boiler, combining the functionality of cooling for warmer months.

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