11 Years of Home ASHP, Kingston-upon-Thames London

11 Years of Home ASHP, Kingston-upon-Thames London


Air Source Heat Pump


We have to install millions of heat pumps in the UK to meet climate targets, likely mostly ASHPs. What are they like to live with in reality? This quick case study from a home in suburban London near the Thames getting a CoP/'efficiency' of 300% gives an idea.

Basic information

The heat pump is a Daikin Altherma EKHB016AB3V3, installed April 2009.

Annual total energy use is about 5500kWh for 120m^2 of internal floor area. The house configuration is inherently poor with lots of wall and roof area for its volume.

The heat pump has run for 11 years to 2021 with only one trivial fault (leaky air bleed valve). The first major failure appeared this December, which was a sticking valve solenoid in the compressor. Diagnosis of this took a long time. The 3kW back-up heater kept the house warm enough.

In reality, there is no routine preventive maintenance to do — the device just works until something fails.

The radiator output has been increased by 68% from the original installation for the gas combi boiler.

Extra Insulation

The customer fitted extra insulation in various places, improving performance significantly.

The Coefficient of Performance (CoP) over the year is about 3.

Internal refrigerant to water unit. Note insulation on pipework. Extra insulation is also installed on the innards of both the outside and inside units.
Hot water tank with extra insulation. Main/water unit top left.

Improved Radiators

When reducing flow temperatures from a typical gas boiler, putting better radiators in is common.

'Better' may mean physically taller and wider, or extra layers and fins. This allows more heat into the room, ie a greater heating power, for a given flow temperature above room temperature.

Historically people have run radiators as high as 85°C, which can injure anyone touching them.

Heat pumps work more efficiently the lower the flow temperature, so 45°C or even below body temperature (~35°C) is good.

Cast aluminium high-output radiators, tall and squat.

CoP and Temperature Setting Graphs

Weather compensation hand-drawn graph.
CoP vs lift hand-drawn graph.

Further Detail

The house was designed in the late '80s, completed in 1990. It was a private development for an end user. The architect was based in Dorking.

Just before completion, the original intended owner backed out of the deal, leaving the builder to complete the work.

The house is traditional construction with brick and concrete tile pitch roofs. The inner skin of the external walls is 125mm low density Thermalite blockwork.

The house was modified by:

  • adding cavity insulation
  • installing triple-glazed argon-filled low-emissivity windows
  • adding heat-recovery ventilation
  • adding loft insulation
  • using LED lighting
  • draught proofing, particularly to light fittings
  • adding a well-designed conservatory which acts as a solar collector
  • added 3.85kWp of solar pv on south-east facing roof.

The increase in radiator capacity came from a simple linear mathematical model based on the original radiators for the gas boiler, and to keep the internal temp at 20°C. The principle was that the ASHP should run 80% of the time. The 20% is to allow capacity for transients.

  • Energy consumption for space heating and hot water in 12 months up to ASHP = 10,000kWh; total energy consumption (gas and elect.) = 13000kWh
  • Energy consumption for space heating and HW over 12 months after ASHP = 4000kWh; total energy consumption = 7000kWh

The CoP was about 2.5 immediately after installation. After that the ASHP was modified by addition of extra insulation to compressor, heat exchangers, etc, hot water setting temperature was lowered and ambient temp set points were refined by experience, with more radiator capacity being installed. Also, the house was modified giving current total energy consumption of 5500kWh per year.

Full Case Study

All images are provided by the courtesy of the owner. To view the full case study, including references, see 11 Years of Home ASHP, Kingston-upon-Thames London: Case Study - Earth Notes

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