Overview

This article is designed to provide details of a specific installation of an air source heat pump facility, where the implementation is air-to-air rather than air-to-water. The property in question is a fairly typical 3-bedroom detached house (187sq.m.), built in the 1990s, with an offical EPC rating of 64 (D) when purchased in 2014.

Although rarely promoted by suppliers and the UK government, air-to-air is much more cost-effective and quick to install and has been proven over many years across Europe. These kind of systems are now used in Orkney and elsewhere, where other options such as gas, wood burners and oil are largely unavailable or uneconomic.

A "new generation" air-to-air system is ideally suited to properties that are "all electric", i.e. rely on electricity for all their heating requirements, with no central heating system. Typically such homes are heated with electric radiators (often with night-storage devices). With current electricity prices these use huge amounts of power and can be extremely expensive to run. Furthermore, their heat output is poor in the late afternoons and evenings. Air-to-air heat pump system systems are also suitable for homes that already have another form of heating (e.g. gas or oil-fired CH systems), but find these can also be very costly to run and may benefit from augmentation with an air-to-air system. Finally, air-to-air is ideal for new builds and occasional occupancy properties, as they provide almost instant heat in a very efficient manner.

External heat pump unit - approx 250mmx550mmx750mm

External heat pump

Internal unit - approx 200mmx300mmx750mm

Internal unit

How it Works

There are just two elements to an air-to-air heat pump system: an external unit that extracts heat from the air (down to a temperature as low as -10C to -15C) and uses this to heat a fluid to c.90C that is then pumped (when needed) to internal devices that look exactly like air conditioning units. Both internal and external units are almost silent. Photographs of an external unit and internal unit are provided above. The internal units are "smart" and typically controlled by an app (and/or remote controller) that handles the hour-by-hour temperature that you want the room to be heated to. For the system I have had installed there are two internal units and one fairly small external unit (up to 5 internal units can be provided from one external unit). The internal units are powered from the external unit, and each has a pair of small flexible pipes to them (so each has 3 connections - 1 power and signalling, and a to-pipe and a from-pipe for the fluid flow). With our two units that is 6 connections in total, but all are small and hidden in moulded ducting (obtained from ManoMano, see image below) - the external unit only requires a fairly small hole through the outside wall of the house for these connections and the power connection. Installation of the units took place over two days, and disruption was minimal.

Power consumption and cost

The total cost of the installed system was a little over £5000 (0% VAT rated), plus the electrical installation - we used the circuit from one of our now redundant storage radiators for the link to the external unit, which was completed a few days prior to the system installation and took roughly 3 hours.

Each internal unit in my system is set to provide 20C during the day and 17C at night. These settings can be dynamically changed via the app, and their power usage is monitored at all times. In theory they provide up to 5:1 efficiency, i.e. for every 1kWh used a total of 5KWh of heat is provided - this is for the latest technology from Daikin (see linked PDF brochure Daikin_New_Multi.pdf). Other providers such as Panasonic, Vaillant, Samsung etc., may offer similar systems, but most only promote their air-to-water solutions, which are far more expensive and disruptive to install. The power usage of the Daikin system can be viewed on a per hour, day, week and year basis, so it is possible to check the actual power usage, and hence cost, of running these systems. In my case the system was installed in late January, and since then has provided consistent heat for two large areas of the house (essentially most of the ground floor).

Power usage (Updated Jan 2024, after 1 year's usage): Total power consumption over 12 months was just under 1500KWh - at current (high, daytime) UK rates that is under GBP600 (actual cost over this period was lower, owing to much lower unit rates in the first half of the period, and lower rates at night, althought nighttime/off-peak usage is relatively low). Peak usage on the coldest days, when outside temperatures were at -5C, was less than 14KWh, and average usage was under 5KWh.

Some observations regarding the installation and usage should be made: (i) we still have some night-storage radiators running, but have turned off four completely and set the others on lower power settings; (ii) our property is quite well-insulated, but does have a lot of glazing, including three sets of french doors and two windows in the main room downstairs; (iii) at night in the winter we like to use our 5kw wood burner, which augments the evening heat and looks nice!; (iv) the system has a small annual maintenance cost, much as a conventional CH system would have; (v) this does not provide a solution to water heating; (vi) for some reason there is no separate government support for this kind of installation, but as it so much cheaper and less disruptive than "wet" systems, this should not be a major factor when deciding on whether such a system is worth installing; (vii) because air-to-air systems distribute heat via the flow of heated air, they are well-suited to properties with larger rooms or open-plan homes and workplaces. As with all such systems they work best in well-insulated properties.

With other measures to reduce our annual electricty usage (e.g. using LED lighting everywhere, moving to more efficient/low power devices, doubling insulation in the loft area using 100mm rolls of wool insulation) the full effect and value of this and other measures can be calculated, but initial results suggest that overall usage in KWh is already around 21% less when compared to 2021 and 2022 and this is probably an underestimate of the reduction to be achieved. Still plenty of work to do however - monitoring specific devices shows that the highest usage that can be directly monitored is by the tumble dryer. The induction hob and oven cannot be measured separately as they are hard-wired into their circuits. The tariff splits overall over the last 6 months are: 35% daytime (08:00am to 11:30pm), 19% nightime standard (mainly for water heating, but some airsource heat pump usage), and 47% nightime storage heaters - daytime cost is 2x nightime and storage heater cost, i.e. roughly 50% of costs

Power usage 2023 and comparisons with 2022 (from Scottish Power)

power usage

Heat pump ducting - internal

heat pump ducting