How a commercial air source heat pump is installed: siting, plant room and commissioning

A commercial air source heat pump is one of the most common replacements for an ageing gas or oil boiler in offices, schools, care homes, retail and light industrial buildings. It needs no boreholes and no abstraction licence, so it is quicker and cheaper to deliver than a ground source system. What it does need is careful sizing and a realistic look at the building’s heating circuit. This guide covers how the plant is sized, what decides its efficiency, which refrigerants are in use and what to settle on site before anything is ordered. The wider process is covered on the commercial air source heat pump installation page.

How a commercial air source heat pump works

A heat pump moves heat rather than creating it. A fan draws outside air across a coil, a refrigerant absorbs that heat and evaporates, a compressor lifts the pressure and temperature of the gas, and a second heat exchanger gives the heat to the building’s water circuit. The refrigerant condenses, passes through an expansion valve and returns to the coil. Because most of the delivered heat comes from the air, the machine delivers several units of heat for each unit of electricity it uses.

Commercial units differ from domestic ones mainly in scale and in how they are installed. Capacity is usually provided by several modular units in cascade, which gives resilience (one unit can be serviced while others run) and lets the plant follow a variable load. Output is connected to a buffer vessel or low-loss header and into the existing heating circuit, often with the old boiler left in place as a backup or peak top-up.

Sizing: heat loss and real consumption

The correct size comes from two things:

1. A heat-loss calculation for the building’s fabric and ventilation, evaluated at the design outside temperature.
2. At least twelve months of gas or oil consumption, which shows the real demand profile, the base load for hot water and the seasonal swing.

Floor area alone is a poor guide. Two buildings of the same size can have very different heat demand depending on glazing, insulation, occupancy hours and ventilation. Oversizing wastes capital and causes short cycling, which reduces efficiency and shortens equipment life. Undersizing leaves the building cold on the coldest day unless a boiler covers the shortfall.

For larger sites, half-hourly electrical meter data matters too. A heat pump adds electrical load, and the incoming supply has to carry it. The distribution network operator (DNO) must be told about a significant increase in demand, and a supply upgrade can be the longest-lead item in the project. This is why the electrical supply is checked at feasibility rather than after the plant is chosen. The same point is made in the guide on how long a commercial heat pump installation takes.

SCOP: what really decides efficiency

Efficiency is quoted as a seasonal coefficient of performance (SCOP). Manufacturers declare it under BS EN 14825, which weights performance across a heating season using the test points in BS EN 14511. Two cautions apply when reading a datasheet:

• SCOP is declared at specific flow temperatures. A figure at 35 °C is not the figure the building will get if its radiators need 55 °C.
• Real running efficiency also depends on controls, defrost behaviour, pump energy and how well the system is commissioned.

The lever that matters most is flow temperature. A heat pump works harder, and delivers less heat per unit of electricity, the higher the temperature it has to produce. Old boiler systems often ran at 70 to 80 °C. Many commercial buildings can run a heat pump at roughly 45 to 55 °C once the emitters have been surveyed and the weak points upgraded selectively. That emitter question is covered in the guide on whether radiators need replacing.

Refrigerants and F-Gas

Heat pump refrigerants are regulated in the UK under the F-Gas Regulation, which caps and reduces the supply of high global-warming-potential gases and requires certified engineers to handle them. As reference points, the global warming potentials used in the regulation (IPCC fourth assessment values) are about 2,088 for R410A, about 675 for R32 and about 3 for propane (R290).

Propane is attractive for its low GWP and good high-temperature performance, but it is flammable. Its use affects where the unit can be placed and how much charge is allowed in a given space, which is governed by BS EN 378 and a flammable-atmosphere risk assessment under DSEAR. A competent designer settles the refrigerant, charge and exclusion zones early, because they drive the plant location.

Siting, noise and planning

External plant makes noise, and noise is one of the most common reasons a scheme is delayed or complained about after commissioning. Siting questions to settle before ordering:

• Acoustic position: a BS 4142 assessment compares the plant’s noise with the existing background level at the nearest sensitive receptor. It sets what attenuation, if any, is needed.
• Planning status: the planning route depends on the site and should be confirmed with the local planning authority; size, siting and noise conditions apply. Listed buildings and conservation areas need consent.
• Airflow and clearance: units need clear airflow in and out. Recirculation of cold exhaust air cuts capacity.
• Structure and access: roof or ground-mounted plant needs a structural check, crane access and drainage for defrost condensate.
• Pipework runs: long runs between plant and building lose heat and add pump energy.

Hybrid and retained boilers

A retained boiler is often the simplest way to handle a very cold peak or a building that cannot yet run at low flow temperatures. In a hybrid arrangement the heat pump covers the bulk of the annual load and the boiler tops up at the extremes, so the heat pump can be sized for efficiency rather than the single coldest hour. Keeping the boiler live through commissioning also means the building is never without heat during the changeover.

What it costs and how it is paid for

Cost depends on capacity, the state of the heating circuit, the electrical supply and the siting. Rather than rely on a single headline figure, use the ranges and tax treatment set out in the commercial heat pump cost guide. Heat-pump plant is commonly an integral feature of a building, so the Annual Investment Allowance is usually the relevant capital allowance; see capital allowances on heat pumps. Grant schemes change often, so check the status of any scheme before planning around it; the current position is summarised on the grants and funding page.

A short pre-order checklist

• A heat-loss calculation and twelve months of fuel data
• The design flow temperature and an emitter survey
• The DNO supply position for the added load
• A BS 4142 acoustic assessment and the planning route
• Refrigerant, charge and any flammable-atmosphere constraints
• SCOP quoted at the design flow temperature
• A commissioning plan that verifies performance against the design

Frequently asked questions

What size commercial air source heat pump does a building need? The size comes from a heat-loss calculation and at least twelve months of consumption data, checked against the coldest design day. Modular cascades are common.

What is a good SCOP? Ask for SCOP at your own design flow temperature. The lower the flow temperature the emitters need, the higher the SCOP.

Do they work in winter? Yes. Output falls as the air gets colder, and the plant is sized for the design day, with defrost allowed for and a boiler available in a hybrid design.

Are the refrigerants being phased out? High-GWP gases are being reduced under the F-Gas Regulation. Current commercial plant commonly uses R32 or propane, with siting rules for flammable refrigerants.

Is planning permission needed? It depends on size, siting and noise. Confirm the planning route with the local planning authority; listed buildings and conservation areas need consent.

To compare this with the alternative, read the guide to air source versus ground source for a commercial building and the page on commercial ground source heat pump installation. To get a view on your own building, use the form below for a desk feasibility; the reply comes by email.

For how commercial plant differs from home systems, read commercial heat pump vs domestic heat pump; for combined boiler and heat pump designs, read the hybrid heat pump guide for commercial buildings.

For the full process, see the commercial heat pump installation overview.