Commercial hot water heat pumps: when hot water is the main load

When hot water is the main load

A typical office is dominated by space heating. A hotel, care home, leisure centre, commercial kitchen or laundry is different: showers, baths, pools, washing machines and dishwashing run all year, so hot water can account for most of the gas the site burns. In those buildings the useful question is not “what heat pump do I need for the radiators?” but “how many litres of hot water do I use each day, and at what temperature must it be stored and delivered?”.

That changes three things. The load is steadier through the year, which suits a heat pump because it runs for many hours rather than short winter peaks. The required temperature is higher than space heating needs, which pushes efficiency down. And the water sits in storage and pipework, which brings legionella control into the design from day one.

Storage temperatures and legionella risk

Anyone who controls premises with a hot water system has a duty to assess and control the risk from legionella bacteria. The Health and Safety Executive publishes the approved code of practice (L8) and the technical guidance HSG274; Part 2 covers the control of legionella bacteria in hot and cold water systems. Read those documents, and have a competent person carry out your risk assessment, rather than relying on a heat pump supplier’s summary. This page deliberately quotes no storage or distribution temperatures: the figures that apply to your building, including any higher expectations for healthcare and care settings, are in the HSE guidance.

What matters for a heat pump project is how the design behaves against that guidance:

• Can the system reach and hold the required stored temperature? A heat pump that tops out below it needs a top-up stage, such as an electric element or a retained boiler, or a higher-temperature machine.
• Is there a regular full-volume heat-up? Part of a legionella control regime is periodic pasteurisation-type operation. Ask how the controls deliver it and what that does to running cost.
• Are there dead legs, low-flow sections or lukewarm zones? Pipework layout matters as much as the heat source.

Two-stage, high-temperature and CO2 designs

A standard air-source heat pump designed for space heating is efficient at moderate flow temperatures. Making hot water for storage means lifting water to a much higher temperature, and efficiency drops as the lift grows. Manufacturers deal with this in several ways, described here in general terms:

• Two-stage or cascade systems. A first heat pump preheats the cold mains feed, and a second stage (a heat pump, or an electric or gas top-up) lifts it to storage temperature. The first, large temperature rise is made efficiently.
• High-temperature heat pumps. Units with refrigerants and compressors chosen for higher delivery temperatures. Check the efficiency figure at the delivery temperature you actually need, not at the headline conditions.
• CO2 (R744) heat pumps. CO2 behaves differently from conventional refrigerants and can deliver high temperatures from a cold return, which suits a large temperature rise of the kind hot water calls for. Performance depends on a cold return to the unit, so storage and control design matter.

Refrigerant handling in the UK falls under F-Gas rules and must be done by qualified people; R290 (propane) is flammable, which affects where units can be sited. See the high-temperature and process heat pump guide for the wider picture.

Sizing from daily hot water demand

Size hot water plant from the energy needed per day and how quickly storage must recover, not from the size of the old boiler. The energy to heat water is simple physics:

Energy (kWh) = litres × 4.18 × temperature rise (K) ÷ 3,600

or about 0.00116 kWh per litre per degree of rise. Multiply by the number of litres drawn per day (from sub-metering or occupancy data, not a rule of thumb) and by the rise from your cold feed temperature to the storage temperature that your legionella assessment requires. That is the daily thermal energy. Divide by the hours the heat pump can realistically run in a day to get a first estimate of heat pump output, then check that the storage volume covers your peak draw periods, such as morning showers or a kitchen service. Add allowance for standing losses from storage and circulation pipework.

Where a site has several demand profiles (a hotel with a spa and kitchen), use the sum of the profiles with a sensible diversity allowance rather than adding every peak together. The commercial heat pump sizing guide covers that logic for space heating as well.

Dedicated unit, or one machine for heating and hot water?

One machine can serve both, using a diverter valve to switch between space heating and cylinder charging. That keeps equipment count down, but the machine then runs at its highest flow temperature whenever it charges hot water, and heating is unavailable during that time. A dedicated hot water heat pump tends to make sense when:

• hot water is the larger share of the load, so it should be designed around hot water duty;
• the space heating system runs at low flow temperatures, which would be wasted if the same machine had to serve high-temperature cylinder charging;
• demand peaks coincide, such as morning showers during the heating warm-up;
• the building keeps a boiler for peaks, in a hybrid arrangement, and a separate unit covers the base hot water load.

A single machine is more likely to suit a building with modest hot water use relative to heating. Ask each supplier to show their reasoning against your demand data.

Running-cost arithmetic with current prices

Cost per kWh of heat is the energy price divided by efficiency. For gas, assume a boiler seasonal efficiency of 85-90% and state the figure you used. For a heat pump the divisor is the SCOP achieved at the hot water delivery temperature, which is lower than the space heating SCOP. DESNZ Quarterly Energy Prices for Q1 2026 (provisional, published 30 June 2026, excluding VAT, including Climate Change Levy) give medium-sized businesses 25.0p per kWh for electricity and 4.55p per kWh for gas.

So at these national averages a hot water heat pump costs more to run than gas on energy price alone. It can still come out ahead with a cheaper electricity tariff, on-site generation, time-of-use charging into storage, or where the gas boiler is being replaced anyway and carbon matters. Do not take any general statement as your answer: put your tariffs into the running-cost calculator and see the method in the full running-cost guide.