How to size a heat pump for a commercial building
Sizing a commercial heat pump starts from the building’s peak heat loss and annual demand, not from floor area or the size of the old boiler.
Two ways to estimate the load
There are two broad routes to a first kW figure, and they answer different questions.
The heat-loss method calculates how much heat the building loses on a design cold day. For each element, loss in watts is U-value × area × temperature difference, and ventilation loss is roughly 0.33 × air changes per hour × volume (m³) × temperature difference. Add them up and you have the peak load the plant must meet. This is the proper basis for design, and for buildings above 45 kW thermal the design should follow recognised CIBSE and BSRIA good practice.
The consumption method works backwards from your fuel bills. It is quick and uses data you already have, which makes it useful for screening. It is not a design method, for the reasons below.
The consumption shortcut and its limits
The shortcut is: peak heat kW = (annual gas kWh × boiler efficiency) ÷ equivalent full-load hours.
Assume a building burns 300,000 kWh of gas a year in a boiler at 88% efficiency. Heat delivered = 300,000 × 0.88 = 264,000 kWh. If you assume 1,800 equivalent full-load hours, the load is 264,000 ÷ 1,800 = about 147 kW. With an assumed 1,500 hours it is about 176 kW; with 2,200 hours, about 120 kW. All the hours figures are assumptions for the arithmetic, not published values.
The spread in that example, roughly 120 to 176 kW, is the problem. The result depends heavily on an hours figure that you do not know. Other limits:
- Annual gas includes hot water, process and cooking loads that do not follow the weather. Strip them out first or the heating peak is misjudged.
- Past consumption reflects how the building was run: set-backs, a broken zone, closed floors, one mild or cold year. Weather-correct it using degree days.
- It says nothing about the coldest hours. A building can use modest annual energy and still have a high winter peak, particularly with intermittent occupancy and a fast warm-up.
- It cannot tell you the flow temperature your emitters need, which sets the SCOP.
Use it to sanity-check a quote or to decide whether a project is worth pursuing, not to buy plant.
Degree days: from peak to annual energy
Degree days measure how cold a period was relative to the temperature at which a building needs heating. They connect the building’s heat loss to annual energy: annual heat (kWh) = heat loss per degree (W/K) × degree days × 24 ÷ 1,000.
Assume a building loses 5,000 W for every degree of indoor-to-outdoor difference and a year has 2,000 degree days (both assumed). Annual heat = 5,000 × 2,000 × 24 ÷ 1,000 = 240,000 kWh. If the design temperature difference on the coldest day is 21 K (assumed), the peak load is 5,000 × 21 = 105,000 W, or 105 kW.
Comparing the heat-loss result with weather-corrected gas use is a good cross-check: if they disagree widely, one of your inputs is wrong, often the assumed air-change rate or a part of the building that is unheated.
Oversizing, cycling, cascades and turn-down
Boilers tolerate oversizing better than heat pumps. A heat pump sized well above the typical load will switch on and off frequently, a pattern called short-cycling. Each start wastes energy, wears the compressor and holds the efficiency below its rated value. Common causes are sizing to the old boiler’s rating (itself often oversized), adding a generous margin on top of a margin, and low system volume.
The standard remedies are:
- Cascades of modular units. Several smaller machines share the load and switch in sequence, so most hours are served by one or two units running steadily.
- Turn-down. Choose plant with a modulating range wide enough for your lowest load, such as a summer weekend.
- Buffer volume. A buffer vessel gives the controls time between starts. Sizing it is a design task, not a rule of thumb.
- Hybrid. Size the heat pump for the base load and keep a boiler for the few coldest hours. This is explained in the hybrid and boiler-replacement guide.
Remember that the peak and the efficient operating point are different things: a design that covers the single coldest hour with a large margin may run at poor part-load efficiency for the rest of the year.
Hot water add-on and diversity
If the site has a hot water load, add it separately rather than folding it into the heating peak. Daily hot water energy is litres × 4.18 × temperature rise ÷ 3,600, and the heat pump capacity needed depends on how long storage has to recover. The commercial hot water heat pump guide covers that, including the legionella requirements that set storage temperature.
Diversity recognises that not every load peaks together: the warm-up peak, the busiest hot water period and the coldest hour rarely coincide. Applying diversity sensibly can avoid paying for plant that will never run at full output, but it is a judgement the designer should show, not hide. Ask how it was applied.
Data to gather before asking for quotes
- Twelve to thirty-six months of half-hourly or monthly gas and electricity data, and the unit rates you pay.
- Floor plans, building age, fabric details (wall, roof, glazing) and any recent upgrades or known problem areas.
- Heating schedule, occupancy pattern, set-point temperatures and any zones that run on different timers.
- Existing emitter types (radiators, fan coils, underfloor) and the flow and return temperatures they run at today.
- Hot water uses, daily volumes if sub-metered, and storage in place.
- Boiler plant details: make, model, rated output, age.
- Electrical supply details: connection capacity, maximum demand history, and your DNO.
- Location constraints for plant: roof loading, ground space, noise-sensitive neighbours, and planning status.
With that, ask each installer to state the design heat load in kW, the method and assumptions behind it, the design flow temperature and the seasonal efficiency to BS EN 14825. Check any installer’s credentials on public registers such as mcscertified.com. Then run the energy side through the running-cost calculator, and see the break-even guide for how.