Size against a load duration curve
Plotting demand from highest to lowest shows how often each capacity is needed. A unit smaller than the peak may cover most annual heat while running for longer periods. An oversized machine can cycle and operate inefficiently.
The source can be the limit
Available source flow and temperature may cap output before the sink does. The heat pump cannot continuously deliver more recovered heat than the source provides, once electrical input and exchanger approaches are accounted for.
Include production change
Future lines, cleaning schedules, efficiency measures and product mix can all change the duty. Sensitivity analysis is preferable to adding an unexplained safety margin.
Size from coincident duty
The relevant load is the heat demand that coincides with an available source, not the boiler nameplate or the year's largest fifteen-minute peak. Plot source availability and sink demand on the same timeline. The lower of the two, adjusted for the heat-pump energy balance, limits useful output in each interval.
A base-load heat pump can achieve more annual running hours than a machine sized for the absolute peak. Existing boilers can retain startup, peak and outage duties. The optimum split depends on capital cost, tariff structure, turndown and the value of displaced fuel.
Convert the source into deliverable heat
At COP 4.0, three quarters of the delivered heat comes from the source and one quarter from electricity. A stable 750 kW source could therefore support about 1,000 kW of useful heat at that operating point. At COP 3.0, the same source could support about 1,125 kW because source heat represents two thirds of output.
That arithmetic is only a screen. COP and capacity change with the duty, while exchanger limits may prevent the full source flow from being used. Manufacturer selection should confirm every proposed operating point.
Check turndown and staging
A large single machine may cycle during low production. Several modules or compressors can improve turndown and resilience, though they add controls and maintenance items. Buffer storage can reduce short cycling but cannot solve a sustained mismatch between source and sink.
Sizing should include minimum flow, minimum on time, startup current, seasonal source temperatures and planned expansion. The selected arrangement should still operate acceptably when one compressor or one process line is unavailable.
Sources
General technical explanation. No specific external source claims are made on this page.