Heat pump planning

Buffer & Water Volume Planner

Estimate the fluid volume needed to absorb minimum-output surplus for one run, then compare it with the volume already participating.

A volume allowance does not establish heat-pump capacity, safe defrost, hydraulic layout or pump selection. Use manufacturer limits and a system design before choosing equipment.

Circuit flow and manufacturer requirement
Used only to calculate fluid flow. Enter the relevant circuit design duty, not a nominal model number.
Actual circuit design difference, for example 40°C flow and 35°C return = 5K. This is separate from the tank temperature swing.
4.18 is an approximate water value. Glycol concentration and temperature change this: enter supplier fluid data.
Enter the documented minimum for your exact heat pump and hydraulic arrangement, including defrost requirements. Zero means not entered; it is not a safety clearance.
One minimum-output run
Use manufacturer output data at the relevant outdoor and water temperatures. For an on/off unit, use its actual operating output.
Smallest expected active load while the heat pump runs. Closed zones must not be counted. Zero is allowed.
Illustrative 10-minute target. Use manufacturer/control design requirements; this is not a universal setting.
Illustrative 3K temperature band between start and stop. Only a volume that actually experiences this change can be credited.
Include only accessible fluid that participates in the assessed cycle. Exclude isolated zones, DHW potable water and vessel volume that does not mix through this temperature band.

Assumptions and calculation method

Surplus heat = max(0, minimum heat-pump output − concurrent active load). Required cycling volume = surplus kW × on-time minutes × 60 ÷ (fluid heat capacity kJ/litre·K × usable temperature rise K).

Required participating volume is the larger of the cycling allowance and the entered manufacturer minimum. Additional volume = max(0, required volume − existing participating volume). These are concurrent minimum-volume tests; they are not added together.

Fluid flow in litres/minute = circuit kW × 60 ÷ (fluid heat capacity × flow-to-return difference). A heat pump's flow temperature alone does not establish this difference. Pipe resistance, pump curves and manufacturer minimum/maximum flow still require checking.

The model assumes constant source output, constant load and well-mixed fluid over one on-cycle. Stratification, control sensors, bypasses, zone valves and pipe arrangements affect usable volume. Existing-water credit is valid only if that water remains in circuit and participates through the entered temperature band. With no surplus, this model has no minimum-runtime cycling-volume requirement; other requirements still apply.

Defrost energy, minimum allowable return temperature, freeze protection and manufacturer hydraulic requirements are not derived here. If the manufacturer minimum is left at zero, no manufacturer or defrost requirement has been checked. The tool does not recommend two-, three- or four-pipe layouts or issue a plant-sizing pass.

Method reference: Caleffi: design details for air-to-water heat pumps, using the same surplus-energy balance converted to SI units. Minimum output, on-time and temperature band are entered assumptions, not validated data for a selected product.

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