Smarter Heating & Hot Water
Calculators / Buffer Volume Calculation

Buffer Minimum Volume Calculation

Calculate the required buffer vessel volume for heat pump systems to ensure proper cycling and efficiency.

Power Out / In

Minimum heat pump output (kW) - Pmin 0.0
Minimum heat demand (kW) - Dmin 0.00
Excess Power (kW)

Starts per Hour and On Time

Starts Per Hour 1
On Time Based on Starts Per Hour (Mins)

RunTime / DT

Minimum heat pump on-time (seconds) - Tmin
SHC of Water
DT / Maximum allowable temperature variation (°C) - Hmax 5

Volume

Required total volume (liters)
Existing system volume (liters) - Vs 0
Total Buffer Volume Required - Vb
Total Buffer Volume Required
0.00
Litres
Vb = (Pmin - Dmin) × Tmin / (Hmax × 4.2) - Vs
Where:
  • Vb = Required buffer volume (liters)
  • Pmin = Minimum heat pump output (kW)
  • Dmin = Minimum heat demand (kW)
  • Tmin = Minimum heat pump on-time (seconds)
  • Hmax = Maximum allowable temperature variation (°C)
  • 4.2 = Specific heat capacity of water (kJ/kg·K)
  • Vs = Existing system volume (liters)

Hmax Maximum permitted temperature drop (ΔT): After achieving setpoint, the heat source will switch off. The ΔT is the maximum temperature drop permitted before the heat source will need to switch on again. This is also the amount of temperature increase that will be achieved within the minimum run time at the minimum output.

Run time - 3 Starts per hour: 1kW Load with 4kW Power input = on 5 mins, off 15mins

RunTime = (3600/starts per hour) / (power input / power out)

Sources: Some information has come from Heat Geek.

Important: This calculator provides theoretical buffer volume requirements. Always consult with a qualified heating engineer and refer to heat pump manufacturer specifications for your specific installation.