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.