The boiler, with its higher output temperatures and greater delivered energy, heats the water much faster. Consequently, hot water is available sooner from a boiler system when starting from cold. On the other hand, a heat pump takes longer to deliver the same amount of heat. However, the energy cost of a heat pump is significantly lower. In cases where night-rate electricity is available, the cost of heat pump operation can be halved. Unfortunately, there is no night rate available for gas.
Since a heat pump operates at a lower flow temperature, it requires a larger heat exchanger area within the cylinder to maintain good performance. In contrast, a boiler with its higher flow temperatures requires a smaller coil area to achieve similar results. However, this smaller coil is less efficient at transferring heat, resulting in a larger temperature difference and potential heat loss within the system during the return to the boiler.
It’s worth noting that the heat pump considered here is operating in its least efficient regime, as it’s being asked to operate at its maximum rated heat output. Beyond this point, reaching higher temperatures would require an electric immersion heater or an alternative heat source.
In reality, how often do we require hot water at 50°C? For most purposes, such as washing, water is used at more comfortable temperatures below 30°C. Thus, this store of hot water is usually blended with cold water to achieve the desired temperature. A heat pump operates far more efficiently if allowed to work a ta cooler temperature, such as 35°C.