5. Air-source heat pumps at Hethel Church
In February 2018, a set of four Mitsubishi air-source heat pumps (ASHPs) was installed in Hethel Church. The maximum output is 44kW, a figure decided by gathering feedback and information from a small number of churches nationwide that had experience of ASHPs. It seemed that the best rule of thumb for the heat input needed for non-steady-state situations used the volume of the building and a multiplier of between 40 and 50 Watts per cubic metre. Our heaters are mounted 4m above floor level, blowing gently downwards and thus counteracting convection currents that take warm air upwards.
Although we have not been through a full winter, all the indications are good. In mild conditions, the heaters have been on their lowest fan speed, consuming less than 6kW electricity, pumping out somewhere in the region of 24kW. In such very cold weather as we have had (eg the Beast from the East), the system performs well and will keep the air inside the building at 15-18 degrees C, using around 14kW of electricity.
Because ASHPs have a 'coefficient of performance' of 3.5 or more when temperatures are above freezing outside, the energy demand will not be more than 15kW or so. This equates to around 60A, making the church's 100A single-phase supply more than adequate to supply the system. This saves the very considerable cost of installing a three-phase supply if more than this were needed (eg for a 40kW radiant heater system).
The document that was submitted to the Diocesan Advisory Committee requesting a faculty to proceed is available via the link below. This gives a full rationale for choosing this type of system.