Church of England Diocese of Norwich Hethel

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.

Installation cost was about £12500 and we managed to get a grant for one-third of this. This compares well with other types of system.

All the indications to date are good. For example, on a recent cold (+1C) Sunday, the church warmed up in 30-60 minutes and became ever more comfortable after that. Monitored energy consumption is typically 14kW. We buy our completely sustainable electricity from Good Energy at a current (December 2021) price of 23p/Unit, so the running costs are around £3.25/hour - excellent value for totally 'green', zero carbon heating in a country church. Routine maintenance costs us £200 / year + VAT. Possibly the interval could be stretched given the only-occasional use, but we want to protect our investment.

Downsides? A cold floor tends to dominate at feet level, but that would probably be the case with other forms of heating. And there is some noise from the blowers - we tend to throttle back to low or medium fan speed during services (2 or 3 on a 4-point scale) - which we judge acceptable. Please visit us to experience if you're interested.

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.

Report_-_Heating_a_small_church_Sry9iG9, PDF