Gas warm-air systems burn natural gas in a heat exchanger and blow the warmed air through ducts into the building. They were a popular choice for churches from the mid-twentieth century onwards, and many are still running today. The main selling point is speed: a well-sized unit can raise the temperature of even a large nave within an hour or two.
The trouble in a church is physics. Hot air rises, and in a building with a high ceiling most of that expensive warmth ends up well above head height. Destratification fans can help push it back down, but they add cost and noise. The ductwork itself can be a headache in a listed building. Visible runs along walls or through the roof space may need a faculty application, and concealing them is rarely straightforward in a medieval structure.
If your church already has gas warm-air in place and the ductwork is sound, a boiler swap may be the most pragmatic short-term option. But if you are starting from scratch, it is worth asking whether the running costs and carbon footprint justify the investment when electric alternatives now heat people directly without warming the whole air volume. The Church of England's net-zero 2030 target means new gas installations face increasing scrutiny from DACs and diocesan advisory committees.