The church is heated by two reversible air-conditioning units.
I have heard that these work very well when the external temperature is moderately cold, but that in extreme cold they can't cope because they work by "pumping heat" from the outside of the building inside. If it's too cold outside they can't extract any heat.
Well this may be true, because over the weekend it was extremely cold in Bordeaux and on Saturday evening the heating seemed to have little effect. Sunday morning was the same, so I pumped it up to its maximum setting and left it on all day till the evening service - in theory bringing the room up to 31°C.
Well they didn't. Nowhere near ! I think we'll need something extra for cold snaps at least !