Midnight wrote:I'm trying to rationalise the energy input needed for a radiant floor system. The specs for the system rate it as heating at a rate of 100 watts per square metre.

Trying to convert that into BTU's to determine the furnace is turning my head to Swiss cheese.

Total area of the floor is 320 square metres.

I think You're going about ths backwards...

The first think you'll need to calculate is the volume of the room, and use an energy guide rating to determine how many btu it will take to heat that volume. Try this one:

http://hearth.com/calc/btucalc.html
You can then convert btu's to Joules (1 joule = 1 watt/sec to get your requirements.