Can someone check my calculation...
Time to replace 15m^3 of air extracted at 20oC, with air heated by a
650 Watt heater from 0oC to 20oC.
Air-Volume = 15 m^3 Temp-Change = 20oC less 0oC = 20oC Heater-Watts = 650W where 1W = 1J/Sec Air SPH = 716 J/Kg K Air Density = 1.3kg / m^3
Time (secs) = Temp-Change * Air-Density * Air SPH / Heater-Watts * Air- Volume Time (secs) = 20 * 1.3 * 716 / 650 * 15 = 430 secs or just over 7 mins
Repeated 3x a day is 0.37hr @ 0.65kW @ 11p/kWhr = 2.65p or bugger all. That is running a fan 3x a day for 10mins assuming it removes 100% of free flow (which is not likely, so the running time could be as much as 20min to achieve 15m^3 of air). Whatever it costs very little indeed yet enforces a near 40% air change 3x a day.
Is my calculation correct?