Out of interest, why would a heat loss spreadsheet care about the heat source?    I can see that the heating source would be of use in a running cost comparison, as costs vary a great deal, per kWh of heat delivered, between different fuel types, but can't see how the heat source can have any bearing on heat loss/heating requirement (as heat loss = heating requirement for any given set of conditions).   I recently did a running cost comparison, for a typical year for our unde