Hi Nick

I'd have said it was a combination depending on the design stage and complexity of the building.

More and more people obviously calculate them using a thermal modelling tool which they are using for design analysis and this makes sense. But I would always judge the results with a level of sceptical critiscism and hence would check them against rule of thumb or other established guidelines and maybe even carry out the odd hand or hevacomp calc.

Of course early on in the design stages experienced engineers will probably use just a rule of thumb, but as the role of building simulation changes this is also changing and whilst there are some very good experienced M&E engineers that will be able to say "well this should be X w/m2 so we'll go with that", who's to say that the simulation engineer can't improve on that. In fact I've come across jobs just like that where lighting loads were submitted for pre-planning on that basis, then when it came to Part L modelling, of course they fell drastically short of the mark, being much greater than the notional values allowed.

I tend to use two packages to cross check loads when I'm concerned and often now use design builder as it uses the ASHRAE heat balance methodology which I reckon is a bit more accurate than the admittance method. A good check if even if it's not totally watertight from a PI point of view.

Regards
Paul

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