In my last three blogs, I discussed the basics of heat-loss and cooling load calculations. The unfortunate truth about these calculations is that fast methods aren’t particularly accurate, and accurate methods require making measurements, checking specifications, and entering data into a computer program — in other words, a significant investment of time.
So how should builders go about making these calculations? There are several ways:
There are at least two reasons why we need to perform load calculations: to size heating and cooling equipment (ideally, using ACCA Manual S), and to design heating and cooling distribution systems (using ACCA Manual D). Neither Manual S nor Manual D can be used unless Manual J calculations are performed first.
These are valid reasons, so a room-by-room Manual J load calculation makes a lot of sense. If you perform such a calculation, you may save money on your heating and cooling equipment (because it is less likely to be oversized), and there will be a lower chance that the homeowners will have comfort complaints arising from a poorly designed heat-distribution system.
In most areas of the country, a room-by-room Manual J load calculation is required by code. If you don’t have the software yourself, you’ll have to hire an energy rater or engineer to perform the calculations. Very few HVAC contractors are capable of performing an accurate load assessment, so I’d be wary of leaving this task to your furnace guy.
As long as there is no code requirement in your jurisdiction for a Manual J calculation for the type of work you are contemplating, you may not need a Manual J calculation. To understand why, we need to examine two myths that have long been promulgated by energy experts. The first myth is that rules of thumb are inappropriate;…