We do a lot of heating and air conditioning system design at Energy Vanguard. Alexander Bell, who goes by Andy, is our design wizard, and I’ve been getting involved with the process again lately.
When I talk to potential clients, a lot of them tell me their contractor wants to size their air conditioner using a rule of thumb. The rule is usually something like this: Install one ton of air conditioning capacity for every 500 (or 400 or 600) square feet of conditioned floor area. How far off are they? Let’s take a look.
A bit of our Manual J data
True HVAC designs always start with a load calculation. So we can look at the data. The graph below shows data for just a few buildings we’ve done in the past few years. Forty of them, to be exact. We’re in the process of putting all our data into a spreadsheet so we’ll have more to show later.
On the horizontal axis, I plotted the conditioned floor area, in square feet. On the vertical axis, I plotted the cooling load divided by the floor area, or square feet per ton. Remember, when HVAC contractors use rules of thumb to size air conditioners, they usually pick a number between 400 and 600 square feet per ton.
Here’s what our data show.
Note that not a single one of these load calculations was as low as the high end of the typical range used in rules of thumb. The low number on that graph is 624 square feet per ton. The majority of the cooling loads shown here are above 1,000 sf/ton. Only eight are below 1,000 sf/ton.
In case you’re wondering, I threw out the data for cooling loads in cold climates when I plotted this graph.…