It’s a pretty safe bet that the vast majority of air conditioners are oversized. ACCA developed its Manual J protocol for heating and cooling load calculations to help HVAC contractors put in correctly sized equipment, but there are a couple of problems. First, most contractors don’t do the load calculations for every new piece of equipment they install. They use rules of thumb instead.
Second, when they do the Manual J, they sometimes don’t do them correctly. As a HERS provider, I’ve looked over a lot of Manual J reports because the ENERGY STAR homes program actually requires them. ENERGY STAR doesn’t require a whole lot. Mainly they want raters to check that the following info is correct:
- Location – Don’t use Phoenix for a home in Charlotte.
- Outdoor design temperature – Should be 99% design temperature from ASHRAE Handbook of Fundamentals
- Indoor design temperature – Should be 75 degrees F for cooling, 70 for heating
- Infiltration – Set to ‘tight’ or equivalent
That’s not too much to ask, is it? Apparently it is for some contractors. They usually get the location correct, but I’ve seen all three of the other parameters entered incorrectly, usually in a way that makes the heating and cooling loads higher, which requires larger equipment.
Beyond what ENERGY STAR requires, there are a lot of other ways that Manual J calculations are done incorrectly. Here are the most common ones:
- Orientation – Some contractors use ‘worst case’ orientation for every Manual J.
- Windows. – U-values that are too high is another goody. Double-pane, low-e windows have U-values of about 0.3, yet I often see 0.5 to 0.9 for houses that have those high performance windows.
- People – Each person in the house gives off heat. Manual J specifies 230 Btu/hour and that the number of people should be equal to the number of bedrooms plus 1. Recently I got a Manual J that had 23 people in a 5 bedroom house. That’s 17 people too many, resulting in an extra third of a ton of AC capacity.
- Ducts – If the ducts are inside the building envelope, the Manual J should reflect that location and have no load associated with the ducts.
Of course, Manual J load calculations can be screwed up in all kinds of ways, and to determine absolutely if the one you’re looking at is correct, you’d pretty much have to redo the whole thing. Surface areas, for example, have a huge effect, as the more of it a house has, the more load it has.
The most common way that HVAC contractors size cooling systems is by using rules of thumb. In my location, they commonly use one ton of air conditioning capacity for each 600 square feet of conditioned floor area. When I see incorrect Manual J’s, it’s amazing how often the loads calculated come out to about 600 square feet per ton.[If you need an accurate Manual J load calculation, we can help. See our HVAC load calculation page for more info.]