I found out last month that the Air Conditioning Contractors of America (ACCA) is working on a new design manual. You certainly know of some of their other manuals: Manual J, Manual S, Manual D, Manual T… maybe even Manual P. They have quite a few others as well (H, Zr…) but now they’re working on one that will address a part of the market we encounter more and more often. And that would be low-load homes.
ACCA put out a call for volunteers to work on the manual in the spring of this year. (Applications were due in April, so it’s too late to apply now.) The objectives of the new manual, at least as specified in the call for volunteers, are:
1,500 square feet per ton of cooling
At the ASHRAE conference in Houston, I spoke with someone who’s on the task force and was told they’re defining a low-load home as a house that has a house-size-to-load ratio of 1,500 square feet per ton or greater. That sounds about right to me. At the low end, that means a 2,000 square foot house would need only about 16,000 BTU per hour of heating or cooling capacity. That’s an air conditioner or heat pump smaller than 1.5 ton!
And that’s just the starting point for low-load homes. We’ve done load calculations for homes that are in the 2,500 to 3,000 square feet per ton range. (See my 2016 article with data from our results.) Here we’re talking about a 2,000 square foot home that needs less than a ton of cooling. As the objectives above point out, heating or cooling 2,000 square feet with 400 cubic feet per minute (cfm) of air flow is a challenge.