When you enter the world of building science — whether through building a house, becoming a home energy rater/building analyst, or just hanging out in cyberplaces like this — everyone talks about the importance of getting actual heating and cooling load calculations based on ACCA Manual J. A great number of HVAC contractors sell and install oversized equipment with air distribution systems that don’t work because these contractors base their choices on rules of thumb.
OK, but what if you hire a contractor or third-party designer to do Manual J load calculations and you’re not an expert and don’t want to be? Suddenly you’re faced with a bunch of seemingly indecipherable reports. How do you know if they’re accurate or not?
Fear not, dear reader. I’ve got some help for you today.
Don’t confuse load with capacity
I don’t think I can make this distinction often enough. Heating and cooling loads are not the same as the equipment capacity needed. I just did it in my last article, and now I’m doing it again. It’s that important. The first thing you need to know is that the term loads refers to how much heating and cooling the building needs and capacity refers to how much heating and cooling the equipment can supply. Here in the U.S., both are measured in British Thermal Units (BTU) per hour.
When you look at Manual J reports, you’ll see the loads. They’re shown separately for heating and cooling, and cooling is further divided into sensible and latent. When the contractor or designer picks a piece of equipment, they’ll have to go through a “derating” process to match the equipment’s performance specifications with the building’s loads. (For more on this, see “Manual J Doesn’t Tell You Equipment Capacity.”)
I mention this topic here because some Manual J reports can confuse you on this distinction, especially for cooling. Depending on which reports you’re looking at, you may also see something like “Req. total capacity at 0.70 SHR.” That’s just a guess at what equipment capacity you’ll need. If the person who ran the calculations has already gone through the derating procedure and specified the equipment, it may be accurate. Or the designer may have left the default number in there for SHR (sensible heat ratio), in which case you should look at that number simply as a suggestion.
In the end, just remember that the load calculation comes first, and your equipment capacity is going to be a bit bigger than the loads.
Notes on terminology
If you’re going to read Manual J reports, knowing a little about the terms used will help you understand them. Here are a few that you need to know:
- One ton of AC capacity is equal to 12,000 BTU/hr.
- BTUh is the same as BTU/hr.
- Sensible cooling results in lower temperature (technically, dry bulb temperature); latent cooling results in lower humidity through condensation of water vapor on the coil.
- SHR is the sensible heat ratio. It’s obtained by dividing the sensible cooling load by the total cooling load. For homes in eastern North America, the humid side, that number often comes in at 0.8 to 0.9, sometimes even a bit higher. In dry climates, it can be 1.0 when ventilating with outdoor air.
A rule of thumb you can use
I often rail against rules of thumb when it comes to HVAC design (or lack thereof), but that doesn’t mean you can’t use one to your advantage. This is the sniff test you can do to see how close the designer might have come to an accurate load calculation. In the warmer climates where air conditioning is a big deal, the rule of thumb used by many contractors for sizing an air conditioner is usually this:
AC capacity = CFA ÷ (500 sf/ton)
CFA is conditioned floor area in square feet.
Sometimes the rule is 400 sf/ton, sometimes 600 sf/ton. But it’s always right in that neighborhood. So if you get a load calculation report, find the total cooling load (sensible plus latent) and divide it by the conditioned floor area. If it comes out around 500 or 600 sf/ton, the designer fudged the calculations somehow to align them with their preconceived idea of how big the loads should be based on their rule of thumb. (I’m talking about new homes here, or complete gut-rehabs. Existing homes generally have higher loads.)
Don’t believe me? Take a look at our data. The graph shown in Image #2, below, is from an article I wrote in 2016 about the results of our load calculations on 40 projects. (Go read the article for full details.) The takeaways here are that our worst result was 624 sf/ton. The average 1,431 sf/ton.
If you’re building a well-insulated house with a good level of airtightness, double-pane low-e windows, and decent specifications overall — in other words, a house that meets most state energy codes these days — your result should be 1,000 sf/ton or higher. If it comes in lower that, you should see that as a red flag and delve into the details to see if the designer made mistakes.
Delving into the details
Finding the loads. First, identify the results for heating and cooling loads. The two main software tools for doing load calculations are Wrightsoft’s RightSuite Universal and Elite’s RHVAC. The reports look a little different but it’s not too hard to find the results. Both types of software make it clear how many BTU/hr you need for heating and for cooling. And for cooling, they also break it down into sensible, latent, and total. From the total cooling load, you can calculate the sf/ton I mentioned above. RightSuite doesn’t do it for you, but Elite’s software does. In the Project Report, they include a section called Check Figures that includes the sf/ton.
Checking the details. If you suspect that the loads may be too high — or too low or about right — you can check the details to see if the designer got the inputs right. Here are some of the main things to check:
- Indoor design temperatures. The standard indoor temperatures are 70° F for heating and 75° F for cooling (with 50% relative humidity).
- Outdoor design temperatures. The outdoor design temperatures depend on where you are, and you should check to see what was entered versus what should have been entered. It’s pretty easy to find the entries on the reports. To find what should have been entered, you can go to this page on the International Code Council’s website. If the entries in your calculation are off by a couple of degrees, it’s not a big deal. If they’re off by 5 degrees, you should ask for it to be corrected.
- Areas. When the designer enters the various floors, walls, ceilings, windows, and doors, having the wrong areas can make a big difference. This is especially true for parts of the building enclosure that have worse specifications, like windows. A code-built house in IECC climate zone 3, for example, has windows that are about R-3 whereas the walls will be R-13. Entering too much window area is a way to inflate the load. Entering too much of any of the areas likewise inflates the load.
- R-values and U-factors. Check the entries for the floors, walls, ceilings, and floors to ensure the designer put in the correct R-values (for insulation) and U-factors (for assemblies like windows).
- Number of occupants. A common way to inflate the cooling load is to add extra occupants. The rule here is that the number of occupants should equal the number of bedrooms plus one. If they put 23 people in a 5-bedroom house (yes, I really saw that!), they’re adding unnecessary load. At 230 BTU/hr sensible and 200 BTU/hr latent, those 17 extra occupants added more than a half ton of cooling load.
- Infiltration. Did they use a simplified input method? If you’re building a new house and meeting a code that requires 7 air changes per hour at 50 Pascals (ach50) or better, the entry should be tight or maybe semi-tight. Better would be to use an actual blower door number. For example, if your code requires 3 ach50, enter that into the calculation. If you’re going for Passive House certification, enter 0.6 ach50 or 0.05 cfm50/sf of enclosure.
- Orientation. The software gives the designer the option of using worst case for the orientation. Your load calculation should have the correct orientation or you’ll end up with extra load in your reports.
- Duct location. If the ducts are in conditioned space or in an encapsulated attic or crawl space, make sure that gets factored in properly. Doing the load calculations for ducts in an unconditioned attic will result in excess load.
Contractors doing these load calculations often feel compelled to stretch a little bit here and a little bit there. Each litte bit doesn’t affect the overall load that much, but by the time you add them all up, you’re looking at putting in a 4-ton air conditioner where 2.5 tons would work. But here’s the thing: Even when you’re as stingy as possible with things that add load, you still end up oversized. So there’s no need to add extra load. If you’re building or remodeling a high-performance house, make sure the load calculation is correct. It’s worth it.
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