Equipment selection may seem like a pretty straightforward process: look at the data plate, find the tonnage that’s close to your Manual J calculation’s total load, and you’re good to go, right? The problem with that approach is that a 3-ton unit won’t deliver 36,000 BTUs per hour like it says on the data tag. That’s the nominal tonnage, and it doesn’t account for losses in the ductwork, airflow, temperature (climate), or other HVAC design factors and field conditions.

Since we can’t take the nominal capacity at face value, we have to look at performance data. You can get performance data from two main places: the AHRI Directory of Certified Product Performance and the expanded performance data in manufacturers’ literature. The data differs between them, but both of these have their uses and can be helpful in different scenarios.

AHRI Directory of Certified Product Performance

AHRI compiles lots of basic performance data in its directory, which you can access at ahridirectory.org. You can search for equipment based on type (air conditioning, furnace, and even geothermal and VRF varieties) and then input information like the manufacturer and equipment model numbers to pull up a list of search results. 

Some websites also give you an AHRI reference number for outdoor/indoor equipment combinations. You can input this reference number if you already have it. On carrierenterprise.com (for Carrier and Bryant equipment), scroll to the AHRI Matches tab. It’ll take a minute or so to load, but you can see the compatible equipment with some basic performance data and pull an AHRI reference number that you can plug right into the directory. You can also filter models by tax credit and ENERGY STAR qualifications, SEER2 ranges, and more.

The picture below shows what the AHRI Matches tab looks like for a 23-SEER2 Carrier Infinity heat pump (27VNA336A003). We’ll take a closer look at the circled result:

Using AHRI Performance Data

These are the performance stats AHRI gave for the condenser (27VNA336A003) and fan coil (FE5BNBD60L00) combination circled above:

Like the simple table on carrierenterprise.com earlier, the AHRI directory presents lower EER2 and SEER2 numbers than the numbers we see in product listings and advertisements. The capacity is also NOT 36,000 BTUs, and that reduction isn’t based on field conditions, either (which could make it significantly lower in practice). However, AHRI has more data about heating capacities and airflow rates.

Now that we have a slightly more detailed picture of the performance data, we can compare it to a 3-ton, 20-SEER2 Carrier Infinity variable-speed heat pump condensing unit (27VNA036A003) with the same fan coil:

When we put them side by side, we can see some differences in capacities under a few different conditions.

The AHRI directory is a good place to get a decent comparison of the equipment options in a few key areas. But these categories are rigid and don’t account for a range of temperatures, airflow, OR sensible, latent, and total BTUs. Sure, it has data for cooling capacity at 95°F, but how many people are going to use that as their design temperature? AHRI data is not tailored to specific climates, but there is another place that offers that information for a range of climate conditions.

Manufacturer’s Expanded Performance Data

The original ACCA Manual S advises against using AHRI performance data for precise equipment selection, but it’s fine for rated capacity comparisons. If you want a better idea of the delivered capacity, look at the expanded performance data from the manufacturer. 

The picture below comes straight from Carrier’s 27VNA Infinity® Variable Speed Heat Pump product data; this table is for the 3-ton, 20-SEER2 system we selected above:

Source: 27VNA0 Infinity® Variable Speed Heat Pump Product Data (Carrier)

It’s a lot to look at, but we can see that there’s much more information about the equipment’s cooling capacity than what AHRI’s directory could provide. (Though, granted, if you were to compare makes and models with this data, you’d probably go cross-eyed.)

Differences Between AHRI Data and Expanded Performance Data

Here is what the two look like side by side:

I’ve highlighted the similarities between the two, but as you can see, the AHRI data has nowhere near the whole picture that we need to design equipment for specific indoor and outdoor conditions. On top of that, even the 95°F total capacity isn’t the same between the two. That’s because the OEM data is based on a broader set of field conditions (hopefully), not the lab conditions AHRI uses to certify equipment.

In the expanded performance data, we can see how different dry and wet-bulb temperatures of the air before it moves over the evaporator affect capacity, even for the same CFM and even the same outdoor temperature. Then, that data is applied to several different outdoor temperatures and CFM rates.

AHRI also doesn’t show how many of the BTUs are sensible, just the total. The expanded performance data shows the total and sensible BTUs for varied operating conditions. You can subtract the sensible from the total to determine your latent BTUs. That’s a critical piece if you’re designing for a home in extremely humid weather, and the AHRI data doesn’t give it.

Interpreting the Expanded Performance Data

We’re trying to match the capacities to our Manual J load calculations. Each manufacturer will present this data a bit differently, but you’ll always see capacities based on a mix of the following:

• Entering indoor air temperatures (DB and WB)
• Entering outdoor air temperatures (ambient) 
• Airflow rates (in CFM)

There are sensible AND total BTU capacities based on different combinations of these three factors. Single-stage and two-stage units have a few different CFM options, and variable-speed equipment operates over a wide range represented by maximum, intermediate, and minimum values. Each tonnage also has its own chart, so if the expanded performance data of a 2-ton unit falls short of the Manual J load calculation’s heat load, you can turn the page and see if the 2.5-ton unit is a better match for the same conditions.

We want to make sure the total, sensible, and latent loads are met (or at least make up 90% or 0.9x the load per Manual S), but we don’t want to exceed them by too much. Here are the oversizing limits for air conditioners and air-source heat pumps as established by Manual S:

• Single-stage: 15% (1.15x the load)
• Two-stage: 20% (1.2x the load)
• Variable-speed: 30% (1.3x the load)

If you’re interested in learning more about the equipment selection process with ACCA Manuals, Ed Janowiak explained it in great detail in his 2021 HVACR Training Symposium presentation. We also have a tech tip that breaks down the process specifically for humid climates and has a lot of the same content.

If the Design Temperature Isn’t Available…

The condenser entering air temperatures show up in 10-degree increments, typically ending in 5. We can still design for other temperatures by interpolating the data, which really just means averaging it out until you get the number you’re designing for. If we need to design for 90°F, we simply add the numbers for the 95°F and the 85°F entries and divide them by two to find the average.

Again, the expanded performance data may not give you everything you need to know on a silver platter, but between that and ACCA Manual S, you have the tools to select equipment based on your climate and the customer’s unique home.

It’s All Wide-Narrow-Wide

If you’ve been part of the HVAC School community for a while, you’re probably familiar with the wide-narrow-wide diagnostic process: start with a general idea of the system, zero in on the problem, and go wide again to make sure you didn’t miss anything and have the complete picture. The AHRI directory and manufacturer's expanded performance data are part of the same wide-narrow-wide mindset.

Start wide with the AHRI directory by comparing systems that could work for the application. Go narrow with the manufacturer’s expanded performance data once you have the direction and want to hit the exact BTU targets that were given in Manual J. 

What’s the last wide? Being diligent about checking your work to make sure the system design is appropriate and that the numbers all check out. The ACCA Manuals don’t stop at J and S, either; on the residential side, Manuals D (duct design) and T (transitions and airflow distribution) are critical for occupant comfort. 

HVAC design is not easy, but there are plenty of resources to make it as accurate and beneficial for the customer as possible to avoid problems like short-cycling equipment, muggy homes, and other serious home comfort or health problems.