Yeah, I’m talking about a heating and cooling load calculation. Or in HVAC-speak, an ACCA Manual J. It’s a way of calculating how much heating and cooling a house needs. The calculations use the details about the house to determine how much heat leaks out of the house in winter and how much heat the house gains in summer. And yeah, I’m really saying you don’t need them—in certain situations.

“Ah, I knew there was a catch,” you may be thinking. And you’re right. But here’s the thing. You definitely want a properly sized heating and cooling system. And in most cases, you can get that without doing an actual load calculation.

When can you skip the load calc?

Before I get into how to determine your heating and cooling loads without a load calc, let’s discuss the primary factors that matter. The first line dividing the houses that need an actual load calculation and those that can do it another way is age. If the new HVAC system is going into a new construction house, you need a real load calculation. If you’re replacing a system in an existing home, you may be able to do it another way.

Then we hit the second hurdle to get over before making the call to skip the load calc. The current system still has to be doing about as well as it ever has at heating and cooling the house. If it’s in decline and struggling to keep up, like my air conditioner was in 2009, a load calculation is still your best bet.

Finally, there’s one more reason you should stick with a real load calc. If you’re doing other work that might change the heating and cooling loads, you probably need a load calc. But if you do the other work first and then go through at least one heating and one cooling season, you should be able to skip the load calc. That other work could be adding onto the house, modifying the duct system, or insulating and air-sealing the house.

The method

So how do you determine the size of your new furnace, air conditioner, or heat pump without a load calculation? If you’ve gone through the list above and the house meets all the criteria, you simply measure time and temperature. Here’s how:

1. Find the 1% (cooling) and 99% (heating) design temperatures for your location. An easy place to find them for the U.S. is in this document from the ENERGY STAR new homes program (pdf). Here’s a screenshot showing the design temperatures for Fulton County, Ga., where Atlanta is.

   

2. To check your air conditioner in Atlanta, for example, find a day when the temperature is 92°F (33°C).  Then while the temperature is at 92°F, measure how many minutes per hour your AC runs. But keep measuring even after the temperature starts dropping. There’s usually some thermal lag in buildings, so your peak cooling load probably won’t happen until after the outdoors has started to cool down. For heating, do the same thing with your 99% design temperature.
3. Convert the minutes per hour to a decimal. If the AC ran 24 minutes per hour, that would be 0.4 hour per hour.
4. Find the capacity of your heating or cooling system. Nominal capacity will work for this. Let’s say it’s 3 tons, or 36,000 Btu per hour.
5. Multiply the decimal runtime by the air conditioner capacity. In this case, you’d get 14,400 Btu per hour, or 1.2 tons. (The conversion comes from 12,000 Btu/hour = 1 ton of capacity.) In this case, you definitely wouldn’t have to replace a 3-ton AC with another 3-ton AC. A 1.5-ton system should work fine if you’ve measured runtime correctly.

A heating example

We have a client in Savannah, Ga., who wants to electrify her 80-year-old home. She has a furnace and air conditioner and used the method above for her heating system. The heating design temperature for Savannah is 29°F (–2°C). This winter she had some periods with a temperature that low. She also had some weather where it got down to 24°F (–4°C), and the runtime wasn’t much different.

The runtime she measured for design conditions was 25 minutes per hour, or 0.42 hour per hour. The furnace has a nominal rated output capacity of 89,000 Btu/hour. Using this method, then, she was actually heating her house with about 37,000 Btu/hour.

Is that an accurate number? It’s probably close enough. Here’s why. The actual capacity of the furnace is almost certainly not higher than the nominal capacity. It could be lower, though. But lower furnace capacity only means that the 37,000 Btu/hour number is too high. If the furnace only produces 80,000 Btu/hour, for example, she’s heating her home with only 34,000 Btu/hour.

Four ways to measure runtime

1. Use a stopwatch. This one’s time consuming and maybe uncomfortable if you have to sit outside and listen to the outdoor unit in the heat. But it’s straightforward, and you don’t have to buy anything new.

2. Get the data from a thermostat. Some thermostats measure runtime and give you access to the data. If you have an Ecobee thermostat,* look there first.

3. Get the data from an electricity monitor. There are a bunch of these on the market now. I have the Emporia Vue and love it.

4. Install a motor runtime data logger. This is how I first did it with my air conditioner in the condo I used to to live in, and it’s how our Savannah client did it. You just attach the logger close enough to the blower motor in your air handler, and it can give you minute-by-minute data showing whether the motor is on or off. The photo above shows one attached to the side of a furnace blower. (See the article about my runtime data logger measurements to learn more about his method.)

Can you skip the load calc?

Actual data is usually a better way to go when you can get it. We know that ACCA Manual J results in inflated heating and cooling loads. That’s not a terrible thing when it’s done correctly. Modeled results are better than the old rules of thumb that consider only the conditioned floor area.

But with existing homes that meet the criteria I listed above, you may be able to come up with a more appropriately sized heating and cooling system to replace the old one. Sometimes, it’s MUCH more appropriately sized. It’s not uncommon for homes to have double the heating and cooling capacity they really need.

At the beginning of the article, I said that most homes could benefit from not doing a load calculation. That’s because many more air conditioners, furnaces, and heat pumps are sold to replace systems in existing homes. New construction is a small fraction of the total.

Currently the majority of the majority that could benefit from this method don’t get a load calculation or use this method. The HVAC contractor usually installs the same-size system that’s already there. Sometimes they even put in a bigger system.

If you have an existing home with a working heating and cooling system, get your runtime data before it’s too late.

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Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Ga. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.