How Do You Calculate the Size of a Furnace, Heat Pump or A/C?

If you’re buying a new heating or cooling system — or both, if it’s a heat pump — getting the size of the system correct is one of the most important design considerations.

There are drawbacks if your system isn’t sized correctly. If it’s oversized, you’ll be paying too much for your equipment, it will operate less efficiently, and it can actually make your home less comfortable than a smaller system. If it’s undersized, you might end up shivering or sweating during extreme temperature days when the system is unable to keep up.

Unfortunately, many HVAC companies take a shortcut to choosing the size of the heating or cooling system that a home needs by using a rule-of-thumb based on the square footage of the home. For example, you might see advice that a home needs 1 ton (12,000 BTUs) of heating or cooling per 500 square feet. If you Google “HVAC load calculator”, you’ll find many online calculators telling you the same thing.

Rule-of-thumb methods might sometimes get you the right number, but it’s typically only by luck. To properly size an HVAC system for your home, you need a qualified contractor to perform a load calculation. There are actually at least two calculations needed, and sometimes more depending on your setup. I’ll describe those in detail later, but first we’ll learn some basics about why these calculations are needed.

Why companies sometimes get it wrong when designing HVAC systems

As mentioned, some companies may rely on educated guesses to choose the correct size of the heating or cooling system they install in your home. This might be based on similar sized homes in your area, or a rule-of-thumb.

A common rule-of-thumb you’ll find mentioned on the internet is that 1 ton of HVAC is required for every 400 to 600 square feet of living space. While square footage is an important factor of the heating and cooling requirements of a home, there are many more variables involved.

Another reason why HVAC companies might install the wrong size system is that the customers sometimes demand it. Typically, homeowners believe that “bigger is better”. Bigger HVAC systems cost more, which may further bias homeowners who may think that paying a premium means that they’re getting a better system. (In fact, undersized systems will tend to perform better than oversized systems.)

Some less scrupulous HVAC companies might also deliberately sell customers larger and more expensive systems than they really need.

Finally, when homeowners replace an existing system, they may sometimes simply choose a new system with the same capacity as the one it is replacing, even if it’s not the optimal size. The homeowner might tell the HVAC companies that the old system worked fine, not realizing that it was operating less efficiently than a properly sized system would.

A quick side note about what we mean by “efficiency”

I’ve mentioned efficiency a few times in the article, so I should clarify what I mean. HVAC equipment has efficiency ratings that describe how well it utilizes energy for heating or cooling.

For example, a gas furnace with a 96% AFUE rating is able to use 96% of the heat energy contained in the natural gas that it burns for heating your home, and waste only 4% of the energy in the process. Air conditioners and heat pumps have SEER2 ratings that tell consumers how much cooling power the unit will provide for each kilowatt-hour of electricity it consumes.

However, this article is about making sure that your HVAC equipment is correctly matched to the heating and cooling needs of your home. This is about the efficiency of the system design, rather than any individual piece of equipment.

For example, you could have a 99% AFUE gas furnace installed — the highest efficiency available — but if the system is twice as large as your home actually needs, the overall system performance will be poor, even though the furnace itself is burning fuel very efficiently.

What happens when a heating system is too large for your home?

What happens if a heating system has a higher heating capacity than your home typically needs? When your thermostat notices that your home is colder than the set point, it will ask the heating system to turn on. Because the system is more powerful than your home needs, it will deliver a lot of warm air very quickly.

This might seem like a good thing because your house will heat up quickly, but the problem starts when the warm air reaches your thermostat. Once your thermostat detects that the temperature has reached the set point, it will ask the system to turn off.

It’s important to understand the functions that a forced air heating system performs. Not only does it deliver heat, but it also circulates that heat throughout a home. There may also be additional equipment that’s integrated, such as a humidifier and advanced air filtration.

With a short operating cycle, the heating system might not have enough time to deliver conditioned air to every room of your home. This can result in cold areas of your home, especially if it has poor insulation or air sealing.

Accessories such as humidifiers and air filters might also not have enough operating time to do their job properly.

While advanced thermostats such as the Nest and Ecobee that have remote sensors can help ensure that all areas of your home reach the desired temperature, this is a bandage on the problem rather than a true fix.

What happens when an air conditioning system is too large for your home?

Air conditioning systems that are too large can also lead to comfort problems. Similar to an oversized furnace, an A/C that is more powerful than a home requires will turn on, deliver a lot of cold air quickly, and then shut off after a short cycle.

This can lead to temperature differences throughout the home because the system hasn’t had enough time to distribute conditioned air throughout the home, potentially resulting in large temperature differences from room-to-room, especially in poorly insulated homes.

High humidity is also an important risk to be aware of with oversized A/C units. Air conditioning units not only lower the temperature in your home, but also remove humidity. In some ways, removing humidity is the more important function because cool air with high humidity can cause mold to grow, causing health issues or even damage your home.

Air conditioners require time to dehumidify the air. An oversized system that cycles on and off for short periods may not be able to keep the humidity level in your home at a comfortable and safe level.

What happens when your HVAC system is sized too small for your home?

While it’s probably less common, an HVAC company might also install a heating or cooling system that is underpowered for your home. Unless you have a situation where the system is severely undersized and the health of its occupants is a concern — for example, if the homeowners are elderly and the air conditioning isn’t sufficient to keep the home cool enough to avoid heat-related illnesses — this is actually less of a problem than an oversized system.

On a really hot or cold day, an undersized system might run for long periods of time or even operate continuously but still not be able to keep your home at your desired temperature. While this is a problem for comfort, it does not have the issues associated with an oversized system, such as inadequate air circulation or dehumidification.

While you might be concerned that long or even continuous run times could be damaging to your equipment, this actually isn’t a concern. Short cycles where the equipment is frequently switching off and on is harder on HVAC equipment than long run times. In fact, multi-stage heat pumps are designed to operate continuously at the exact power output to needed keep your home at the desired temperature.

What is an HVAC load calculation?

The only way to be really sure that you are getting an HVAC system is that correctly sized for your home is to know how much heat it loses in the winter and gains in the summer. This is called a load calculation.

In North America, the industry standard is the Manual J Residential Load Calculation, which is published by the Air Conditioning Contractors of America¹. The 8th edition of the Manual J is 628 pages long and, according to its description, “provides detailed requirements for producing a residential load calculation per the CLF / CLTD method.”

I’ve had a Manual J performed on my home. From my point of view, it looked a lot like any other energy audit. The contractor spent time in my home taking an inventory of the HVAC system, examining the insulation level in the walls, noting the number of windows and doors, and taking other measurements.

The contractor then input those measurements into software that performed the Manual J calculation and produced an estimate of my home’s heating and cooling needs based on the average climate in my area. The specifics of that calculation are far beyond the scope of this article, but as a way of describing the complexity of the Manual J, here are the relevant section titles from the manual:

• Section 18 - Design Conditions
• Section 19 - Fenestration Load Procedures
• Section 20 - Opaque Panel Load Procedures
• Section 21 - Infiltration Load Procedures / Infiltration Principles
• Section 22 - Internal Load Procedures
• Section 23 - Duct Load Procedures
• Section 24 - Ventilation Load Procedures
• Section 25 - Blower Heat
• Section 26 - Hot Water Piping Load
• Section 27 - Winter Humidification Load
• Section 28 - Moisture Migration Load

This is obviously much more sophisticated than the old-timer’s “1 ton per 500 sqft” rule-of-thumb. If you want to play around with a Manual J calculation yourself, there are some online resources you can find, but I can’t vouch for their accuracy.

What does a Manual J report look like?

The output of a Manual J report will differ depending on the software the contractor uses, but my HVAC company used a package called CoolCalc. Here’s the report he shared with me:

The heating BTUs describes the winter heating requirements, while cooling BTUs and SHR (sensible heat ratio) describe the air conditioning requirements due to temperature and humidity. Other reports may include more technical detail. If you’re interested, the Energy Vanguard blog has a good article describing how to read Manual J reports².

An energy audit and a Manual J calculation are not the same thing

I did briefly mention home energy audits, which are similar in some ways to the Manual J calculation. An energy audit is designed to let a homeowner know where their building is losing heat in the winter and gaining heat in the summer so that they can make wise decisions about where to invest in energy upgrades. That’s where the similarity ends though, because an energy audit is a general purpose tool that doesn’t get into the details of your HVAC system, such as making sure that the ducts are sized correctly.

Home energy audits are definitely a valuable tool, and they are often made available for free or reduced cost through local rebates and tax incentives. However, they aren’t the same thing as a Manual J calculation, so you can’t rely on the results of an energy audit alone to guide the design of your HVAC system.

Other load calculations: Manual S, T, and D

A Manual J load calculation is an important step in determining the correct size of HVAC system for your home by quantifying the building’s heating and cooling loads, but it doesn’t tell the contractor which equipment will meet those requirements. For that, your contractor will need another calculation called the Manual S, or the Residential Equipment Selection guide, also published by the ACCA.

Depending on the type of HVAC system being installed, there might be even more calculations. There is a Manual T: Air Distribution Basics for Residential and Small Commercial Buildings, and Manual D: Residential Duct Systems.

As a homeowner, while it’s not necessary to ask your contractor about the details of each of these calculations, it is a good practice to ensure that your HVAC system is being designed according to these calculations, rather than guesswork or rules of thumbs.

How to choose an HVAC company that will properly design your HVAC system

Two houses that have the same square footage can have vastly different heating and cooling requirements, even if they of a similar age. Unless your contractor has x-ray vision, they simply can’t know what’s between the exterior walls of your home.

Asking for a Manual J calculation is a great way to eliminate contractors who you don’t want to work with. If they refuse or come up with excuses for what that’s unnecessary — maybe they claim that their experience trumps need for doing all of the steps prescribe by the Manual J — that’s a great indicator that you should select someone else.

I can tell you from personal experience that the contractors I spoke with sounded a little surprised when I asked for one, and you might experience the same. It’s a technical thing to ask for, and most homeowners simply just want an HVAC system installed at a reasonable price. However, hopefully I’ve made it clear that it’s worth taking the extra steps to make sure that pick a contractor that will properly design a system that heats and cools your home. Not only will a well-designed system be more economical, but it will have health and safety benefits too.

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