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Air Conditioner Sizing: Load Calculations vs Rules of Thumb

Air Conditioner Sizing: Load Calculations Vs. Rules Of Thumb

It’s time to revisit one of my favorite topics:  air conditioner sizing.   Many contractors use rules of thumb to decide what size cooling equipment to install.  Usually, the rule is based on the amount of conditioned floor area, and contractors in many areas generally use the rule of 1 ton of air conditioning capacity for each 400 to 600 square feet of floor area.  The rule is usually specified in square feet per ton (sf/ton).

The rule doesn’t account for the window type, orientation, or overhang.  It doesn’t account for airtightness or the insulation levels.  It doesn’t account for any of the features of the house that actually have an impact on how much cooling it needs.  It just scales the air conditioning system to the size of the house.

Rules of thumb vs. load calculation results

First, a quick note on the numbers.  When we talk about square feet per ton, higher numbers mean lower cooling loads or lower air conditioner capacity.  Cooling loads and air conditioner capacities are actually given in the amount of heat gained or lost per unit time.  In the US and Canada, it’s mostly BTU per hour.  In the rest of the world, it’s kilowatts.  Sticking to BTU/hr, we can divide the cooling load by 12,000 to convert to tons.

Then, to factor in the size of the house, we could divide the load or capacity by the floor area, but the numbers would be tiny.  For example, a 3,000 square foot house with a 3 ton air conditioner would have 3 ÷ 3,000 = 0.001 tons per square foot.  By inverting, we get a nicer number 1,000 square feet per ton.

What we find is that most newer homes—even in hot climates—have square feet per ton numbers significantly higher than 400 to 600. Even 1,500 square feet per ton or higher isn’t unusual.  That means a contractor using 500 square feet per ton is installing an air conditioner that’s three times larger than it should be.  In a 3,000 square foot house, for example, the rule-of-thumb installer might put in 6 tons of capacity.  Using the Manual J and S protocols, they probably would find the house needs only 2 to 3 tons of air conditioning capacity.

My rule of thumb is 1,000 square feet per ton for newer houses.  I don’t mean that’s how I do air conditioner sizing. That’s just my quick check to see if a system might be oversized.  If that calculation comes out close to 500 square feet per ton, I assume the installer didn’t do a load calculation, or if they did, they fudged the numbers.  If it’s 1,000 square feet per ton or more, it may be okay. But that applies to the above-grade parts of a house. Basements are often closer to 2,000 square feet per ton—or higher!—for newer houses.

Load calculation data from our jobs

In a 2016 article, I showed some of our load calculation data from our HVAC design jobs.  I pulled out 40 homes in hot climates and found the average cooling load was 1,431 sf/ton.  I don’t recall how I filtered the data to exclude cold climate homes.  Also, I didn’t look at the equipment sizing data.  Recently, though, I went through our jobs from 2021 and redid the analysis.

The graph below shows the breakdown for some of the load calculations we’ve done at Energy Vanguard in 2021.  The data come from 75 homes, and the numbers plotted here are for the 167 individual zones in the houses.  All the homes included here are in hot climates, which I defined as having an outdoor cooling design temperature of 90 °F or higher.  We use Wrightsoft’s RightSuite Universal to calculate the ACCA Manual J loads.

Cooling load by zone for some of our HVAC design jobs in 2021
Cooling loads expressed as square feet per ton for houses in hot climates

Of the 167 zones, only 53 came in lower than 1,000 square feet per ton.  A mere 20 zones were below 700 square feet per ton.  That means only 12% of the zones in this group might have been close to having right-sized cooling equipment using the 400 to 600 square feet per ton rule.  The average load for all 75 homes, as you can see on the chart, was about 1,200 square feet per ton.

Equipment sizing data from our jobs

Now let’s look at the actual air conditioner sizing numbers.  Most of the time, the air-conditioner capacity will be larger than the cooling load because of two things.  First, you have to meet both the sensible and latent cooling loads, not just the total load.  Second, air conditioner capacities don’t always line up perfectly with the cooling loads.

Using the same group of homes as above, I’ve plotted the actual air conditioner capacity we specified.  The graph below shows the results of that analysis.  This time there are 63 homes included in the data.  (It’s not 75 because some of those in the previous chart were jobs where we did only the load calculation and did not select equipment.)

Air conditioner sizing usually comes out higher than the cooling load.
Air conditioner sizing usually comes out higher than the cooling load.

In those 63 homes, we selected 151 individual heating and cooling systems.  As you can see, the columns have shifted to the left here, indicating that the air conditioning equipment we chose was larger than the loads.  That’s why the average capacity of the systems we selected was 856 square feet per ton, making our average capacity 28% higher than the average cooling load (1,192 square feet per ton).

But there’s a catch. I used the nominal capacity for the systems we selected.  The actual capacity from the manufacturers’ expanded performance data would show less capacity.  The real square feet per ton number here would probably be about 1,000.  And remember, even right-sized systems still will be oversized because Manual J loads are 10 to 20 percent higher than the actual cooling loads.

Air conditioner sizing key points

We can summarize with a few key points here.

1. Using a rule of thumb like 400 to 600 square feet per ton will work for very few houses.  Sizing an air conditioner to that metric will almost always result in an oversized air conditioner.  Sometimes grossly oversized.

2. Actual cooling loads found by using the ACCA Manual J protocol are much lower than the rule of thumb numbers.  That means the square feet per ton numbers are higher.

3. Actual air conditioner sizing is also much lower than the rule of thumb.  In the batch of jobs I analyzed, we’re at 856 square feet per ton.

4. Our air conditioner sizing numbers are actually better than shown above because I used nominal capacity instead of finding the actual capacity.

5. Another factor that makes our air conditioner sizing larger is that in most of our HVAC design jobs, we specify variable capacity equipment.  Oversizing doesn’t hurt as much with that type of equipment because it can ramp down to meet the load.  (Just be sure you know the turndown ratio of the equipment.)

Designing an HVAC system starts with proper sizing. Look at the square feet per ton number you get to see if you’re in the ballpark. If the number is less than 1,000 square feet per ton for newer homes, either the number is wrong or the house isn’t as efficient as it should be.

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.

 

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This Post Has 12 Comments

  1. Obviously “right-sizing” is the goal. What are the negatives of oversizing vs undersizing?

    oversizing cons:
    more initial cost
    shorter cooling cycles will decrease lifespan of system and comfort
    less humidity removal because of shorter cycles

    under sizing cons:
    not being able to recover on the hottest days.

    I’m sure i missed some, as i’m just a homeowner, would love to hear what i missed.
    Also an opinion on if its better to slightly oversize or undersize

  2. Thanks for this article Allison – HVAC sizing is always a keen topic of discussion! Question for you – I believe the commercial code as it relates to HVAC generally targets a minimum of 4 ACH – so you should have ventilation equipment capable of handling that. Does residential have such a requirement? I do not believe it does, though I am not the expert like you are. If you can continuously filter indoor air at 4ACH it would seem like you have a pretty good system – but then for simply moving air a basic HVAC installation would not be sufficient for a low load home since it’s going to be sized fairly small (assuming it’s a tight house). I’ve heard the number that generally speaking a ton of HVAC is moving around 400 cfm – 2 tons would get your 800 cfm, etc. But if that HVAC is not running all the time then obviously you’re not moving any air and you have stale air. Thoughts?

    1. @Charles, that’s an excellent question. I believe you’re conflating ventilation, filtration and internal air changes. Let’s start with ventilation…

      As an example, the residential standard for ventilation (ASHRAE 62.2) stipulates a 2,000 ft2 home with 3 bedrooms needs a ventilation rate of 90 CFM (minus the verified infiltration credit). That works out to 0.3 ACH, assuming a volume of 18k ft3. If the ventilation air is distributed by the main air handler, it can be set to run on low speed 24/7, or x minutes per hour. Likewise, one can improve air filtration by running the air handler off-cycle (potentially leading to condensate re-evaporation in humid climates). But these are separate considerations from internal air changes, which is what you appear to be asking about.

      There’s no code requirement for internal air changes, at least not for residential (nor for commercial, AFAIK). ACCA’s Manual LLH (Low Load Homes) refers to air turnovers per hour (ATH) and “air loading factor” (ALF = system airflow / conditioned floor area). These are somewhat controversial concepts driven mostly by diffuser considerations, i.e., register throws. It’s interesting to see how Manual LLH dances around these old-school concepts that might still apply to homes built 20 or 30 years ago.

      I’ve been designing mechanical systems specifically for low load homes for more than 15 years. Some of my projects (including my own home) have ALF’s as low as 0.15 cfm/ft2 and air turnover rates less than 1 ATH, and here’s what I’ve learned:

      As we ratchet down the cooling load, register throws (and air mixing in general) become less and less important. For low-load homes with normal sized rooms, you only need to get the supply air to the room. I would argue that most homes built (and verified) to meet today’s energy code would qualify as low-load homes. In that context, there are more important things to worry about than register throws and internal air turnover.

      More important when designing an A/C system is that your airflow achieves the desired sensible-latent split based on the loads. For example, 350 cfm/ton increases latent capacity, whereas 450 cfm/ton increases sensible capacity (and efficiency).

  3. Good point about distinguishing design load vs equipment nominal capacity vs equipment capacity at design conditions. Going further, one must consider sensible vs latent vs total LOAD, and sensible vs latent vs total CAPACITY — the meat of the Manual S procedure (i.e., S for equipment selection).

    Almost forgot: when specifying a heat pump, one must consider how much oversizing is acceptable. For example, in cold climates with dry summers, the penalty for oversized cooling capacity is relatively small compared to the savings from less supplemental heat demand. In warm, humid climates, the penalty for oversized cooling capacity is large. In between those extremes, it becomes a judgement call. “Cold climate” heat pumps allow closer sizing in cold and mixed climates that have humid summers.

  4. Here in England we’re getting a heatwave too — but there’s heatwaves in Northern Europe and there’s heatwaves in Atlanta! Not quite the same thing. But still.

    Sizing in an all-electric house like I’ve remodelled mine to be is tricky. We’re a heating-predominant climate so I had to size for the heating load. This is problematic as the three installed mini-splits (to provide three zones, one for upstairs, one for the kitchen/breakfast nook, one for the rest of the ground — sorry, US-speak needed I guess for his audience! —“first” floor) are, combined, spot-on for the heating load calculation but oversized for the more modest cooling load. It gets especially bad in the 1-and-a-quarter ton unit for the bulk of the first floor (rating is one-and-a-half tons for heating) is in a space with a design cooling load of barely three-quarters of a ton.

    However. Being inverter (VFD) mini-splits which can modulate way, way down, everything seems to work okay. We do get humidity here (it’s what in the US would be a marine climate zone) but the mini splits ability to spin down to very low outputs gives a smidge of sensible cooling and a long enough runtime to maintain dehumidification.

    But yes, when I spent time in southern California, typical fixed-speed US HVAC was (from what I saw) plagued by oversizing, short cycling and poor comfort. Florida if anything seemed even more poorly designed as a generalisation (sorry fine folks of FL, I’m sure there are some contractors who do things right, I just never visited any houses where you’d worked!) with homeowners resorting to arctic blasts from the registers as thermostats were set low to try to keep on top of the humidity.

  5. We moved from the hot, humid South to 4000 ft elevation in northern AZ, only to discover that the house that we bought had a failed heat pump. We contacted several contractors, both locally and in Phoenix, and the usual “rule of thumb” was exactly what you said: 400 sq ft per ton, or 6 tons in our case (!). That was more than 50% of the total heat pump capacity for our 3X larger Atlanta area house! With no dehumidification or related effects! I quickly estimated our load for year-round use here, which the long-term data says should include summer highs of perhaps 100F and winter lows of perhaps 25F on occasion, tempered with my previous experience in specifying the sizes (always smaller than the contractors proposed) for our homes during our lives in the East and South, and also keeping in mind that the air handler and most of the ductwork (mostly flex) are in the attic. The heat pump is still a bit over-sized, but with the changing climate and significantly hotter summers plus some extra-cold winters, 4 tons with a 2-speed compressor and variable-speed air handler plus 10kW resistance heat add-on keeps us comfortable and on-target year-round. If and when we need to replace it, we’ll want to do the full-scale calculations that you use as well as use the latest weather data, but I’m guessing that we’ll end up deciding between a 3 ton and 3.5 ton variable-speed compressor plus a variable-speed air handler (if we need to replace this one), to make it even better and more efficient. I hope that your articles will inspire more contractors, architects/designers, and homeowners to avoid using rule of thumb shortcuts and instead use valid, more-detailed, and appropriate calculations.

  6. Great info, as always, Allison. I was happy to see you noted inverter technology with respect to slightly oversizing ac equipment. Initially, we had two 1.5-ton ac systems in our new home. Recycled equipment from a replacement job we performed. Eventually one condenser compressor passed on to the freon Heaven in the sky and the cobbler’s children went barefoot for one summer. Retreat to the comfort side of the house as needed!

    When we landed a large church hvac renovation, 28 mini splits were going to be ordered and the added reduction in price was the inspiration needed to add two more that would be for our home. Only one problem: my bride was also our office manager and bean counter. She immediately understands my intentions and stated – very emphatically- they better not be for “her” house! Turned out she didn’t like the look of wall or floor mounted inverter mini splits! Yikes!!! No matter, the two extra were sold within days.

    Concealed mini split air handlers were just coming out, but had ultra low abilities to overcome resistance to airflow. I had designed for .03 & .05 (S/R). Solution? Convert all returns to supply ducts and cut in return air filter grills.

    The manufacturer told me if I did that, there would be no warranty. Their national trainer said if anyone could pull this off, it was me. More than a decade later my inverter mini splits with the concealed air handlers are performing perfectly. At the time, I elected to also replace the still working second system and micro zoned the home to four instead of two zones.

    You asked about run times. Given that they are inverter systems, they seldom are off. Our home is 100% hydronic radiant heating with a high eff NG boiler that is also modulating/condensing. When I first installed the inverter mini splits, I heated with them alone to see specially how our comfort level would be in our living room with its vaulted ceiling and was pleasantly surprised to find the comfort level a close second to radiant. Long story not so short, the inverter mini splits serve as backup for heating.

  7. How does one go about collecting the information needed for manual J (and the other manuals) in and old home where you often don’t know what’s in the walls, the floors; not to mention the invisible measurements like leakage?

    1. @Bob, there are some tricks for discovering the info needed to model an existing home, old or not. For example, a lighter or match will reveal if the glass has a low-e coating (google ‘flame’ and ‘low-e’). You may be able to tell what’s in the walls by removing a few outlet or wall switch plates until you find one with a gap between the drywall and electrical box. A thermal (IR) camera will reveal how well the insulation was installed so you can estimate the effective R-value for the assembly. If ducts are outside the envelope, you can usually inspect most if not all of the insulation. One advantage of modeling an existing home: you can do blower door and duct leakage tests to measure actual leakage rates.

      Having said all that, the most accurate way to size a replacement A/C, heat pump or furnace is through cycle timing. This assumes the existing system is in good working order and you have the luxury of time to wait for a hot day (or cold, if sizing a furnace). Also, if the system is multi-stage or variable capacity, it must be locked into high stage. And unless it’s your own home, a data logger (indoor & outdoor temp) is usually the best way to get the data.

  8. Allison, thanks for the broad insight of sizing a/c systems. It strikes home because my sister-in-law(and her husband) are building a home next to ours as I speak. I’m going to ask what the a/c sizing is because the construction is ICF(insulated concrete form) and that will undoubtedly have a bearing on it.
    Another factor and one that was NOT discussed is that of the Value and Function of the powered attic ventilation
    with regard to the sizing the a/c and the the ultimate effectivity of same. We have NO eaves in our home, only low hips on the roof pitch of about 2/12 so I installed two auto. turtles(1000+/- cfm each), at about 25% and 75% the length of the roof included the garage(2600sf).
    Point being that the fans pull ambient air all over across the insulation blanket.
    (the eaves have vented soffits every 6 ft)
    Since much of the solar radiant roof heat
    is re-radiated downward it heats the air AND the insulation blanket(the hot roof mass also heats the air thru conduction), the insulation gets hot on top first then conducts said heat thru to the ceiling according to thermal characteristics and thickness of the blanket. If there is hot, 140°-150°(un-cooled) air on top of the blanket contributing, the heat will continue to build and the ceiling will, over time, become so warm that the “correctly sized” a/c system will not be able to keep up. I experienced this when we were gone for 3 wks. The ceiling was 85° F and the otherwise sufficient a/c unit took 72 hrs cont. to bring the temp down to 76°. With ambient air on top of the blanket by auto thermostat the ceiling temp would never have gotten out of control. The air would draw both the hot top temp down closer to ambient AND cool the underside of the hot roof matl the same to reduce the re-radiated heat that was adding to the problem. Some have said that pwrd attic vents are not useful. Seems to me that ambient air is far cheaper than air conditioning the attic. Would you agree?
    Your thoughts?

  9. Thanks Allison for another fine paper that exposes the problem with oversizing. If we paid attention to air flow requirements for Manual S, D and T (stop using nominal tonnage on the packaging) we would have properly sized efficient comfortable homes.

  10. Thanks for your update on load calculations vs rules of thumb, and the apparently rampant oversizing in AC! However your cooling load chart leaves me unsure whether 500 SF/ton is plausibly correct near the high end of the range of design temperatures (90 to 116) of the data points used to construct the chart. While it seems obviously wrong to apply the same rule of thumb across the USA, where I live, in the heart of central California’s San Joaquin Valley, the temp exceeds 105 a few days each summer. Could you plot your calculated cooling load (SF/ton) data vs design temperature? Has someone already done this, or otherwise used Manual J calculations in various climates to get an improved rule of thumb / “sanity check” relating SF/ton to design temperature in “newer houses”.

    Also, what qualifies as “newer” or what insulation specs does this imply? My home was built in 1999 but about all I know about it is that it has dual pane windows.

    Thanks for any advice, and thanks for your education efforts!

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