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5 Tons Is Never the Right Answer

5 Tons Is Never The Right Answer!

We do a lot of HVAC design (third party, all over the US) at Energy Vanguard.  I’ve also written a lot about it here in this blog and in my book.  And if you’ve been reading any of those pieces, you probably know I like rules of thumb.  Well, today I’m going to tell you about one of our rules of thumb.  Yeah, the one in the title:  5 tons is never the right answer.

Why would we, who hate rules of thumb, use one ourselves?  It’s not hard to explain actually.  But first, if you don’t already know what the heck I’m talking about, let me explain.  Air conditioner and heat pump capacity is often given in tons.  One ton of cooling capacity is equivalent to 12,000 BTU per hour, so 5 tons would be 60,000 BTU per hour.  (There’s an interesting history behind the ton of cooling capacity.)

Here are our two reasons why 5 tons is never the right answer.

Most houses don’t need 5 tons

As I showed in our data, our load calculations average out to one ton for each 1,200 square feet of conditioned floor area.  That drops to about 900 square feet per ton for the equipment sizes we specify.  So, using the second number, we’d need 5 tons only for a 4,500 square foot house.  We do houses that large and even larger, but the average house size in the US is less than 2,500 square feet.

Take Bill Spohn’s brand new net zero energy high-performance home in Pittsburgh, Pennsylvania.  The photo above is the outdoor unit for his heat pump.  He’s heating and cooling 4,400 square feet with only a two ton heat pump.

What about older houses, you ask?  Well, I live in one of those.  It was built in 1961.  The total floor area is 3,900 square feet.  I have a 2 ton system serving most of the main floor.  I have half-ton ductless unit in the sunroom.  And when I get to that part of the basement, I’m going to install a 1 ton heat pump.  That’s 3.5 tons in 3,900 square feet.  That’s 1,100 square feet per ton for a pretty good old house!

Houses that need 5 tons need more than one system

For larger houses that do need 5 tons of total capacity, we don’t specify a single 5 ton system.  We typically specify two or more systems—sometimes many more.  And that’s true even for smaller houses.  My house will have three separate systems when I’m done with the basement.

Putting in multiple systems provides some good advantages.  For example, it makes heating and cooling different parts of the house differently possible.  Like the bedrooms cooler?  Just set that system’s thermostat lower and leave the other ones set higher.

It’s also great for resiliency.  But to get the full resiliency of multiple systems, you have to put in one outdoor unit for each indoor unit rather than one outdoor unit connected to multiple indoor units.  The latter will provide some extra resiliency because one indoor unit going out doesn’t knock out all your heating and cooling.  If you have only one outdoor unit, though, you lose all your heating and cooling when it goes out.  (See my article on one-to-one versus multi-splits for more.)

Finally, multiple smaller systems makes it a lot easier to design duct systems.  With a single 5 ton system, you’ve got to distribute air over what’s probably a very large house.  If you’ve ever done duct design for a large house, you know that’s not an easy task.  There’s inevitably a beam or stairs or some other obstacle that makes it difficult.  So you architects designing houses, think about giving us space in more than one place for the air handlers.

The nitty gritty

So, yeah, 5 tons is never the right answer.  It’s possible to make it work if you really have to, but there will almost certainly be air distribution and comfort problems in the house.  You could try zoning to make it work, but zoned systems often have problems.  (And of course, never use a bypass duct if you do zone.)  Keep your equipment small, your ducts large, and your thumbs free of rules.


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 is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to our newsletter and follow him on LinkedIn.


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This Post Has 28 Comments

  1. You nailed it Allison. In addition to the reasons you gave for not using a 5-ton, there is at least one more. Ducted equipment manufacturers optimize their designs around 3-ton equipment since that is the largest sales volume. If you look at a given product family (2-5 tons) for a given manufacturer, the 5-ton model usually has the lowest rated capacity. The larger sizes required for 5-ton units also makes installations tougher in restricted spaces.

    I hope your next blog is on how to arrange multiple units in a home to make the most of the advantages that you listed.

    1. Roy: Thanks for adding your approval and the additional reason to avoid 5 ton units.

      Regarding how to arrange multiple units, I mentioned that in my book tour talks this week because we recently had a job for a big spread out house where the architect gave us a central mechanical room. In houses like that, we really need multiple mechanical rooms or conditioned attic or crawl space or basement areas where we can place additional systems. I’ll add this topic to my list, but it probably won’t be my next article.

  2. I support this article’s information and have been preaching the “no 5 tons” in residential applications for years. Two systems are a much better solution. Comfort and long-term cost of operation are the best way to convince folks to spend a bit more for two systems. Over the years we have solved many comfort issues by using two smaller systems vs. one oversized system. Thanks for sharing.

  3. 1961 is an “older” house!? My three houses are 1889, 1910, and the newest was built in 1920. What would you call these houses? “Much older”, “very old”, or maybe a euphemism such as “historic.”
    Heating, cooling, and even insulating these homes provides more challenges than your “old” 1961 house.

    1. Rob: Good point. The issues that I deal with in my 1961 house are different from yours in those “much older” houses. For one, I have drywall on the inside, not plaster.

  4. Since we have opposable thumbs, I oppose rules of thumb! The HVAC in my newly constructed home was “designed” using multiple “rules of thumb” none of which were valid for modern construction. It was amazing how difficult it was just to get the builder to finally agree to downsize the furnace to the proper size! The fact that the “rules of thumb” all violated code as defined in the ACCA Manuals fell on stubborn deaf ears. The local jurisdiction blindly allows these “rules of thumb” to continue.

  5. In teaching Manual J, we never ever tell students about x number of sq ft per ton. I purposely tell them to nananananana with their fingers over their ears when anyone starts talking about per sq ft.

    Regions are different and locales are different. Different temp swings, different humidity.

    I just never ever want to hear my students saying oh the house is just 2200 sq feet so it will need x.

    We would not need Manual J then.

  6. Totally off-topic, what is the tan brown colored box on the right in the photo?
    That does sound like a nice house – outdoor areas, according to the book A Pattern Language are a transition zone that make you feel more comfortable entering and exiting. There is something especially nice about a dog run.
    Thanks for this wonderful article today – sums up a lot of recent info.

    1. I’m intrigued by that brown box too – maybe a compost bin, or a rainwater collection device? Or perhaps an old heat exchanger that got overloaded. 😀

      1. It has what could be a hose outlet and there is a hose nearby so I was thinking water something too. It isn’t very big though, couldn’t store much water with it. The odd hexagonal openings are a mystery though. These sorts of deep inquiries are what keeps everyone on there toes…

    2. Yes, that was my first reaction: what is the SpongeBob-SquarePants-looking box for?

      1. I know! It’s a pedagogical tool. Now we’ll all remember this blog about pitfalls of HVAC sizing by rules of thumb.

    3. Kelly: I knew someone would ask about that. It’s a rain barrel with a different shape. And yes, Bill and his wife read A Pattern Language and incorporated the concepts into their design.

      Sorry for the delayed response. I’ve been on the road for the first week of my Northeast book tour.

  7. I believe multiple hvac units is the best investment you can make outside of a well done wall assembly. Anyone who has ever lived in a multi-story home with one unit knows what it is like having significant temperature differentials between floors. Ok, zoning is possible but after the extra expense of that just get multiple units with more simple duct design, right?

  8. Beyond fantastic article. Thank you. This should be required reading for all HVAC companies.
    Got into a discussion with a National company that bought a local company,
    they adamantly say 1 ton per 500-600 sq ft.
    My own new home, built 2019, Topeka, KS, has 3,892 sq ft on the main level and two – 2 ton of air, split house left and right. 14 foot and 18 foot vaulted and cathedral ceilings.
    Walk out basement, 10 foot ceiling, facing east has 3,136 sq ft and has a 2 ton A/C, Usually only needed during holidays with the 2nd kitchen and lots of people.
    Total 7,028 sq 6 ton.
    That is one ton for each 1,000 sq ft. Systems still cycle with the recent 100 degree weather.
    Also has a whole house dehumidifier to keep humidity at 44-48 percent.

    1. Greg: Sounds like you did a great job getting the systems downsized from what many contractors want to install. I’ll bet it’s a lot more comfortable, too.

  9. So this is off topic but from a efficiency standpoint why aren’t there some refrigerators designed that have coils that run outside the wall for better efficiency? Seems like the refrigerator is running in your house and then you’re using your HVAC to cool your house. So basically you’re spending money twice to get rid of that heat.

    1. The refrigerator’s condensing temp is designed for room temps… ahaaaaaa

      Take it outside and it could struggle on Peak Load days.

    2. Glenn: What you’re talking about is a split system refrigerator. It’s certainly possible to do that, but then you’d have an appliance that suddenly needs more maintenance. Current refrigerator design makes it a package unit, with everything in the same box. No installer can mess it up by making refrigerant line joints that leak or putting in the wrong amount of refrigerant. But if you turn it into a split system, you introduce the potential for a lot more maintenance.

      The benefits just wouldn’t justify the extra cost and potential drawbacks. Modern refrigerators are pretty darn efficient these days, using only about 500 kWh per year or even less. It doesn’t add a lot to the cooling load, but it does help with heating.

      And as Chriss Heenan said, you can’t just put the standard fridge outdoors because it’s not designed for those temperatures.

  10. Great stuff Allison! I led a study at my old job to dispatch rule of thumb sizing when I found our subcontractor using the old 500 sqft/ton rule in the first two homes I walked. We were building high-performing homes (tight envelope and ducts, better insulated, better windows, radiant barriers), and not getting any credit for it in our load calcs. This was 2013, and most of the HVAC contractors we reached out to weren’t interested in doing things right. Credit to Reliance Heating and Air for teaming with us and trusting the process. Thankfully many others have come around since then. We found that across different plan types (1-story to 3-story, 2000 to 5000 sqft), we could consistently expect 1000 to 1100 sqft per ton. While we didn’t default to this new rule of thumb, it did help us verify proposals and know when to dig deeper into the data. The numbers don’t lie.

    1. Thomas: That’s great to hear that you’re finding more contractors willing to go beyond the oversimplified square footage calculation. People often ask me—in fact it happened at one of my book tour events this week—if things are getting better. When I hear this kind of stuff from builders like you, it gives me more reason to answer in the affirmative.

  11. So how ‘never’ is ‘never’?

    We’re building a large custom home and one ‘expert’ HVAC bidder recommended three 5-ton systems and one 3-ton system, for a total of 18 tons… the home is 13,700 sf, so wouldn’t a few 5-ton systems be necessary to cover the square footage?

    We also have a bid for a geothermal system with one 6-ton pump and one 4-ton pump to cover the entire home… conveniently missing the 5-ton no-go zone… but interestingly concluding that 10 tons are sufficient, compared to 18 tons for the other guy.

    Man it’s hard to get apples-to-apples comparisons…

    1. Timothy, with any project, you want to have an HVAC blueprint, and that will give you apples-to-apples bids. For a project like yours you would like to have an HVAC design professional (like Energy Vanguard) on board, which will get you the right size and type of systems for your structure and climate.

      Even better, if your architect/builder did not put emphasis on performance of the home’s design, firms such as EV will be able to suggest changes in your building envelope that might bring you down a few tons, and get you more comfort, even if you already started construction (well, it depends). Comfort /= system size (that’s a not-equal sign), almost never.

      And before you go all natural gas for your heat/hot water, open your mind to other options and talk to experts (your builder, bless his/her heart, may or may not be one).

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