How to size a Mini Split or other HVAC system
(aka air conditioner sizing, heating system sizing)
Last update: August 2, 2024
The Basics
Sizing simply means figuring out what size equipment you need to heat and/or cool your home or room. We calculate how much heating/cooling we need, then we buy equipment that supplies that capacity. That's true no matter what flavor of heating/cooling equipment you want (central HVAC, mini split, window unit, etc.).
Sizing is important because a unit that's too small won't heat and cool your space well, and a unit that's too big will cost more than necessary (and may have other possible problems). Most contractors will try to sell you a system that's too big, either because they're trying to make more money, or because they don't know how to properly do the sizing work. (sources 1, 2, 3, 4)
The best way to size your system is to have a "Manual J" calculation done on your space. Manual J is the gold standard for sizing, taking into account things like how much insulation you have, what kind of windows and what direction they're facing, and everything else. Many utility companies will do this for free (check with them), and if not, you can hire an energy auditor. Do not go with an HVAC contractor for the Manual J (who has an obvious conflict of interest in wanting to sell you a bigger system than you need), go with your utility or an energy auditor so you can trust that they did it right. They'll tell you exactly what size you need, and then you can shop accordingly.
When you buy your equipment, I suggest you match your Manual J to the "Max" figure of the equipment, not the "Rated" figure. For example, let's say your Manual J says you need 24,000 BTU, and you have these options for equipment:
Shopping Example | ||
Equipment | Rated BTU | Max BTU |
Option 1 | 16,000 | 25,000 |
Option 2 | 24,000 | 37,000 |
You'd go with Option 1, matching your Manual J to the Max figure, not the Rated figure for Option 2. There's a detailed explanation of why, below.
If you have ducts, you should also have your them tested for leaks, because installing properly-sized equipment won't do any good if you're going to gain or lose lots of BTUs through your ductwork. The national average performance of ducts is only 57%. (source) If your ducts are a problem, then have them serviced. Of course, if your ducts are running through conditioned space, then duct losses are not a problem, because you "lose" the air right into the area that you were trying to condition in the first place.
The problems with oversizing are cost, and possibly comfort
Buying a system larger than what you need is just a waste of money. That's why I steer you to get a Manual J report so you don't buy equipment that's too big.
Oversized systems might also be less comfortable, but that's becoming less relevant as modern units are moving to inverter technology, which can ramp the output up or down depending on the load. Without inverter tech, the system might blast you with too-hot air in the winter, and they cause uneven temperatures throughout the house, because it won't run as much as a right-sized sytem, so the air isn't circulated as much.
If you're satisfied with that answer you can skip to the next section. Otherwise, you can keep reading here about other potential problems with oversizing.
Conventional wisdom is that oversizing also creates other problems besides cost and less comfort, but I consider those to be mostly myths.
- MYTH: Oversized AC's don't
control humidity as well. The theory here is that
oversized systems don't run long enough to dry the air.
This is only sometimes true. First, modern units
with inverters keep running even at low loads. Second, a
university study of homes where grossly oversized units were
replaced with right-sized units didn't result in lower
humidity. The researchers theorized that the longer
running time of the right-sized units meant that the fans they
were drawing in more moisture into the house through the
building envelope and the attic. (source
PDF) And Martin Holladay, an icon in the
building science field, says, "Oversized air conditioners aren't
really as terrible at dehumidification as many energy experts
claim." (source)
However, I know of at
least one case in which an oversized system reportedly did
result in more humidity.
- MYTH: Oversized systems wear
out faster because of short-cycling. The theory
here is that big systems heat or cool the house so quickly that
they shut off after just a few minutes, and the constant on/off
cycling is hard on the equipment. There are a few problems
with that: First, with modern inverter tech, cycling is
even less than older systems that are sized perfectly.
Second, it's hard to believe that the extra cycles would really
make a big difference in equipment life. The AC will
likely be up for replacement just because it's old before it
dies completely. Third, any stress added by extra cycles
should be balanced by the fact that the equipment doesn't run as
long.
- MYTH: Oversized systems use lots of extra energy. The theory is that AC's need to run for several minutes to get efficient. But the study above found that they don't use more energy overall. However, for mini-split systems, especially heaters, oversized systems can indeed use a lot more energy. (source)
BTU: The measurement of heat
Heat is measured in BTU's (British Thermal Units). For example, a heater might supply 24,000 BTUs per hour to a house when it's operating. To size a heating system, we figure how many BTUs of heat the house loses to the outside, and then get a system big enough to replace the BTUs that were lost.
We use BTUs for cooling, too, to measure how much heat an air conditioner removes from a house or a room. For example, a small window-unit AC will remove about 5000 BTU per hour from a room. When we size for cooling, we'll be figuring out how many BTUs of heat enter the house, and then get a system that can remove that much heat.
AC equipment is also measured in tons. This has nothing to do with the weight of the equipment, it's just a measurement of heat. One ton is 12,000 BTU. So a 2.5-ton system would be 2.5 tons x 12,000 BTU/ton = 30,000 BTU.
Match to the “rated” capacity or the “maximum” capacity?
Confusingly, equipment specs show
both a "rated" BTU capacity as well as a "maximum" capacity.
For example, a mini split I bought has a "rated" cooling volume of
6000 BTU, and a "max" volume of 11,184 BTU. (source)
Which figure do you use when shopping for equipment to match your
Manual J?
The short answer is that in most cases, max is preferable. If that satisfies you, skip to the next section. Otherwise, keep reading this one.
Some things to know about equipment efficiency:
- “Efficiency” refers to how much electricity is used per unit of heating/cooling delivered. We measure efficiency as "COP". The higher the COP, the less electricity used for the same amount of heating/cooling. A COP of 3 is better than a COP of 1.
- Modern equipment uses inverters which can ramp the output up or down depending how much heating/cooling you need. That means, they run harder when they need to make a big difference in temperature, and slower when they're close to the set temperature.
- Equipment is most efficient (unit of electricity used per unit of heating/cooling) when running at the rated volume. For example, at 95°F outside, the 6000-BTU unit mentioned above has COP of 3.66 at the rated level, but only 2.73 at the max level. Likewise running below rated volume is also less efficient than running at rated. The penalty isn't the same, though: running above rated is generally worse than running below rated.
- Manual J is based on season extremes. Not the absolute extreme that has ever happened, but covering 99% of the most extreme hours based on a 30-year average. (source) So, if you buy equipment based on its rated volume, then most of the time (i.e., not extreme weather), your equipment will run below rated volume, which is less efficient than running at rated volume.
- Because temperatures vary a lot during the heating or cooling seasons, there is no way to get ideal efficiency. Whether you match your Manual J to the rated or the max volume, sometimes your system will run higher or lower than rated, which is less efficient than running exactly at rated.
A table probably makes that last concept easier to understand. Say your Manual J comes back at 12,000 BTU. You're looking at two different models, one that's 7,000 BTU rated and 12,000 BTU max, and another that's 12,000 BTU rated and 20,000 BTU max.
Choosing equipment for 12,000 BTU Manual
J |
||
7/12k BTU eqp. | 12/20k BTU eqp. | |
Most days | More efficient | Less efficient |
Extreme temp. days | Less efficient | More efficient |
Let's walk through that table.
- Let's say you buy the first unit, 7/12k BTU. The 12k from your Manual J is for the seasonal extreme temperature, most of the time you'll need less than 12k BTU. So, on those less-extreme days, your equipment may run closer to its 7k rated volume, which is its most efficient. So, your equipment will be more efficient on most days, and less efficient on the extreme days. Also, if the temps happen to go way beyond the assumed extremes, then you might not have enough heating/cooling.
- Let's say you buy the second unit, 12/20k BTU. Most of the time (i.e., not extreme temperatures), it'll be oversized, and running below rated volume, which is less efficient than running at the rated volume. But on the extreme temperature days, it'll run close to the rated volume, which is more efficient.
Easy mistakes
to make when sizing gas heaters
First off, think twice before buying a gas heater,
or replacing an old one. Modern heat pumps
almost always save energy versus gas, even in cold climates. (source)
However, if you're dead-set on going with gas, then keep
reading.
Many homeowners who buy gas-fired equipment are bitten by a couple of big misunderstandings. The first is that they don't know that gas furnaces are rated by their input, not their output. So a "60,000 BTU" gas furnace that's 80% efficient will supply only 48,000 BTU to your home. If you've figured that you need a 60k BTU of heating and you buy a 60k BTU gas furnace, you're going to wind up with only 48k BTU of heat which is not enough. So you need to calculate the output when you buy a gas furnace. (Oil furnaces, on the other hand, are rated by their output, so a 60,000 BTU oil furnace will supply 60,000 BTUs.)
The other mistake is to ignore the efficiency when replacing old equipment. Let's say you have a 70k BTU gas furnace at the end of its useful life. Your contractor points out that your old unit was only 80% efficient, but he can sell you a new 70k unit that's 95% efficient and it will cost only $350 more, promising that you'll make the difference back quickly with the energy savings.
But regardless of the energy savings, the unit is bigger than what you need, so if you buy it you'll be paying more than you need to. Your old unit put out 70k BTU x 80% = 56k BTU. The new one puts out 70k BTU x 95% = 66.5k BTU. So the new one is effectively 10.5k BTU larger than what you need—and you'll be paying for it. Here again, you must shop by output, not input. If you need only 56k BTU of heating and you go with a 95% efficient unit, then you should buy one in the neighborhood of (56k BTU ÷ 95% = ) 59k BTU.
Methods of sizing HVAC equipment
METHOD 1: Manual J calculation (good)
A "Manual J" calculation is the gold standard for sizing HVAC equipment. This is something you hire an energy auditor, local utility, or engineer to do. (Again, don't get an HVAC company to do it. Contractors frequently churn out bad Manual J's by using the wrong inputs—either because they're trying to upsell bigger equipment, or because they sincerely don't know how to use the software properly.) You can't really do a Manual J yourself. Even if you bought the software, without training on how to use it, the chance of making a mistake is high, and as they say, garbage in = garbage out. (If your input data is bad, then the result is worthless.)
METHOD 2: Size it to your existing system (questionable)
If you're shopping for an HVAC to replace an existing system,
you might think you can just buy the same size. The
problem is that your existing equipment is probably already
oversized. A properly-sized system will run continuously (or
nearly so) on the hottest and coldest days of the year. If
your unit is shutting off even in the most extreme weather, it's
oversized. Also, if you've installed energy efficiency
upgrades (e.g., more insulation, double-paned windows) since you
bought your system, it's probably oversized. If you don't know
whether your existing equipment is sized properly, get the Manual J.
If you're certain your existing equipment isn't oversized
(because it runs continuously on extreme weather days), then you
can buy the same capacity equipment to replace it. This
might not work if you're replacing one kind of system with a
different kind of system, though:
- Central HVAC > Mini Split. Central systems are usually bigger to account for duct losses.
- Window Units > Mini Split. Window units are leaky and poorly-insulated on the sides, so a house with mini splits will have a smaller BTU load.
Even if you know the size you want, you might have a hard time getting it. I had a small house that could be cooled well by three window units, which were 5000, 5000, and 6400 BTU respectively. So I didn't need more than 16,400 BTU of cooling capacity for the new system. But even though I told that explicitly to the contractors I invited to bid, they gave me quotes for 24,000- and 28,000-BTU systems. Sadly typical.
METHOD 3: Online calculator (uncertain)
There are various calculators on the net that try to help you size your system. They are not as accurate as a good Manual J, and they also require that you enter all the data perfectly. If you make a mistake, garbage in = garbage out. I tried several but the interfaces and/or accuracy were so bad I wouldn't recommend any of them.
METHOD 4: Go by square feet alone (poor)
The worst way to size a system is by square feet, though many contractors do just that. The problem is that two homes that are the same size can have drastically different needs, depending on climate, insulation, shading, foundation, etc. An engineering company which does Manual Js pulled the data for the last 40 reports they did for buildings in hot climates, and for the buildings that were <3000 sf, the results ranged from 624 to 1900 square feet per ton of cooling—quite a range. (source)
But for what it's worth, which is not much, the formula is 1 ton (12,000 BTU) of cooling for 1200 square feet in hot climates (based on my eyeballing the median from the engineer's data mentioned above). Contractors typically oversize by recommending something like 1 ton of cooling for only 500 sq. ft.
We're throwing lots of numbers around so let's put them in a list:
- 1900 sf/ton: Best example from a sample of ≤3000 sf buildings in hot climates as calculated by an engineer
- 1200 sf/ton: Median for above engineer's data
- 624 sf/ton: The worst result from the engineer's sample
- 500 sf/ton: What you're likely to hear suggested by a contractor
Note that you could invert the figures if that's more comfortable: Another way of saying 1 ton for every 1200 s.f., is 10 BTU per square foot.
POOR way of figuring heating load (BTU/sf) | |||||
Blue |
Green | Yellow |
Orange |
Red | |
6-30 BTU/sf | 5-27 BTU/sf | 6-26 BTU/sf | 5-23 BTU/sf | 4-20 BTU/sf | |
See
article for why this is a poor way to figure heating
loads.
Map from DoE. Heating values from my own calculations, |
METHOD 5: Go by square feet + climate (poor)
If it's bad to estimate the cooling load by square feet (and it is), then estimating heating by square feet is even worse. As you can see from the table, the range between efficient and inefficient homes is huge. This underscores how useless it is to estimate heating loads by house size alone.
You might be tempted to just pick the number in the middle for your house. Don't. Get the Manual J.
Other Technical Topics
BTUs: Technical definition
A BTU is the amount of heat required to raise the temperature of
one pound of water by one degree from 60° to 61°F at a constant
pressure of one atmosphere. (Wikipedia)
Different
substances require different amounts of heat to change their
temperatures because each substance has a different molecular
makeup. It takes about four times as much heat to change the
temperature of water as it does for air. The property of a
substance that dictates how much heat is required to change its
temperature is called specific
heat.
BTUs to heat air
I wanted to know how much energy is required to heat and cool air, so I could figure things like the penalty for running a clothes dryer considering that the dryer is sucking conditioned air out of the house, and unconditioned air in, which then has to be heated or cooled.
Incidentally, the figure for the energy required to condition air isn't really useful for figuring the energy needed to heat or cool a home itself. For that, you don't figure how much heat is required to change the temperature of the air in the home, because the inside air is generally always conditioned to a certain temperature. What you need to know is how much heat is entering or leaving, which is all the HVAC system has to deal with. And if the inside temperature is not maintained (e.g., you left the AC off all day and now you want to turn it on because it's very hot inside), I presume we'd have to figure the amount of heat to add or remove from all the objects in the house too, not just the air, which would be very difficult to estimate. (We could assume X lbs. of stuff per 100 sf, and figure that the specific heat of the objects is around half that of water, but those are just guesses.)
So here's my stab at calculating the BTUs required to heat air, and this is what I got.
- Air weighs 1.184 kg/m3 at room temperature (Engineering Toolbox)
- So air weighs 2.6 lbs./m3 (1.184 kg x 2.2 lbs/kg)
- So one pound of air is 1/2.6m3 = 0.385m3
- So one pound of air is 13.59 ft3 (0.385m3 x 35.3 ft3/m3, Google calculator)
So it would take one BTU to raise the temperature of 13.59 ft3 of air by one degree if water and air had the same specific heat, but as we saw earlier, they don't. The specific heat of water is 4.14 times greater than that for air. So, we should be able to heat (13.59 ft3 x 4.14=) 56.3 ft3 of air by 1°F with one BTU.
I calculated that because I couldn't easily find a reference figure for how much air could be heated with one BTU. But after doing the calculation I found a reference: 55 ft3, from this 1909 book by the American Technical Society. That's extremely close to the 56.3 ft3 figure we calculated above. Good for us.
Cost to remove heat generated by appliance use
It can be useful to know how much energy is required to remove heat from electrical appliances (like light bulbs), and how much it costs.
- 1 kW = 3412 BTU
- A 2.5-ton AC uses 3.5 kW and removes (2.5 tons x 12,000 btu/ton =) 30,000 BTU/hr
- So it's 3.5 kWh per 30,000 BTU. That's 3.5 kWh/30,000 BTU ÷ 3.5 = 1 kWh/8571 BTU
- To remove the 3412 BTU generated by 1 kWh of appliance use, we'd use 1 kWh/8571 BTU x 3412 BTU = 0.4 kWh
- 0.4 kWh x 16¢ kWh = 6.4¢ worth of AC to remove the heat generated by 1 kW of electricity use
- If we wanted to be really thorough, we'd calculate the energy required to remove the heat we created by running the AC to remove the original heat, which is a recursive issue of course.
Converting horsepower to tons of cooling
By a moderator at Refrigeration Engineer:
- 16 EER means 16 Btu/hr of cooling for every watt of power used
- 1 watt = 3.412 Btu/hr, so
- 16 EER unit provides 16 ÷ 3.412 = 4.69 watts of cooling for every watt of power used. This is the coefficient of performance (COP).
- 1 ton refrigeration = 4.712 horsepower, electric
- 1 hp electric = 4.69 / 4.712 = 0.995 tons refrigeration