Mini Split Considerations: When considering a robust heating/cooling system, it would be prudent to consider a mini split and familiarize yourself with some of the details and issues that go into determining your equipment and size requirements.

Heat Pump or Air Conditioner only?

What kind of equipment do you want? There are two types of mini split, one is air conditioning only, and the other is a heat pump used for both heating and cooling.

If you purchase an AC-only mini split, it will not be able to heat your home. If you live in a very warm area this may be what you want, or if you already have a heating system you are happy with. But it will never be a heating system.

You can’t change it without changing the whole system! If one did try to change the functionality, not only would they have to change the expansion valve to a reversible expansion valve, but the tube size, pressure rating and insulation would have to change throughout the system to accommodate the difference in pressures.

And of course the computer control system would have to be changed. You will certainly be better off buying and installing a whole new system.

A heat pump, on the other hand, is designed to swing back and forth as an air conditioner and then as a heater. The reversible expansion valve allows the refrigerant to change direction depending on functionality or mode. When it is working as an air conditioner, the heat from the inside coils is moved to the outside, and when it is working as a heater the heat from the outside coils is moved to the inside.

If you compare an expansion valve to the nozzle on a spray bottle, a reversible expansion valve allows the spray to change directions! That would be necessary with a heat pump swinging back and forth between air conditioning and heating, but an air conditioner would never need to change directions.

What is a BTU rating?

BTU, British Thermal Unit, is a measure of power, like the Watt. Just as a lightbulb may be rated at 100 Watts, a Mini Split may be rated at some number of BTUs. One BTU is equal to 293 watts, the amount of energy required to change the temperature of one pound of water onedegree fahrenheit.

One gallon of water weighs about 8.33 pounds. If one were to put a gallon of water at room temperature (70°F) on the stove and heat it to boiling (212°F) it would require about 1100 BTU. On the other hand, an 8oz cup of tea would require 71 BTU to bring to a boiling point.

When an engineer is designing an HVAC system they balance the number of BTUs lost with the number of BTUs added through the heating system, and vice versa through the cooling system. A well designed HVAC system maintains a constant temperature (as much as possible).

Power and Efficiency

When we talk about power, we need to be conscious of time. Clearly it takes more power to move something quickly than slowly. Imagine carrying a sack of potatoes up a mountain – it’s a lot harder to run than to walk slowly. BTU is normally an abbreviated measure of BTU / Hour, BTUh, or BTU-Hour.

Generally when we say that an air conditioner unit is 25,000 BTU, we mean that it puts out the amount of power it would take to change the air temperature. In other words, the BTU rating is in terms of output from the appliance. If we compare that to the number of watts the appliance uses (input) we get the EER efficiency rating BTU/Watts (output/input). More output compared to less input gives higher efficiency – that’s good (or vice versa – that’s bad).

PRO TIP: Some manufacturers give you one BTU rating on their system. In this case they may be giving one number rating that includes both heating and cooling. Obviously, that rating must incorporate two different figures. Other manufacturers combine these two numbers into one overall rating but also give numbers for both individually. Each value will reflect a different value regarding the unit’s capacity to heat and the unit’s capacity to cool.

Understanding Heating vs Air Conditioning BTUs

The goal of the HVAC system is to maintain some comfortable temperature during both hot and cold months. Essentially the HVAC system must move heat in order to combat the outside temperature. The difference between the person’s comfortable temperature and the outside temperature must be overcome. Note that the cold in cold times may be further from the ideal than the hot in warm times.

For example, if someone lives in Kansas the average winter low is around 20°F, but the average summer high is about 93°F. If this person is comfortable at 70°F, then he wants to heat a 50°F difference, and cool only a 23°F difference. All else being equal, it will require twice as much energy to heat in the winter as it does to cool in the summer.

If our Kansan moves to Georgia then the summer average high is 90°F (close the the same in Kansas) and the winter average low is only 34°F (16° warmer than Kansas), that is only 36°F from the ideal 70°F. In Georgia the values are much closer together. Summer would require about the same cooling power, but winter would not require as much heating power.

Moving Air for Heating and/or Cooling

In general, the split mini system (or any HVAC system) is always moving heat from a colder space to a warmer space. The physics of the situation demands that we move heat. There is no such thing as moving cold. That is, it is moving heat to a place where heat is more concentrated from where it is less concentrated. Again, moving heat from higher concentration to lower concentration – working against the natural movement.

When it is cold outside, a furnace will light a big fire, heat up the air, and blow it into the ducts throughout the house. It is hot in the furnace and the heat moves through the house where it is cold. With a split mini the same pumping action happens, but there is no fire or furnace. Rather heat is moved from outside to inside and then blown into the house. The heating action happens by compressing and condensing the refrigerant.

On the other hand, when it is hot outside, the split mini moves heat from inside the house and carries it outside, where it is released into the environment. In both of these cases, the refrigerant is compressed and condensed so that it carries the heat to the other side (outside for air conditioning, inside for heating) and then the refrigerant goes through the expansion valve into the evaporator where the heat is released. Everything hinges on the expansion valve.

Heating or Cooling?

The condenser and the evaporator are both coils. One coil is used to condense the refrigerant and encapsulate heat, the other coil is used to expand refrigerant and release heat. The refrigerant goes from one to the other through the expansion valve. The awesome thing about a split mini is that the expansion valve is actually reversible! That means the refrigerant goes in one direction to heat and then the other direction to cool! When it is summer the refrigerant goes from the coil inside to the expansion valve and releases heat in the coil outside the house. When it is winter the refrigerant goes in the other direction from the coil outside the house to the expansion valve and then to the coil inside where it releases the heat.

The reversible expansion valve (in tandem with the control board and the rest of the system) allows the refrigerant to change direction and change function!

Determining Split Mini Size

PRO TIP: The best way to get an accurate sizing estimate for your split mini is to contact a professional. There are many variables, and they can easily affect each other. That being said, we are going to now discuss many of the factors that go into determining the correct size.

Size is Important

If you buy a unit that is too small, it will be running constantly and it will never get the room (or zone) heated or cooled sufficiently. On the other hand, if you buy a unit that is too big, it will short cycle. This means it will turn on for a short period of time, get the room to the proper temperature, and then turn off. Although this might sound good, the unit is designed to be most efficient by running continuously. Short cycling will ruin the unit, waste a lot of energy, and generally require a lot more maintenance.

PRO TIP: Remember, with any mechanical system starting and stopping cause the most wear and tear on the parts, and usually are the most inefficient parts of the cycle.

Determining the Size of Your Mini Split

When we discuss moving air to heat or cool a room there are many variables that play into the equation.

Room Size

The size of the room is usually the most influential variable to consider. All else being equal, the bigger the room, the more air, and the more BTUs you will need to change the temperature.

The rule of thumb is to multiply the floor size (in square feet) by 25. For example, if your room is 40 feet by 50 feet your floor size is 2000 square feet, and you will need a 50,000 BTU unit (40 x 50 x 25 = 50,000). If your floor is only 20 x 30 you will need less power and our calculation comes to 15,000 BTU ( 20 x 30 x 25 = 15,000). This makes sense, again, the larger the room the more BTUs you need.

Ceiling Height

Our rule of thumb works well for a room with a standard eight-foot ceiling, but if you have higherceilings you will have to increase the BTU rating, and of course for lower ceilings you should decrease the BTU rating.

In newer houses in the US the ceiling is often nine feet on the ground floor and 8 feet on other floors. That being said, ceilings in general can range up to 11 or 12 feet. You may want to add about 10% onto your estimated BTU rating for each additional foot of height over eight feet. For example, if you are sizing your 40’ x 50’ foot room (2000 ft²) and you measure your ceiling and find it is 10’, then you should add 20% to your 50,000 BTU figure and install a 60,000 BTU system.

Other Considerations

Other factors that may play an integral part in acquiring a correctly sized unit are climate (or outdoors temperature), quality of insulation, sun exposure, and general usage of the room.

BTU Sizes based on Climate Zones

The US Department of Energy has created this map with climate zones according to temperature and humidity for your convenience. If you find the location you are interested in on the map below, determine which zone you are in, then you can easily estimate how many BTU you need for your system. Notice the zones are numbered 1 to 7, from warm to cool.

Now find a zone on the chart and get the number of BTUs needed for air conditioning or heating. The chart shows air conditioning and heating requirements per square foot

How many BTU’s do I need for a mini split AC

In the chart we give you total BTU requirements for air conditioning in a 1000 square foot room in each of the climate zones.

How Many BTU’s do I need for a mini split in heating mode?

In the chart we give you total BTU requirements for heating in a 1000 square foot room in each of the climate zones. Notice the zone numbers have been flipped.

House Insulation

Most of Alaska is in zone 7. Hawaii, Guam, Puerto Rico,and the Virgin Islands are all in zone 1.

If you are installing a mini split in order to keep your house temperature different from temperature outdoors, your insulation will play a major role. If your insulation is not maintaining the temperature you would like, you will have to increase the power to your heat pump.

To get a sense of how insulation affects your power requirements, imagine boiling a pot of water. Maybe your pot of water will take 10 minutes to reach boiling point. If you cover the pot, maybe you can drop the time to 8 minutes. That would be an example of better insulation.

If, on the other hand, you replaced about a quarter of the water with cold water every 5 or 6 minutes, the water would never boil. That is essentially the effect of low quality insulation. You may be replacing temperate air with air from outside.

There are a few ways to determine the quality of the insulation of your home. Just like the water in the pot, if your home never reaches the temperature you set on the thermostat that could very well be a function of poor insulation. Another sure sign is feeling air moving through your walls, or closed windows and doors. In older houses check also for air moving through the floors and ceiling.

Often the age of the insulation can indicate the problem. The older the insulation is the more likely it is damaged, missing, or just not as effective as it once was. Insulation deteriorates over time. It may be lost due to the elements or to animals.

PRO TIP: In general, if your home was built within the last 20 years and you know it was insulated with a house wrap or vapor barrier – then you can assume it is still in pretty good shape as regards the chart below.

PRO TIP: If your house is a little older than 20 years, check your windows and window frames. If you feel slight drafts or air movement around the apertures you can consider your insulation to be about average.

PRO TIP: And if your house is older than that, your windows and doors feature real drafts and you feel the air coming in then you may consider the house to be poorly insulated.

As you size your Split Mini, you can accommodate each of those conditions with an additional percentage of energy in your installation. These are all very general numbers. If you feel that your insulation is somewhere between “good” and “average” you may want to increase only 5%.

The chart may reflect a moderate climate.

Sun Exposure

Just as climate and temperature affect the amount of power one needs, so does the amount of sun exposure which affects the temperature. If your room is on the south side of the house and gets a lot of sun you may well feel the warmth of that sunshine. This means you will need fewer BTUs to heat and more BTUs to cool. On the other hand, if the room is on the north side of the house, and is well shaded from the sun getting no extra heat you will need fewer BTUs to cool and more BTUs to heat.

In order to use this exposure in your calculations, determine how much difference there is between this space and a more moderate space in the house. For example, if this space is generally cooler than a more moderate space of the same size in the summer by an average of 5 degrees with no difference in heating/cooling units, you may want to scale down the air conditioner capacity by a percentage.

If your moderate room is being heated by 30,000 BTU, and the temperature difference from the outside in winter is 50ºF, then 5º is 10% of 50º so you should raise the BTU rating by 10% and use 33,000 BTU heating capacity in the shady room. If the rooms are different sizes you should use the BTU per square foot measure.

Type Of Room

It is also important to consider the type of room you are heating and cooling. What activities happen there? How many people are there at the same time normally? What kind of machinery is running?

A human being’s temperature is generally 96.8°F. That means, if you have one person in the

room they are generating heat. If you have a room that is normally occupied by more people, like a theater, the room may need to be air conditioned even in the winter. If the people are expending a lot of energy, as in exercising or dancing, they may be generating more heat.

Kitchens are notorious for heat producing appliances. The oven, the stove, the back of the refrigerator – all these appliances produce heat. All this should contribute to your split mini size calculations.

Energy Efficiency

SEER stands for Seasonal Energy Efficiency Ratio.

HSPF stands for Heating Season Performance Factor.

HSPF is basically the SEER rating for winter heating energy performance/efficiency.

As we mentioned above this is the ratio of the cooling output of an air conditioner over a typical cooling season divided by the energy input it uses in Watt-Hours.

The more efficient the mini split, the more expensive it has to be. That means there will be a trade off somewhere – you will acquire the efficiency you need. This leaves us with the question how much efficiency do you need, and how do you think about this?

One way to approach the question is my looking at the climate you are living in. The more extreme the climate, the more important the efficiency is. If you are spending a lot of time and energy on your climate control you will want efficiency. On the other hand, if your climate is more moderate than the efficiency may not be as important to you.

On the other hand, many people will buy the most efficient system possible in order to engender sustainability.

It may be important to keep all these ideas in mind while speaking to a contractor about your mini-split. There is a lot of data that goes into a proper sizing decision and the technician may be willing to explain the choices they make, as long as you know what questions to ask.