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A mini-split’s size is determined by the BTU rating (square footage x 25) for the basement, but size isn’t the only factor influencing how strong the mini-split must be. The basement’s finished state, insulation, type of appliances, and difference between current and desired temperature must be considered.
I wish that I could just buy a mini-split, stick it in my basement and that all my temperature and humidity troubles would disappear. The thing is, in order for that to happen, there needs to be some precision.
Thankfully, while the terminology and phrasing around calculating the required size of a mini-split can seem complicated, the actual calculation is relatively straightforward.
Factors Influencing the Necessary Mini-Split Size
There are several influences on how big or small your mini-split should be. Some of these are more obvious than others, but I will walk you through what you need to be thinking about.
Before we discuss these factors, it will be helpful to know the following:
- BTU is the abbreviation for British Thermal Units and refers to the amount of heat that is needed to heat 1 lb of water by 1 ℉.
- A mini-split is both a heating and cooling unit. The strength of the unit (which is determined through the heating capability) influences its ability to heat, cool, and dehumidify a room.
Size of Basement/Basement Sub-Division
One of the primary considerations for what size mini-split to purchase depends on the size of your basement or the section of the basement that you want to condition. It also depends on whether it will be used for single or multi-zone conditioning.
You need a certain amount of BTU according to the amount of square footage (ft2) of your basement.
I will explain how to calculate your basement’s BTU per hour rating in a later section, but let’s look at what other factors play a part in sizing your mini-split.
Finished or Unfinished?
The state of your basement, finished or unfinished, makes a difference.
A finished basement has conditions similar to the upper living areas in terms of insulation values and the integrity of the thermal envelope.
An unfinished basement is going to be more difficult. An unfinished basement is likely to be considered outside of the thermal envelope as an unconditioned space. If certain conditions are met, your unfinished basement can even be exempt from the need for insulation.
A finished basement will need a mini-split with a lower BTU rating compared to an unfinished basement as a result of its better temperature regulation. In fact, the two are not really comparable as it is very difficult to control the temperature of an unconditioned or uninsulated space.
R-value refers to the thickness of insulation materials and, therefore, their efficiency at preventing heat transfer. Good insulation will result in a lower required BTU for the mini-split.
While insulation does not heat or cool the space directly, good insulation will make it easier to heat and cool a space. This is because insulation helps you to gain control of a room’s temperature through isolation from external influences. It prevents cold and heat from entering and escaping the house’s thermal envelope.
You need to ensure that the BTU rating of your mini-split can compensate for any BTU loss through the insulation.
The BTU loss rate for insulation can be calculated with the following formula:
T2 refers to outside temperature while T1 is inside temperature; you subtract to find the difference between the inside and outside temperature. Surface area refers to the square footage of the wall, and we use this to find out the rate of loss per 100 ft2 of a wall.
It is important to remember that the appliances in the room make a difference to the temperatures. Thus, they are a difference to the BTU rating that you need for your mini-split. An obvious example of such a room is the kitchen in which you find stoves, fridges, and pretty strong lighting, all of which give off a lot of heat.
Basements are another of those rooms that generally have strong lighting and often contain appliances such as furnaces and boilers, which produce a lot of heat (within reason, of course, so if your basement is unbearably hot, you may have a leak).
Now, while this means that you shouldn’t need as strong a mini-split for heating purposes, you still need to account for the cooling side. The cooling part of the mini-split needs to be able to stand up to the heat of the room, so you can’t really size down in a combination unit.
Difference Between Current and Desired Temperatures
If there is a greater disparity between what temperature you want the room to be compared to what it is, you will have to go for a bigger, more powerful mini-split.
A smaller unit may be able to compensate for a few degrees hotter or colder in the basement, but if you are looking at bigger changes, you will need something capable of combating the natural temperature of the basement.
Basements are already prone to colder temperatures than the rest of the house, which will also be impacted by the state of your basement. Whether it is insulated (and the R-value) or if you are looking to condition a space surrounded by unconditioned areas.
Calculating Required BTU
To work out the square footage of your space, you need to measure the length and width of the room or area you intend to use the mini-split for. Multiplying these values will give you the space in ft2.
Multiply the square footage by 25 to determine the BTU rating for the room.
(Length of room × width of room) × 25 = BTU rating
Mini-Split Sizing Guide
Ensure that this table, which should be a quick guide, reflects the BTU rating you need according to the BTU losses the room is subject to.
I advise that you consult a professional for accurate calculations to get the most appropriate mini-split and the best results!
The basic formula for calculating your BTU rating is found by: ft2× 25 = BTU.
|Size of basement (ft2)||BTU per hour|
If you see product information that talks about “tons” for the mini-split unit, don’t worry, that’s not how much it weighs! 12,000 BTU/h is considered a ton, so it refers to BTU rating.
In-Between: Oversize or Undersize?
What happens if your basement size requires a mini-split BTU rating that is in between the ratings available to you? Well, you have the option to oversize or undersize, each of which options comes with issues that you will have to consider and weigh.
If you go smaller, the unit will not condition your room sufficiently. It requires more time for cooling and heating to take effect, meaning it will run for longer periods, and you will have higher utility bills as it uses more energy.
An oversized mini-split will be able to drop the temperature in the room quickly, so that the dehumidifying process won’t keep up. That, along with the short cycling (frequently running for short periods), will result in humidity and condensation problems. You will also have to turn it off frequently to counteract the unit’s capacity that would overpower the room.
However, some mini-split models, like those from Mitsubishi, can operate at a low enough percentage of their capacity or have special short-cycle programs that can compensate for use in a smaller room. As the unit can moderate its power to suit the room, oversizing might be negligible. In which case, you could up-size to solve your BTU troubles.
Ultimately, if your BTU is not easy to accommodate, I recommend consulting an HVAC specialist. A professional will have the knowledge and experience to help you problem-solve according to your circumstances and local climate conditions.
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