Sizing a heat pump seems like a daunting task. How do I calculate what size heat pump do I need? How many BTU heat pump do I need? This is usually an estimate that you need an HVAC expert for.
We are going to simplify how to calculate what size heat pump do you need. That will enable everybody to roughly estimate the size of the heat pump (be it a mini-split heat pump or ground-source heat pump). We will do that in 3 key steps:
- First, we’ll look at how HVAC experts size heat pumps (using 8 factors from Manual J; the method was established by Air Conditioning Contractors of America).
- Then we’ll simplify these 8 factors with a useful rule of thumb (boiling down 8 complex rules into 1 simple general rule of thumb so that everybody can roughly estimate how big a heat pump should they get). Based on that, we designed the Heat Pump Size Calculator (check further below; you just input square footage and ceiling height, and it will give you roughly BTU you need). We also included a table of how big your mini-split heat pump should be given certain square footage.
- To demonstrate how Heat Pump Size Calculator works, we’ll solve 2 examples; ie. calculating what size heat pumps do you need for a 2,500 square foot house.
In the end, you should be able to roughly estimate (and calculate) how big a mini-split heat pump or how big a ground-source heat pump you need for your house.
Note: Do keep in mind that these are only rough estimations that can help you get a feeling of what size heat pump you require. An HVAC expert is required on-site to map out your home, determine heating/cooling needs, and so on for a specific situation you have.
Let’s look at how HVAC experts size a heat pump:
How HVAC Experts Calculate Heat Pump Size (Using Manual J)
Every HVAC expert calculates what size heat pump you need using the same set of principles. When you need heat pump sizing, they all know to look at the Manual J (the alpha and omega book of HVAC sizing) and follow the 8 rules.
These 8 rules include all the main and secondary factors when it comes to sizing any heat pump. They work both for sizing air-source heat pumps (these are mini-split heat pumps) and for sizing ground-source heat pumps.
Here are the 8 rules or factors from Manual J that HVAC experts should follow when sizing a heat pump:
- Determine the local climate (+ how many days a year you need heating/cooling). Obviously, if you live in Chicago, you will need a bigger heat pump than if you live in Miami, Florida. In general, colder climates require higher heating output (measured in BTUs or kW).
- Total square footage; one of the most important determining factors for sizing a heat pump. Consider the room distribution and general home’s layout as well.
- Windows; how many they are, where are they located?
- Occurrences of air infiltration; where it is and air infiltration quantification.
- Insulation quality; how well insulated is the house, does it meet the region’s efficiency rating?
- People; how many people live in the house?
- Temperature preferences; what is the ideal temperature of the home for homeowners?
- Heat generating appliances; which appliances generate additional heat (oven, fridge, washing machine, etc.)? Sum all of them up and quantify the overall effect on the indoor temperature.
Now, it’s rather complex to determine the effect of all of these factors. These 8 rules were meticulously put together by Air Conditioning Contractors of America and it’s a standard part of Manual J.
Don’t be taken aback by the complexity of everything you need to check to size a heat pump; even HVAC experts who have been in the field for 10+ years use certain simplifications.
Let’s boil these 8 factors down into 1 simple rule of thumb:
How To Approach Heat Pump Sizing On Your Own? (1 Rule Of Thumb)
Some of the factors in Manual J cancel each other out. Example: You can have a badly insulated home but don’t have many windows, and have several 1,000W+ home appliances (oven, washing machine, etc.).
The key determinant of how big a heat pump you need is how big your house is. The bigger the house, the bigger the heat pump you need, right?
Considering all the factors, we can roughly boil them down into a single rule of thumb. This rule of thumb comes very useful when you want to adequately determine the size of the heat pump you need.
Here’s the 1 rule of thumb:
30 BTU of heating output per 1 sq ft of living space.
This heat pump sizing rule is fairly simple to use. It roughly incorporates the averages from the rules found in Manual J. This rule is akin to the EPA’s rule of thumb for sizing air conditioners and it also relates to the heating BTU calculator.
For every sq ft of living space, you need about 30 BTU of heating output. That means, for example, that for a 1,000 sq ft home, you would require a 30,000 BTU heat pump (that’s a 2.5-ton heat pump).
We can use this simple rule to create the Heat Pump Size Calculator:
Heat Pump Size Calculator (Just Input Square Footage)
Another key parameter is the ceiling height. 30 BTU per sq ft rule of thumb is for standard 8 ft high ceiling. If you have higher ceilings, you will need a more powerful heat pump, and vice versa. Here is the calculator:
Using this calculator, everybody can roughly estimate what size heat pump they need. This is primarily a mini-split heat pump size calculator but can be used to roughly estimate the size of ground-source or even water-source heat pumps.
The heat pump size calculator outputs the resulting size of a heat pump in BTUs (British Thermal Units). You can simply convert that into:
- Tons (in the US, the capacity of heat pumps is usually measured in tons). Use 12,000 BTU = 1 ton conversion, or make use of the BTU to tons converter here.
- kilowatts or kW (in Europe, Asia, and rest of the world, the capacity of heat pumps is usually expressed in kW). Use 3412 BTU = 1 kW conversion, or make use of the BTU to kW converter here.
Note – how to make the estimate even more precise: If you live in the cold north (Canada, Illinois, Minnesota), it makes sense to add up to 40% to the total output heat pump capacity calculated by the Heat Pump Size Calculator. If you live in the hot south (Florida, Texas, South California), you can reduce the total output heat pump capacity BTUs by as much as 30%.
Using the calculator, we can create a table that specifies how big a heat pump you need depending on how big your house is (ie. square footage):
Heat Pump Sizing Chart By Square Footage
|Home Size:||Heat Pump Size (In BTUs):||Heat Pump Size (In Tons):|
|300 sq ft||9,000 BTU||0.75 tons|
|500 sq ft||15,000 BTU||1.25 tons|
|750 sq ft||22,500 BTU||1.88 tons|
|1,000 sq ft||30,000 BTU||2.5 tons|
|1,500 sq ft||45,000 BTU||3.75 tons|
|2,000 sq ft||60,000 BTU||5.0 tons|
|2,500 sq ft||75,000 BTU||6.25 tons|
|3,000 sq ft||90,000 BTU||7.5 tons|
You can see for the heat pump size table that, for example, a 2,000 sq ft house requires around 60,000 BTU or a 5-ton heat pump.
Let’s solve two examples to illustrate how you can calculate what size heat pump you need manually and via the calculator:
What Size Heat Pump Do I Need For A 2,500 Square Foot House? (Example 1)
Let’s say you have a bigger 2,500 sq ft home and you want to buy a heat pump for it. How do you adequately pinpoint how big a heat pump you need?
Well, you should call an HVAC expert, and he or she will use the 8 factors in Manual J to calculate the required size of the heat pump. For you to get a feeling of how many BTU heat pump you should be considering, you can use the simple 30 BTU per 1 sq ft rule to estimate the size of the heat pump for a 2,500 sq ft home.
Let’s do some manual calculating:
For 1 sq ft, you would need 30 BTU of heating/cooling output.
How many ton heat pump (or BTUs) you need for 2,500 sq ft?
Heat Pump Size (2,500 sq ft) = 2,500 sq ft * 30 BTU per sq ft = 75,000 BTU
You would need about 75,000 BTUs. If you convert that into tons, that’s a 6.25-ton heat pump. If you convert that into kW, that’s a 22 kW heat pump.
In short, for a 2,500 sq ft home, you would require a 6.25-ton heat pump.
Let’s see if the Heat Pump Size Calculator also gives us the 75,000 BTU result for a 2,500 sq ft home:
The calculator confirms that our manual calculation was correct.
What Size Heat Pump Do I Need For A 1,500 Square Foot House? (Example 2)
In this example, we have a 1,500 sq ft home and would like to buy a mini-split heat pump for it. Obviously, the first question is what size mini-split heat pump do you need? Once you determine that, you can check out our article about the best mini-split heat pumps currently on the market here.
An HVAC expert on-site will determine the size of the mini-split heat pump exactly but we can estimate what the result will be by applying the 30 BTU per sq ft rule of thumb and manually calculating what size heat pump you need for a 1,500 sq ft home.
Here’s the calculation:
Mini-Split Heat Pump Size (1,500 sq ft) = 1,500 sq ft * 30 BTU per sq ft = 45,000 BTU
For a 1,500 sq ft home, you would need about 45,000 BTU heat pump. Let’s convert that to tons and kWs; that’s 3.75 tons (about 4 tons) and about 13 kW.
In short, you would need about a 4-ton mini-split heat pump for a 1,500 sq ft home.
We can confirm this manual calculation using the mini-split heat pump sizing calculator:
As you can see, the heat pump BTU calculator produces the same result: 45,000 BTU.
Always remember that for adequate sizing, you would need an HVAC expert that does the on-site sizing. The heat sizing here can serve as a rough reference.
If you are in the market for a heat pump, you can contact vetted HVAC experts in your area using this form here. You will get up to 4 free quotes on heat pumps and they will help you out with proper heat pump sizing as well.
Hope all of this helps a bit.
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