How To Calculate The Air Conditioner Size For Your House

So it’s time to either install air conditioning in your house for the first time, or to replace your old run down unit. But what size do you need? Well I’m not going to lie and tell you the answer is easy like so many other sites will.

In fact, there are so many variables involved in choosing the correct size central air conditioner that you would be mad to buy one without talking to a professional first. But what we can do is arm you with as much information as possible, and this will help you make sure that:

  • You don’t get ripped off
  • You can go and speak to a professional with at least some knowledge of what you need
  • And most importantly, even on the hottest of summer days, you will stay comfortable and cool

At the end of this article, you will find a handy calculator that will give you quick calculation of the size of the AC Unit you might require, but I can’t stress enough that you really should get an HVAC contractor out to calculate it properly.

Anyway, let’s get on with the actual useful info!

How To Calculate The Air Conditioner Size For Your House


Central Air Conditioner Size Calculator

Use the below calculator to give you an indication as to the size of the air conditioning unit you might need.

Read on to learn more about picking the right model.

Central Air Conditioner Size Guide

As I have already mentioned, there are many different factors that a pro will take into account when calculating your ideal air conditioner size.

Just some of these factors are:

  • Size of the house
  • Number of windows
  • Type of insulation
  • Height of ceiling
  • Heat transfer of walls
  • Average/max summer temperatures
  • Color and type of roof
  • Is there a basement or slab?
  • + much more

See what I mean? There are so many variables that it would be madness (and probably a waste of money) to try to calculate it yourself.

But you can at least know what you are talking about when the professionals come knocking.

Let’s take a look at what you should know.

Important HVAC Terms To Be Aware Of

So here are some of the terms and phrases that you might hear thrown about. Keep in mind that you don’t need to 100% understand all this stuff, just having a basic grasp of what each term means will ensure you aren’t bamboozled by jargon.

We have put together a quick infographic below to help. Read on for a more detailed explanation.

air con jargon busterManual J or J Load Calculation

Let’s start with the heavy stuff hey! The Manual J or J Load Calculation is a term that any air conditioning professional that you speak to may or may not mention to you – but you can guarantee they will be thinking about it as they walk around your house.

The Manual J calculation is used to determine the heating and cooling loads of a particular house or building. It takes into account many different factors, some of which we mentioned previously in this article.

While it is possible to calculate this manually, it is quite a time-consuming task. Luckily, HVAC contractors have access to many different software solutions to calculate this for them in a fraction of the time that it would take to do it by hand. Your HVAC specialist might contact you prior to your quotation appointment to go over some of the questions they need to ask you to plug into their software in order to save time when they are actually at your house.

It is not really necessary to go into this in more detail here, as this really is a calculation that is primarily used by professional contractors.

Air Conditioner Tonnage

Unlike portable units, central air conditioning units are measured in “tonnage”. So that is how much the weigh right? Nope…..wrong!

Confused yet? Im sure they do this just to confuse people!

When we talk about air conditioning, a “ton” is the measurement of a units ability to cool. If you have a one ton AC unit, it means it has the ability to cool 12,000 BTUs an hour.

Residential central air conditioners come in a range of sizes from 1.5 ton to 5 ton models. If you need a unit that has more than 5 ton cooling capacity, then it is likely you will need multiple AC units.

Hang on….. BTUs? I guess we should discuss that too!

BTUs (British Thermal Units)

British Thermal Units is a measurement of heat. 1 BTU is the amount of heat required to increase the temperature of 1 pound of water at sea level by 1°F.

That’s nice isn’t it…but what does that have to do with air conditioners?

Well, when we talk about BTUs in relation to AC it refers to the amount of heat that the unit can remove from the air per hour. So a model with a higher BTU rating has more cooling power. When we talk about air conditioner tonnage, a one ton unit is equal to 12,000 BTUs

So you should just get the model with the highest BTU rating right? Nope – incorrect!  We will go into it in more detail shortly, but it is vital that you do not purchase an AC unit that is too big.

Bigger Isn’t Necessarily Better

While you may think that the more powerful the AC unit, the better – this is certainly not the case.

There are numerous reasons why installing an air conditioner that is too powerful is a bad idea:

  • The upfront costs to purchase the unit are higher
  • Larger units use more electricity (even when working less)
  • The number of times that the unit will turn on and off is much higher, causing wear and tear and higher maintenance costs
  • House remains humid because the unit does not run long enough to remove the moisture from the air

Consult A Professional

hvac contractor

I’ve lost count of how many times I have said it, but I will say it again – get a professional HVAC contractor in to calculate the correct size unit. And obviously to install it too.

You might even find that you’re existing unit has a problem that is simple for a contractor to diagnose and repair.

If you find that your air conditioning system has low airflow, there are some simple things you can check yourself that might solve the issue. You can read our guide on fixing low air flow here.

Don’t waste your time on a DIY installation, it will just end up costing you more in the long run.

Before Replacing - Get a Pro To Tune-Up Your AC
  • Service provided by a local pro
  • Inspection of all parts and levels
  • Condenser coil cleaning
  • Lubrication of all moving parts
  • Final report



Pricing last updated on 2018-11-18 at 03:04 / affiliate links - Details

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