HVAC Sizing Guide: What Size HVAC Unit do I Need?

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HVAC Sizing Guide What Size HVAC Unit do I Need -3

How to Size HVAC System 

Properly sizing HVAC systems is vital for both homes and business alike. So how do we determine the size that’s right for our space? We put together an HVAC sizing guide to help homeowners understand a little more about proper HVAC sizing.

How many BTU per square foot?

It starts with the square footage of your home or business as well as understanding the btu demands of that space. BTU stands for British thermal units and are used as a way to measure an HVAC system’s cooling and heating ability. In most cases, a home or busines will need 25-30 BTU for every square foot of space. Keep in mind that this equation is just a rough estimate as there are many factors that affect the amount of BTUs needed.

Although, HVAC units are sold by their cooling capacity in “tons” not “BTUs”, it doesn’t affect much. Selling HVAC units in tons allows HVAC contractors to find the right size depending on the BTU range a unit can handle. Generally, 1 ton AC unit can remove 12,000 BTUs of heat per hour. 

What Size AC Unit do I Need? 

This is a difficult question to answer as there’s no definitive groupings to categorize our homes and buildings into. Each space has it’s own unique heating and cooling needs. So to find the proper AC unit size, experts recommend an inspection instead of replacing with the same size tonnage. This allows the homeowner and HVAC contractor to determine if the current unit is under, over, or perfectly sized. This inspection, also known as a Manual J Calculation, goes beyond simply estimating the AC tonnage from square footage BTUs.

A Manual J Calculation is one of the most common ways to evaluate your home’s characteristics and help determine the size of AC unit you need. Other than square footage, factors which impact the cooling and heating ability of an HVAC system include:

    • Climate Zone (Four States area is located in Climate Zones 2 and 3)
    • Exterior Materials (Brick, Siding, Ect)
    • Amount of Sun Exposure
    • Insulation
    • Age, Amount and Size of Windows/Doors
    • Ceiling Height

For Cooling:

    • Type of AC unit (Window, Central, Mini-split)

For Heating:

    • Type of Heating Unit (Furnace, Heat pump)

What Size Heat Pump do I Need? What Size Furnace do I Need? 

Similar to the previous question, homeowners replacing heat pumps or furnace heaters will also greatly benefit from a Manual J Calculation. While BTUs are often used to describe how much heat a unit can transfer, Heat pumps and furnace heaters also use BTUs. Heat pump BTUs describe the amount of heat absorption and transfer while furnace BTUs refer to heat production. In other words, the capacity and energy needed to efficiently heat your home or business.

Better to Oversize or Undersize AC? 

Properly sized heating and cooling systems make a significant difference. Problems of improperly sized HVAC systems include:


    • Run Constantly
    • Higher Energy Bills from Running Constantly
    • Need Frequent Repairs
    • Higher Risk of System Failure
    • Shorter Overall Lifespan


    • Short-cycles (stresses the system and increases risk of frequent repairs)
    • Higher Energy Bills from Continously Turning Off/On
    • Higher Humidity Levels
    • Hot and Cold Spots Throughout the House

Furthermore, there are additional problems that can occur such as pipes freezing and bursting with inadequate heat sources. 

HVAC Sizing Rule of Thumb 

However, there are unique situtiations when you need to buy an a system that can handle more than your home requires. Nonetheless, the recommended maximum for cooling systems is only 15% more than the BTUs needed. The max for furnaces is 40%, and Heat pumps, which handles both heating and cooling, is 25%. This allows the system to be prepared for days with extreme temperatures without being overpowered.

Commercial and Residential HVAC Services with Barnard Heating and Cooling

After a Manual J Calculation and discovering how which tonnage will handle your BTU needs, the shopping can begin. This will allow you to compare similar systems across multiple brands to find the best price. There are many different factors in determining the HVAC size you need. This is where a reputable, local HVAC professional can help guide you through the process. In addition to providing maintenance tips and suggestions for prolonging the lifespan of your HVAC system.

Need more Advice? Reach out to one of our local HVAC experts today! At Barnard, we work with our clients to help them understand the various factors that affect cooling and heating their home. Let us guide you through finding the proper HVAC size for your home’s needs so you can feel confident in your system.

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