Dave’s Take | Detriments of Under and Over-Sizing Your HVAC System

In Dave's Take by Allan WLeave a Comment

If you’re planning on buying, replacing or adding any piece of HVAC equipment to your home, it is essential to keep in mind that the size/output of a home’s HVAC appliance is vital to it’s effectiveness. A system that’s too large will continually cycle on and off wasting energy and in some cases require larger ductwork wasting space, money and potential energy savings. A duct system that is set for an original 3 ton furnace which then is replaced by a 5 ton furnace with no ductwork modification can cause real problems with your system. A furnace that is trying to move 5 tons of air through ductwork that can only support 3 tons causes a back up potentially freezing your coils and overheating and eventually cracking your heat exchanger. An undersized system will fail to warm or cool your home optimally (or ‘area’ in some cases) when it’s really needed. As a general rule, it’s always better to go a little smaller than a little bigger than necessary.

Let’s look at furnaces as an example. Common furnace sizes are 40,000-, 50,000-, 60,000-, 80,000-, and 100,000-BTU input capacities. When deciding which size to select for your home remember, bigger isn’t always better. Your best option in the quest to find the perfect furnace fit for your home is by asking a qualified central air conditioning contractor or dealer. A qualified heating contractor should use the Air Conditioning Contractors of America Manual J to properly size a unit; if yours doesn’t, it might be wise to select another contractor or call us at  (800) 880-0199 . You cannot size a unit based solely on your home’s square footage. Nor can you just replace an old furnace with a new high-efficiency one that’s the same size unless you match output capacities; high-efficiency furnaces often have smaller input capacities so be sure to keep these variables in mind.

Sizing your HVAC system correctly

Whether heating or cooling and whether it is a boiler, furnace, A/C, or mini split, it is essential that your system is sized properly and in accordance with your needs.

It is essential that your contractor knows this as well. Essential not only that he knows about the importance of sizing your system, but also that he knows that there are different methods of sizing each system. Again, if you’d like to go it alone you are more than welcome to call us at  (800) 880-0199 and we would be happy to walk you through the process.

Determining Your Heating Needs *Rule of Thumb Model


Red- Zone 1

30-35 BTU’s per square foot

Orange- Zone 2

35-40 BTU’s per square foot

Yellow- Zone 3

40-45 BTU’s per square foot

Green- Zone 4

45-50 BTU’s per square foot

Blue- Zone 5

50-60 BTU’s per square foot


Always choose the higher of the 2 numbers if your home isn’t insulated properly OR if you are not sure.

Now that you know which zone you are in multiply the appropriate factor above by your home’s total heated square footage to arrive at your approximate required heating capacity. As an example, if you are in the Green Zone and are figuring in your BTU needs at 50, you multiply this by your home’s square footage. If your home is 2,000 sq/ft than your furnace should be at least a 100,000 BTU furnace. NOTE: Any space below grade (I.E Basement) is not considered conditioned space and should not necessarily be counted as square footage when factoring your conditioned air needs. Variables such as these are why it’s a terrific idea to give experts like us a call at  (800) 880-0199.

2000 square feet

X 50 heating factor being in the green zone

100,000 BTU required to heat your home

When you’re looking online or in-person at furnaces you’ll undoubtedly notice the advertised BTU rating. HOWEVER, there is an important consideration to take note of, you need to determine the actual BTU output. For example: a 100,000 BTU furnace with an 80% AFUE will actually output 80,000 BTU’s of heat.

100,000 BTU input

X .80 efficiency

80,000 BTU real output

Measuring for a boiler

Gas boilers use natural gas or propane to heat water to its boiling point. The hot water is then transferred through pipes in the home, feeding individual radiators in each room. Boilers offer an alternative to traditional forced-air furnaces, particularly in areas of the country where the air is dry. When shopping for a gas boiler, you want to find one that fits the needs of your home. Buying a boiler that is too small will not provide enough heat and will end up constantly running. A rule of thumb for boiler calculations is to take your square footage and multiply it by 50. The rule is that you should at least have 50 BTU of output per square foot. This will vary and ultimately increase depending on multiple factors just like in a furnaces case, these factors being location in the house, your house’s location in the country, your level of insulation, the age of your windows and so on.

Insulation Variables

Variables such as your insulation, type and number of windows, number of stories, construction type, etc. will greatly affect the required BTU’s per square for both heating and cooling. A general rule of thumb is that if your home is well insulated with newer style windows, you can select the smaller size system that falls within your total square footage.

If your home is two story it will place less of a load on the system in the downstairs area as the second floor acts as additional insulation. If your home is not well insulated, has older style windows, and/or a larger than average number of windows, you will want to select the larger system which falls within your square footage range. The less insulated and more windows within the environment, the more likely you will experience greater air and heat loss.

Decisions Before You Buy

As you can tell, there are a number of variables and an almost dizzying amount of factors that come to play when sizing the right piece of equipment for your home. Luckily, you have us at Younits to answer any questions you may have and we are always happy to help you make the best, informed choice when it comes to your HVAC needs.



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