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A Guide for Residential HVAC Unit Sizing

Sizing an HVAC unit basically means deciding on the size of the system that is required to cool your home properly. Residential HVAC unit sizing is extremely important because a unit that is too big will cost you more than what you should have incurred whereas an air conditioning unit that is too small will not cool your house properly. Learning about proper AC sizing is important because it helps you know if you are buying the right conditioner for your home.

Most HVAC systems are meant to cool and supply heat as well and as a result, you must size for both cooling and heating. First of all, you will need to figure out the amount of cooling you will need and after that, you can figure out the heat. Some systems will require a lot of cooling and not so much heat whereas others will need a lot of heat and very little cooling.

The ideal way to size your HVAC unit is by having a “Manual J” calculation conducted on your house. This is usually the gold standard for HVAC unit sizing and it takes into account aspects such as the amount of insulation in your house, the kind of windows and the direction they are facing and many other factors. A majority of utility companies will perform this calculation for free, and if not, you can seek the services of an energy auditor. An energy auditor will perform all the necessary calculations and inform you of the exact size you need for your home so that you can shop accordingly.

It is also important to ensure that the efficiency of all your ducts is tested because installing accurately-sized HVAC units will not be of any good if you will end up gaining or losing a lot of BTUs through your inefficient ductwork. The average performance of ducts is roughly 57% and if your ducts are a problem, you must ensure they are services before installing your new HVAC unit. However, if your ducts are running through a conditioned space, then the duct losses should not be a big problem because you will “lose” the air into the part of your house that you are actually trying to condition.

Why Is Residential HVAC Unit Sizing Essential?

When it comes to heating, ventilation and air conditioning, the size of the unit matters. The common notion with many homeowners is that the bigger the unit, the better it is which is totally wrong. The reason why many people still think that bigger is better is because the idea was based on old central ACs from the 80s which were huge and used a lot of energy to cool a small room. Not forgetting, electricity was cheap back then and insulation was non-existent and if it did, it was at its minimal best.

If The HVAC Unit Is Too Big – If you buy an HVAC unit that is too big for your home, it should not be a surprise to realize that it runs more like an HVAC unit from the 90’s – turning on quickly, cooling your house rapidly and eventually shutting down. Your house shall still be cool and nice to live in but since it hasn’t undergone the whole cycle it was designed for, heat will quickly infiltrate and you will have to turn on your system. This constant on and off-cycle will defeat the purpose of its energy-efficient design and skyrocket your monthly electric bills.

If The HVAC Unit Is Too Small – If your HVAC unit is sized too small, the chances are that it will run constantly and eventually spend most of the time trying to make up for where it should be. Once again, your house shall be cool and nice to be in but the problem is that your small air conditioner will have a hard time cooling the entire house because it is too small and has to work slowly. The result is that this system will run constantly and end up running up your electric bill.

Mistakes Made During Residential HVAC Unit Sizing

Many homeowners are often faced with several misunderstandings. The first misunderstanding is that they are not aware of the fact that gas furnaces are normally rated by their input and not their output. As a result, a “50,000 British Thermal Units” gas furnace with an efficiency of 80% will only supply 40,000 British Thermal Units to your house. If you have figured that you will need a 50k BTU of heating and you go ahead and purchase a 50k BTU furnace, you will end up with only 40k BTU of heat which is definitely insufficient for your home. It is therefore important that you calculate the output when buying a gas furnace.

Another grave mistake when sizing HVAC units is ignoring efficiency when replacing your old unit. For instance, you might want to replace your 60k BTU, 85% efficient gas furnace but an inexperienced salesman sells you a new 60k BTU unit that is 92% efficient at a higher cost with the promise of better air conditioning capabilities. The truth is that things shall only turn sour for you so it is important that you replace your unit with another with similar specifications.

With HVAC units, sizing has absolutely nothing to do with the physical dimensions but rather the unit’s ability to produce cooled air. Different HVAC systems are manufactured with different capabilities so it is important to understand how the sizing is measured.


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