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Choosing the Right Size HVAC System for Your House

Posted on October 8, 2019

Choosing the right HVAC size for your house.Whether you are building a new house or just replacing old climate control equipment, one of the most important — but often overlooked — factors in selecting heating and air conditioning components is the size of the system in relation to the size of the house. To ensure optimum operational efficiency with minimum energy usage, it is crucial to match the structure’s heating load with HVAC units of the appropriate heating capacity.

What is heating load?

Heating load is simply the quantity of heating or cooling required by any given building in order to maintain a comfortable climate inside. This figure is calculated using three factors: design load, extreme load and part load.

Design load simply refers to the heating/cooling needs of the building based on static data, such as its physical size, the amount and type of insulation used in its construction, building materials and orientation to the sun. These things are unchanging and help set a baseline for the amount of climate control needed for the space. Extreme load refers to the amount of heating and cooling necessary to maintain comfort in the most adverse extremes of hot and cold weather. And part load refers to the level of climate control needed to keep the temperature steady with minimal energy expense, once that temperature has been reached.

How to calculate heat capacityWhat is heating capacity?

Heating capacity describes the quantity of heating or cooling a specific HVAC component — whether an air conditioner, furnace or heat pump — is capable of providing. This number is more straightforward than heating load, but that seeming simplicity may lead to errors when choosing equipment for an existing or new house or commercial building. Too often homeowners (and even a surprising number of contractors) select equipment rated to meet only the design load needs of a building, without considering how the system will function at part load or, more crucially, under sustained periods of extreme load.

How do I find my building’s heating load?

Your overall load factor is affected by many variables, not mere cubic footage. How much of your house is occupied? How densely furnished is it? Is your basement and/or attic finished? Do nearby trees provide shade in warm weather? To make sense of all this data, it’s a good idea to contact your local HVAC professionals and schedule an appointment. A technician will come out to your property and make note of a wide variety of data, which they can then use to tabulate your heating and cooling needs, from the lowest to highest demand. This is as much an art as a science, as your heating load may vary by as much as 25% just from being rotated 90 degrees in its own footprint!

Isn’t it always better to just buy a bigger/more powerful system?

On the surface, it is easy to imagine that the old adage “too much ought to be enough” might apply to your HVAC needs. But counterintuitively, a system that is too big for your house, office or shop may actually end up making you less comfortable than one more closely tailored to your building.

One problem with oversized air conditioners is that they cool small spaces very quickly — which does not afford them the chance to dehumidify the air. To remove the moisture from your indoor environment, your AC must run longer cycles, and an overly potent system will cool the house so rapidly it never gets the opportunity. You may find your humidity remains at 60% or higher and never goes down; this means you are still sweaty even though the thermostat shows the room is in fact at the desired temperature.

On the other hand, a too-big furnace blasts extremely hot air into the room, triggering the thermostat to shut off quickly — resulting in hot spots and cold spots all over your house as the furnace kicks on and off over and over again all day. Additionally, sitting or sleeping near a vent means being roasted every time the furnace starts a new warming cycle!

A properly sized HVAC system will operate on a “slow but steady” basis, bringing the temperature and humidity gradually to the desired levels, then maintaining that climate through efficient operation at part load.

Can’t I just replace the old one with a new unit of the same capacity?

It’s not unreasonable to expect that an older air conditioner or furnace can be swapped for a new one rated for the same output. But unless you installed the previous units, you don’t know that those were actually properly suited to the space then. Not to mention the fact that any of the variables listed above — occupancy, furnishings, growth of nearby trees, etc. — may have changed since then. It’s always best to call your HVAC experts in order to make sure you know your actual current heating load.

A little time spent in advance dialing in the right equipment for the needs of the space may save you money and discomfort both now and in the long run, preventing the purchase of more expensive overpowered components and instead installing efficient units that will save energy on an ongoing basis for years down the road.

The Importance of Correctly Sizing HVAC Systems – The ACHR News

SIZE IT: Accurately calculating heating and cooling loads is an essential element of properly sizing an HVAC system.

To do this, an HVAC contractor must perform a heat load calculation, which considers changes in the home — additions, a finished basement, adding insulation to the attic, etc. New furnaces should be sized based on the unit’s output capacity, which is greater for high-efficiency furnaces than for lower-efficiency units.

A growing number of homeowners across the U.S. are choosing propane, as it delivers energy savings and comfort at an affordable cost.

High-efficiency propane furnaces offer ratings from 90 to 98 percent and can provide space heating for both small floor plans and large custom homes — making them a valuable addition to a variety of projects.

Every house is different, so it’s important that heating and cooling systems are designed for the correct load. Even the same house, rotated 90 degrees, could vary in cooling load by 25 percent or more. HVAC calculations require time and attention to detail, but it will make a world of difference in a home’s performance and a customer’s satisfaction.

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