Its extremely important that the sizing of your Heating and Air Conditioning Systems be done correctly.

Without a doubt, there is nothing more important than correctly sizing a new heating and cooling installation. It is easy to both oversize and undersize the installation. Correctly sizing is extremely important in regards to comfort, efficiency, and lowest operating and maintenace costs over the lifetime of a completely new system installation.

Systems normally run 12 to 15 year range before requiring replacement, but once they reach the 10 year point, they often become unreliable and less efficient than the new modern systems as improvements in technology is occurring on a daily basis.

Several national surveys have reported that more than half of the heating and cooling installations are not sized correctly by the HVAC contractors.

The most common sizing mistake is in oversizing. This error in judgement not only makes the new system cost more to install, but also causes it to operate inefficiently, fail more often, and costly to operate. IF the heating equipment is oversized, it often creates large temperature swings in the house. This can cause an uncomfortable atmosphere and wasted energy. When the  air conditioner (and heat pump) do not run long enough to properly dehumidify the air, this can result in the "clammy" feeling and mold growth which is unhealthy and is present in many air conditioned houses.

It is the installer/contractor's job to perform the correct sizing calculation for the building. Many installers only check the "nameplate" (the label on the unit that has the Btu per hour output among other things) of the existing system and sell you one just like it, or even worse, one that's larger. This is a not a correct sizing method and not in your best interests!  Rest assured, that a Smiley Heating and Cooling installation will be done in a professional manner and a thorough analysis of you needs will be accomplished before anything else is done.

Before homes became tightly constructed as they are now, it was not unusual to install air conditioners and furnaces that had two, three or even four times the required capacity. Additionally many people have installed new windows, caulking,insulation and weatherstripping to their homes. Due to all the possible improvements made by the homeowner, nulls the contractor's ability to use the nameplate to determine what size system is required.  If the name plate is used then it is likely to result in a system that is oversize. Improvements made to reduce heat loss in the winter and heat gain in the summer should permit you to install a smaller system. The more efficient smaller system while remaining comfortable, will allow saving large amounts of energy over a period of many years which can result in a substantial savings in dollars.

Many replacement installations are done simply by replacing the existing configuration with similar equipment without a  thorough evaluation to determine if the original package was optimum for the home. Due to many improvements in the efficiency rating of new systems, many times to replace the existing unit with one of the same size would result in a system larger than what is required. This is just as bad as undesizing a system. It is important that all relevant factors be considered in the new or replacement installation.

A huge number of factors must be considered when designing a new installation: some of the most important include:
Some of the most significant factors for correctly sizing a cooling and or heating system include:
The weather temperature extremes in you particular area.
The amount and type of insulation installed in the dwelling.
Number of windows, window sizes, their locaction and particular type.
Infiltration rate of the air throught the house
Number of occupants, their ages and needs such as asthema etc.
What the occupant actually prefers in regard to furnace or heat pump.
Number of occupants, their ages and needs such as asthema etc.
Ready availability of utilities such as using a heat pump when gas lines may be too expensive to install
Trees size and shape and orientation in regard to whether or not they block sun rays.
What type of lighting and what major appliances will be giving off heat.
In some cases you may be able to get a correct sizing calculation before signing a contract for the gas or electric utilites for little or no cost to you. This will give you some assurance that the HVAC contractor is doing right by you.

When the evaluation is in progress, pay close attention as a good estimator will measure ceilings, walls, floor space, and  and all windows to be able to determine the room volumes. Additonally there should be an assessment the R-value of the home's insulation, windows, and building, to also include the  materials used in the construction of the home. The rate of air leakage in the building is a valuable tool to determine if additional insulation and sealing is required for the best efficiency overall.

One of the very best methods of determining air leakage in a home is a test called blower door test.

If the contractor walks in, and simply looks at the existing unit, and recommends that the replacement unit be the same size,  or larger, then he obviously is not taking into account any changes or improvements that have been made to the house nor is he looking at the possibility of an original error in the sizing the original unit.

Occasionally the HVAC contractor may ask you how many square feet in the house. If he offers a unit size based on square feet, he is 'sizing by Square foot' and is one of the most commonly used methods which is very inaccurate. It is not unusual for a contractor to use a rule of thumb that says a one ton (12,000 btu) for each 500 square feet of area. Interestingly enough, the next contractor may be using a different rule of thumb and come up with a very different requirement.

Author and Heating and Cooling expert
Tom Chambers