Sizing Systems

Replacing or installing a new heating and air conditioning system first requires accurate calculation of the cooling and heating requirements of your house. Accurate heating and cooling requirements are necessary to select the correct size system. Energy efficiency, comfort and longevity can never be achieved without proper sizing. Correct sizing holds more importance than the system you chose or how it is installed. The biggest problem facing the entire HVAC industry is oversized heating and air conditioning systems. Many professionals not performing the proper calculations make homeowners believe calculated accurate sizing to be unimportant. Many homeowners believe the biggest system gives them the best protection. Driven out of fear of a heating or air conditioning system that can't provide sufficient capacity causes most homeowners to chose larger than necessary equipment. Yet every reliable source on the internet from the Department of Energy to manufacturers' organizations, engineering and contractor organizations will tell you the biggest problem in the HVAC (Heating Ventilation and Air Conditioning) Industry is grossly oversized systems.  Given the choice between two competing contractors a homeowner will always favor the contractor proposing the biggest system.

Oversizing of heating equipment started at the beginning of heating systems. Large behemoth heating systems were manufactured and installed with large heat exchangers made of masses of metal. In industry, schools, hospitals and homes there were large massive heating systems. Some of us as children can remember the huge octopus in our basements or gigantic hot metal boxes called boilers. They consumed whole basements or large sections of the house. Firing up these giant heating machines took a lot of energy. Extra heat radiated off these Energy Hogs was enough to turn everybody's basement into a sauna. The energy lost through the chimney was sometimes sufficient to heat the house. There was little engineering involved with sizing or capacity and no regard to efficiency. Once fired in the morning the residual heat would usually provide enough capacity to maintain the temperature in the house for many hours or the remainder of the day. Continual operation of these systems would result in overheating of the space and even more wasted energy. They were by every definition Energy Hogs. All heating systems were oversized and continual operation meant excessive energy bills and overheating. So began the concept bigger is better. Short operating cycles were defined as energy conservation and efficiency. 

Bigger is not Always Better

Heating system designs are very different today. The true concept of energy efficiency is smaller is better with long operating cycles. Longer continual operation of an ultra efficient furnace produces more stable temperatures and almost perfect comfort. Furnaces and boilers are also microscopic in size in comparison to their mammoth ancestors of yesterday. Weighing in at 70 pounds for a wall hung 98% efficient boiler to 160 pounds for an ultra 95% efficient condensing gas furnace. Compared to yesteryear's behemoths 50% efficient boilers and 60% efficient gas furnaces weighing 1,500 to 2,000 pounds. Gone are the days of those large massive metal  furnaces. But what has not left is the logic that accompanied the dinosaur and dragonian masses of metal. It's easy to understand where the logic began believing bigger is better and shorter run cycles use less energy. The same logic accompanies turning up a thermostat 15 degrees instead of 3 or 4 degrees will heat the space quicker with hotter air. It is also the same logic to want the biggest vehicle with the ever growing appetite for larger SUVs. That logic continued until skyrocketing energy costs seriously affected our wallets enough to tell us our appetites are not conducive to energy conservation. Let's take a closer look at this thinking. The quicker we want to heat our homes the larger capacity the system is required. And the heat produced is hotter. It's a matter of speed rather than sufficiency. It's the same thinking that goes with an SUV. We have more horsepower and a larger vehicle. The argument is the larger vehicle is safer and the extra horsepower engine will get us there faster. A smaller mid size car can get us there just as fast and cost a lot less to purchase and use a lot less gas. If the situation was a race we would want the larger SUV engine in the smaller mid size car. The SUV and the mid size car can both take us from point A to point B just as fast with the only difference being the amount of energy consumed. Driving within the speed limits will increase efficiency and use less fuel. Heating our homes isn't a race and getting to a required temperature needs to be accomplished with the least amount of energy. Like the SUV if we don't really have a logical reason or need to get the temperature in lightning speed the smallest furnace will use the least energy and be more efficient. To produce comfort we want to blanket the area with a warm not hot air. Hotter air increases stratification resulting in warmer air at ceiling levels but cool air at floor levels. Using milder warm air will gently blanket the space and dissipate the heat more evenly producing greater comfort and less stratification. Considerably less energy is consumed and a much smaller heating system is required. Less energy consumption, increased comfort, smaller equipment, less expense and longer equipment life are the advantages of accurate sizing. Where are the advantages to oversizing? Oversizing creates more service, more failed systems and significantly increased energy consumption helping HVAC contractors and manufacturers and the oil/gas industry. But there's no advantage to you as a homeowner.

Oversizing air conditioning systems creates the same problems. Comfort and efficiency are sacrificed as the unit never operates long enough to remove humidity from the air. Heating and air conditioning systems require 12 to 15 minutes per run cycle to reach operating efficiency. An air conditioning system requires this time before the coil is cold enough to remove moisture and effectively dehumidify the air. Oversized cooling systems result in cool clammy conditions with wide temperature fluctuations. Shorter operating cycles result in expensive service calls and premature equipment failures.

HVAC systems aren't sized for nuclear winters and volcanic summers or to achieve temperature conditions at the fastest possible speed. With energy efficiency as our goal, comfort follows equally.

Oversizing has become so epidemic the Federal Government's Department of Energy has taken action providing warnings to state governments, manufacturers, homeowners and contractors. They have mandated professional energy audits and sizing of systems in the worst affected areas of Florida and California.

To achieve your goals of energy efficiency and comfort, system sizing must be based on an exacting proven method. A method accepted by the HVAC industry, the engineering community and the Department of Energy. The method takes into consideration the size, construction, and location of the house and the weather conditions within that area. Each house has a specific energy efficiency determined by the construction materials, style and size, insulation values, thermal barriers and door and window types and sizes. R values or insulating values are calculated for each external area. The position of each wall, door and window to the sun is calculated for solar gain and prevailing winds for heating losses. The specific climate conditions are determined. The climatic conditions vary for each part of every state with it's own specific design winter and summer temperature, humidity, length of time of weather conditions at various temperatures and temperature swings. Finally the indoor conditions required are plugged into the calculations to arrive at specific heat gains and losses for the design conditions. The heat and moisture gained by the house during the maximum peak summer temperature and humidity is called the cooling load.  This is the energy defined in btus required to remove the maximum amount of heat and moisture to obtain the desired indoor temperature and humidity conditions. For heating the heat load of the house is the amount of energy in btus required to obtain desired design temperature. Humidity removal is an energy consideration for cooling but not for heating. 

Let's first understand certain basics and fundamentals of HVAC system and sizing. Sensible heat is the energy required to increase or decrease temperature. Latent heat is the energy required to remove or add moisture to air. Enthalpy is the total amount of energy in air including specific and latent heat. The capacity of a heating or air conditioning system is rated in btus and tons for air conditioning. A BTU is a British Thermal Unit which is the amount of heat required to raise one pound of water one degree fahrenheit in one minute. In HVAC design and systems btus are the amount of energy rated in one hour time increments. For example referring to 12,000 BTUS for a cooling or heating load is 12,000 btus removed or added to a space in a one hour time period or shown as 12k BTUH. 12,000 btus equals one ton. Tons are a more simplistic rating of BTUS for heat pumps and air conditioning systems. Residential furnaces or boilers refer to btus but not tons. Your house has a specific heating and cooling requirement rated in btus. All HVAC units have a rating of btus or tons for capacity. The summer design temperature for calculating cooling loads for most areas of the country averages 95 degrees outside air. Air conditioning system capacities are consequently rated at 95 degree outside air temperatures. Below 95 degrees cooling system capacities increase and above 95 degrees capacities decrease. Air conditioning system capacities are rated in sensible cooling, latent cooling and total cooling capacities shown as btus per hour or BTUH.

In the HVAC (Heating Ventilating and Air Conditioning) industry there is a method developed by the HVAC engineering community and universally accepted over the years to accurately determine heating and air conditioning requirements referred to as Manual J. As more requirements develop due to newer construction materials, techniques and ventilation, Manual J continuously evolves. Today Manual J-8 is the latest version for determining heating and air conditioning requirements. When Manual J is correctly calculated and applied the final heating and cooling requirements referred to as the loads are accurate for any house in any location. This is the method used by DESCO Energy and SEER - Solutions for Energy Efficient Results. For this reason we provide a 100% guarantee on the results provided through our online sizing form. We are the only online source providing accurate Manual J8 load calculations at no charge for our potential customers.

Many times homeowners will tell our technical support associates at DESCO Energy they know the size system they need. They may have had a contractor or several tell them the same size system is required. But when we do a load calculation we usually find those estimates were grossly oversized. It's not unusual for many contractors from the same area to use the same rule of thumb method and arrive at the same oversized system. Oversizing is an epidemic problem and not an isolated problem. It is more likely you will end up with an oversized system from anything less than a Manual J calculation rather than the right size. Realize that similar size quotes from several contractors are not an indication of accurate sizing. You can usually expect the results of an accurate Manual J load calculation to indicate you require a much smaller system than you presently have or thought you may have needed. According tot he Department of Energy over 70% of existing HVAC systems are oversized and 50% are grossly oversized.

Typical Example
Here is a typical example that occurs every day from oversizing of heating and air conditioning systems. Our example Homeowner has an older house. 15 years ago our homeowner's HVAC system was installed and at that time the system was oversized for the house. In the past 15 years our homeowner has increased the energy efficiency of the house. Windows were replaced with upgraded windows from single pane to double insulated low e argon gas filled windows. Insulation was added to the attic and new more efficient doors were installed. Aluminum siding was removed and insulated vinyl siding was installed over a new new thermal radiant barrier. Essentially our Homeowner has increased his house efficiency by 100% and decreased the heating and air conditioning load by 50%. The original heating and air conditioning system was oversized for the original inefficient house 15 years ago. Now that same system is grossly oversized. Our homeowner now decides to upgrade his heating and air conditioning equipment. Along comes our typical HVAC contractor and determines as always to beef up the new equipment. The HVAC contractor tells the unsuspecting homeowner it would be a good idea to install a larger system just in case and because other homes this size use larger equipment he always recommends. Our contractor tells our unsuspecting Homeowner there has never been one complaint from other customers. His customers always have more than enough heating and air conditioning capacity. And then he plays on your fears and asks, "Do you want to take a chance with a smaller system and end up without enough heat or cooling. Remember that volcanic summer 3 years ago or that nuclear winter we had 2 years ago? Not one of my customers complained. They all said they had capacity to spare and their equipment was still cycling in the hottest and coldest days. Isn't that what you want? I'll put in the smaller system but don't call me when you're too cold or hot." At this point our homeowner should say no and tell his contractor to take a hike. The contractor's fear scheme worked again and the choice was to go with the larger equipment. Our homeowner puts his trust in the contractor's recommendation and has now installed a severely grossly oversized heating and air conditioning system. The system does exactly what the contractor promised. The furnace only runs for a short period of time before it shuts off again with hot blasts of intense air. The air conditioning system comes on and the homeowner feels the cold air blasting out of the grilles. The cooling system shuts off in less than 5 minutes on the hottest days. What our homeowner doesn't see or know will hurt him. The furnace is so grossly oversized that instead of cycling on the thermostat, it cycles on the safety limit because the duct system isn't large enough to accommodate the air flow required. The furnace produces short blasts of very hot air. The oversized furnace wastes a lot of energy in the stack and from the continual short on and off warm up cycles. Not only is this a wasteful situation, it's also unsafe. A safety limit is a limit that should only trip during emergency situations. But the limit is tripping because the air flow and duct system are undersized for this mammoth furnace. And our homeowner has no idea any of this is happening. When our homeowner runs his air conditioning with short intense cold air cycles our homeowner has no idea the destruction and wasted energy that is occurring as a result. The cooling system never has difficulty keeping the house cool with the short run cycles but never runs long enough to dehumidify the air. As a result the house is always cool and clammy. Because the system was so oversized the contractor had to remove refrigerant so it would operate satisfactorily. Otherwise the system would freeze up and the coil would be one big ice ball. A common practice for our example contractor because there's no understanding of the basic principles of heating and air conditioning systems or how to size systems. Our Homeowner is now on borrowed time. For the next several years the equipment requires numerous service calls. In less than 5 years the compressor and furnace have had major failures. Before the equipment is 10 years old our Homeowner is now replacing the system and blaming the equipment manufacturer. The homeowner is convinced the problems are all a result of defects in the equipment and it is nothing but junk. In addition to this catastrophic failure energy costs are double what they should have been. This saga isn't an isolated incident and happens more frequently than not. The resultant failures occur in epidemic proportions. 97% of all warranty claims are a result of oversizing. Our homeowner's problems had nothing to do wit the manufacturer or the quality of the equipment. It had to do solely with an incompetent contractor and an unknowing homeowner.

Online Problems of Oversizing
The problem of oversizing by incompetence is serious enough without the need for online sellers adding to this dilemma. Here's a perfect example found on several websites. The plant chart shown is used for planting seeds but never to be used for determining heating and air conditioning loads or selecting equipment.

Adding to the  epidemic problem of oversizing are online sellers of HVAC systems having little or no experience. It would be better to provide no assistance rather than misleading plant charts or tables showing so many square feet per ton. After understanding the methods, requirements and calculations used to accurately determine the heating and cooling loads of your house you see how seed planting charts and the little calculations offered will provide a misleading impression of how accurate sizing is obtained. HVAC manufacturers are extremely concerned about online seller's misleading practices. It is for this and many other reasons the online sales from these websites are usually void of manufacturers' factory authorized warranties or support.  To learn more about this and other associated problems go to our Buyer Beware Section.

To eliminate and avoid oversizing DESCO Energy created SEER Solutions for Energy Efficient Results. Following the guidelines and requirements of SEER avoids problems with sizing. Follow the sizing form link below to go to the only online sizing form using accurate Manual J8 calculations and results. SEER recommends all homeowners use the online sizing form to accurately determine their heating and cooling load requirements.

You will need to take measurements of your house, windows and doors. In addition the amount of insulation in walls and ceiling or roof areas, present equipment model numbers, type of system you presently have and other pertinent information.

SEER recommends completing reading the entire Start Here and Solutions Section.  Knowledge provided by SEER gives you the power, confidence and direction to achieve maximum energy savings, comfort and system longevity. Following SEER's guidelines you can be assured of a successful HVAC do it yourself installation.

Click Here to go to our Online Sizing Form.

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