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We talk a lot about system design, and that's because it's important. It's actually one of the most important parts of getting a new heating and cooling system. Back in the olden days we just said you needed 1 ton of cooling for every 400 sq/ft of house. But we have gotten better at building houses, and a house that was built in the 70’s is not the same as a house built in the 90’s which is not as well built as a house today. So we can't just walk in and say you need 1 ton for every 500sq/ft of house since we build them better now. 

Believe it or not, there is an actual science to properly sizing a heating and cooling system, picking a unit out, and getting the air to the rooms where it's needed, in the proper amount and velocity that is needed. We call this the ACCA design series after the ACCA books. For the basics, we use three books, Manual J for how many BTU’s your home needs, Manual D to size the ductwork, and Manual S to pick the correct unit. All of these are needed to properly design a system. 

Unfortunately, there are a large majority of firms that simply ignore this step in the process of replacing equipment. Whether it is a lack of knowledge, lack of software, or a lack of care, we see the wrong systems installed on a daily basis. The largest majority of systems we see are severely oversized, on undersized ductwork. The results are short run times, higher than normal noise from the duct and units, and shorter life span of the equipment. As with many things, the toughness and durability of equipment is not what it used to be 30 plus years ago. And while it might not be built as well, we are paying more for those items than ever before. That is why we believe it is crucial to ensure your heating and cooling system is designed to meet the needs of your home. 

One possible reason some companies do not go through the design process is time. We all know there are only 24 hours in a day, and a proper design can take up to 8 hours to perform. Those hours also need to be included in the job, and can potentially make the bid higher than a contractor not going through the design process. Another reason may be a lack of proper software. There are many different programs out there approved to run the calculations, but they all have an associated cost. Additionally, there is a large learning curve that comes with the design process. Not only is there the initial learning that may take weeks or months, but the continuing education that is needed to keep current with the ever changing field.

In addition to shorter life span of equipment, and increased maintenance and strain on the units, oversized equipment could be contributing to uneven heating and cooling in certain rooms. Let's take a simplified look at how oversized equipment can lead to an uncomfortable room. Lets first point out that a load calculation (Manual J) is based on the 98% extremes of summer and winter. That means 97% of the summer your a/c is too big, 1% of the summer it's perfect, and 2% of the summer it's slightly too small. The same applies for heat as well. 

Our simple version will be a house that needs 50,000 btu of heat but we have a 100,000 btu unit installed. On a design day, a 50,000 btu furnace would run all day and maintain the thermostat set point. A 100,000 furnace only runs half the day in 15 minute on cycles and 15 minute off cycles (contrary to belief, the unit running half the time is not more efficient, even though it only runs half the time). If you have long runs of duct to those cold corner rooms, a 15 minute cycle may not be enough time to deliver enough heated air to that cold (or hot) room before the furnace turns off. A unit that runs longer is more efficient and creates a greater level of comfort. Think of your car on a cold (or hot) day. Once you reach your desired comfort, you adjust the blower and thermostat down to just enough of a trickle to keep that comfort level. You typically do not turn it on to full blast and then off once comfortable, only to repeat the on off cycle each time you start to get comfortable. The same concept applies to our home heating and cooling systems. 

The design process is quite fascinating and somewhat complex. I could go on for days about the subject, but I know most people don't share the same level of passion I do for system design. But, if you are considering a new heating and cooling system, make sure your contractor is following the proper steps to ensure you're getting a system designed around you and your home. 

If you would like to learn more, please feel free to email or call me, and I would love to talk about this subject more. 

Have a question about heating or air conditioning. Send us an email and we will pust it and our answer right here for everyone to see. 

Ben Haws 

JBH Heating and Air Conditioning | JBHEnergy.com