Properly sizing an HVAC system

You have chosen your new system for you home, office, etc. Now is the critical and one of the most important steps for your contractor to do is chose the size or tonnage of your new system. Can you trust that they will make the right choice? Well it is sad to say probably not. 


If you are changing out an existing system then most of the time the contractor will simply replace your old system with the same size. You may ask why is this the incorrect choice? Most systems in California are oversized according to Southern Ca Edison.


If you have an older home and have upgraded windows for example from single pane to a double pane, or added insulation to your attic then you have reduced the load (the amount of cooling or heating it takes to condition your home) which in turn lowers the amount of cooling and or heating needed. 


A system that is oversized or too large will cool or heat your home too quickly in which case your thermostat cannot react fast enough, and in the end it gets colder or hotter than the setting. Imagine a rollercoaster, with the ups and downs, and thats what the temperatures in you home are doing. For example if the air conditioining gets to cold and turns off then it warms back up and starts all over again. 


A properly sized system will run longer cycles, which stabilizes the temperture in the home by more gradually cooling it, then turns off. This creates a more level comfort. If you go with multistage equipment that can run at different levels it makes it work even better. The Trane XV20 for example can provide up to 700 stages of cooling (and heating with a heat pump)


How can you properly size your system? A qualified contractor can perform load calculations by using a software program like Wrightsoft. Basically the house is measured, all the windows, the amount of insulation in the attic, the walls, and even the orientation or how it is positioned to the sun, as well as the climate data based on where your home is located. This will tell you exactly how much heating and cooling your home needs. 


When a system short cycles, turning off and on, is will shorten the life of your equipment by causing more wear and tear and increase your electric bill due to the amount of power it takes to start your system. 


Finally with load calculations you can also figure out how much air it takes to properly cool each room as well. Guessing how big a system needs to be, or using old rules of thumb like 400 sq ft per ton should be obsolete but unfortuneatly this is still the practice of many companies. 


Case in point. A home owner in Riverside, Ca. that we ultimatley installed a sytem in, wa quoted a 5 ton system by a large company in Southen Ca. When I met with the home owner I explained the need to do load calcs and in the end the house required 3 tons, not 5. Imagine oversized by 2 tons (24,000 BTU) not only will the house stay more comfortable and run more efficient, he also saved money on the equipment and ducting costs as well.  


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