This is a rule of thumb

You can really get screwed over with the size of your HVAC. Size matters in terms of heating and cooling. If the HVAC is too small, it will run all day long. The HVAC will work and work until it burns itself out. Then a hefty HVAC repair is needed or the HVAC is just plain done for. Super expensive or cheaping out with tiny HVAC? Too large of HVAC equipment is no better though. The HVAC is then constantly cycling on and off. The powering up and down is bad for the HVAC unit. It will also cause damage in this process. It also is bad in terms of efficiency. It is better to keep a steady stream of heating or cooling. Powering totally on and shutting all the way down takes way more power to do. The HVAC will end up costing more when it runs half the time. Most HVAC providers try to size up with HVAC equipment too. What they do is called rule of thumb. Basically the HVAC guy walks around a home and takes loose measurements. He eyeballs it and then pads his numbers up for error. Then, the HVAC professional recommends a size for the home. The bigger the HVAC, the more commission. So those numbers get padded quite high and the homeowner usually gets stuck with way too much HVAC to handle. Do a little research before making the big leap on HVAC. Decide what type of HVAC you want and what is the recommended size for your square footage. Take the HVAC contractor totally out of the equation.