For the heating and cooling system, the size is actually what matters. You need to obtain a unit that is properly sized for your home. Units that are too large tend to overwork themselves. For those who have a larger central HVAC unit for the home, it can quickly bring up and lower the indoor temperature. Now that sounds like it could be a good thing, right? Actually no, central HVAC units work in addition to work until they reach a pair temperature on a thermostat. Generally if the unit can quickly reach this temperature, then it shuts off. The system then will be put into an endless cycle of extreme heating/air conditioning and then being off. The turning on/off function rapidly is bad for your unit. The unit can overheat and is also not energy efficient. You are better off using a unit that is smaller in proportions that will steadily heat or air condition your home to the right temperature. HVAC technicians should know what size it should be for your home. They come in and take measurements of the home. They do not bring measuring tape and really try during the process however. The technicians use the rule of thumb instead. They give the house a simple look over and provide a loose figure on how large the house is. Then those numbers are added up just in case error. They then take the new numbers and choose a unit for your needs. This sounds like an easy approach to get stuck purchasing a more costly and not needed unit. You need to know the size you wish to purchase.