Are you considering whether you need to replace your home’s central heating, ventilation, and air conditioning unit—commonly known as your HVAC system? As a general rule, if your existing HVAC system is more than 15 years old, it’s only a matter of time before it starts to cost more to maintain than it would to replace it with a new system. In addition to higher energy bills brought on by a system that can no longer function at peak efficiency, repair costs will also start to add up as parts are replaced and the ventilation system undergoes major overhauls.
An important component of natural ventilation is air change rate or air changes per hour: the hourly rate of ventilation divided by the volume of the space. For example, six air changes per hour means an amount of new air, equal to the volume of the space, is added every ten minutes. For human comfort, a minimum of four air changes per hour is typical, though warehouses might have only two. Too high of an air change rate may be uncomfortable, akin to a wind tunnel which have thousands of changes per hour. The highest air change rates are for crowded spaces, bars, night clubs, commercial kitchens at around 30 to 50 air changes per hour.[17]

The condensed, pressurized, and still usually somewhat hot liquid refrigerant is next routed through an expansion valve (often nothing more than a pinhole in the system's copper tubing) where it undergoes an abrupt reduction in pressure. That pressure reduction results in flash evaporation of a part of the liquid refrigerant, greatly lowering its temperature. The cold refrigerant is then routed through the evaporator. A fan blows the interior warm air (which is to be cooled) across the evaporator, causing the liquid part of the cold refrigerant mixture to evaporate as well, further lowering the temperature. The warm air is therefore cooled and is pumped by an exhaust fan/ blower into the room. To complete the refrigeration cycle, the refrigerant vapor is routed back into the compressor. In order for the process to have any efficiency, the cooling/evaporative portion of the system must be separated by some kind of physical barrier from the heating/condensing portion, and each portion must have its own fan to circulate its own "kind" of air (either the hot air or the cool air).


Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration", with each approximately equal to the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. The value is defined as 12,000 BTU per hour, or 3517 watts.[32] Residential central air systems are usually from 1 to 5 tons (3.5 to 18 kW) in capacity.
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High-efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main differences between a conventional and condensing furnace are the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these ways, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than does a conventional furnace; both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.
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