Your home's system as a whole

Most people think of heating and cooling first, but the system does more than change the temperature. An HVAC system includes the equipment that ventilates, heats and cools a building, the ductwork that moves air around it, and the parts that filter and clean the air. Heating and cooling systems also remove moisture and help keep indoor air quality up by taking out particles and spreading ventilation air. In other words, comfort is temperature, humidity and clean air together.

That is why a problem that seems to be about the air is often about comfort. Many problems people take for air quality problems are really comfort problems, such as temperature, humidity or air movement that is too low or too high. Defects in the system can keep it from holding comfort, indoor humidity and air quality where they should be. When something feels wrong, it helps to think about the whole system, not one box in the basement.

Source: U.S. EPA · U.S. Department of Energy

Homes heat in a few different ways, and each way moves heat to your rooms differently. A furnace heats air and uses a blower motor and air ducts to spread warm air through the house. A boiler heats water to make hot water or steam, which then travels through a series of pipes. A heat pump pulls heat from the surrounding air to warm the house, and it can cool the house too.

Forced air, a furnace sending heat through ducts and vents, is the most common type of home heating system. Other homes heat with radiators. Chicago's Heat Ordinance can be met with any permanent heating system the Chicago Mechanical Code recognizes, such as steam or hot water radiators, other water-based systems, warm-air furnaces or heat pumps, but not fireplaces. Some homes pair two sources: dual-fuel or hybrid systems combine the efficiency of a heat pump with the reliability of a gas furnace.

Source: U.S. Department of Energy · City of Chicago

Cooling equipment comes in a few common layouts. A central air conditioner circulates cool air through a home using a system of ducts and registers. Many are called split systems because they have an outdoor unit, which holds the condenser and compressor, and an indoor unit, which is often connected to a furnace or heat pump. Systems that hold the evaporator, condenser and compressor in one unit are called packaged systems.

Homes without ducts have other choices. A ductless mini-split air conditioner gives zoned cooling without ductwork. Ductless heat pumps can stand in for radiator or baseboard heating and replace window units for cooling, with no ductwork needed. In a ductless system, refrigerant carries the heat straight to the indoor heads, which deliver heated air to the room. The page on how heat is moved explains what happens inside these units.

Source: U.S. Department of Energy · ENERGY STAR

Whatever makes the heat or the cooling, something has to carry it to your rooms. In a forced-air system, a blower moves air over the cooling coil and spreads the cooled air. Air filters sit in the return air duct or plenum and work whenever air circulates through the ducts. Supply and return grilles that are sized correctly help air flow well through a home.

Because the parts are linked, one weak part can affect the rest. How much air the system moves depends on the furnace or air handler, the motor speed settings, the ducts, the air filter and the condition of the air delivery system. Leaky ducts, or too little return air, can pull outside air into the home and add to the heating and cooling load. Humidity problems, a lot of dust or rooms that never get comfortable can also come from poorly insulated ductwork. When some rooms are too hot or too cold, the cause could be the equipment's operation, the ducts or too little insulation.

Source: U.S. Department of Energy · ASHRAE · ENERGY STAR

The thermostat is the part you touch most, and it tells the rest of the system when to run. Programmable thermostats control heating and cooling equipment and can change temperatures on a set schedule. Smart thermostats can adjust based on whether people are home or active. The thermostat you buy needs to be compatible with your heating and cooling system. If you have a heat pump, you may need a special programmable thermostat to get the most savings all year.

Where the thermostat sits matters too. The EPA advises placing it away from registers, appliances, lights, doorways, fireplaces, skylights, windows, direct sunlight and drafts, and says interior walls are best. Some homes are split into zones, each with its own control. A typical zone control system uses automatic duct dampers that open and close based on each zone's thermostat.

Source: U.S. Department of Energy · ENERGY STAR · U.S. EPA · AHRI

The third part of HVAC, ventilation, is about the air itself. The EPA names three ways to improve indoor air: reduce the sources of pollution, bring in fresh air to dilute pollutants and send them outdoors, and use filtration and air cleaning to remove pollutants. Some homes also have a ventilator. A heat-recovery ventilator (HRV) moves heat between two separate airstreams, and an energy-recovery ventilator (ERV) moves both heat and moisture.

Moisture is part of the picture as well. Poor humidity control can lead to mold and even damage to the building. The pages on ventilation and humidity go further into both.

Source: U.S. EPA · ENERGY STAR · U.S. Department of Energy