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How A Central Air Conditioning System Works

In the typical split system design, the furnace contains a fan that forces air through the system during both the winter and summer months. A variable speed fan in your furnace improves heating and cooling efficiency, provides better comfort and indoor air quality control - always get a furnace equipped with a variable speed fan! Once indoor air is pulled to the HVAC system, the following happens (assuming the above HVAC design):

1) The air is filtered. Air cleaners are positioned between the return air ducts and the furnace. The air cleaner has a high MERV rating (used to rate the effectiveness of air cleaners), with the ability to filter microscopic elements out of the air. These work much more effectively than normal furnace filters, and some models can even capture and kill the common flu virus! (for more information see: indoor air quality)

2) The air is cooled. The air is then pulled into the furnace by a variable speed furnace fan, and forced through the evaporator coil. An evaporator coil is a series of piping containing chilled refrigerant from the air conditioner (at a low pressure). The cold piping absorbs heat and causes moisture to condense, thus cooling and dehumidifying the air. This conditioned air is pushed on by the fan while the heated refrigerant is sent back outside to the air conditioner (condensing unit). The refrigerant is pressurized in the condensing unit which removes the heat. Then the cycle repeats.

3) Mold is killed & coil efficiency kept high. In the damp interior of an evaporator coil system, mold can grow which can affect not only the health of your family (mold spores in the air), but the health of your HVAC system by decreasing the efficiency of the evaporator coil. In the HVAC design example above, a UV lamp is mounted within the evaporator coil to prevent this. UV lamps are able to kill and prevent mold from growing, thus keeping your home healthy and your cooling efficiency high.

4) Cooled air sent back into the home. After leaving the evaporator coil, the cool, dehumidified air is pushed back into the home. In extremely humid climates, a separate dehumidifier can be installed to remove more humidity from the air than is removed by the evaporator coil.

5) Air is exchanged. At the top left of the above HVAC design sits a ventilator. Ventilators are quite amazing. They are part of complete indoor air quality solutions as they exchange fresh outdoor air for stale indoor air. They are able to do this with minimal loss of energy, and they retain the cooled and dehumidifed air your HVAC system worked hard to produce!

How The Above Scenario Changes When Heating

A central air conditioning system is usually combined with a heating system because they share some of the same devices, and of course the same ductwork for distributing conditioned air:

1) The Air is filtered. As with air conditioning, the air is filtered before entering the furnace.

2) The air is heated. The gas (or oil fired) furnace heats the air by igniting (from the pilot) flames and heating a metal unit called the heat exchanger. This heat exchanger then heats the air as it is forced through the system (via the same fan used to bring air through the evaporator coil when cooling in the summer). Note: as mentioned earlier, a variable speed fan in a furnace produces higher efficiency for both heating and cooling and is able to move at different speeds to efficiently keep air moving through your home even then the furnace is not running. This improves home comfort and humidity levels when indoor air quality products are in place like air cleaners, humidifiers, UV lamps and ventilators.

3) The air is humidified. Since air is normally dry in the winter, and heating will zap more moisture, a whole home humidifier can be mounted after the furnace to properly humidify the indoor air. This helps the air feel warmer which means you can keep the temperature lower, resolves the health related irritations of dry air, and protects paintings, woodwork and instruments in your home!

4) Air is exchanged. As in the summer, an energy recovery ventilator can exchange outside air with stale indoor air without losing the energy used to heat and humidify the home.

Heat Pump or Cooling Only System

When using a heat pump to heat and cool your home, or when you live in a climate where only air conditioning is needed, an air handler replaces your furnace and evaporator coil. An air handler is also referred to as a fan coil (usually based on installation design), but in a very basic sense, an air handler is a combination of an evaporator coil and the variable speed fan from a furnace. In a residential home it is normally installed in the same space as a furnace/evaporator combo. How it works:

1) Cooling in the summer. For "air conditioning" with a heat pump and fan coil, everything works the same as with a normal A/C unit, evaporator coil and furnace configuration. The same refrigerant, the same process, etc.

2) Heating in the winter. During the winter, the heat pump reverses the cooling process. Though it may not feel like it, even in cold weather air retains heat. And as long as the refrigerant is colder than the outside air, heat can be extracted. A heat pump absorbs this heat, heats the refrigerant and then sends the heated refrigerant back inside to the fan coil where the coil then heats the air. The balance point is a term used to describe the point where the outside air temp and heat pump capability matches. Below this point supplemental heat is required because the heat pump cannot pull enough heat from the air outside.

Hybrid Systems

A hybrid system is designed to offer higher heating efficiency during colder months. Heat pumps, unless geothermal, are all electric and can be less expensive to run when the outside temperature is 40 degrees and above. A gas furnace is typically less expensive to heat with when temperatures drop below 40 degrees (this of course varies based on local gas and utility prices).

1) Cooling with a hybrid system. There is no difference to cooling when a hybrid system is in place. During warm weather the heat pump performs just like an air conditioner by pressurizing and cooling the refrigerant while the evaporator coil and furnace operate like example one.

2) Heating with a hybrid system. In the winter, the heat pump and gas furnace switch on and off depending on determined fuel prices and temperature. When the air is cold but not bitter cold, the heat pump haply heats your home for less money. When temperatures dive to freezing and below, the furnace takes over. And you pay less on monthly winter utilities!

energy_starHVAC Energy Efficiency

Heating and cooling accounts for as much as half of a home's energy use. We believe in offering our customers products that have achieved ENERGY STAR approval for efficient heating and cooling.

ENERGY STAR qualified products prevent greenhouse gas emissions and cut down on how much you spend on your monthly utility bills.

The ENERGY STAR Guide to Sealing Your Home:

Just like a vehicle, your home comfort system requires routine maintenance to keep it running at its best. Without routine service, heating and cooling systems waste energy and are more likely to break down. However with the proper attention, they can keep you comfortable year-round.

Heat pumps and oil-fired furnaces and boilers need a yearly professional tune-up. Gas-fired equipment burns cleaner; it can be serviced every other year however yearly scheduled maintenance is still ideal.

Step 1
A close inspection will uncover leaks, soot, rust, rot, corroded electrical contacts and frayed wires. In furnace (forced-air) and boiler (hot-water) systems, the inspection should also cover the chimney, ductwork or pipes, dampers or valves, blower or pump, registers or radiators, the fuel line and the gas meter or oil tank, as well as every part of the furnace or boiler itself.

Step 2
Next, the system should be run through a full heating cycle to ensure that it has plenty of combustion air and chimney draft. Contractors use smoke pencils to check for sufficient draft and also test the air for carbon monoxide.

Step 3
Finally, it's time for the down and dirty task of cleaning the burner and heat exchanger to remove soot and other gunk that can impede smooth operation. For the burner, efficiency hinges on adjusting the flame to the right size and color, adjusting the flow of gas or changing the fuel filter in an oil-fired system. A check of the heat pump should include an inspection of the compressor, fan, indoor and outdoor coils and refrigerant lines. Indoor and outdoor coils should be cleaned, and the refrigerant pressure should be checked. Low pressure indicates a leak; to locate it, contractors feed tinted refrigerant into the loop and go over it with an electronic detector.

Tuning up the distribution side of a forced-air system starts with the blower. The axle should be lubricated; blades cleaned and lower motor checked to insure the unit isn't being overloaded. The fan belt should be adjusted so it deflects no more than an inch when pressed. Every accessible joint in the ductwork should be sealed with mastic or UL-approved duct tapes. Any ducts that run outside the heated space should be insulated. On a hot-water system, the expansion tank should be drained, the circulating pump cleaned and lubricated and air bled out of the radiators.

While thermostats rarely fail outright, they can degrade over time as mechanical parts stick or lose their calibration. Older units will send faulty signals if they've been knocked out of level or have dirty switches. To recalibrate an older unit, use a wrench to adjust the nut on the back of the mercury switch until it turns the system on and, using a room thermometer, set it to the correct temperature. Modern electronic thermostats, sealed at the factory to keep out dust and grime, rarely need adjusting. However, whether your thermostat is old or young, the hole where the thermostat wire comes through the wall needs to be caulked, or a draft could trick it into thinking the room is warmer or colder than it really is.

A neglected in-duct humidifier can breed mildew and bacteria, not to mention add too much moisture to a house. A common mistake with humidifiers is leaving them on after the heating season ends. Don't forget to pull the plug, shut the water valve and drain the unit. A unit with a water reservoir should be drained and cleaned with white vinegar, a mix of one part chlorine bleach to eight parts water or muriatic acid. Mist-type humidifiers also require regular cleaning to remove mineral deposits.

Most houses with forced-air furnaces have a standard furnace filter made from loosely woven spun-glass fibers designed to keep it and its ductwork clean. Unfortunately, they don't improve indoor air quality. That takes a media filter, which sits in between the main return duct and the blower cabinet. Made of a deeply pleated, paper-like material, media filters are at least seven times better than a standard filter at removing dust and other particles. An upgrade to a pleated media filter will cleanse the air of everything from insecticide dust to flu viruses.

Compressed, media filters are usually no wider than six inches, but the pleated material can cover up to 75 square feet when stretched out. This increased area of filtration accounts for the filter's long life, which can exceed two years. The only drawback to a media filter is its tight weave, which can restrict a furnace's ability to blow air through the house. To insure a steady, strong airflow through the house, choose a filter that matches your blower's capacity.

Duct Cleaning
A maze of heating and air conditioning ducts runs inside the walls and floors of 80 percent of American homes. As the supply ducts blow air into the rooms, return ducts inhale airborne dust and suck it back into the blower. Add moisture to this mixture and you've got a breeding ground for allergy-inducing molds, mites and bacteria. Many filters commonly used today can't keep dust and debris from streaming into the air and over time sizable accumulations can form - think dust bunnies, but bigger.

To find out if your ducts need cleaning, pull off some supply and return registers and take a look. If a new furnace is being installed, you should probably invest in a duct cleaning at the same time, because chances are the new blower will be more powerful than the old one and will stir up a lot of dust.

Professional duct cleaners tout such benefits as cleaner indoor air, longer equipment life and lower energy costs. Clean HVAC systems can also perform more efficiently, which may decrease energy costs, and last longer, reducing the need for costly replacement or repairs. Cleaning has little effect on air quality, primarily because most indoor dust drifts in from the outdoors. But it does get rid of the stuff that mold and bacteria grow on, and that means less of it gets airborne, a boon to allergy sufferers.