LEARN ABOUT How HVAC Systems Work

(Gas-fired, up flow model)

Sequence of events:

Thermostat (1) calls for heat and checks all safety devices

(2). Ignitor (3) (or safety pilot) is energized and gas valve (4) opens.

Burner (5) lights.

Heat accumulates in plenum (6), energizes blower control (7).

Blower (8) drawing return air (9) over air filter (10) gains heat from passing over heat exchanger (11) and is distributed thru supply system (12).

When thermostat (1) is satisfied, the sequence is reversed. Gas valve (4) closes; burner (5) shuts off. Air gradually cools and blower control (7) shuts off blower (8).

Observation: 

Unlike what many people think, an air conditioner does not ADD cool air to an area but instead REMOVES heat from the existing air, leaving the remaining air cooler.

Theory:

Oversimplified, the miracle of cooling is accomplished by the use of (1) refrigerant; (2) a compressor and (3) an evaporator.
Refrigerant acts like a sponge. “Squeeze” (compress) it and the heat is expelled; let it expand and it will absorb heat.
If you squeeze it outside, the heat is expelled into the air; bring it back into the house and let it expand and it will soak up more heat from within.

A Visual Sequence Of Events:

Thermostat (1) simultaneously energizes indoor blower (10), outside condenser fan (5), and compressor (3). Refrigerant (2) enters compressor(3) as a low temperature (LT), low pressure (LP) gas. Compressor’s (3) “squeezing” action converts LT/LP gas to high temperature (HT), high pressure (HP) gas as it leaves compressor and enters condenser coil (4). Condenser fan (5) transfers heat from HP/HT gas to the outside air (6); this reduces gas to HP/LT liquid (7). HP/LT liquid (7) leaves condenser coil (4) To Activate This System and enters evaporator coil (8) through a narrow orifice (expansion valve). As liquid expands, it loses pressure reverting back to a LT/LP gas. This LT/LP gas, flowing through the evaporator coil (8), absorbs heat from the return air (9) circulated over the evaporator coil (8) by the indoor blower (10) and returns (2) to the compressor (3) to start a new cycle.

Observation:

In the cooling cycle, a heat pump is just a conventional air conditioner. For a detailed explanation and a visual sequence of events for a cooling cycle, click here to see our web page “How Does A Split System Air Conditioner Work?”

Theory:

In the heating cycle, the “brain” of a heat pump (the reversing valve) reverses the flow of the refrigerant. Now the refrigerant, instead of extracting heat from within the structure and expelling it outside, extracts heat from the outside and brings it inside.
Heat can be extracted from the outdoor air as long as the temperature of the refrigerant in the outdoor coil is lower than the temperature of the outdoor air. When the outdoor air temperature is 0 degree Fahrenheit it still contains 90% of the heat which it contained at 70 degree Fahrenheit.

A Visual Sequence Of Events Of The Heating Cycle Follows:

Thermostat (1st stage)* (1) simultaneously energizes indoor blower (10), outdoor condenser fan (5), and compressor (3). Refrigerant (2) enters compressor (3) as a low temperature (LT), low pressure (LP) gas. Compressor’s (3) “squeezing” action converts LT/LP gas to high temperature (HT), high pressure (HP) gas (7) as it leaves compressor (4) and enters the indoor coil (8). Indoor Blower (10) causes transfer of heat from this HT/HP gas as it circulates return air (9) over the indoor coil (8). As the HT/HP gas loses its heat to the air surrounding the indoor coil, it is reduced to a HP/LT liquid. HP/LT liquid leaves the indoor coil (8) and enters the outdoor coil (4) through a narrow orifice (expansion valve), it literally “boils off” causing it to lose pressure and again becoming a LT/LP gas. This LT/LP gas, flowing through the outdoor coil (4), absorbs heat from the outside air (6) circulated over the outdoor coil by the condenser fan (5) and returns to the compressor (3) to begin a new cycle.

* A heat pump is most efficient at temperatures above 32 degrees (often called the “balancing point”). When the house requires more heat than can be extracted from outside air below the balancing point, electric strip heaters (2nd stage), usually built into the air handler, are energized by the thermostat.

NOTE: Occasionally, heat pumps must remove the frost that forms on the outdoor condenser coil. By going into a “defrost cycle”, heat pumps automatically rid themselves of the frost by switching from heating to cooling for a few minutes to melt this frost. The resulting “steam” that comes from the top of the condenser, accompanied by the sound of the reversing valve, sometimes alarms homeowners unfamiliar with the process.