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I started this as an answer to another thread and it got out of control. So for those of you who want to know how an A/C system works, here you go. I’ve been an HVAC tech since 1984 in one form or another. Hospitals, cars, light rail, houses, factories, houses, printing plants, houses, wafer fabs, nuclear power plants, houses, everything but the White House. I like to think I’m honest, I don’t rip off my customers. Won’t work for a shop that works that way. I’m sure I haven’t answered all your questions below and I’ll be happy to do so. Just realize I don’t check on SDMB every day, but I’ll do my best.
I’ve been meaning to write a Heating and Air Conditioning FAQ type article for a while now. Here’s my shot at starting it. Please read and critique. Some one else will have to translate USA English (what I’m Writing) to Rest of the World English.
If I get too deep into Thermodynamics, I apologize, but this is the STMB and I assume some intelligence on the reader’s part. I am not trying to talk down to you.
Air Conditioning usually refers to mechanically cooled air circulated by a fan powered air moving device. Many other varieties exist. I am referring to the home you probably live in.
A typical system would be comprise of: Evaporator Coil in a box, Fan in a box, some sort of air heating device in a box, refrigerant compressor in a box, condenser coil in a box. These boxes can be all separate, or some combination of all of them. Most common is Outdoor Compressor and Condenser coil in one box, refrigerant lines connecting to Evaporator Coil, Air Heating Device, Indoor Air Fan, Control Boards and Circuitry in a second box. The Indoor Box will have a Main Voltage to Control Voltage step down transformer. Control voltage will typically be 24 VAC nominal. That is sent the the Thermostat (R) terminal.
The Thermostat enables: the Compressor through the Y Terminal, Fan by thermostat G terminal, allowing the compressor and fan to run. If you have a heat pump then there is a terminal to energize the reversing valve. That will be O or B. O is energized in Cool and B is energized in Heat. In the early days of heat pumps there was no standard. Now days most systems energize the reversing valve in Cool “O”. But check your system. When the compressor contactor is energized by the Y terminal, the outdoor unit Contactor (the Brits call this an Amplifier) is engaged, switching High Voltage Main Power to the Compressor. The low voltage signal energizes the contactor which in turn delivers the high voltage power to the high amperage load of the Compressor. The Compressor is a hermetically sealed device. That means it’s in a sealed can you may not get into. It contains an electric PSC* motor and a refrigeration compressor head sealed into one unit. The Inlet or Suction Line and the Discharge or High Pressure Line are soldered to the outer steel shell of the Compressor. The refrigeration compressor head is one of two varieties: reciprocating or scroll. A reciprocating compressor is much like a 2 stroke gasoline engine, with pistons and reed valves. An Electric motor drives the compressor pistons through the crankshaft, rather than exploding gasses driving the crankshaft. The Scroll is like FM Radio: Fricking Magic. Much better design. Also left as an exercise for the curious. Either way, you have an electric motor turning a device that takes in refrigerant vapor from the evaporator, compresses it into a high pressure, high temperature gas, ( compressing a gas increases its temperature) where it is piped to the condenser coil, usually fan cooled by ambient outdoor air, condensed into a liquid by giving off its heat to the outdoor air, the now high pressure liquid refrigerant is fed to the inlet of the indoor Evaporator coil, through the metering device, into the low pressure evaporator, evaporated into a vapor, absorbing heat from the air passed over the evaporator coil by the fan, and drawn back to the compressor to be compressed and pushed through the cycle again.
Clear as mud, right? Here is what you have to get your head around. Physics is Physics. If a liquid anything is introduced into an environment where the ambient pressure is below its vapor point, it will boil, in turn ** causing it to absorb heat** or energy from its environment. We all remember that heat is energy, right? That is a crucial point. In physics, there is no such thing as COLD. There is only HEAT, More Heat, Less Heat, Sensible Heat, Latent Heat, but NO COLD. 32ºF or 0ºC is cold to us, but to Physics, it’s just another data point. Anything below human body temperature SEEMS cool or cold, any temp above that seems warm or hot. Just another data point to Mother Nature.
In an air conditioning system the evaporator coil lives in the air stream defined by the Duct Works, the Fan, and Air Heating Device. Evaporators a designed to absorb X Btu’s from Y CFM (Cubic feet per minute) of airflow. Current ASHRE standards are 350 to 500 CFM @ 12000 BTU’s per hour. These are the variables that define the EER rating of your AC unit or the SEER of your Heat Pump Unit.
12,000 BTU’s per hour is One Ton of Air Conditioning, a legacy of the old days when Ice (Water Ice) was the medium of Air Conditioning, 24,000 BTU’s is 2 tons, 60,000 BTU’s is 5 tons, etc. Do The Mathif you’reInterested.
So when your thermostat (2) says “Hey, it’s too warm in here, Air conditioning – turn on”, what is does is send one signal to the Fan Relay to energize the fan (G Terminal) and one signal to the Compressor Contactor (Y Terminal) to energize the Compressor.
(1) Compressor: This implies a device which draws in on a suction side and pushes out on a higher pressure discharge side. Sounds simple, but this is one thing that eludes many people. What is a compressor? In the world of Air Conditioning and Refrigeration, it is a device that sucks and blows. Sucks in from the Evaporator at a low pressure and Blows out at a high pressure to the Condenser.
(2) Thermostat: No matter how fancy or how simple your thermostat is, it is just a switch. It turns on FAN, COOL, or HEAT. That is all it can do. It can do that at programmed times, at programmed temperatures but that is all it can do. If your house is at 62ºF and you set your thermostat at 85ºF, it will not heat your house to 70º one microsecond faster than setting your thermostat to 70ºF. Why? you ask. Because it is simply an On/Off switch. No Magic. No Variable Heat. No Bull. Just On or Off. It is not like the burners on your stove. It is on or it is off. Same for Cooling. The system runs On or Off until it reaches the temperature set on your thermostat.
(3) Heat Pump: To Satisfy the Pedants: All Mechanical Refrigeration Systems are technically Heat Pumps. Common Parlance though has decided that Refrigerators and Air Conditioners are Refrigerators or Air Conditioners and Two Way Heat Transfer Systems are “Heat Pumps”. In a Heat Pump the Hot, High Pressure gas is piped to the indoor coil where it is “cooled” or condensed, thereby giving off its heat to the indoor side of the system, and the outdoor coil is the evaporator, absorbing heat from the out side air. That is the Heat Mode. In Cooling Mode, the Hot, High Pressure gas is sent the outdoor coil to be cooled and condensed into liquid while the cooler high pressure liquid is metered into the indoor coil where it is evaporated into a vapor, absorbing heat, or cooling the indoor air. Yes, it is Magic. Or Physics. Or Thermodynamics. It works. Most places, most of the time. YMMV.
So I know this is not a complete treatise on HVAC but I hope it answers some questions.
*Permanent split capacitor. Very fascinating, Googling it is left as an excise for the student.