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What it is: The amount of air the blower moves in one minute. It’s measured in cubic feet per minute (Cubic Feet per Minute). Imagine air like water in a pipe—bigger flow = more water moving. Same idea with air.
Why it matters: Airflow affects everything—cooling, heating, dehumidification, noise, and how hard the system works. If airflow is too low, the indoor coil can freeze in cooling or the furnace can run hot. If it’s too high, the air may feel drafty and not dehumidify well.
The air that is already around you—room air (indoors) or outside air (outdoors). When the SIM says “ambient 75°F,” it just means the surrounding air is about 75°F.
Inside the indoor coil (evaporator), the liquid refrigerant is kept at a low pressure. At low pressure, it boils at a low temperature—turning from liquid into vapor. During this change of state, it absorbs heat from the house.
Key idea: Most of the “heat moving power” happens during state change (boiling/condensing). That’s why saturation temperatures are a big deal.
What it is: The change of state where refrigerant turns from vapor into liquid inside the outdoor coil (the condenser). During condensing, the refrigerant gives up a large amount of heat to the outdoor air without much change in its own temperature at that moment—this is heat released during a change of state (latent heat).
Where it happens: In normal cooling, hot high‑pressure vapor leaves the compressor and enters the condenser. First it desuperheats (sensible cooling until it reaches the condenser saturation temperature). Then it condenses (changes vapor → liquid while releasing latent heat). Finally, the liquid may be subcooled a few degrees before heading to the metering device.
Why it matters: Efficient condensing keeps system pressures reasonable and capacity high. If the outdoor air is very hot or the condenser is dirty or has poor airflow, the condenser must run at a higher saturation temperature (and pressure) to dump the same amount of heat—this raises compressor work and reduces efficiency.
The saturation temperature inside the condenser where refrigerant is changing state from vapor to liquid. It reflects how hot the condenser must run to reject heat to the outdoor air. Comparing this temperature to the actual liquid line temperature tells you subcooling.
A common target for airflow. Many techs aim around 350–450 cubic feet per minute for each ton of cooling (1 ton ≈ 12,000 BTU/hr). The exact sweet spot depends on climate, humidity goals, and equipment.
A quick way to express how full the system is with refrigerant compared to the correct amount (100%). Too little or too much hurts performance and can damage parts.
Reality check: There’s no onboard sensor that “knows” exact percent. In the field, we infer “fullness” from performance indicators (subcooling for thermostatic expansion valve/electronic expansion valve, superheat for fixed orifice), plus line sight glass (on some systems), weight in/out, and context.
The “heart” of the refrigeration circuit. It pulls in cool vapor from the evaporator and compresses it to a higher pressure and temperature so it can reject heat in the condenser.
Keep it safe: The compressor should only get vapor back from the evaporator. Liquid returning to the compressor is called floodback and can break things.
Liquid refrigerant reaches the compressor when it should only receive vapor. This can wash away oil, cause metal wear, and lead to failure.
The outdoor coil. Hot high-pressure vapor enters, gives its heat to outdoor air, and turns back into liquid (condenses). A fan blows outside air through the coil to help reject heat.
What’s happening inside: Hot, high-pressure vapor enters and first desuperheats (gives up sensible heat until it reaches Condenser saturation temperature). Then it condenses (releases a large amount of latent heat as it changes from vapor to liquid). Finally, liquid may be subcooled a few degrees. Air moving across the fins carries that heat away.
Why subcooling matters here: Adequate subcooling confirms you have solid liquid leaving the condenser. If subcooling is too low, part of the “liquid” line may still have bubbles, which hurts feeding at the metering device.
The temperature difference across something. In air conditioning, we often mean Return Air Temp – Supply Air Temp. Example: 75°F return → 55°F supply gives 20°F ΔT.
Why ΔT changes: With the same sensible capacity, higher airflow spreads the same heat removal over more air, so each pound of air cools less → lower ΔT. Lower airflow concentrates the heat removal into less air → higher ΔT — but too low airflow risks coil freeze and poor dehumidification control.
When the outdoor coil is covered in lint/dirt, it can’t reject heat well. The high-side pressure rises, amps go up, and capacity and efficiency drop.
Dirt on the indoor coil blocks air and heat transfer. Airflow falls and the coil may get too cold and start icing. Clean coils = happy system.
Clogged filters create a big pressure drop that the blower must push against. Many systems have low airflow problems simply because the filter wasn’t changed.
An electronically controlled motor that tries to keep a similar torque as pressure changes. Airflow still moves with restrictions, but it sags less than a PSC under the same blockage.
Behavior under restriction: It increases electrical input to maintain torque as static pressure rises, so airflow falls more slowly than PSC — but power draw and noise can increase, and at some point it still loses airflow.
The indoor coil. The refrigerant boils here at a low temperature, pulling heat out of the air. The blower pushes room air across the coil fins; the coil gets cold; air leaving the coil is cooler and drier.
The saturation temperature inside the evaporator coil. At this temperature (and its matching pressure for the system’s refrigerant), the refrigerant in the indoor coil is changing state from liquid to vapor. In plain words, this is the refrigerant’s boiling temperature inside the evaporator.
What it tells you: It shows how cold the evaporator is operating and whether the coil could approach freezing. When you compare the evaporator saturation temperature to the actual suction line temperature, the difference is the system’s Superheat (how many degrees the vapor is warmer than its boiling point inside the evaporator).
How to find it: Read the suction (low‑pressure) gauge and use a pressure–temperature chart for the specific refrigerant to translate that pressure into the corresponding saturation temperature.
A simple metering device with a fixed-size hole. Flow depends mostly on pressure difference. Because it can’t adjust itself, charge is set by superheat.
Small bubbles that appear when liquid refrigerant loses pressure or picks up heat and some of it “flashes” into vapor. Too much flash gas reduces how much usable liquid reaches the metering device.
Causes: Pressure drop in lines, heat gain into the liquid line, or too little condenser capacity can all create vapor bubbles in the liquid stream. This starves the metering device and reduces cooling capacity.
The “hidden” heat absorbed or released during a change of state without a big temperature change. Refrigerants absorb a lot of heat while boiling in the evaporator and release a lot while condensing in the condenser. This is the engine of air conditioning.
The pressure measured on the condenser side. It’s used with a PT chart to find Condensing (Saturation) Temperature. High head pressure stresses the system and often points to dirty condenser, overcharge, or very high outdoor temps.
Why hotter days raise head pressure: The condenser can only dump heat into the outdoor air if its Condenser Saturation Temperature (Condenser saturation temperature) is higher than outdoor temperature by some margin (often called the approach or condensing split). As outdoor temperature rises, the coil needs a higher Condenser saturation temperature to maintain enough temperature difference (ΔT) to push the same heat out. Because pressure and saturation temperature are linked by the refrigerant’s PT curve, a higher Condenser saturation temperature means a higher head pressure.
Physics snapshot: Heat rejection rate is roughly proportional to the temperature difference between the refrigerant inside the condenser and the outdoor air (plus airflow and coil effectiveness). When the air outside gets hotter, that temperature difference shrinks unless the refrigerant temperature rises too. Raising refrigerant temperature requires raising pressure → head pressure climbs.
How hard the house is to cool or heat right now—people, cooking, sun through windows, doors opening, insulation, and leaks. Load changes SIM readings like ΔT, superheat, and subcooling.
The actual temperature of the smaller liquid line coming out of the condenser. Compared against Condenser saturation temperature to calculate subcooling.
Line Set Matters: Long or sun-heated line sets can add heat to the liquid, lowering effective subcooling by the time liquid reaches the metering device. Good insulation and routing reduce this effect.
How hot or cold it is outside and how hard the condenser has to work to reject heat. Hotter days raise head pressure and reduce how much heat the condenser can throw away.
Permanent-Split-Capacitor motor. Simple and reliable, but airflow drops a lot when the system’s resistance rises (dirty filter/coil, small ducts). Your SIM shows this clearly when you move the airflow restriction sliders.
Behavior under restriction: As static pressure increases, PSC airflow drops quickly because motor torque is nearly fixed; it simply slides down its performance curve. That’s why dirty filters hurt PSC systems so much.
Pressure units. pounds per square inch is pounds per square inch. pounds per square inch gauge is gauge pressure (compared to outside air). Refrigeration gauges usually read pounds per square inch gauge.
The working fluid that moves heat by changing state. It boils at low temperature in the evaporator (absorbing heat) and condenses at higher temperature in the condenser (rejecting heat).
PT Chart: Every refrigerant has a pressure–temperature chart that links its saturation temperature to its pressure. Techs use PT charts (or digital tools) to translate gauge readings into Evaporator/Condenser saturation temperature.
How full of moisture the air is compared to how much it could hold at that temperature. Lowering RH usually makes air feel more comfortable. AC systems remove moisture as air cools below its dew point on the cold coil.
Dew Point: The temperature where air becomes saturated and water starts to condense. AC coils work by cooling air below its dew point so water condenses on the fins — that’s dehumidification.
Sensible vs. Latent Heat: Sensible heat changes temperature; latent heat changes moisture state. Cooling a room requires removing both sensible and latent heat. Airflow and coil temperature determine how much of each you remove.
Air pulled from the rooms back into the equipment to be cooled or heated again. “Return” → equipment; “Supply” → rooms.
The special temperature where refrigerant is changing state—liquid ↔ vapor—at a given pressure. There are two you’ll use all the time:
Why PT matters: For each refrigerant, pressure and saturation temperature pair up (PT chart). If you know one, you can find the other. This lets you translate gauge pressures into the “change-of-state” temperatures that make superheat and subcooling possible to calculate.
The push the blower must overcome from filters, coils, ducts, and grilles. Higher static = harder to move air = lower cubic feet per minute for the same blower. Your SIM shows how dirt and duct resistance increase static and reduce airflow.
Why it rises: Filters get dirty, coils clog, flex duct kinks, undersized trunks, too many bends — each adds resistance. The blower must push harder; if it can’t, airflow drops. PSC motors suffer most because their torque doesn’t adjust much; ECM (constant torque) compensates better but still has limits.
Measuring basics: Use a manometer on the return and supply sides (external to the furnace/air handler cabinet) and add them to get TESP. Compare to the blower’s rated maximum. If you’re over the rating, airflow is likely low and the system is stressed.
Definition: How many degrees the liquid refrigerant is cooled below its condensing (saturation) temperature in the condenser.
How to think about it: The condenser’s job is to turn hot vapor into solid liquid and then cool that liquid a bit more. That “bit more” is your subcooling. If you don’t have enough, you might still have bubbles (flash gas) in the liquid. If you have way too much, you might be overcharged or over-rejecting heat.
More detail: A condenser that only just reaches Condenser saturation temperature might produce liquid at exactly the saturation temperature — but any pressure drop or added heat can immediately create bubbles. Subcooling adds a safety margin by cooling the liquid below saturation so it stays liquid until it reaches the metering device. On thermostatic expansion valve/electronic expansion valve systems, subcooling is a more stable indicator of charge because the valve is already regulating superheat.
Definition: How many degrees the vapor refrigerant is heated above its boiling (saturation) temperature in the evaporator.
How to think about it: Once all liquid has boiled away inside the evaporator, the remaining vapor can warm up a little before it returns to the compressor. That extra warming is superheat. If it’s extremely low (near zero), there’s a floodback risk. If it’s very high, the coil might be starved—low charge or low feed.
More detail: In a healthy evaporator, most of the coil length is used for boiling (state change) and a short tail end is used for superheating the remaining vapor. Very low superheat suggests liquid may still be present at the outlet (floodback risk). Very high superheat usually means the coil is starved (not enough liquid feed or too much load/too little coil surface).
The actual temperature of the larger insulated line returning to the compressor from the evaporator.
How it’s used: Together with Evaporator saturation temperature (from suction pressure via a PT chart), it lets you calculate Superheat (superheat):
Why it matters: If superheat is near zero, there may be liquid reaching the compressor (floodback risk). If superheat is very high, the evaporator might be starved (low charge, restricted feed, or low load/airflow).
The air blowing out of the vents after the equipment cools or heats it.
The pressure on the evaporator/compressor inlet side. Use it with a PT chart to find Evaporator saturation temperature and then figure out superheat using the suction line temperature.
The total push the blower must overcome outside the equipment cabinet—filter + coil + supply/return ducts + grilles. Compare measured TESP to the blower’s rating to judge airflow health.
A sizing number in air conditioning. 1 ton ≈ 12,000 BTU/hr of cooling. Many homes are in the 2–4 ton range, but good design matters more than “bigger is better.”
thermostatic expansion valve: A thermostatic valve that tries to keep superheat steady by adjusting how much liquid goes into the evaporator. electronic expansion valve: An electronic version controlled by a board.
Charging tip: With thermostatic expansion valve/electronic expansion valve systems, techs usually judge charge with subcooling (not superheat) because the valve is already manipulating superheat.
How a thermostatic expansion valve “knows” what to do: A sensing bulb strapped to the suction line near the evaporator outlet feels the suction line temperature. Inside the valve, forces from the bulb pressure, spring, and evaporator pressure balance to open/close the valve and hold a target superheat. An electronic expansion valve uses an electronic sensor and a stepper motor to do the same job under control of a board.
Hunting: If the valve overshoots and then undershoots repeatedly, superheat will swing up and down — that’s hunting. Causes include poor bulb placement/insulation, bad charge in bulb, sticky valve, or unstable load/airflow.
Most of the “heavy lifting” happens during a change of state. Boiling (evaporation) soaks up a lot of heat without much temperature change; condensing gives off a lot of heat. That’s why saturation temperature temps are central in the SIM and why superheat and subcooling are such powerful clues.