AC Maintenance: Superheat & Subcooling
Video Transcript
Maintenance Process
Dean: Alright, we’re gonna go over superheat and subcooling. Superheat, is what?
George: So, superheat is the amount of heat that that freon has picked up out of your home. So you have a suction pressure, and based on that suction pressure, that’s what’s called saturation temperature.
Dean: Now, we’re getting a temperature reading on the suction line coming back?
George: Yeah, you want to take a contact temperature of the line itself, the copper line coming back to the unit.
Dean: All right, stop, so we need gauges and we need a temperature probe.
George: Yeah, you need–
Dean: This ain’t ya mama’s turkey baster probe, right?
George: No.
Dean: I mean this is a nice probe.
George: Yeah, you need a pipe clamp temperature probe.
Dean: Okay, so we gonna check the temp, we gonna check the pressures, and then we’re gonna let it run 15, 20 minutes, and then we’re gonna come up with an educated guess?
George: No, no, you’re not gonna be doing any guessing.
Dean: No guessing, okay, not beer can cold like my daddy taught me?
George: No, you’re gonna look at your saturation temperature on your gauge versus what that line is and look at what the manufacturer says under the conditions it’s operating–
Dean: What do you mean what the manufacturer says? What do you need for that?
George: You gotta take a wet bulb reading in the house, you gotta know the temperature outside, so and all manufacturers have a chart that tells you–
Dean: Where’s the chart at?
George: They’re in the paperwork, on the system.
Dean: Keep your paperwork handy. And you read the chart, and you just kinda spend about 15, 30 minutes out here looking, weighing everything out and analyzing, and go, baby, we’re on.
George: And then the same thing with the subcool reading, the subcool reading is gonna tell us that the system is charged properly also, but the subcooling is how much heat we got rid of.
Dean: Okay.
George: So, next comes in the factor of your condenser coil.
Different Types of Freon
Dean: Is there any difference between R-22 and 410A as far as checking which way?
George: Yeah, so, there’s a big difference in the pressures between R-22 and 410A.
Dean: Okay.
George: R-22 runs at a lower pressure than 410A. You take R-22 on a system that you may be running anywhere from 200 to 250 PSI head pressure. Well, you go to R-410A and now we’re talking 275, 325, 350, sometimes even higher than that.
Dean: Okay, so we’re running at a higher pressure at 410A.
George: That’s right.
Dean: Now, 410A’s a whole lot cheaper than R-22 cause they’re out dating R-22, R-22’s gonna be like buying gold.
George: Yep.
Dean: So if you got an R-22 system out there, this is where you can get educated, this system’s an R-22, it’s 18 years old, and the compressor goes out. You really need to look into putting in another system. You are crazy if you repair this and put R-22 back in it, because one leak, you’re talking 8, 900 bucks just to put the freon back in it. Plus, the leak search, sometimes you gotta be smarter not harder, you know what I mean?
George: That’s right.
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Summary
Superheat measures the temperature of a gas above its boiling point. Subcooling measures the temperature of a liquid below its boiling point.
Superheat
– What is Superheat?
Superheat measures the temperature of a gas above its boiling point. Measuring refrigerant superheat in an AC unit tells us the amount of heat that the Freon has absorbed from your home.
– How to Calculate Superheat
Warning, we’re about to get SUPER technical!
- We measure the suction pressure of the compressor in PSI (Pounds per Square Inch).
2. Depending on the suction pressure, we use a pressure chart to calculate the saturation temperature, also known as the boiling point – the temperature at which liquid Freon evaporates.
- The higher the pressure, the higher the boiling point.
- Freon “boils,” or evaporates, at a much lower temperature than water (-13°F at sea level, compared to water’s boiling point of +212°F). This makes Freon very effective to use as a refrigerant.
- In the AC compressor, Freon is under greater pressure – so it evaporates at a higher temperature than normal, which means it stays a liquid in the system for longer.
3. We will let the unit run for 15-20 minutes to get the best results.
4. We use a temperature probe to measure the Freon temperature as it exits the evaporator, subtract the saturation temperature, and voila!
If you ever wanted to know how to calculate superheat (and who hasn’t), there you go.
– Potential Problems with Superheat
Calculating superheat can tell us how far liquid Freon is traveling through an evaporator coil.
- If superheat is too high, there’s less liquid and more gas in the coil. This can cause overheating.
- If superheat is too low, more liquid and less gas are in the coil. This can cause flooding and damage in the compressor.
Subcooling
– What is Subcooling?
Subcooling tells us how cool the liquid Freon gets in the condenser coil. A subcool reading can help us make sure the cooling system is properly charged with Freon. Superheat measures the Freon in a gas form – how far ABOVE its evaporation point it is. Subcooling measures Freon in a liquid form – how far it is BELOW the evaporation point.
– How to Check Subcooling
- We use gauges and a temperature probe to measure subcooling, just like when measuring superheat.
2. We check the temperature of the liquid line near the condenser coil.
3. We’ll check the head pressure at the condenser, comparing the readings to the manufacturer’s standard.
4. Then we subtract the two numbers to calculate subcooling.
– Potential Problems with Subcooling
- Low Subcooling – If the Freon’s temperature isn’t far enough below its evaporation point, that could indicate that there’s an insufficient Freon charge in the system.
- High Subcooling – If the temperature is too cold, Freon could be backing up into the condenser. There could be too much Freon in the system, or the metering device, used to drop the temperature of the Freon, is failing.
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