Help! new A/C tech having charge probs?

my problem is that i am running spring maintenance calls as a new hvac tech. with txv, and newer install piston/captube metering devises i am having no problems. on older piston/cap tubes is where im struggling according to everything ive been taught on how to check superheat i can seldom get it to work. i check the temp on the line near the service valves and compare it to the pressure converting it to temp using a tp chart or whats listed on the gauge. 9 times out of 10 there is no superheat reading which indicates to me overcharge or plugged up coil. ive cleaned coils condensor and pulled cleaned evaporator, checked orifices in the metering for plugs. ive removed freon and the pressure converted to temp using a tp chart matches the exact temperature of the the suction line meaning 0 superheat, it rides even. now old school method suction line beer can cold i have no problems. the lines are not freezing the compressor is not making funny noises. just no superheat reading.

Help! new A/C tech having charge probs?
Different manufacturers suggest diferrent means.Carrier uses a wet bulb temp. inside cross refrencing suction line temp. pressure and ambient air temp. outside.This tells you what the suction line temp.should be. This works well,if the temp.is too warm you add to the charge. If temp. is too cold you reduce the charge. This works great. Adjusting charges with this method i have dropped the temp.at the vent 8 to 10 degrees.So it works for me. This method is used on piston cap tube units.I have not serviced tex or thx units i believe they take a temp just past the valve and you can adjust the valve itself to get super heat and proper charge. An easier method to use on piston cap tube units is to look at your suction pressure on the guage,drop down to the temp.scale add 10 degrees and your suction line temp. should match up.So if you have 70 psi on the guage your r-22 temp is 42 degrees add 10 degrees and your suction line should be 52. This works very well,one of the best ac trainers in the country taught me this. I have checked it against the wet bulb method it is within a degree or two.WORKS GREAT. heat pumps are what i service,they are very sensitative to accurate charge a couple oz's. makes a difference. another tip is to get an accurate suction line temp.you must use a digital thermometer with a wire sensor.I pull the armor flex back and tape the sensor to the suction line then pull the armor flex over the sensor and tape it. This gives you an accurate temp. You do not get accurate temps.with infared on suction lines the air movement throws them off.Also it takes 20 minutes for a charge adjustment to show up on your guages and temp. readings so it is not a fast process to do correctly.One more thought,older units have corroded inside the metering device and cap tubes are probably smaller than a newer unit. Early in the season the air temp. outside is not that warm. When you have partial jug of 22 it charges slowly. When a unit is close to full or correct charge it gets real slow.Most guys turn the the jug over and liquid charge.Not suppose to do that.With the cooler temp. the 22 will freeze at the metering device then your suction pressure will not show a change eaven though your adding refrigerant.Next the suction line freezes,and the compressor makes all kinds of noise, Your suction pressure will fall off as if you have a stopped up line then the compressor kicks out on over load. Sound familiar?Actually were not suppose to liquid charge anytime,I know your thinking everyone does it and they do.In the cooler weather you need to get a 5 gallon bucket half full of hot water.put your 22 in that to heat it up right side up so you vapor charge this will save you a lot of extra work.Sorry to be so lengthy but there are whole books written on this subject.
Reply:Pretty good answers around the board....check your high side pressures...should be equal to 30 F plus ambient air temp...if less than undercharged..more than overcharged...Use 20 F for 410-A systems with the larger condensor coil.





also check amp draw on the compressor....overcharge puts more load on the compressor





TXV's will give you a correct superheat so be sure to check subcooling too.





oh- most importantly make sure all panels are in place!!!!!!
Reply:What kind of pressures are you running? What is the outside ambient and the inside ambient temp? Are you getting good airflow across the evap coil? It sounds like you are still overcharged. I M or e-mail and I will try to help
Reply:superheat is only for txv and piston metering devices. sub-cool for cap tubes.
Reply:Low load situation, too cool inside and out.
Reply:Overcharging is pretty common for a lot of techs. They like to read the gauge and keep dumping gas in until they get 62 psi suction. Beer can cold is still a bit cool for the suction line. It should stay pretty consistent with a txv, but will change a bit with an orifice (piston). The rule of thumb I learned when I started is 10 degrees superheat, 20 degrees temp drop across evaporator, 30 degrees temp pickup across condenser. This assumes that evap and cond coils are reasonably clean. Anymore, I don't use the gauges unless there's a problem. If you are getting airflow across both coils, your suction should be cool, but not ice cold....your liquid line should be ambient, or just a bit warm....you should feel heat coming from the condenser, and get a good temp drop across the evap. In many cases, there are a lot of variables (installation) which change the exact pressures and temps that the system will run at.





I guess the bottom line is that you have to have some superheat, or you'll kill the compressor, either sooner or later. Confirm that airflow, don't settle for less than 10 degrees superheat, 15 or even 20 is better yet.





It can be confusing at first, but try to imagine what the refrigerant is doing inside the pipe. If you understand the refrigeration process, it'll get easier with practice.


Hope this helps....Best of luck to you.


20+ years HVAC tech.