New to superheat and subcooling

[HeathT]

Hi all. Have been working in a/c just over 4 years but never really been taught any fundamentals or done any training with my current employer. just learning from experience and this website as been a goldmine for picking up tips and knowledge. I have been reading alot of threads about superheat and subcooling. Is it something you would check at every service or just use it for troubleshooting. Also is a simple air on air off coil temp reading suffice to ascertain if a unit is working correctly?

[Slatts]

Hi all. Have been working in a/c just over 4 years but never really been taught any fundamentals or done any training with my current employer. just learning from experience and this website as been a goldmine for picking up tips and knowledge. I have been reading alot of threads about superheat and subcooling. Is it something you would check at every service or just use it for troubleshooting. Also is a simple air on air off coil temp reading suffice to ascertain if a unit is working correctly?

Gedday heath, Superheat should be set when the system is commissioned. If your system is performing OK and your compressor is not overly hot or excessively cold I wouldn't go playing with it.
Further to that, I've seen a few people who are new to refrigeration, on seeing a low temp compressor covered with ice, rush off and close down the TX valve.
Consider this.
If you have a -30 C saturated suction temperature and -10C suction line temp at the compressor, you have a total suction superheat of 20K.
At -10 suction temp, you will have ice on the compressor. Live with it.
But I've just re-read your post and now realise you're talking about aircon.

For a general service I'd be more interested in the function of the condenser fans, the cleanliness of the condenser and the liquid line sight glass. Make sure your condensate drains are clean and clear, especially the traps. There's nothing more embarrassing than having a ceiling fill up with water the day after you've serviced the aircon.
Make sure the air filters are clean and in good condition and the evaporator is clean.
Check the condition of any V belts and bearings in the system.
If your air on and off temps are in the ball park, don't go worrying about superheat and subcooling beyond a quick feel of the pipes to make sure they're not excessively hot / cold.
Of course, if the system is not performing normally, check everything properly.

[nike123]

I have been reading alot of threads about superheat and subcooling. Is it something you would check at every service or just use it for troubleshooting.

That depend on do you put your gauges on system or not. If you put them, than reason is not to read pressures. Reason is to read pressure to convert it to saturation temperatures to be able to determine superheat and subcooling, and that you need to be able to tel how system is performing.

Also is a simple air on air off coil temp reading suffice to ascertain if a unit is working correctly?

No, you need humidity reading and air flow reading and performance table or graph of tested unit to compare with calculated cooling or heating capacity.
And still, something could stay undisclosed with that.

[Slatts]

Hi nike, from what heath wrote, I'm assuming he is talking about a routine maintenance service.
If this is the case, ball park checks should suffice.
check belts, bearings, coils, fans, filters, pressures, oil levels, gas level, drains, traps,tactile temperatures, check for oil on joins, hot electrical connections and general operation.
I think a total system performance test is a tad over the top.

[nike123]

Hi nike, from what heath wrote, I'm assuming he is talking about a routine maintenance service.
If this is the case, ball park checks should suffice.
check belts, bearings, coils, fans, filters, pressures, oil levels, gas level, drains, traps,tactile temperatures, check for oil on joins, hot electrical connections and general operation.
I think a total system performance test is a tad over the top.

That is why I wrote:

That depend on do you put your gauges on system or not.

[El Padre]

Hi,
Subcooling will indicate the quantity of liquid in the condenser, and superheat will indicate the quantity of refrigerant in the evaporator, they are essential values to know if you think that there is a refrigerant flow or charge problem, you could have low suction pressure and high superheat and think that the system is short of refrigerant but if the subcooling was correct it would probably be a blocked drier or expansion valve problem. Most chillers etc. will tell you the saturated suction and condensing temperatures on the controller, I would not personally put gauges on a split system on a maintenance visit if it was operating correctly, especially as over here we have R410A.

Cheers

[Gary]

Hi nike, from what heath wrote, I'm assuming he is talking about a routine maintenance service.
If this is the case, ball park checks should suffice.
check belts, bearings, coils, fans, filters, pressures, oil levels, gas level, drains, traps,tactile temperatures, check for oil on joins, hot electrical connections and general operation.
I think a total system performance test is a tad over the top.

With the exception of checking pressures, I agree wholeheartedly. On a routine maintenance call (no problems reported) if all of the above checks out and the suction line is cold, the system is working reasonably well and there is no need to look further... and no need to put gauges on the system.

In particular, I would recommend air on/off temps be checked on both coils.

[Slatts]

With the exeption of checking pressures, I agree wholeheartedly.

Point taken Gary. i do mostly commercial refrig and some aircon maintenance on larger ducted systems. They (the aircon systems) mostly have suction, oil and discharge gauges mounted on them. I don't normally hook up my gauges to small splits on a service and have had only limited exposure to R 410a.
R410a is fairly common here now, in fact I don't think they're selling new R22 systems anymore here about, so I guess I won't be able to hide from it much longer

[HeathT]

Thanks for all the input guys, I really appreciate it and like I said before this site has been a goldmine for learning more about A/C

[Gary]

An important thing to remember here is that poor airflow through either coil will substantially alter your subcooling and superheat measurements... so always check airflow first.

People tend to underestimate the importance of good airflow... it is absolutely essential.

[HeathT]

Hi nike, from what heath wrote, I'm assuming he is talking about a routine maintenance service.
If this is the case, ball park checks should suffice.
check belts, bearings, coils, fans, filters, pressures, oil levels, gas level, drains, traps,tactile temperatures, check for oil on joins, hot electrical connections and general operation.
I think a total system performance test is a tad over the top.

Generally i do all the checks mentioned on a maintenance visit. How would you check oil levels, would it be done on the sight glass on the compressor?

[Slatts]

Generally i do all the checks mentioned on a maintenance visit. How would you check oil levels, would it be done on the sight glass on the compressor?

yes. obviously if there isn't one, don't worry about it.
In most compressors, if the oil is between 1/4 and 3/4 of the way up the sight glass on the compressor it should be OK.
Carlyle make a range of semihermetics for which the recommend the oil level is between 1/8 and 3/8 of the way from the bottom of the glass. They recommend the level is checked with the compressor off.
To high an oil level can result in it being airrated (refrigeratrated?) by the compressors crank, causing a loss of lubrication. It can also cause a hydrolic lock up of the compressor, which is what we call a BAD THING.
It can cause such problems as broken or bent con rods or cranks, blown valve plate gaskets, broken valves etc.
If the level is low it could be caused by poor pipe sizing or bad installation (lack of oil traps in risers etc), not checking and adjusting the oil level when the system was commissioned, dirty air filters or evaporator coil causing either flood back, which airrates the oil and causes it to be lifted out of the sump, or lack of suction velocity which is needed to push the oil that makes it to the evap back to the compressor.
Oil level is important. check it if you can when you service a system.

[HeathT]

Thanks slatts very informative.

[Slatts]

no worries