9 superheat & subcooling scenarios

• LOW CHARGE = High superheat. Low suction pressure, indoor TD, subcooling, head pressure & compressor current draw.
• OVER CHARGE = Low superheat. Normal indoor TD. High suction pressure, subcooling, head pressure & compressor current draw.
• LOW INDOOR AIR FLOW = Low superheat & suction pressure. Low to normal head pressure. High indoor TD. High to normal subcooling. Not much effect on current draw.
• MILD LOW OUTDOOR AIR FLOW = Low superheat & subcooling. High suction & head pressure. Normal indoor TD. Normal to high current draw.
• SEVERE OUTDOOR LOW AIR FLOW = High superheat, suction pressure, head pressure & current draw. Low indoor TD & subcooling.
• MILD RESTRICTION (factory charge) = High superheat. Lowsuction pressure, indoor TD & current draw. Normal to high subcooling. Normal to low head pressure.
• MILD RESTRICTION (with excess refrigerant) = Normal superheat, suction & indoor TD. High subcooling & head pressure. Low current draw.
• SEVERE RESTRICTION (with excess refrigerant) = High superheat, subcooling, current draw & head pressure. Low suction pressure & indoor TD.
• WEAK COMPRESSOR VALVES = Low superheat, current draw, head pressure & indoor TD. High suction pressure & subcooling.

Thanks Nike, i'll print that off. It will come in useful i'm sure:)

...concise, precise, useful. Thanks!

Thanx for that will keep that close.

Sir, I thank you very much.

these parameters are really useful (when i understand them they will even more lol)

Quote:

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Originally Posted by frostedflake

Sir, I thank you very much.

these parameters are really useful (when i understand them they will even more lol)

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What you are dont understand?
Ask, and we will help.

please also see attached on this topic,it was posted on here a while ago and may be of some use

nice & accurate Nike,
going to print it and keep it in Van.
thnx Ice

Does anyone want to add more scenarios? Maybe situations with multiple troubles.

NIKe thanks so much for these very importan points i will aske u some questions later.
best regaed

Quote:

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Originally Posted by Noori

NIKe thanks so much for these very importan points i will aske u some questions later.
best regaed

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It is not my creation. I just found them on one document I downloaded earlier an there is no author indicated.
I found them very handful and that is why I posted them.

Regarding your questions, I will be glad to answer (if I knew answer;)) to them.

Another useful list of scenarios is found here:
http://sporlan.jandrewschoen.com/Jun...%20ANSWERS.pdf

And another one:
http://sporlan.jandrewschoen.com/Col...0Servicing.pdf

And another one:
http://sporlan.jandrewschoen.com/Col...t%20Finder.pdf

Great documents Nike123, thanks!! :)

On 4 and 5. "outdoor air flow"

dose this mean condenser air flow? or low ambient air?:confused:

Quote:

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Originally Posted by Phase Loss

On 4 and 5. "outdoor air flow"

dose this mean condenser air flow? or low ambient air?:confused:

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Condenser air flow!

Great!
Thanks

Just one more scenario.............

How all these parameters will be affected if there is air or moisture in the system?

That question arises after cleaning my vacuum pump from water from a 300 KW water chiller.........;)

Air=Noncondensables
Water=Restriction

http://img161.imageshack.us/img161/9...otguiderw4.jpg
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OK im still in school and trying to tinker with my own home system a bit. Because although ive got my 20degree temp difference i am getting no condensation and it is way to humid out to not be gettin any. My superheat is low also at 4 degrees and manufacture specs call for 17-20 in current conditions. ideas?

Quote:

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Originally Posted by Cjmcatch

OK im still in school and trying to tinker with my own home system a bit. Because although ive got my 20degree temp difference i am getting no condensation and it is way to humid out to not be gettin any. My superheat is low also at 4 degrees and manufacture specs call for 17-20 in current conditions. ideas?

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I am also in a first year apprentice, but i would like to try to help and possibly some more experienced will correct me so i can learn also.

Is this a TXV or fixed metering device? Symptoms are low charge, but check airflow first. Check condenser fan amps, and blower amps. Check insulation in the blower cabinet to make sure it has not come off and blocked airflow. Never assume low charge before checking airflow first.

What is your actual subcooling? If you have a TXV then your subcooling is the only way to truly figure out whats going on in the system. Superheat is a good way of troubleshooting because if the load is high it opens, and if the load is low it closes. It's a good way of checking a TXV, but not good at checking charge of a system.

I have a 3 ton residential split system with 20 subcool and 20 superheat, manu specs 10-12 subcool its a ducan evap and cased coil with txv and R22. Will super heat always be low with a overcharge? I ask because I dont see how there could be a restriction, I installed the system and took care to avoid any contaminants. The txv could not be funtioning right also I suppose, any ideas?

This is a very useful thread and posts. Thanks.

Another scenario,if anyone can answer.
Checked a chiller and found low subcooling( well the manufacturer did) with an ambient of 9°C, he added some R134A, system was at 100% load.

Two weeks later another manufacturer employee went out and said it was over charged, reclaimed the gas and said it had been overcharged by 2kgs(64kg charge).
I was not sure what the ambient or pressures were as i did not attend.
Someone explained that as the ambient was lower the subcooling coud have been low and as the ambient was probably higher the 2nd time around and had been overcharged the subcooling would have been low again because of the liquid backing up in the condenser.
How can you have effectively have the same scenario ( low subcool) and have two different answers.

hi wozza

I think if the condensor is backing up with liquid subcooling would be high not low as there is more liquid rather than beforehand where the subcooling is lower cos there is less liquid.

Subcooling is the difference between SCT and liquid line temp. The liquid temp cannot drop below its surrounding temp (9C). Given cold ambient, the SCT can come down close to ambient (9C). Subcooling can be very low in cold ambients, regardless of charge.

Low subcooling is not a valid symptom. I block off the condenser, check for high subcooling, which actually means something... and then I check the superheat.

In the second scenario, the subcooling would be high, not low.

The thing to mention in the Original Posts list of scenarios,

If your system is capillary or fixed orifice, superheat is the measurement you need to get right,

For t/x valves you need to get your subcool right.

Quote:

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Originally Posted by Aircondmech

The thing to mention in the Original Posts list of scenarios,

If your system is capillary or fixed orifice, superheat is the measurement you need to get right,

For t/x valves you need to get your subcool right.

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I would say you need to get both right in any system.

Cheers Gary.
It makes a bit of sense now.

The chiller had a large approach temp of 9-10°C, but it also had a low superheat of 2°C.
This meant the shell+ tube evap was not producing anough heat transfer which concluded was due to the concentration of glycol which the customer could not tell us and i didnt have the correct tool.

What i dont understand is - if you have low subcool it means the liquid is not staying in the condenser long enough to cool, would this also mean you have a low head pressure? But if you have inverter fans controlling head pressure then this should be o.k. resulting in normal subcool?

I think i,m starting to confuse myself again.

I totally understand. Suction superheat and discharge superheat but somehow cannot get to grips with subcool 100%.

Anyone shed some light?

The important thing to watch for in subcooling is that liquid is not backing up into the condenser (high subcooling).

This becomes difficult to get an accurate measurement in cold ambients, because it is physically impossible for the SC to exceed the cond TD (SCT minus air in). With fan control and/or physical blockage, we can raise the TD and get a more accurate SC measurement.

Similarly, (assuming the TXV bulb is subjected to refrigerated space temp) it is physically impossible for the bulb SH to exceed the evap TD (air in minus SST) .

I had a preventative maintenance done on the A/C last friday. This tech measured the suctions and temperatures and wants $500 to enlarge the intake duct. Prior techs haven't bothered to measure the temperatures and pressures. Suction is 50, Discharge is 225. Subcool temp is 14, superheat is 36 (Fahrenheit I assume). Is it really the evaporator being starved of air? The house was built in 1979. The unit is 11 years old and it has recently had its condenser fan motor replaced.

According to these links, the suction is low, discharge is normal, Subcool and superheat are high.

http://appliancejunk.com/forums/index.php?topic=2924.0
http://www.1coolair.com/houston_ac_n....php?BlogID=10

More numbers: Compressor amps: 12.6, rated 20.7; Indoor motor: 1.6 amps (no rated written down); Outdoor motor: 1.9 amps, rated 2.6, supply temp 52, return temp 70, temperature split: 18 at unit, 16 indoors (Fahrenheit I assume). Prior techs measured the amps, but not the temps & pressures. The unit is a 3.5 ton Tappen.

If the evaporator were starved for air, the split (delta-T) would be high and the superheat would be low. The evaporator is starved for refrigerant, but the condenser is not, therefore the liquid flow is partially restricted.

Is the system TXV or fixed orifice?

I was thinking it was lining up with the description of 'Restricted metering device'.

Quote:

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Restricted Metering Device
A restricted metering device acts similarly to a liquid line restriction that occurs after the receiver. A restricted TXV will cause the evaporator, compressor, and condenser to be starved of refrigerant, causing low suction pressures, high superheat, low amp draws, and low head pressures

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I don't know on txv vs fixed. I'm not a A/C tech, just a homeowner that is willing to look up the numbers. The unit is a 3.5 ton Tappen installed in 2000 (it is 11 years old). I was told that Tappen is a cheap air conditioner brand. The stickers are ripped off, so there is no serial number or model number.


The A/C guy says if anything is wrong with it its not worth fixing because of the unit's age. He really wants to sell me a new unit.

TXV and fixed orifice are different types of metering devices... and that's where the restriction would most likely be. Most TXV's have an inlet screen which may be plugged.

I would consider up to 15F to be normal subcooling. You might try adding a little refrigerant to compensate for the restriction and lower the superheat, but don't go much beyond the 15F subcooling limit... definitely keep it under 20F subcooling. Keep in mind that raising the subcooling adds to the energy consumption and shortens system lifespan. It's a trade-off, but may be the best choice if you can't afford to fix it. Much better to fix the restriction.

Or it may make more sense to replace the system... especially since the new systems are more energy efficient.

Does the fact that when he did the inspection when it was 65 outside and 75 inside and that the unit hasn't been run in several weeks skew the pressure and temperature numbers?

What is the rough ballpark figure to fix a metering device restriction?

This isn't a heat pump in heating mode, is it?

It is a heat pump but it was inspected in cooling mode. I live in Arizona (southwest USA). It hardly needs to be used late-February through early April.

Hi guys new to this but here is what I have. I got a r-22 spit system a/c. manufatured in 03/05. I was told 2 years ago that I have a slight leak in the system and the tech added some refigerant. It has been cooling great after the repair and all last season but this year it is running longer and not seaming to cool so good.

I have a suction pressure of 70, converted temp of 41, suction line temp of 51, head pressure of 250, converted temp of 117, head line temp of 96. Air entering condenser is 82 and exit is 100. Air entering evaporator is 76 and exiting is 52.

If I have my theroy right I have a low superheat of 10 and a high subcooling of 21.

Ambiant temp outside is 81 and all temps are in F.

Insufficient airflow through the evaporator... and probably overcharged 2 years ago.