It’s not rare to see high superheat high subcooling problem in AC diagnostics. When both superheat and subcooling are elevated, there are only 2 possible culprits. We will look into these culprits and lay out how to fix high superheat high subcooling AC problem.
For this situation, we need to have:
- Actual measured superheat is by 2°F or more degrees higher than the target superheat; this is considered high superheat.
- Actual measured subcooling is by 3°F or more degrees higher than the target subcooling; that is considered high subcooling.
Namely, we have covered high superheat causes here and high subcooling causes here. If we put all of these causes in a neat table, we can quickly see what is causing high superheat and high subcooling in air conditioning units:
| High Superheat Causes: | High Subcooling Causes: |
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As you can clearly see from the bolded causes in high superheat and subcooling, this particular problem is caused by either:
- Restriction in the liquid line (usually ice).
- Metering device (TXV, AEV, or piston) is underfeeding.
We are going to check if you have high superheat high subcooling due to restriction or due to metering device underfeeding and how to fix each of these issues.
Note: We can also get wrong measurements/calculations. If you need to check if you have done everything as you should, you can check how to measure superheat here (10 steps) and how to calculate superheat here (with R22, R410A, R134A examples), as well as how to measure subcooling here (9 steps) and how to calculate subcooling here. This target superheat chart (R22, R410A, R134A examples) and this target subcooling temperature can also be useful.
Alright, let’s start with the more common cause of high superheat high subcooling and how to fix it:
High Superheat High Subcooling Caused By Restriction In the Liquid Line (1st Cause)
Restriction in the liquid line is usually used by a piece of ice. Now, refrigerant lines are a closed system; nothing goes in, nothing goes out. However, in very limited quantities (usually), water can enter into the lines (humid air, condensation). That’s why we have a filter dryer that absorbs this.
There are two cases when this filter dryer doesn’t do its job adequately:
- There is something wrong with the filter dryer itself.
- The amount of water entering the refrigerant lines is too high for the filter dryer to absorb it all.
In either of these cases, the excess water in the lines freezes over (temperature of liquid refrigerant can go below water’s freezing point of 32°F). This forms a piece of ice that can partly block (restrict) the liquid lines.
The metering device now gets less liquid freon, and feeds less freon into the evaporator coils. This less-than-needed refrigerant in evaporate coils in quickly turned into 100% vapor, and the vapor increases in temperature more than it should. This results in high superheat.
The ice blockage partly restricts the flow of liquid refrigerant. That means the refrigerant is being packed behind the ice; thus there is more refrigerant there. Since the outdoor condenser coil cannot adequately cool so much refrigerant, the temperature of the refrigerant cannot sufficiently fall. This results in high subcooling.
Here is how we can diagnose if you really have an ice block causing the high subcooling high superheat situation (ACHR News has a good article about mistaking restriction for undercharged AC unit here):
You should see a temperature drop over the ice block. In normal operation, all the liquid lines should have the same temperature (let’s say 40°F). If you do have an ice block, you should measure a higher temperature before the block (let’s say 42°F). After the block, the temperature should drop (let’s say 38°F). An HVAC technician may also use a freeze test or thermal imaging to locate the restriction.
If you don’t see this temperature drop, then you know the high superheat and subcooling are caused by the 2nd cause: metering device underfeeding.
If you have this ice restriction in the liquid line, here is how you can fix it and normalize superheat and subcooling:
All liquid line restrictions should be fixed by a licensed HVAC technician. This is not a DIY we can do ourselves. The best course of action if you suspect liquid line restriction is to call your HVAC guy. He or she will locate the restriction, see if it can be removed or, alternatively, replace the part of the line containing the said restriction.
Note: It is also important to check if everything is OK with the dryer filter. If it’s not doing its job correctly, we run a risk of again getting a piece of ice in the lines, and eventually we will end up with the restriction in the liquid line again, increasing superheat and increasing subcooling.
Metering Device Is Underfeeding (2nd Cause)
The job of the metering device (located on the liquid line) is to feed an adequate amount of liquid refrigerant into the evaporator coil. If it is underfeeding, we will have less refrigerant in the evaporator lines, it will get hotter than it should, resulting in high superheat.
In the outdoor unit, the underfeeding metering device, such as TXV, fixed orifice (piston), or AEV, will restrict the normal flow of refrigerant. We will have a build-up of freon in the outdoor condenser coil. Cooling down a bigger amount of refrigerant will be slower, resulting in high subcooling.
This metering device underfeeding happens quite a lot in units with TXV valve, and the high superheat and high subcooling is the 1st symptom we usually see.
Here’s how to fix a metering device underfeeding, such as TXV, that is causing high superheat high subcooling:
In units with a TXV valve (TEV valve), we have to check the sensing bulb. Usually we get two scenarios:
- TXV valve is not adequately insulated.
- TXV valve is not properly secured.
Check the insulation and secure the TXV valve. After that, you should see the high superheat come down to normal superheat and high subcooling come down to normal subcooling.
If you have a piston as a metering device, there are also two options for why such a piston might be underfeeding the refrigerant:
- Piston is not correctly sized. If we have an undersized piston, it will be underfeeding the AC evaporator coil, resulting in a high superheat high subcooling situation.
- Piston is missing.
When you repair the TXV valve or piston, the elevated superheat and subcooling will normalize.
In short, when you see high superheat high subcooling, immediately think about two things: restriction in the liquid line, and metering device underfeeding. Once you diagnose which one of these problems is causing the high superheat high subcooling, you can fix that cause, and see both of these key metrics normalize within a few cycles.