Low superheat low subcooling is quite a paradoxical AC problem. How can you have low superheat and low subcooling at the same time? We will explain what causes low superheat low subcooling issue in air conditioners, and how to fix it.
Here is what low superheat and low subcooling means:
- We get low superheat when the actual measured superheat is 2°F or more degrees lower than the target superheat. You can read how to measure superheat adequately here, and how to read the target superheat from the chart here.
- We get low subcooling when the actual measured subcooling is by 3°F or more degrees lower than the target subcooling. You can read how to measure subcooling here, and how to find out target subcooling for your AC here.
Now, to understand why both of these low superheat subcool states exist, we have to check the causes for both of them and cross-match common causes. Here is a chart that contains low superheat causes and low subcooling causes:
|Low Superheat Causes:||Low Subcooling Causes:|
As we can see, the low superheat low subcool is not caused by a low refrigerant charge. A low refrigerant charge can result in low subcooling, but it will not decrease superheat; rather, it will increase superheat, leading to the high superheat low subcooling issue we discussed here.
We can see that there is only 1 low superheat low subcooling cause (bolded letters in the chart above). That’s the metering device like the TXV valve overfeeding. Let’s look at why this happens and how to fix low superheat low subcooling:
Low Superheat Low Subcooling Caused By Metering Device Overfeeding (TXV, AEV, Piston)
Here is how metering device overfeeding leads to low superheat and low subcooling:
- Low superheat. The metering device (TVX, piston) is letting through too much liquid refrigerant. When a bigger-than-normal amount of refrigerant enters the evaporator coil, the heating load from the indoor warm air flowing over the evaporator coil is not sufficient to turn all of it into 100% vapor and increase the temperature of that vapor sufficiently. Basically, it’s harder to heat up more refrigerant, and we are left with low superheat.
- Low subcooling. As a result of low superheat, we will have a cooler-than-normal vapor refrigerant entering the compressor and then into the condenser coil, resulting in low subcooling.
Here is how to fix low superheat low subcooling due to overfeeding metering device:
The problem is the metering device. Now, we can have a TXV valve (Thermal eXpansion Valve, or TEV) or a fixed orifice like a piston.
If you are experiencing low superheat low subcooling with a TXV valve, the most common problem is that the sensing bulb (key part of the TXV valve) is either:
- Not insulated properly.
- Not secured to the diaphragm properly.
We need to check the TXV insulation and if the valve if adequately fixed on the diaphragm. Once we insulate and secure it properly, we should see low superheat go back to normal superheat and low subcooling go back to normal subcooling.
If you have a piston metering device (fixed orifice), you also have to check for 2 things, namely:
- Is the piston correctly sized? If the piston is too big (oversized metering device), you will get low subcooling.
- Is the piston even there? A missing piston can cause overfeeding and result in low subcooling.
Once you get a correctly sized piston that is feeding the normal amount of liquid refrigerant, you should see both low superheat subcooling normalize.
Overall, whenever you see low H low S (low superheat low subcooling), your first thought should be that there is something wrong with the metering device. Only overfeeding TXV or piston causes low superheat low subcooling, and you have to fix it to normalize the temperatures.