Subcooling, explained: charging a TXV system by the numbers
On a TXV system the valve holds superheat steady, so subcool is your charging number. Here is how subcool works, how to read it, and how to charge to a target without chasing your tail.
If superheat tells you what is happening on the low side, subcooling tells you what is happening on the high side — and on a TXV system it is the number you charge to. Get subcool right and a thermostatic expansion valve has a solid column of liquid to meter, the coil gets fed properly, and the system runs at the efficiency it was designed for.
What subcooling is
Subcooling is the number of degrees the liquid refrigerant has cooled below its saturation (condensing) temperature at the pressure you measured. It is the mirror image of superheat:
The formula
Subcool = saturation temperature − liquid line temperature. Read the liquid line pressure, convert to saturation temperature on the PT chart for that refrigerant, then subtract the measured liquid line temperature from it.
Subcool exists because the bottom rows of the condenser are doing more than just condensing — once the refrigerant has fully turned to liquid, the remaining coil keeps pulling heat out of it, dropping it below its boiling point. A healthy stack of subcool means you have a full, solid column of liquid leaving the condenser. That matters because the TXV needs liquid, not a frothy liquid-vapor mix, to meter accurately.
Why subcool is the TXV charging number
A thermostatic expansion valve actively modulates to hold evaporator superheat within a narrow band. That is its job. Because the valve is constantly adjusting superheat, you cannot use superheat to judge charge on a TXV system — the valve will mask both an overcharge and a moderate undercharge by simply opening or closing. Subcool, on the other hand, climbs as you add refrigerant and falls as you remove it, so it is the honest charging signal.
Rule of thumb
Most manufacturers spec a target subcool somewhere in the 8 to 14°F range. But target subcool varies by manufacturer and model — always charge to the number printed on the unit's charging chart or data plate when one is present, and only fall back to the typical band when it is not.
Reading the number
- Low subcool (under ~5°F): usually undercharged — there is not enough refrigerant to back up a full liquid column in the condenser, so liquid is leaving warm. A restricted or dirty condenser, or a condenser fan that is not moving air, can fake the same low number, so confirm the coil and airflow first.
- High subcool (over ~20°F): on a TXV system this points to overcharge — excess liquid is stacking up and backing into the condenser, flooding coil surface that should be condensing. High subcool also raises head pressure and hurts capacity and efficiency.
- On target: subcool sitting at the unit's spec means you have a full liquid line and the valve has what it needs. Pair it with a superheat reading in the valve's normal range and you have a system that is both correctly charged and metering well.
Charging to subcool, step by step
- Confirm conditions and airflow first. Subcool charging assumes a clean condenser, correct indoor airflow, and a stabilized system. Fix airflow and a dirty coil before you touch refrigerant, or you will chase a number that keeps moving.
- Find the target. Read the target subcool off the charging chart on the unit. Only use the 8 to 14°F band if no chart is present.
- Measure and compare. Take liquid pressure and liquid line temperature, calculate actual subcool, and compare to target.
- Adjust in small steps. Add refrigerant to raise subcool, recover to lower it — a little at a time, letting the system stabilize for several minutes between checks before you read again.
Before you connect
Refrigerant work requires EPA 608 certification, and recovering refrigerant to the atmosphere is illegal. Always charge against the unit's chart when it exists — the typical band is a guide, not a substitute.
Try the tool
Charge by the numbers with the Subcool calculator
Enter your refrigerant, liquid pressure, and liquid line temp to get subcool and the saturation temperature on the spot, with undercharge and overcharge flags. It runs offline, right at the condenser.
Open the Subcool tool