High superheat is an AC problem that won’t go away by itself (except due to the #4 cause). We have to fix high superheat. In order to do that, it’s particularly useful to understand what causes high superheat in air conditioners. There are 6 common high superheat culprits that we always check, and we’ll go over all of them one-by-one.
Namely, a high superheat is a state where the actual measured superheat is more than 2°F higher than the target superheat. Here is how we measure superheat and here is the target superheat chart that will help determine if your AC has high superheat. Here’s how this looking in an equation:
High Superheat = Actual Measured Superheat – Target Superheat > 2°F
Example: Let’s say we have a 3-ton 16 SEER air conditioner that runs on R-410A refrigerant (on a 80-degree). We measure that the actual superheat is 17°F. Then measure the indoor wet bulb temperature (we get 60°F), and we consult the target superheat chart; the total superheat for R-410A on an 80-degree day is 10.0°F. Hence we have a 7°F difference; we obviously have high superheat.
Now, if we were to continue to use this AC unit, we would encounter the following high superheat problems:
- Loss in capacity. Despite this being 3-ton AC unit, it will produce 2.5 tons or even only 2 tons of cooling output.
- Loss in energy efficiency. This is a 16 SEER unit, yes, but at high superheat, the efficiency is downgraded to 14 SEER or even 12 SEER, and we will pay the difference in electricity costs.
- Overheating the compressor (most important issue). The compressor should be getting 10°F superheat vapor. Now it’s being hit by a hotter-than-usual 17°F vapor. This can cause the compressor to overheat and break down. And, as we know, failed compressor = failed AC = You need to pay $1,000s for a new AC unit.
In order to understand why our air conditioner has high superheat, let’s first look at what causes high superheat:
What Causes High Superheat?
In most basic terms, superheat happens when the liquid-vapor saturated refrigerant in the evaporator coil is evaporated too quickly (turns into 100% vapor prematurely). These are two main reasons why this can happen:
- Not enough refrigerant is entering the evaporator coil. Causes #1, #2, and #5 below can cause the lack of sufficient refrigerant entering the evaporator.
- AC extracting too much heat via the evaporator coil. Causes #3 and #4 below deal with excessive airflow (CFMs) and excessive load that will result in high superheat.
Let’s look at all 6 culprits that can lead to either of these two eventualities and cause high superheat in our air conditioner. We will start with the most common one:
1. Low Refrigerant Charge (Most Common Cause)
The most common cause for high superheat is low refrigerant charge. Namely, if our AC unit doesn’t have enough freon, you will see that the metering device will underfeed the evaporator coil. Now, there is not enough refrigerant in the evaporator coil.
That means the available freon will be vaporized more quickly, and its temperature will start increasing. By the time vaporized refrigerant exits the evaporator coil, it’s temperature can be way above the target superheat. Hence, we have high superheat due to a lack of refrigerant.
Here’s how to fix high superheat due to insufficient refrigerant charge:
Charge the AC unit with freon immediately, right? Well, no.
There must be a reason why we have a low refrigerant charge. The refrigerant lines are a closed system; no refrigerant should ever escape. If we see that we have low refrigerant charge, there are only 2 explanation:
- Refrigerant lines were insufficiently charged from the beginning. This will cause high superheat in new air conditioners; it’s an installation mistake (this can’t happen with pre-charged refrigerant lines in DIY mini splits like Mr Cool, for example, since the charge there is exactly right).
- You have a leak in refrigerant lines.
In most cases, we have a case of leaking refrigerant. That’s why we have to fix low refrigerant by:
- First, locate the leak and seal it.
- Only after we seal the leak, we can charge the air conditioner.
If we don’t seal the leak adequately, and charge the system, we will get normal superheat at first. However, the freon will start leaking again, and the actual measured superheat will start to increase. This will again lead to high superheat.
Basically, we have to fix the leaking ship by first sealing the cracks in the hull, and only then we pump the water out of the ship.
Locating and sealing the leak, as well as air conditioner charging, is a job best left to the professionals. In this case, it makes a lot of sense to call your HVAC guy for the job; it’s not an exactly easy-peasy DIY job.
2. Restriction In Liquid Line (Ice Due To Moisture)
Another cause of high superheat is a restriction in the liquid line. The liquid line feeds the liquid refrigerant to the metering device, which should then properly feed the evaporator coil.
However, if there is a piece in the liquid line blocking the free flow of refrigerant, the metering device will get less liquid refrigerant. That means that the amount of refrigerant the metering device will feed into the evaporator coil is also lower. That smaller amount of freon in the evaporator coil will vaporize quickly, and will have time to gain too much heat; the result is high superheat.
The restriction or partial blockage in the liquid discharge line is usually a piece of ice. Namely, if moisture entered our refrigerant lines, that water will quickly freeze; producing a piece of ice that will block the free flow of liquid refrigerant.
How to differentiate between high superheat caused by low freon charge vs. high superheat caused by a restriction in the liquid line?
If you suspect you have a restriction in the liquid line, you should use a clamp-on thermometer and measure the temperature along this discharge line. At the point of restriction (usually at the entrance of capillary tubes or at expansion TXV valves), you will see a sudden drop in temperature. This confirms you have a restriction that is causing the high superheat.
Here’s how to fix high superheat due to restriction in the liquid line:
You have to remove the restriction, and you will get back to the normal superheat. This is much much easier said than done. For restriction removal, you will also need a help of a professional HVAC technician.
3. Too High Indoor Evaporator Airflow (High CFMs)
This is a situation when we have adequate evaporator coil feeding (enough refrigerant). This refrigerant should be steadily vaporized by the incoming warm indoor air over the evaporator coil.
However, if the indoor fan is running too much warm airflow over the evaporator coils, the refrigerant will evaporate quickly, it will increase in temperature, and by the time it exists in the evaporator coil, we will have a superheated vapor (above the target superheat).
Example: The indoor AC unit can handle 300 CFM airflow. If we were to set indoor airflow way above that – say 500 CFMs – we can get high superheat because there is just too much air carrying too much heat to the indoor evaporator coils.
How to fix high superheat due to excessive CFMs:
The easy thing to do is to reduce the airflow. That means we should set the indoor fan to Medium instead of High setting. Figuring out why there the airflow over the indoor coils is higher than it should be is quite difficult: again, an HVAC technician is needed to diagnose the root of the problem.
4. Too High Load (Very High Indoor Temperature)
If you measure high superheat, the best case scenario is that it is caused by an excessive load. That means that we are blowing very hot indoor air over the evaporator coils. As a result, the saturated refrigerant will turn into 100% vapor quicker, and we will have high superheat.
Excessive load is primarily caused by very high indoor temperatures. This can happen on very hot days (100°F day or hotter), if we have a lot of appliances producing heat indoors (kitchen appliances, washing machine, etc.), lots of people in that room, and so on.
Example: If the indoor temperature is 80°F and we set the thermostat to 72°F, we usually won’t get high superheat (only 8°F above thermostat temp). However, if the indoor temperature is 90°F and we want to cool it down to 72°F, the air hitting the evaporator coil can be too hot for the AC unit to handle (we’re talking about 18°F above thermostat temp difference).
Here’s how to fix high superheat due to excessive load:
In most cases, we don’t have to do anything. The running AC unit will reduce the indoor temperature eventually (if it’s not undersized, of course), and the load will be reduced. The superheat will decrease from high superheat to normal superheat. This is the easiest high superheat cause you can fix because you only have to bring the indoor temperature down a bit.
5. Inadequate Metering Device Feed (In Liquid Line)
We have 3 types of metering devices: TXV valve, AEV valve, or capillary tube (piston). The only job of a metering device is to adequately feed the evaporator coil with sufficient refrigerant. That’s it.
If we have a faulty metering device – be it TVX, AEV, or piston – the evaporator coil feed won’t be correct. If the metering device is feeding too much refrigerant (overfeeding), we will get low superheat. If the metering device is feeding too little refrigerant (underfeeding), we will get high superheat.
Namely, will less available refrigerant in the evaporator coil, it will be quickly vaporized to 100% vapor and start gaining temperature to target superheat temperature, and above. This results in high superheat.
Here’s what to do if you have high superheat due to a metering device problem:
We have to check the metering device. A trained HVAC technician will be able to tell what is wrong with the TXV valve, for example, and act accordingly. When the metering device feeding is normalized, we will return to normal superheat.
6. Incorrect Superheat Measurement (Human Error)
As always, high superheat can also be a result of incorrect superheat measurement. We all make mistakes, and it’s not all that rare to see a wrong superheat calculation.
The error can happen at 3 different stages:
- Measuring actual superheat. You should check this 10-step-by-step guide on how to measure superheat.
- Wrong measurement of wet bulb and/or dry bulb temperature for target superheat calculation, or error in reading the target superheat off the target superheat chart.
- Mistake in calculating the difference between actual superheat and target superheat.
In all these cases, it’s best to redo all the measurements and calculations. In too many cases, we will see that we actually don’t have high superheat; we have normal superheat, but we just make a mistake the 1st time over.
Hopefully, you can use these 6 high superheat cause explanations to figure out why you have high superheat. Fixing high superheat will ensure that you get optimum performance and efficiency from your air conditioner, and avoid the risk of overheating the precious compressor.