- Tech Tips
- Podcast Episodes
- Tools & Products
Let’s Start with the basics.
Water freezes at 32° Fahrenheit and 0° Celsius at sea level and atmospheric pressure. When any surface is below that temperature and the air around it contains moisture, ice / frost will begin to form. In some situations ice is to be expected, such as in refrigeration evaporators and exposed portions of refrigeration suction line and the metering device outlet in refrigeration applications. Frost and freezing is also expected in heat pumps operating in heat mode. The outdoor coil on a heat pump becomes the evaporator coil and during low outdoor ambient conditions it is expected that the outdoor coil will eventually freeze and require defrost.
The reason that frost / ice is inevitable in some applications is simply due to the laws of thermodynamics (moving heat). In order to get heat from one place to another you need to have a difference in temperature. So inside of a freezer where you hope to get the box temperature to 0°F (-18°C) the coil temperature needs to be BELOW 0° to transfer heat out of the freezer and into the coil and then down the suction line to the compressor. If you have a freezer with a design coil temperature difference of 10° and a design superheat of 8° the coil will be at -10°F and the suction temperature at the evaporator outlet will be -2°F.
On a heat pump running in heat mode you will commonly find an evaporator (outdoor coil) that runs 20° – 30° colder than the outdoor temperature. This means that if it is 30° outside the outdoor coil temperature could easily be 0°F. In these cases, ice is normal and periodic defrost is expected and required.
Some systems we work on and install will freeze. Air conditioning is not one of them.
In an air conditioning system we must keep the evaporator temperature above 32°F. We can easily know the evaporator temperature by looking at our suction saturation temperature (suction gauge temperature for the particular refrigerant). For R22 32° is 57.73 PSIG at sea level, R410a is 101.58 PSIG and R407C is 67.80 PSIG. If we don’t keep our evaporator coils above these coil pressures / temperatures the system will freeze. The rate at which it will freeze is a function of –
The ice buildup always starts in the evaporator and works it’s way outside. If you have a frozen compressor you have a REALLY frozen evaporator. When you find a frozen system, take your time and get it fully defrosted. Take care to manage the ice melt water and keep it away from motors and boards where it can cause damage and a shock hazard. Some towels and a shop vac are great to have handy when defrosting a unit. When possible allow it to defrost slow and easily to prevent damage.
So what circumstances can result in low coil temperature?
Low Evaporator Load
Low load is often equated with low airflow… and it usually is low airflow, but there is a bit more to the story than that.
An air conditioning system has one final design result, one big end goal that we are shooting for. Matching the refrigeration effect to the evaporator load.
We must match the quantity of refrigerant moving through the evaporator coil to the amount of heat the evaporator coil is absorbing
That is our mission, and that is the primary reason we measure superheat. Superheat gives us a look at how well we are matching refrigerant flow to heat load. High superheat means underfeeding, low superheat means overfeeding.
There is an issue though, we could have a correct superheat and still have a coil temperature of under 32°, and this is not acceptable in an air conditioning system. When the coil absorbs less heat than designed the coil temperature and suction pressure drop. In cases where a TXV or EEV is controlling suction superheat the suction pressure will drop even further as the valve attempts to keep the superheat from plummeting.
This is why we must size a system and it’s ductwork appropriately for one another as well as for the space, climate and even altitude. If we install a system that requires 1200 CFM of airflow to properly balance the refrigeration effect to the load at 75° design indoor temperature and that system is only receiving 900 CFM of airflow, you run a very good likelihood of freezing. This is especially true when the outdoor temperature drops or the customer decides to drop the thermostat lower than normal.
Low load is often due to low airflow, low indoor ambient conditions and equipment oversizing. Low load conditions will have symptoms of low suction pressure, low superheat and high evaporator Delta T. Start by looking for the obvious, dirty coils, dirty filters, dirty blower wheels, blocked returns, mismatched equipment, improper blower settings, closed registers and undersized ducts. You can then move on to performing static pressure tests to locate more difficult issues.
Low load is the most common cause of persistent freezing and should be top of mind when a technician is diagnosing a freezing system
Low Refrigerant In the Evaporator
System undercharge or underfeeding due to restricted refrigerant flow (restricted filter driers, plugged screens, failed expansion valves or undersized pistons) can also result in freezing over time. Low refrigerant can result in fewer molecules of refrigerant in the evaporator coil which results in lower coil pressure because the coil contains both saturated liquid at the beginning of the coil as well as superheated vapor towards the end. This type of freezing requires time because less refrigerant in the coil equals less refrigeration (cooling) effect.
If the coil temperature is below 32° in an undercharged situation the coil will simultaneously build frost as the beginning of the coil after the metering device AND underfeed the coil resulting in high superheat. Over time as the frost builds it will start to block the beginning of the coil which blocks airflow and insulates that portion of the coil from airflow which reduces the coil load. Eventually once the coil is totally blocked with frost almost all of the load is removed from the coil and you have a low refrigerant issue that LED to a low load issue that resulted in a complete freeze up.
Once you defrost the system and test you will find that low refrigerant charge conditions result in low suction, low subcool, high superheat and low head pressure. Refrigerant restrictions will be low suction, high superheat, high subcooling.
Often once you resolve the charge issue you may also find another low load issue as well that contributed to the freezing. In many cases when low charge is the cause the customer will notice the issue before the system is FROZEN SOLID.
Low refrigerant will often result in a partially frozen coil more than a full block of ice. Remember, low COIL refrigerant can be restriction related or low charge, but if it’s low charge you will have low subcooling, if restriction it will have high subcooling.
Low Outdoor Ambient
When a cooling system is operated during low outdoor temperatures the condensing temperature and head pressure will drop. If the head pressure drops low enough the suction pressure will also drop resulting in freezing. The only way to resolve this cause is to install some type of head pressure control such as fan cycling or fan speed control to keep the head pressure from dropping significantly.
If the indoor blower shuts off the coil temperature will drop. Sometimes a blower motor will have internal issues or controls issues that cause it to shut off periodically. This can cause intermittent freezing that can be hard to diagnose. Checking controls, belts, blower amperage, bearings and motor temperature can all help in diagnosing these issues. Sometimes leaving an amperage data logger on the motor along with a coil or supply air temperature sensor can give you the ammunition you need to pinpoint an intermittent issue.
When diagnosing a freezing situation don’t jump to conclusions, get all the ice defrosted before making a diagnosis and keep a sharp eye out for airflow and design issues. Freezing is often due to more than one issue combined that act to turn your customers air conditioner into an ice machine.
First I want to give credit where credit is due. This post is made possible by the fantastic demonstration video by Neil Comparetto that I embedded below.
Before you get bored and stop reading I want to give some conclusions. Ice can form in a vacuum, but I still advise pulling a fast, deep vacuum in most cases. Now… keep reading to see why.
The statement that is often made by techs is that pulling a vacuum “too quickly” can result in freezing of the moisture inside the system and reduction of evacuation speed. This conversation usually occurs when another tech is demonstrating a SUPER FAST evacuation, or by a tech who is advocating for the consistent use of triple evacuation.
Neil’s video proved that pulling a deep vacuum quickly can result in freezing with even a small amount of liquid water present. He also demonstrated that this is possible with a typical HVAC hookup and that it can pull down to 500 microns with substantial ice present. This was done in 50 degree shop, with a glass jar (insulator), with relatively low internal volume, pulled down using a vacuum pump direct connected with large hoses.
So he proved that under certain circumstances, ice can form and cause a real problem with evacuation speed and even trick a junior tech into thinking they pulled a proper vacuum when they did not.
So what causes this and how do we prevent it?
Water, like all substances, changes state due to the molecular density and configuration based on the pressure surrounding the molecules and the temperature of the molecules (average molecular velocity). When we pull a vacuum on water there are two opposing forces, on one hand we are DECREASING the pressure which leads to evaporation and then boiling, but as the water boils it begins to lose heat because that molecular energy of the highest velocity molecules are being evaporated and removed by the vacuum pump, thus reducing the average molecule velocity (temperature). The key reason why it can freeze in these experiments is because the heat rejected through evaporation / boiling is significantly higher than the heat ADDED through the sides of the jar which is why it starts by flashing, then boiling, then back to liquid then freezing. You are WATCHING the change in energy state in real time as the available heat in the jar and added through the walls is overcome by the heat REJECTED from the boiling liquid and out the pump.
Now, if the pump was left on the ice long enough, the ice would eventually all SUBLIMATE (change directly from solid to vapor) but the rate at which that would occur is based solely on the amount of heat being added to that ice through the walls of the jar. This addition of heat is equal to the differential in temperature between the ice in the jar and the temperature around the jar.
The deeper the vacuum pulls the colder that ice will get, which will increase the differential between the ambient around the jar and the ice temperature inside.
If we hit the jar with a heat gun, the ice would melt quickly because we are ADDING a huge amount of heat very quickly. In the same way, if the ambient temperature in Neil’s shop was higher or if the vessel was made of copper (conductor) instead of glass (insulator) it would be less likely that ice would form and if it did form it would sublimate more quickly.
Here are my current conclusions based on this video (and many others), good science and practical field experience
Here is the video-