HVAC Water Leak - Unit Icing, a Common Cause

This time of year in Southern California brings that fantastically mild weather which gets everyone out of their houses and into the cool sunshine. These are the days which make Los Angeles the place everyone wants to be. On such days I like to go hiking in the San Gabriel Mountains outside Pasadena, or drive down to Orange County to do a little shopping, or perhaps enjoy a day at the Getty Museum in Santa Monica, gazing out over West LA and Hollywood.

I would enjoy these days more if it weren't for those pesky emergency weekend calls from commercial clients. That is because, within the HVACR industry, this is also the time of year for the dreaded water leak call. Water leaks in air conditioning equipment typically take two forms: the first form is usually due to an overflowing drain pan which is typically caused by a plugged drain line. But it's the second form which we see the most this time of year. The call typically comes in as follows, "My air conditioner was working great yesterday now it is blowing hot air and dripping water on the sales floor". This description is the telltale sign of a unit "icing over".

Air conditioners have a condenser coil and an evaporator coil. The purpose of the evaporator coil is to extract heat from a space. The purpose of the condenser coil is to expel that heat to the atmosphere. Between the two coils a refrigerant transports that heat. Air conditioner systems are designed to exacting specifications and are delicately balanced between evaporator and condenser; the coils have to be the right size, the air flow has to be in the right amounts, the refrigerant has to be charged to specific amounts. If any part of the system gets off balance the unit operates inefficiently, or sometimes, too efficiently.

This can be the case in a fall season problem like I mentioned above. Most units these days have digital programmable thermostats. Digital programmable thermostats are meant to save energy by turning units off during times when it is expected that a building would not be occupied. They also protect units from operating outside their ideal parameters. But there are times when you want to override the program, say perhaps because you are staying late at work one night and you want one or more units to run while you are still there. This typically isn't a problem but it can become one if you override the thermostat to run the unit continuously 24/7. It's not unusual for this to happen in commercial air conditioning applications such as big box retail stores. The manager or an employee overrides the program and then forgets that they have done so. The unit is left to run continuously.

As mentioned above, to run properly, an air conditioner needs to have all its loads in balance. When a unit runs at night there are very few loads. The space inside is relatively cool because the lights are off, the people are gone, and the computers are off. So the evaporator doesn't have to work very hard to remove heat because there is less heat to remove. Outside it is very cool so the condenser is also finding it very easy to expel heat to the atmosphere. So the unit is running very efficiently, in fact too efficiently. With so little load, the evaporator coil gets colder, so cold that it can often drop below freezing. Cold air cannot hold as much moisture as warm air so it is typical for evaporator coils to condense water out of the air and when an evaporator coil drops below freezing that evaporated water starts to freeze. Essentially the evaporator coil becomes an ice machine! Ice will continue to build as long as the thermostat is on. Eventually the ice block becomes so large that air can no longer pass through the evaporator coil properly. With no air blowing across the evaporator coil the unit can only blow outside air into the space. Morning comes and it warms up outside, now the unit is blowing warm outside air, the ice starts the melt, and presto! You have a water leak and a unit blowing warm air.

The point of the story is to explain to users that HVACR equipment, unfortunately, are still not fool proof. That can be made more reliable at an added expense, for example with a fan cycling switch, but most units do not come with these devices standard. Better for a user to understand the proper use of their thermostat. The two rules to follow are:

1. Never leave your equipment running all night long if no one will be occupying the space. (The one exception would be computer rooms because the heat load is generated all night long.)

2. Never turn your thermostat below 68 degrees (even during the day). This will not produce colder supply air it will only insure that the unit runs for a longer period of time. If you turn the set point too low, the room may never reach that temperature and so the unit will run 24/7.

Follow these two rules and you will avoid icing you unit up in most cases. The only other situation where HVACR units ice up involve proper maintenance. That I will save for another article.