BlogHVAC RepairApril 30, 2022Why Does A Thermostat Screen Go Blank?

It’s a hot day, and it feels hotter inside your home than it does outdoors. You inspect the thermostat — it’s blank! What do you do?

First, don’t panic — or assume your air conditioning unit is broken. Blank thermostat screens are a common problem, especially in Florida. “They are probably the cause of one out of every four customer questions we receive,” says Alan Wilson, Millian Aire’s Vice President of Technology and Training. “Fortunately, the fix can be pretty simple.”

Why Am I Getting A Blank Screen?

Blank thermostat screens are typically caused by a lack of power to the entire HVAC system, or to the thermostat alone. Assuming the power isn’t out for the entire house, the most common reason for the HVAC system losing power has to do with too much water in one of the HVAC system’s condensation drain pans. There are two pans (one internal and one external), each with float switches.

If either drain pan becomes filled with water, it triggers the float switch to automatically turn the system off. While this is a nuisance in terms of preventing normal HVAC operation — and creating that annoying blank thermostat screen — there’s a benefit to the homeowner, as the system is preventing more condensation from forming. This keeps water from overflowing from the pan and causing damage.

Drain Pan Overflow

Why do drain pans get filled and overflow? Here are the two most common causes:

Line Gunk
“The overwhelming cause of drain pan overflow is the slow buildup of gunk in the drain line,” says Alan. “Even in a closed system of pipes, matter can build up in the lines. This can happen with the change of seasons, as the drain line dries out, calcifies, and bacteria can build up. That builds up over time, reducing the line’s inner diameter. That reduces the amount of water the system can process quickly.”

Especially in a hot and humid state like Florida, the HVAC system can produce a gallon of condensation every hour. This can expedite the drain pan reaching capacity if the line emptying the pan becomes compromised.

Severed Power Lines
Another possible, though less common, reason for the HVAC system shutting down is the severing of a power line. This can be caused in various ways — for instance, with a rat or mouse chewing through the electrical wire.

It can also happen during yard work, especially if someone accidentally severs the line with a weed whacker.

Wilson says he’s also seen HVAC systems shut down by occurrences seemingly unrelated to the HVAC system, like a homeowner hanging photos on a wall. If a nail accidentally pierces the thermostat wire, which is a low voltage wire that runs from the air handler to the thermostat, that can be the culprit.

How Do I Avoid Overflow?

Since the most common cause is line gunk, there are fairly easy ways to avoid the dreaded blank thermostat screen. These don’t require special training, are easy to perform, and can save you the cost of a service visit by a technician.

  • Change Filters
    This easy-to-remember piece of advice — just two words — will not only help prevent the problems that can lead to blank thermostats but will contribute to the overall efficiency and power of your system. By catching more particulate matter in the HVAC system, new filters help reduce the amount of residue that can eventually lead to drain lines compromised by gunk. Wilson recommends changing filters monthly.
  • Eliminate The Overflow
    To really be proactive about drain overflow issues, Wilson recommends regularly eliminating the water in the drain pan using a shop vac. “Just hook the shop vac up to the drain line outside and let it run,” he says. “In a humid state like Florida, I recommend doing it monthly. You can’t overdo it!”

Finally, a reminder of what not to do to keep your system running — don’t pull the float switch out! Remember that under humid conditions, the HVAC system will continue producing excess condensation – as much as a gallon of water an hour. It will overflow the drain pan and can create a much larger problem.

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