Why Your Thermostat Is Clicking But Your A/C Won’t Turn On


Why Your Thermostat is Clicking But Your A/C Won’t Turn On

If your thermostat clicks whenever you set it to “cool,” but your air conditioning doesn’t come on, the condenser may not have enough refrigerant to runThe thermostat is operating as it’s supposed to, but something is telling the safeties in your central air unit not to run.

However, a lack of refrigerant is just one of several possible faults that may prevent your A/C from turning on:

  • A dirty air filter will staunch the amount of air the unit will have to circulate through your home, causing the coil containing the refrigerant to freeze up.
  • The A/C’s drain pan is full, and the float switch told the unit not to run. This may come across as an odd setting, but it exists so your house doesn’t get flooded with condensation.
  • A dirty evaporator or condenser coils could cause the A/C to shut down.
  • If you have a ducted central air system (more on that later), and that system is zoned with multiple thermostats, it’s possible the zoning board wasn’t wired correctly.
  • The A/C’s motor unit stopped working and needs to be replaced.

Again, a clicking thermostat is (largely) an indicator that the thermostat itself is working properly. If, however, the thermostat is clicking continuously, start with diagnosing the thermostat. A damaged wire, cracked mercury bulb, or broken relay may be the culprit.

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What Type Of A/C System Do You Have?

Answering this question is necessary to determine why your A/C won’t turn on when your thermostat calls for cool air.

Generally, there are three types of systems: split-system central A/Cs, packaged A/Cs, and ductless “mini-split” A/Cs. We’ll provide a basic overview of how each one works, as doing so will allow you to better identify the issue in your home.

Split-System Central A/Cs

Split-system central A/Cs have four major components:

  1. An indoor evaporator coil that sits within an air handler.
  2. A condenser and compressor that sits outside of the home.
  3. A blower motor that sucks in warm air from your conditioned spaces while simultaneously delivering cool air to your rooms.
  4. A filter that scrubs warm air returning from areas throughout your home.

As the name implies, a split-system central A/C uses ductwork to deliver cool air throughout your home. If you have registers (grated boxes mounted to your ceilings and/or floors) you have a ducted distribution system.

A ducted system contains two trunks: a supply and a return. While the supply sends cool air to the rooms throughout your house, the return sucks hot air through a filter and back to the evaporator coil, which uses refrigerant to cool the air before it enters the supply. It’s just cycling the air within your home.

As the warm return air passes over the evaporator coil, the refrigerant inside the coil will start to heat up, turning it into a gas. That warm, refrigerant gas travels through a copper line to the compressor located outside.

The compressor’s job is to pressurize the gas and send it through the condenser’s outdoor coils. As the refrigerant runs through the coils, a large fan expels heat from the refrigerant, cooling it again and turning it into a liquid. That liquid refrigerant then runs back to the indoor evaporator coil.

What’s Prone To Failure In Split-System Central A/Cs?

A clogged air filter, low refrigerant, broken blower motor, dirty evaporator coil, or dirty condenser coils can prevent your A/C from starting up. We’ll discuss each of these issues in detail below.

Packaged A/C Systems

If you live in a single-family home or condo it’s unlikely that you have a packaged A/C system. A packaged A/C system contains the condenser, compressor, evaporator coil, and blower all in one unit.

These are more common in commercial buildings. Ever drive past your office and see those big giant boxes on the roof? Those are packaged A/C systems. Just like split-system central A/Cs, packaged systems also use ductwork to deliver cool air and suck in warm air.

What’s Prone To Failure In Packaged A/C Systems?

The only major difference between a packaged A/C system and a split-system is the location of the condenser and compressor. As such, the two systems share many of the same common faults.

Ductless “Mini-Split” A/Cs

Ductless mini-split systems have grown more popular over the years, and are great options for those living in older homes.

Like their ducted counterparts, mini-splits also contain condensers and compressors located outside of the home. Copper lines carry refrigerant to one or multiple indoor units. These units are typically mounted high on walls. How they work is a little more complicated than a ducted split or packaged system. Most mini-splits are capable of heating as well as cooling conditioned spaces. As such, there are a few more components that are prone to failure.

Like ducted systems, ductless mini-split A/Cs have condensers, compressors, and evaporator coils. In addition, they contain reversing valves. A reversing valve contains four ports entering a main tube: three on the bottom and one on the top.

Inside the tube is a seat-and-slider assembly which controls how the refrigerant flows across two of the three bottom tubes. Which tubes are charged depends on whether you’re trying to heat or cool your home.

What’s Prone to Failure in Ductless A/C Systems?

The reversing valve, condenser, compressor, and the wall-hung indoor units themselves are all prone to issues, one of them being low refrigerant.

Low refrigerant is typically indicative of a leak somewhere throughout the system. In mini-split systems, the wall-hung “heads” (those units which actually deliver cool air to your rooms) have flared copper connections that are prone to leaks.

A flared copper connection consists of two components: a threaded brass “male” insert and a piece of copper tubing expanded at its end. The tubing sits inside of a brass nut, which pulls the copper against the brass male, forging a tight connection.

Maybe the flared copper has burrs around the edges. It’s possible that the flared end is too large, and catches the threads of the nut. Either of these issues would prevent a strong flared connection and lead to a leak.

4 Simple Reasons Why Your A/C Isn’t Turning On

There are dozens of possible reasons why the A/C won’t run after your thermostat tells it to condition your space. However, there are four causes you should diagnose off the bat.

1. A Dirty Air Filter

This is easy to check if you have a ducted, central A/C system.

Suppose you’re not getting cool air to any of the rooms in your second floor. You turn the thermostat on, hear it click, and nothing happens. Go up to the A/C unit sitting in the attic. You should see a large steel box called an air handler. It contains the blower motor, fan, and condenser described earlier.

A return box with a filter compartment should be attached to the bottom of the air handler. Lift up the lid for the filter compartment, and remove the filter. If it’s covered in lint, dust, hair, and other debris, this is likely the reason why you’re not getting A/C.

When the filter’s clogged, the blower can’t pull enough return air over the evaporator coil. Over time, the refrigerant inside the coil will freeze, and the A/C won’t turn on. Even if the coil doesn’t freeze up, the air handler’s safety switches will likely sense low pressure, and fail to turn on as a result.

2. The A/C’s Drain Pan is Full

Every HVAC system in an attic contains a safety pan for the condensate drain.

If the line carrying condensate from your evaporator coil ever fails and leaks water into the pan, a sensor will register the water and tell the A/C unit not to run. Doing so ensures the unit doesn’t produce more condensation, pour over the pan, and flood your attic. If your A/C is running on the second floor, then a full drain pan likely isn’t the issue. Drain pans are only installed in attic units.

3. Leaking Refrigerant

As stated earlier, leaking refrigerant will most certainly cause an A/C system to shut down. You know what could be causing refrigerant to leak in a mini-split system, but what about a central system?

Central systems are a little trickier to diagnose. The copper lines may have corroded over time, a contractor may have sent a nail into a line during a remodel – the list goes on. At this time, call an HVAC technician to perform a pressure test on the system. He or she will be able to determine if a leak exists. If one does, they’ll be able to identify where it’s coming from and recommend an appropriate repair.

4. The Blower Motor Has Failed

If your HVAC system was ever making loud, funny noises before it failed, a broken blower motor is likely at the heart of the issue.

After hiring a technician to confirm that this is the problem, speak with him about the cost of installing a new system. Depending on the age of your system, replacing a blower motor won’t be worth it in the long run. Other components will start to break, and you’ll end up spending more down the line.

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Related Questions

Is An Improperly Wired Zone Board The Problem?

This could be the issue, but look at how many thermostats are in your home. If you have a thermostat for every room, and one zone is getting cool air while another isn’t, then check the dampers on the ductwork. One may have failed.

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