What Does an AC Float Switch Do?
Have you ever wondered why a toilet tank fills only up to a certain level every time? A float switch inside the tank moves up and down to precisely open or close the toilet fill valve.
That is also the same concept applied in HVAC and especially in condensate float switches. But why does an AC needs a float switch? Importantly, what is a float switch and how does it work?
An AC float switch is an electronic sensor which detects the condensate level in the AC drip pan. If the condensate drain line is clogged, water in the pan will rise, forcing the float to move up, break contact between 2 wires effectively turning off the AC, preventing more condensate from being formed.
An AC float switch is also known as a condensate overflow switch or AC safety switch. Some folks even refer to it as an AC safe T switch.
An air conditioner float switch turns off the AC unit when the condensate drain line is clogged. That prevents the formation of more condensate (water) which would then fill and overflow the drip pan causing water damage.
You could therefore say that an AC float switch is the AC system’s “failsafe”. Air conditioners can generate anywhere between 5 and 20 gallons of water in a day and with a clogged drain line all that water would end up damaging your ceiling, floor, drywall and other sections of your house.
When your air conditioner is not working, there is always a big chance that the float switch has turned it off. Draining the condensate drain pan and unclogging the drainpipe will fix the problem.
To reset an AC float switch, drain the water in the drip pan and unclog the condensate drain line. A shop vac can be used to suck out the clog and drain the water from the pan. Once the water in the pan drops to the right level, the AC system will turn back on.
To really understand how an AC float switch works, I believe you need to first understand how an air conditioner, drip pan and drain line work together.
Relationship between AC Evaporator Coil, Drip Pan, Drain Line and Float Switch
An air conditioner system is made up of 4 main components:
- Evaporator coil
- Condenser coil
- Expansion valve
All the 4 components are connected together using copper tubes and a refrigerant/coolant (commonly known as Freon) circulated between them. The evaporator coil is the one located inside the house and that is where cooling actually happens.
Condenser coil and compressor are located outside the house. The refrigerant absorbs heat from the indoor air and releases it to the outside inside the condenser coil.
What most people forget is that an air conditioner’s job is to not only to cool the indoor air. It also dehumidifies it.
The refrigerant enters the evaporator coil as a cold liquid. A fan pulls warm and humid air from the house and forces it across the coil.
When the warm air comes into contact with the cold coil, the refrigerant absorbs heat from the air and that is how cooling happens. Cooled air is then sent back to the house through the supply air ducts.
When the water vapor in the air (humidity) is subject to the cold coil temperatures, it condenses (becomes a liquid). That liquid has to go somewhere otherwise it will drip on the floor/ceiling and cause extensive water damage.
For that reason, there is a pan placed under the evaporator coil known as a condensate drain line or AC drip pan. The drip pan collects the condensate from the coil but it should not stay in it for long.
A condensate drain line is connected to the drain pan to continually drain the condensate to the outside. When everything is perfect, this system works very well.
Where is My AC Float Switch Located?
The location of your AC float switch will depend on what kind of AC unit you have. There 2 types of HVAC units:
- Vertical units
- Horizontal units
In vertical units, the evaporator coil is located on top of the furnace or air handler. Warm air enters the unit at the bottom and cold air leaves from the top.
In horizontal HVAC units, air moves sideways. Warm air enters from one side while cold air leaves from the other end. These installations are found in attics or crawlspaces where there isn’t enough space for vertical installations.
Thanks to gravity, the condensate in vertical units can only flow downwards. As such, vertical units have only one condensate drain located below the evaporator coil.
Things are however a little different for horizontal units. The condensate has the potential to flow sideways, outside the area covered by the drip pan.
For that reason, horizontal units have a primary drip pan (located under the evaporator coil) and a secondary drip pan located underneath the entire unit. The secondary drip pan acts like a backup for the primary pan.
If you have a vertical unit, the float switch will be located somewhere on the condensate drain line. On the other hand, if you have a horizontal unit, the AC float switch will be located on the secondary drain pan.
How an AC Float Switch Works
An AC float switch comprises of a very simple mechanism. There is a small piece of plastic (the float) at the end of the switch which can slide up and down easily.
This float is also connected to 2 wires. The 2 wires are connected to the AC electronic system and specifically the thermostat.
When the float is at the bottom of the assembly, the 2 wires have a good contact, meaning that current is flowing through the system and hence the AC works normally.
The reason why the float stays at the bottom of the assembly is because there is nothing to move it up. As such, the contact between the 2 wires remain intact.
However, when the AC drain line is clogged, the condensate cannot be drained hence water level in the pan starts to rise. As the water rises, the float keeps sliding upwards relative to water level in the pan.
When the water level in the drip pan reaches the critical or set point, the float breaks contact between the 2 wires’ terminals. When that happens, the electric circuit path will have been broken and the AC will turn off.
As you can imagine, when the AC is turned off, warm and humid air will not be pulled from the house meaning that no more condensate will be produced. The drip pan will therefore not overflow, protecting your house from water damage.
How Do You Reset an AC Float Switch?
If your AC float switch has tripped, you will only need to drain the water in the condensate drain pan to reset it. That will however not fix the problem because as soon as the pan is full again, the switch will trip.
In fact, if your AC float switch keeps on tripping, it is a sign that the drain line is clogged. To prevent it from tripping again and again, you will need to first unclog the drain line.
Note: Some secondary AC drip pans have a drain line while some don’t. If your secondary drain pan has a drain line, you will need to inspect it as well as the primary drain line.
To quickly drain a condensate drain pan full of water, use a shop vac to suck out the water. When that is done, you will also need to unclog the drain line.
To unclog the AC condensate drain line, connect the shop vac’s house to the drain line end outside the house and run it for about a minute. You can use a rag or tape to create a better seal between the vac’s hose and the drain line.
How to Prevent AC Float Switch from Tripping
To prevent your air conditioner float constant tripping, you have to ensure that the condensate drain line does not clog. Because of the humid nature of the evaporator coil, the condensate drain line is a perfect breeding ground for mold, mildew, algae and other such organisms.
Over time, these organism will multiply and completely clog the drain line. Failing to change the AC filter or using the wrong size of filter can also cause the drain line to clog.
Ideally, an AC filter should be change after 90 days but inspected monthly. You should also use the correct size to prevent dirt particles from passing and clogging the drain line.
Here however is how you prevent mold from growing and clogging your drain line:
- Remove the AC drain line cap
- Pour about ¼ cup of vinegar down the drain line
- Wait for about 15 minutes then flush the line with boiling water.
- Put the cap back on.
Apart from cleaning the drain line with vinegar, you can also regularly (after about 3 months) connect a shop vac to the AC drain line and pull out and gunk that can have the potential to cause a clog.
Float Switch vs Wet Switch
Unfortunately, float switches are not foolproof. Sometimes they do not work as designed. And that is why your HVAC technician may recommend something known as a wet switch.
As I mentioned, float switches uses a float to trigger a switch and turn off the AC. On the other hand, wet switches use water to complete a circuit between 2 metal probes.
In terms of effectiveness, wet switches are more effect that float switches. They will promptly turn off the AC when as much as a drop of water gets to them. On the other hand, there will need to be a high water level in the pan for the float switch to turn off the system.
Wet switches are more expensive than float switch but is really nothing compared to the cost of fixing water damage related problems.
And that is everything in as far as Air conditioner float switches are concerned. I hope that this post was helpful.