The condensate drain is the source of many a service call in Florida. A lot of the time, the problem is just a clogged drain; bacterial zoogloea (drain snot) loves humidity, and the all-aluminum coils don’t do us any favors. Other times, there are some clear drain design or installation flaws that present hazards to the property (or even people). Apart from the annoyance of tripping float switches, condensate in a poorly designed or installed drain causes problems when water can accumulate in certain places. 

Indoor Design Considerations

In Florida, we usually route drains into a chase that goes underground, but the trap is still accessible indoors. Fan coils put the drain under negative pressure, so the trap keeps a pocket of water in it at all times to keep the blower from sucking air out of the drain and into the air handler. This negative pressure has a heavy influence on drain design, especially when it comes to trapping, venting, and installing cleanouts.

Cleanouts always come before a trap. They should be accessible so that we can get water, cleaners, or brushes into the drain line for cleaning. These are on the negative side of the drain, so they must be capped when not in use.

Vents, on the other hand, come after a trap. Not all systems have these (for example, some drains may have an outlet that ties into the sewer system; we would NOT want a vent to allow sewer gases to come into a building). However, when systems do have vents, they must always come after a trap and stay uncapped to allow air to escape the drain.

Unintentional traps and poor design can cause drain backups. If the float switch is installed appropriately, those backups may cause the HVAC system to stop cooling quite often. If installed poorly, the water may back up in the pan and cause damage without cutting power to the equipment or its components. 

Proper Trap Depth and Height

Traps need to be deep enough to prevent the blower from sucking air in through the drain. That means it needs to be built to overcome static pressure, and the equipment manufacturer will help you design drains accordingly.

However, there is a rule of thumb that we can use in the case of fan coils and air handlers: measure the static pressure and multiply it by two to see what your minimum trap depth should be in inches. For a reading of 0.5”wc, the minimum trap depth should be one inch. 

Even then, bigger is better. Static pressure is partially determined by airflow restrictions, so it can increase as a filter clogs up over time. In that case, a trap that was sized for 0.5”wc might be too shallow if that static pressure bumps up to 0.7”wc. 

In addition to creating a trap that’s deep enough, we need to make sure the outlet is lower than the inlet by at least the diameter of the tubing. In other words, if you use ¾” PVC and create a trap, the trap outlet should be ¾” lower than the inlet. Typically, the top of the piping on the outlet will be level with the bottom of the piping on the inlet.

Avoid Double Traps

Double traps are a common problem in designs where the tubing drops into an underground chase. The pipes need proper support to prevent sagging, which creates an unintentional double trap. If there are two pockets of water in the same run of pipe, there will be an air bubble between those pockets. This air makes it harder for water to pass through the drain, causing it to back up instead of draining out. 

Now, some systems may have multiple traps without having the same issues as a double trap. The difference is that the traps are designed properly and have a vent between the traps to prevent an air bubble from blocking the water flow. 

It turns out that vent design is also important for preventing water damage…

Vents SHOULD Be Tall

Short vents are a common design mistake we see in some neighborhoods in our Central Florida service area. We want vents to be taller than the drain pan. That way, if a drain has a double-trap or other blockage, water will only back up in the drain pan before the vent and trip the float switch. If that vent isn’t taller than the pan, water will just back up in it and spill out of the top.

You also do NOT want to cap your vents. Cleanouts, which come before the trap, get capped; that’s where we can add water during cleanings. Vents always come after the trap and exist to let air escape, and air can’t get out if a vent is capped. 

Float Switch Placement and Redundancy

Float switches have an important job: to stop the refrigeration cycle before the drain pan overflows and causes major water damage. These should be wired into the secondary drain line, and there may even be one in a secondary pan underneath the primary pan.

We want the float switch to trip if there is a problem, so we need it to be positioned below the drain pan. If the float switch is positioned above the drain pan, water will back up into the pan and spill out before it reaches the float switch. Sometimes, the float switch is positioned straight out from the secondary drain port, which is slightly higher than the primary drain port. Other times, it will be at the end of a relatively short run of tubing along the side of the air handler. Both configurations are shown below.

Installing multiple float switches is ideal if you can make it work and the customer agrees. You could have one float switch connected to the secondary drain port and then another mounted to a secondary drain pan underneath the primary. (Note about secondary drain pans: they should be larger than the primary drain pan so that any water in a backed-up or cracked drain pan can be caught by the secondary pan.) That way, your customers have extra protection if one float switch isn’t working or there’s another issue, like a cracked primary drain pan.

If you do install multiple float switches, wire them in series. That way, one tripped switch will trip all others in a circuit and kill power to the thermostat or contactor (depending on how you wire it).

Outdoor Design Considerations

Even if the drain has been fabricated perfectly, it can cause trouble when it allows water to pool in one area (i.e., at the outlet). Slip hazards are relatively self-explanatory, but water damage could also happen because the drain outlet is too close to the building.

We think of water vapor as a common enemy on the building science side of the industry, but bulk water (such as from puddles of standing water) is so much worse, especially for building materials. Just as brazing alloys are melted and drawn into hot tubing joints, standing water can get drawn into the building materials in spite of gravity. 

We have to do our due diligence as HVAC contractors to prevent any problems caused by bulk water from the condensate drain.

Outlet Positioning

Outdoors, we have to be mindful of the drain outlet. Ideally, it should be at least two feet away from the home and the unit and face away from those. We also want to make sure that it doesn’t drain into any areas like gardens, places where soil erosion is a concern, or where people may slip and fall while walking.

Slip hazards often aren’t apparent immediately. If water dumps onto a concrete walkway, it’ll just dry up. The main problem is that the microbes in the drain also come out. As that happens repeatedly, microbial growth will occur where the water dumps out and accumulates, and THAT can get very slippery when it’s wet. 

Fabrication

Planning is just one part of the process. We can have a perfectly functional drain design fall flat on its face with improper installation practices.

Be Careful When Connecting to Drain Ports

A drain line will have multiple ports: primary (lower) and secondary (higher). These will typically require male threaded adapters, though there may be exceptions. This fitting type is prone to cracking if installed improperly (such as when tightened too hard or the wrong thread sealant is used). 

The threads should be coated in a non-hardening pipe dope, not a glue or pipe dope that could harden—at Kalos, we personally like the silicone-based Nylog White by Refrigeration Technologies. Then, it should be inserted and tightened with your fingers; using tools to overtighten the fitting could also lead to cracking.

Proper Pitch

Local codes may vary, but the IMC requires at least ⅛” of fall for every foot of horizontal run. Some AHJs have slightly steeper requirements. In any case, the techs at Kalos are trained to fabricate drains with a pitch of at least ¼” of fall for every foot of horizontal run, and we consider that to be best practice. 

This minimum pitch also applies to the short run between the air handler and the trap, not just the tubing between the trap and the outdoor drain outlet.

Dry-Fit Your Connections First, Then Glue Them

Condensate drain lines may have several connections: elbows, traps, the float switch, etc. These will need to be piped together, but we should first dry-fit the connections to make sure the design will work with the clearances around the unit. Sometimes, you’ll find that a piece of tubing is too long and bumps a wall or other nearby object, so you will know to cut the PVC before gluing anything. You can also use a level on the horizontal tubing pieces to confirm that you can achieve the pitch you need for your local AHJ.

You can glue the connections once you’ve confirmed that the tubing fits within the space and meets code. You’ll apply a primer to the male end of the tubing first and then go back over it with a medium-grade PVC cement before connecting the tubing and fittings. The only fitting you do NOT want to glue is the cap on the cleanout. You can glue the cleanout pipe to the top section of the tee that connects your trap, but the cap itself needs to stay unglued because you, another contractor, or the customer will eventually need to take the cap off to clean the drain.

Install your float switch(es) according to the design you and your customer have chosen.

Insulate the Horizontal Sections and Trap

Codes don’t require you to insulate the drain, but we find that it’s best practice in humid climates like ours in Florida. Condensate is relatively cold when it runs off the coil and drains out, so the pipes will be cool (read: below the dew point). Moisture in the air around the drain line could condensate onto the pipe’s surface, and we all know what happens next…

That’s an extreme case, but we still want to prevent any kind of water accumulation, staining, or growth. Insulating the horizontal portions and traps keeps those sections warmer and reduces the risk of a moisture problem (and the destructive consequences that follow).

Finishing Up

After assembling the drain, we need to set it up to remove condensate properly. That means we need to be able to confirm drainage, make sure our traps are full, set up our float switches, and test the unit.

Prime the Trap and Confirm Drainage

When you finish installing your drain, open the cleanout and pour about a gallon of water. This step achieves two things:

• Primes the trap with water
• Allows you to confirm drainage

A trap doesn’t need that much water, so the excess will drain out. You should be able to see evidence of drainage at the outlet, such as wet grass. You could also put a small rag underneath the drain outlet and see if it’s wet after pouring water down the drain.

Wire Float Switches

You can wire float switches in two main ways: break Y or break R. The option you choose will ultimately depend on your climate. We have a tech tip explaining the merits of each, but in short, R is better for humid climates, and Y is better for dry climates. 

Breaking R shuts power to the thermostat—everything. Breaking Y stops power to the contactor but keeps the blower running, which allows backed-up condensate to evaporate back into the air. That’s helpful in Phoenix, AZ, but makes for a nightmare scenario in Florida.

Again, regardless of which wire the float switch is designed to break, multiple float switches must be wired in series. If one trips, they all trip.

Run the System 

Always finish up by running the equipment. Same as before, you will want to make sure the condensate drains out while the unit is running and that there are no leaks or condensation on the outside of the tubing. Test the float switch(es) to make sure the customer can depend on them if there’s a blockage, too.

A problematic condensate drain line is pretty hard for a customer or contractor to ignore, either because of tripped float switches or unfortunate water damage. It’s always good to check the drain line and note any issues with pitch, insulation, strapping, and general design whenever you’re looking at a system for the first time, whether that’s on a maintenance job or a service call. 

If something seems wrong, bring it to the homeowner’s attention. Being able to catch and rectify issues with the drain as early as possible can prevent a lot of heartbreak for the homeowner. Even if they don’t want to pay you to fix it, there is a record of them declining the service, and you will know that you are not responsible for any future drain issues.