When we think of air leakage, a few common scenarios probably come to mind: disconnected ducts at connections, poorly sealed connections, and air leakage around vents are ones we’ve mentioned quite a bit in tech tips and videos over the years. However, there are a few that we haven’t really talked about, particularly for fan coils: leaks at the air handler (such as around unused ports) and around the filter slot, especially if there’s a media filter cabinet underneath the air handler.

Signs of Air Leakage at the Air Handler

Unlike a furnace, a fan coil’s blower is above the coil, meaning there’s negative pressure that will suck air in underneath the blower. That’s where the leakage comes from. We also see instances where air bypasses the filter, bringing all of its pollutants with it.

Air leakage at the air handler can come from a few different locations (we can throw filter bypass in the mix here, too, since the effects are the same). There are a few main things to look out for when you do a visual inspection of the air handler:

• Noises (or noise complaints near the unit)
• Dust buildup in a concentrated area of the air handler exterior
• Dirty blower and coil
• Poor filter fit

The blower sucks in air, so you’ll hear a whistling noise near the air handler when the unit is running if there’s a return air leak. If the customer complains about hearing a whistling noise whenever they walk past the air handler closet or when they’re in the garage, you’ll know to pay attention to the other signs, too. Now, this noise may not be noticed by the customer if the unit is in an attic or crawl space, so we need to be observant and catch any odd noises.

As the blower sucks in air that doesn’t pass through the filter, that air comes with dust, moisture, and other nasty particles. These end up on your coil and blower, so an exceptionally dirty coil or a blower with growth on it could indicate an air leakage problem. On the exterior, dust will tend to build up around small air leaks, just as we see wet dust around leaky vents. As air is sucked in, not all of the dust will be able to make it through the leak, so some of it will deposit onto the surface (unless the leak is huge).

How to Seal Air Leaks at the Air Handler

One of the most common places for air leaks at the air handler is around unused drainage ports. Air handlers may come with several out of the box, as the manufacturer wants to cover all bases from an orientation standpoint (a vertical unit would use different drainage ports than a horizontal one). We only use one set of ports for the condensate assembly, and then the rest just stay plugged.

When the unit is installed in its permanent orientation, you can open or remove the air handler panel and apply silver tape or silicone around the edges of the unused ports. 

Air Leaks Around Useful Gaps

There will be gaps around the areas where the panel opens. We can’t just seal those gaps for obvious reasons; the unit will need service eventually, and we won’t be able to access it!

In those situations, using a quality tape that isn’t permanent on the outside of the unit will be your best bet. Masking tape is a favorite of ours because it comes off so easily. The main objection that a customer would probably have is that masking tape isn’t pretty, but this objection can usually be countered by communicating the benefits: the air and the internal components stay cleaner. Foil tape achieves the same purpose and looks a little less ugly.

Air Leaks Near Media Filter Cabinets

Media filters are common add-ons in our area, so we often come across cabinets that have been added underneath the air handler but not properly sealed to it. Since the gap is immediately upstream of the media filter, none of that air that gets sucked in has any chance of being filtered; it partially defeats the purpose of installing a media filter.

Due to the small space between the air handler and filter cabinet, silicone caulk is usually the best type of sealant to use. If you don’t have silicone caulk or don’t feel comfortable using it yet, you could also use silver tape. The drawback of using silver tape is that there usually isn’t a lot of space for it to make contact with the surfaces; you may have to fold it a little and get crafty to make it fit the tight space.

You can also seal up old 1” filter slots if they will no longer be used after installing a new media filter cabinet. That will prevent any air from slipping through the gaps around the slot door.

How to Stop Filter Bypass

Filter bypass is a common problem whenever you have filters that are too small or non-gasketed filter back return grilles. 

The first thing to consider is the size of the filter. The filter should fit snugly in the slot; there should NOT be gaps around it. If the filter has a gap and bows in, that’s a sign that the filter is undersized. At that point, the first order of business should be to find the proper filter size and find one that suits the slot.

Then, once the size is okay, we should determine whether there’s a gasket that creates a seal around the filter and prevents bypass. If not, we should use some masking tape to seal the filter into place and prevent any air from sneaking around it.

Best Practices for Tape and Silicone Caulk

Tape and silicone caulk are great tools to help seal cabinet leaks, but some care and finesse are required to get them to set correctly. 

Taping Best Practices

Whether you tape the inside of the air handler around unused condensate drain ports, the gap where the panel comes off, or even any kind of ductwork, you must have three things on hand for best results: a bottle of denatured alcohol, a rag, and a tape squeegee.

Tape has a hard time adhering to dirty surfaces, and dust buildup is just a fact of life with small air leaks. The alcohol can be sprayed right onto the surface, and then you can use a rag to make sure it’s applied to the entire area. Then, with the area clean and dry, you can apply the tape. After applying the tape, use a tape squeegee to get rid of any air bubbles and press the adhesive against the surface. Seriously, that tape squeegee makes a huge difference.

Silicone Caulk Sealing Best Practices

Silicone caulk application can be tricky and requires a bit of practice. It’s best to practice application techniques BEFORE you seal around anything with silicone if you don’t have much experience with it. You’ll want to get a feel for how it behaves when spread around gently.

As with the tape, a rag and some alcohol will come in handy. Apply the caulk to the area you intend to seal. Then, using a little spritz of alcohol and a rag (or your finger), gently spread it over the area to make sure it adheres to the surface and fills the gap(s).