When A Roof Leak Is Really A HVAC Leak

When a building occupant spots water dripping from the ceiling, he doesn’t know or even care what the source of that water is. To him, it’s a roof leak.

We would like to address one common source of so called “roof leaks”: HVAC units mounted on the rooftop.

Bottom discharge vs. side discharge:
Bottom discharge HVAC units typically are mounted on a raised curb on the rooftop, and the ducting is mounted to the bottom of the unit and penetrates down through the curb. No ductwork is exposed to the weather in this situation. Side discharge units have with the ductwork exiting the side of the unit and continuing some distance along the rooftop until it penetrates the building.

How HVAC systems work:
A greatly simplified example of how an HVAC unit works will eliminate a discussion on how the refrigerant cycle works and concentrate on air flow. This starts when warm air from the interior is drawn into the return plenum, which then proceeds to the unit itself. At this point, the warm air flows through the filters and condenser coil, which looks like a car radiator and serves a similar function in cooling the air that flows through it. Air then flows through the blower fan, and finally down the supply ducts to provide cool air to the interior of the building. Since the return and supply sides of the unit are on opposite sides of the blower fan, the air pressure will be lower on the return side and higher on the supply side. What this means for us is that the return (lower pressure) side wants to suck in air (and any adjacent moisture), while the supply (higher pressure) side wants to blow air out. That is why most leaks occur on the return side.

Investigating Leaks:
Investigating leaks at HVAC units will start just like any other leak investigation, by visually checking conditions at the leak location inside the building. If the leak is at an HVAC unit, the return and supply plenums should be readily evident. The first step should be to determine exactly where the water is coming from. For example, is the water dripping down the outside of the unit from where it penetrates the roof deck? This may indicate that the leak is coming from the roof itself, but it is still possible the unit is the source. On the other hand, if water is dripping out of the bottom of the unit, and there is no water on the outside of the unit, the source will almost certainly be the unit.  Many times this investigation can be done while the unit is not actively leaking by following water stains. If the entry point cannot be determined by following stains, a water test may be needed. If the unit is  suspected of being the source of the leak, it would be a good idea to get the HVAC service tech involved, so he  can shut the unit down and the inside of the unit can be safely accessed. This will also prevent any disputes about the findings of the investigation. A cooperative approach is the way to go. After all, even if the HVAC unit is leaking, it’s usually not the service tech’s fault. His priority is that the unit heats and/or cools as it is designed to do.

At this point, the investigation will proceed onto the roof. Of course, this is the time to check the roofing components, flashings etc. When checking the unit, the place to start is the return section. This will usually mean opening the filter access panel, and may involve removing the filters to get a clear view. A common source of leaks is the seal at the access panel, and many times water is evident as soon as the panel is opened. It may be necessary to check the return itself. Reaching into the unit with a digital camera and taking pictures will sometimes address this issue. If the exposed ducts on a side discharge unit are suspected, they simply need to be sealed properly.

Making repairs:
Once the source of water entry has been determined decisions will need to be made on what to repair and who will do that repair. As a rule, if the leak is coming from inside the HVAC unit, i.e. from a bottom discharge unit, we prefer that the HVAC contractor perform the repair as he has more experience with the mechanics of the unit. However, if the leak is coming from exposed ductwork outside the unit, then we can certainly perform the repair, however either party you select can stop this as a leak source.

Three Very Common Sources of HVAC Leaks

HVAC Condensation Pan Leaks:
When air conditioning units operate, they collect condensation. HVAC units typically have internal sheet metal pans that collect and drain off the condensation water. Because this is a continuing process, the sheet metal pans have a tendency to rust out. When this happens, the water inside the HVAC unit can leak into the building. This is a common source of leakage, particularly in older units. It represents a significant percentage of the reported leak calls that we receive. If you experience “roof leaks” when it’s not raining, the chances are that it’s an air conditioning problem.

HVAC Air Duct Leaks:
Sometimes HVAC units have sheet metal air ducts that run across the roof. These ducts have overlapping seams, where they are connected together. These seams are sealed to prevent air from leaking in and out. But, they are also sealed to prevent water from getting inside. Unfortunately these seals can crack and wear out. When the HVAC units operate, a high vacuum occurs in some of the ducts. If the duct seals are cracked or broken, the vacuum can pull rain water inside. Once inside the ducts, the water can be blown through the system. If you experience leakage that is seen directly below air ducts or ceiling air vent openings, the chances are that this is the problem.

Vacuum induced Equipment Leakage:
Most HVAC units create significant interior vacuum. If there is a small hole or opening in the HVAC unit, the vacuum can pull rain water inside the unit. Sometimes roof leakage occurs on brand new roof systems directly underneath HVAC units. When this happens, we conduct intensive water tests, to determine the source of the problem. In many cases we find that the roof is not leaking. The HVAC unit is leaking internally.

HVAC Roof Leaks

Most roofs have some sort of roof mounted HVAC equipment, frequently mounted on curbs. Roof mounted HVAC units contribute to leaks if not properly maintained. The two most common problems with HVAC units are improperly piped condensate drains and outright leakage of the units. Improper handling of the condensate can also shorten the life of the roof.

HVAC units typically have a “wet” side and a “dry” side. The wet side is exposed to the sky and readily allows rain water to enter. It is designed to conduct this water out of the unit without leaking into the building. The dry side, on the other hand, is not designed to be wet and if water does enter it can and will get down into the building. Wet sides, however, sometimes do leak into the building and dry sides often get wet (especially when their access doors are not properly secured and come loose.)

When there are repeatedly roof leaks in the vicinity of an HVAC, it’s time to think it’s likely not the roof. RAMCON technicians are trained to differentiate between roof leaks and HVAC leaks and will assist you in identification.

Most curb mounted HVAC units dump their condensation through a pipe on the side of the unit. These outlets require a “P-trap” to be installed to ensure proper operation of the drain. Without a trap it is possible that the condensate drain will not function and the condensate pan will overflow into the building below causing what appears to be a “roof leak”. Often this drain problem is not constant, so the unit drains properly sometimes and sometimes it does not. This makes it difficult to isolate the source of the problem and identify the true source of the perceived “roof leak”.

Once condensate gets out of the unit it is either dumped directly onto the roof or conducted away in piping. At RAMCON, we strongly recommend that it be piped to the roof drainage areas, as we consistently see roof deterioration from the corrosive fluids of the HVAC units. This is especially true of built-up roofing systems, as the consistent presence of these corrosive fluids on the granulated cap sheets and aluminized smooth surfaced roofs accelerates the deterioration of the protective surface and significantly shortens the life of the roof.

See the photograph above. The “staining” shown by the arrow is actually a thinning of the granules because the roof has been continuously wet over prolonged periods of time. The water coming out of the P-trap should be piped all the way to the drain. This roof now needs restorative repairs which would otherwise not be required had the condensate been handled properly. With gravel surfaced roofs condensate handling is less of a problem, however, the gravel tends to hold the condensate and become a breeding ground for plant growth which can harm the roof. These conditions are a major factor here in Florida, where AC units run almost continuously.

RAMCON can assist you in installing these condensation lines and making sure they are secured to the roof properly.