How to Prevent Condensation in HVAC Grilles, Registers, and Diffusers (GRD)

When the difference in temperature between the supply air and the room air is too great, one may observe small water droplets forming on the surfaces of metal HVAC grilles and diffusers. If these droplets persist and grow over time, water may eventually drip from the metal components onto the floor or even onto occupants in the space. This phenomenon is known as condensation.

The air we breathe is a mixture of various molecules, with nitrogen and oxygen being the predominant components. Additionally, water is present in the air, with its concentration fluctuating between 3-5% depending on temperature and pressure. While water molecules in the air typically remain in the vapor phase, they can condense back into liquid form when warm, moist air comes into contact with a cold surface.

Most HVAC grilles, registers, and diffusers are constructed from steel or aluminum, both recognized for their excellent heat conductivity. Consequently, as conditioned air flows through these components, the metal surface temperature gradually changes until it approaches the supply air temperature. For example, in scenarios where cool supply air, such as 65°F, is introduced into a space, the metal surfaces will progressively decrease in temperature over time until they closely match the 65°F temperature.  Insulation can slow down this process, but when cold air directly contacts metal, the metal surface will transfer heat to the airstream by convection and the temperature will decrease over time.  If the metal surface temperature gets too low, droplets of water can form on this cold surface.

Condensation is a frustrating issue that can cause discomfort to occupants, or even cause water damage to walls or other surfaces. If you notice condensation in the building, or you fear it might occur, there are a few tactics you can employ to ensure it doesn’t happen. Since the problem is caused by a combination of high humidity and low supply air temperatures, there are a few ways to avoid the issue.

Solution 1:  How to prevent condensation by decreasing the humidity level in the space

The first solution to protect against condensation would be to ensure that the relative humidity in the space is as low as possible before the air conditioning is turned on. Maintaining the relative humidity level between 30 and 50% is optimal and will result in most occupants feeling thermally comfortable. If the building is exceptionally large, it will be difficult to reduce the overall humidity of the entire space. But for offices or small spaces, a dehumidifier can work well to reduce the overall relative humidity. In small rooms, desiccants such as Damp Rid, or rock salt can also be effective at absorbing water from the air to reduce the relative humidity level. Once the humidity level drops, the dew point will also drop. At a relative humidity of 50% and temperature of 85°F, the dew point will be lower than 55°F. This means it’s highly unlikely that the supply air temperature will be cooled beneath this dew point which subsequently means that the metal products will not become cold enough to condense water droplets. Therefore, keeping the space humidity low (30-50%) is a surefire way to stop condensation.