Dehumidification strategies for commercial buildings

Most commercial buildings in Canada spend more hours worrying about dry winter air than damp summer air, so dehumidification is easy to overlook. Yet humid summers in southern Ontario, Quebec and the Maritimes, coastal moisture in Vancouver, and high internal moisture loads in pools, arenas, kitchens and densely occupied spaces all make humidity control a real design problem. Too much moisture leads to clammy occupants, condensation on cold surfaces, mould growth and damaged finishes.

Why humidity control matters

Air temperature is only half of comfort. Two spaces at 23 °C (73 °F) can feel very different if one is at 40 percent relative humidity (RH) and the other at 65 percent. ASHRAE Standard 55 uses humidity as one of the inputs to thermal comfort, and ASHRAE Standard 62.1 requires mechanical systems that dehumidify to be designed so that space RH stays at or below 65 percent at the design outdoor dew point and design indoor latent load.

The bigger risks are physical. Sustained high humidity supports mould growth on surfaces and in porous materials, and moist air reaching a cold surface below its dew point condenses. For more on that failure mode, see condensation and mould prevention.

A typical comfort target for occupied commercial spaces is roughly 30 to 60 percent RH, with many designers aiming to keep summer conditions near or below about 55 to 60 percent RH. Special spaces such as museums, cleanrooms, operating rooms and data centres have their own targets set by their governing standards.

Where latent loads come from

Latent load is the energy needed to remove water vapour. The main sources in commercial buildings are:

• Outdoor air brought in for ventilation, usually the largest latent load on humid days
• People, who release moisture through breathing and perspiration; the latent share rises with activity level
• Infiltration through the envelope, especially in buildings running under negative pressure
• Processes and open water, such as pools, kitchens, dishwashers, showers, ice resurfacing and indoor plants

The ratio of sensible to total cooling load is called the sensible heat ratio (SHR). Offices with high lighting and equipment loads may have an SHR of 0.8 or higher, while schools, assembly spaces and pools can drop well below 0.7. The lower the SHR, the harder it is for a conventional cooling coil to handle the moisture. The sensible and latent heat guide covers these terms in more detail.

How a cooling coil dehumidifies

A chilled water or DX coil removes moisture when its surface is colder than the entering air's dew point. Water vapour condenses on the fins and drains away. The leaving air is close to saturated, typically 90 to 95 percent RH, at a temperature set by the coil.

To deliver space air at about 50 percent RH and 23 °C, the supply air dew point usually needs to be in the range of about 10 to 12 °C (50 to 54 °F). That means the coil must cool the air to roughly that temperature. When the sensible load is low, for example on a mild humid day or in a lightly occupied zone, that cold air overcools the space. The controls then reduce coil output, and dehumidification stops. This is the core problem every strategy below is trying to solve.

Dehumidification strategies

Overcool and reheat

The coil is controlled to a fixed leaving air dew point, and a downstream heating coil or zone reheat brings the air back to a comfortable supply temperature. It is simple and effective but wasteful. NECB and ASHRAE 90.1 place limits on simultaneous heating and cooling and on reheat, generally allowing it where the reheat energy comes from recovered or site-recovered sources or under specific exceptions. Check the applicable edition and jurisdiction before relying on new-energy reheat.

Recovered heat reheat

The same approach, but reheat comes from a recovered source: hot gas reheat on DX units, condenser water heat recovery, a heat recovery chiller, or a heat pipe or wraparound loop that transfers heat from the entering air to the leaving air. Wraparound heat pipes precool the air before the coil, which increases moisture removal, then reheat it after the coil at no added energy cost other than a small fan pressure penalty.

Dedicated outdoor air systems

A dedicated outdoor air system (DOAS) conditions only the ventilation air and dries it to a dew point below the space target. That dry air absorbs the latent load from people and infiltration, leaving terminal units such as chilled beams, fan coils or VRF indoor units to handle only sensible load. Separating latent and sensible control is one of the most reliable ways to hold humidity in buildings with variable occupancy.

Desiccant dehumidification

Desiccant wheels adsorb moisture directly from the air rather than condensing it. Active desiccant wheels use heat to regenerate the desiccant and can reach very low dew points, which suits ice arenas, cold storage, pharmaceutical spaces and some cleanrooms. Passive desiccant wheels, often called enthalpy or total energy wheels, transfer moisture between exhaust and outdoor air streams and reduce the latent load of ventilation air without regeneration heat.

Dedicated dehumidifiers

Packaged dehumidifiers use a refrigeration circuit to cool air below its dew point, then reject the condenser heat back into the airstream or to an outdoor condenser, pool water or a hydronic loop. They are the standard approach for natatoriums and are also used in basements, archives and storage areas.

Variable airflow and coil control tactics

Lower fan speed across a coil lengthens contact time and improves moisture removal. Many DX units offer a dehumidification mode that slows the indoor fan, and chilled water plants should limit warm chilled water resets during humid weather.

Comparing strategies

Designing and controlling for humidity

A successful design starts with an honest latent load calculation at the right outdoor condition. Peak dry bulb design conditions do not produce the peak latent load; a humid, mild day at the 1 percent dew point condition often does. ASHRAE Fundamentals publishes dehumidification design data for Canadian stations alongside the cooling and heating values that appear in the NBC climate tables.

Practical steps:

1. Calculate latent loads at the design dew point condition as well as at peak dry bulb.
2. Determine the supply air dew point needed to absorb the space latent load at the design airflow.
3. Confirm the coil, refrigeration system and controls can reach that dew point at part load.
4. Choose a reheat or decoupling strategy that complies with NECB or the locally adopted energy code.
5. Locate humidity sensors where they represent the space, away from supply diffusers and exterior doors.
6. Specify how humidity control interacts with economizers, ventilation reset and night setback in the sequence of operation.
7. Verify performance during commissioning on a humid day, not only on a design-day spreadsheet.

Economizers can bring in large amounts of humid outdoor air on mild days, so a dew point or enthalpy high limit matters in humid climates. RH sensors also drift, so plan for periodic calibration.

If you are planning a new system or troubleshooting a humid building, our HVAC design team can help evaluate options.

Key takeaways

• A cooling coil only dehumidifies when its leaving air is below the target dew point, which often overcools spaces at part load.
• Outdoor air is usually the biggest latent load, so ventilation air treatment drives most humidity strategies.
• Recovered heat reheat, DOAS and desiccant systems control humidity with far less energy than new-energy reheat.
• Size equipment using the design dew point condition, not just peak dry bulb.
• Energy codes such as NECB limit simultaneous heating and cooling, so confirm the reheat strategy complies.
• Sensor location, calibration and economizer high limits are as important as equipment selection.