Sensible and latent heat: why the ratio matters

Every cooling load has two parts. Sensible heat changes the temperature of the air. Latent heat is the energy tied up in water vapour, and it is removed only when moisture condenses out of the air. A space can be at the right temperature and still feel clammy if the latent load is not handled.

The balance between the two, expressed as the sensible heat ratio, determines what kind of equipment can serve a space well. Many humidity complaints in otherwise well-sized buildings come from a mismatch between the space ratio and the equipment's capability.

Definitions

• Sensible heat is heat that raises or lowers dry-bulb temperature without changing moisture content. Solar gain, lighting, computers and conduction through walls are almost entirely sensible.
• Latent heat is heat associated with a phase change, in HVAC usually water evaporating or condensing. People exhaling and perspiring, cooking, open water, plants and humid outdoor air are latent sources.
• Total heat is the sum of both.

The sensible heat ratio (SHR) is:

SHR = sensible load / total load

An SHR of 1.0 means a purely sensible load. An SHR of 0.70 means 30% of the cooling effort must go into removing moisture.

Typical sensible heat ratios

Values vary with occupancy, ventilation strategy and climate. The ranges below are indicative for room loads (not including outdoor air) and should be confirmed by calculation.

When outdoor air is added, the coil SHR can drop further, especially in humid summer climates such as southern Ontario. In much of Alberta and the BC interior, summer outdoor air is drier, and outdoor air may even reduce the latent coil load.

Calculating sensible and latent loads

At standard air conditions:

• Sensible: Qs (W) = 1.23 x L/s x delta t (K), or 1.08 x cfm x delta t (°F) in Btu/h
• Latent: Ql (W) = 3010 x L/s x delta W (kg/kg), or 0.68 x cfm x delta W (grains/lb) in Btu/h

Internal latent loads come from tables. ASHRAE Handbook, Fundamentals gives heat gains for people by activity: a person doing light office work gives off roughly similar amounts of sensible and latent heat, while heavier activity shifts the split toward latent. See internal heat gains from people, lighting and equipment.

Why the ratio matters for equipment

A cooling coil removes moisture only if its surface is below the dew point of the air passing over it. The colder the coil and the slower the air, the more moisture it removes. Each piece of equipment has a natural SHR that depends on coil temperature, airflow per unit capacity and fan speed.

Equipment SHR

Packaged rooftop units and split systems are often designed with high airflow per tonne for efficiency, which gives a relatively high SHR, often in the 0.70 to 0.80 range at rated conditions. That is adequate for many offices but may not remove enough moisture in a densely occupied space.

If the space SHR is lower than the equipment can deliver, the unit satisfies the thermostat before it has removed enough moisture. Temperature is fine; humidity creeps up.

The oversizing trap

Oversized constant-volume equipment makes this worse. It reaches the setpoint quickly and cycles off. Moisture that condensed on the coil can re-evaporate into the airstream when the compressor stops while the fan keeps running. The result is a cold, damp space. Right-sizing, discussed in rules of thumb in HVAC sizing, is the first defence.

Reading the ratio on a psychrometric chart

On the psychrometric chart, the room SHR sets the slope of the room ratio line. Supply air must lie on this line to meet both the sensible and latent load. A low SHR line is steep, which requires a lower supply dew point and often a lower supply temperature. Many ASHRAE charts include a protractor to draw this slope; see our psychrometrics guide for the procedure.

Strategies for low-SHR spaces

When latent loads are high relative to sensible loads, options include:

1. Lower the coil leaving temperature so it removes more moisture, then reheat if needed. Effective but energy-intensive unless reheat comes from recovered heat.
2. Reduce airflow across the coil with variable-speed fans, which lowers coil temperature and increases moisture removal.
3. Use a dedicated outdoor air system (DOAS) that dries outdoor air to a low dew point, so the space units can handle mostly sensible loads. See dedicated outdoor air systems.
4. Add a desiccant dehumidifier where very low dew points are needed.
5. Use hot gas reheat on packaged equipment, recovering condenser heat to reheat dehumidified air.
6. Reduce latent sources, such as covering open water or improving kitchen exhaust capture.

Diagnosing humidity complaints

Ask these questions before changing equipment:

• What is the actual space dew point, not just relative humidity?
• Is the unit cycling frequently?
• Does the fan run continuously while the compressor cycles?
• Has occupancy or use changed since design?
• Is outdoor air flow higher than design, for example due to a failed damper actuator?

Trend data from the building automation system often answers these quickly. A retro-commissioning review can identify control changes that restore humidity control without new equipment.

Frequently asked questions

Can a unit remove too much moisture?

In winter or in very sensible-dominated spaces such as data rooms, yes. Unnecessary dehumidification wastes energy, and in data centres excessive dryness raises static electricity concerns. Coil temperatures can be raised when latent loads are low.

Does a lower thermostat setpoint fix humidity?

Rarely for long. Occupants may feel better briefly, but the root cause is usually a mismatch between equipment SHR and space SHR, or cycling.

Is relative humidity a good control variable?

It is useful for occupants to understand, but dew point is the better control variable because it does not change with temperature.

Key takeaways

• Sensible heat changes temperature; latent heat is moisture that must be condensed out.
• The sensible heat ratio sets how much moisture removal equipment must provide.
• Equipment with a higher SHR than the space will leave the space humid.
• Oversized constant-volume equipment is a common cause of humidity problems.
• DOAS, variable-speed fans, reheat from recovered heat and desiccants handle low-SHR spaces.