Where NOT to locate the room thermostat  (C) Carson Dunlop Associates Warm Air Stratification Indoors
Comfort & Heating / Cooling Costs, Problems, Solutions

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Stratification of air indoors as a source of occupant comfort or IAQ complaints:

This article describes the basic causes & cures for un-wanted air stratification indoors that affects the comfort of building occupants.

Page top sketch provided courtesy of Carson Dunlop Associates, a Toronto home inspection, report writing and education company.

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Indoor Air Stratification Can Cause Uneven Room Temperatures

Honeywell room thermostat

Even when your room thermostat is properly-located and even when a warm-air heating system is operating normally, some rooms or areas within some rooms may be uncomfortable as heat may be distributed unevenly.

Air stratification during the heating season can leave warm air collected near the ceiling with air nearer the floor uncomfortably cool. Solving this problem by turning up the thermostat or moving it lower on the wall can "work" but is likely to increase heating costs unnecessarily.

This article supports THERMOSTAT CALIBRATION where we discuss room thermostat location and adjustment to improve the comfort of building occupants when heating or cooling air are not distributed satisfactorily or when indoor temperatures are uneven.

Causes of Warm Air Stratification: warm air buoyancy, low supply air velocity, short circuits

Effects of Warm Air Stratification in Buildings

Principal methods for curing warm air stratification

Prozuments (2017), quoting in turn the ASHRAE Fundamentals Handbook (2009) section on "Space Air Diffusion" and also excerpting from Bragnanca (2016) offers this description of air stratification in discussing studies of warm air supply systems that focus on nozzle or jet air supply systems.

As described in the 2009 ASHRAE Handbook-Fundamentals, the airflow pattern in cooling mode from a flush-mounted ceiling diffuser is projected downward to the floor and follows it, producing a stagnant region near the ceiling.

In heating mode, however, the airflow reaches the floor and folds back toward the ceiling. If the downward air does not reach the floor, a stagnant zone appears near the floor, causing issues discussed in the background section.

Based on these features, modern commercial multi-cone diffusers were developed to mechanically adjust the inner cones, to switch from vertical flow pattern in heating mode, to horizontal pattern in cooling mode.

Braganca, et al., find in their innovative study that thermal comfort was significantly improved using the diffusers with inserted lobes compared to those without lobes, without increases of pressure drop and sound pressure levels.

Contradicting this view, Prozuments continues by noting that Krajcik (2012) all came to the opposite conclusion [probably because they were not focused on nozzle or jet air supply systems - Ed.]. Those authors found that

... at the set air supply temperature and flow rate the air mass stratification occurs with the temperature gradient above 1.8°F/m (1°C/m). Supply air temperature did raise the temperature in the occupied zone to the desired level. However, it also raised the temperature gradient...

Both sets of authors go on to point out that in essence, making the occupants warm-enough by simply raising the supply air temperature is a more-expensive way to heat the occupied space than if proper mixing of stratified air could be obtained.

Prozuments continues

Even though the warm supply air has gradually heated up room air, the difference between the air temperature right below the supply device and the occupied zone was nearly 5.4°F (3°C), which is the maximum acceptable differential in modern ventilation systems.

... good thermal comfort is described by low-temperature gradient (< 1.8°F/m <1°C/m) and low deviation from the room setpoint temperature. - Op. Cit.

The bottom line: these authors conclude that reducing the temperature differences at different heights above the floor in a heated space (reducing warm air stratification) by increasing the supply air flow rate is desirable for comfort, air quality, and economy.

Other factors in perceived heated area comfort include the level of activity (metabolic rate) of the occupants, their choices of clothing, and the humidity level.

Research & References on Indoor Air Stratification Causes & Cures

Audel Oil Burner Guide page 303 on two stage thermostats to reduce indoor air stratification (C) InspectApedia.comOur excerpt from our copy of the 1946 edition of Audel Oil Burner Guide illustrates that the comfort issue with indoor air stratification is not a new discovery. [Click to enlarge any image]


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