BEAM Plus Existing Buildings

BEAM P

LUS FOR

XISTING

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NDOOR

NVIRONMENTAL

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1.2

15 T

HERMAL

OMFORT IN

ATURALLY

ENTILATED

REMISES

Page 134

b) Performance with air-conditioning

The measurement locations shall include at least one representative

sample of each type of premises (occupied spaces) as defined by the

type of HVAC system used, design occupancy density, nature of usage,

zoning, etc. The measurements shall be undertaken in a normally

occupied period. The sensors used in the measurement survey shall

have an accuracy that complies with ISO 7726 [2] or equivalent. To earn

credit the results shall demonstrate compliance with the prescribed

design criteria within the prescribed limits, for a minimum of 90% of the

prescribed locations.

ACKGROUND

Thermal comfort standards such as ISO 7730 [ 3 ] and ASHRAE 55

establish relatively tight limits on recommended indoor thermal

environments, and do not distinguish between what would be considered

thermally acceptable in buildings conditioned with natural ventilation.

Field studies and research has demonstrated that occupants of buildings

with centralised HVAC systems become finely tuned to the very narrow

range of indoor temperatures provided, developing high expectations for

homogeneity and cool temperatures, and soon became critical if thermal

conditions do not match these expectations [4,5]. In contrast, occupants

of naturally ventilated buildings are more tolerant of a wider range of

temperatures. This range extends beyond the comfort zones established

for air-conditioned buildings, and may more closely reflect the local

patterns of outdoor climate.

Analysis of the available data has established that behavioural

adaptations, such as changes in clothing insulation or indoor air speeds,

could account for only half the observed variance in thermal preferences

of people when in naturally ventilated buildings. Given that physiological

adaptation is unlikely to play much of a role; it is suggested that the rest

of the variance is attributable to psychological factors

[4]

. Relaxation of

thermal expectations may be due to a combination of higher levels of

perceived control and a greater diversity of thermal experiences in a

naturally ventilated building.

For the purposes of ASHRAE 55-2004, occupant-controlled naturally

conditioned spaces are those spaces where the thermal conditions of the

space are regulated primarily by the occupants through opening and

closing of windows. The ‘Optional Method for Determining Acceptable

Thermal Conditions in Naturally Conditioned Spaces’ is intended for such

spaces. In order for this optional method to apply, the space in question

must be equipped with operable windows that open to the outdoors and

that can be readily opened and adjusted by the occupants of the space.

Allowable indoor operative temperatures for spaces that meet these

criteria may be determined from Figure 5.3 in ASHRAE 55-2004. This

figure includes two sets of operative temperature limits—one for 80%

acceptability and one for 90% acceptability. The 80% acceptability limits

are for typical applications and shall be used when other information is

not available.

The allowable operative temperature limits in Figure 5.3 may not be

extrapolated to outdoor temperatures above and below the end points of

the curves in this figure. If the mean monthly outdoor temperature is less

than 10°C or greater than 33.5°C, this option may not be used, and no

specific guidance for naturally conditioned spaces is included in this

International Standard Organization. ISO 7726, Ergonomics of the thermal environment — Instruments for measuring

physical quantities. 1998.

International Standard Organization. ISO 7730. Moderate thermal environments – Determination of the PMV and PPD

indices and specification of the conditions for thermal comfort.

de Dear R, Brager G S, Reardon J, Nicol F et al. Developing an adaptive model of thermal comfort and preference/

Discussion. ASHRAE Transactions. Vol. 104. 1998. pp 145-167.

Brager G S, de Dear R. A Standard for Natural Ventilation. ASHRAE Journal. October 2000. pp 21-28.