TM52 The limits of thermal comfort Cardiff
Transcript of TM52 The limits of thermal comfort Cardiff
TM52 The limits of thermal
comfort: avoiding overheating in
European buildings
Fergus Nicol
Oxford Brookes and London Metropolitan Universities
Outline
• Outline the current issues
• Overheating task force work
• Adaptive comfort
• Theory of criteria
• Why criteria were revised
Current Issues
Current Issues
• The energy message of heat saving in winter using highly insulated and airtight buildings also means there is a danger of overheating in the summer months
• Overheating has been an increasing concern and CIBSE, organised an Overheating Task Force to look into the problem
• The Task force was drawn from the concerned bodies – CIBSE, the Universities, Health and Safety Executive, and engineering and architectural consultants
Task Force Members
• Andy Ford (Chair) (Mott MacDonald Fulcrum – now University of the South Bank)
• Fergus Nicol (Oxford Brookes University)
• Michael Humphreys (Oxford Brookes University)
• Brian Spires (Technical Consultant)
• Andrew PG Moore (HSE)
• Hywel Davies (CIBSE)
• Gay Lawrence Race (CIBSE)
• Max Fordham (Max Fordham LLP)
• Jake Hacker (Arup)
• Anastasia Mylona (CIBSE)
TM52 ContentsComfort and discomfort
Our thermal sense
How can we judge if a building is overheating?
Behaviour and discomfort
Predicting discomfort
The basics
Investigating and modelling thermal comfort
Predicted mean vote (PMV) and predicted percentage
dissatisfied (PPD) using the heat balance model of comfort
The basis of comfort standards
Existing standards
Problems for standards
Risk of overheating
Problems with a single temperature overheating limit
Discomfort as a function of a deviation from comfort temperature
CIBSE recommendations for identifying overheating
CIBSE guidance
Overheating in mechanically cooled buildings
Overheating: CIBSE 2006 approach
• In the past the CIBSE approach to overheating
was based on not exceeding a single limiting
temperature
• 25oC for > 5% of occupied (working) hours
• 28oC for > 1%
• There was no acknowledgment that the extent
of overheating was a concern as well as its
duration
Problems
• The existing guidance was found to be inadequate, taking little account of the type of building and the climatic dependence of comfort on outdoor temperature
• In the case of naturally ventilated buildings it was at variance with international standards
• The definitions on which it was based took little account of the actions of building occupant
Adaptive comfort
• The adaptive approach to thermal comfort is
based on the findings of field surveys in
workplaces and other building types
• It is now accepted as the standard approach
to specifying target temperatures in naturally
ventilated buildings where indoor conditions
are less easy to control
• The general approach applies in all buildings
Occupant Building
Comfort is achieved by
the occupants adapting to
the building
Or by the occupants
adapting the building to
suit them
This has to be done within the existing climatic, social,
economic, architectural and cultural context. Buildings
should be designed to provide acceptable conditions and to
allow occupants to control environment 11
Lisbon, PortugalLisbon, Portugal
Field studies can be used to judge the overall
effect of the environment
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Definition of Thermal Comfort
• That state of mind which expresses satisfaction with the thermal environment
• ASHRAE Scale Bedford scale
• +3 Hot 7 Much too warm
• +2 Warm 6 Too warm
• +1 slightly warm 5 Comfortably warm
• 0 Neutral 4 Comfortable neither cool nor warm
• -1 slightly cool 3 Comfortably cool
• -2 Cool 2 Too cool
• -3 Cold 1 Much too cool
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Called the Comfort
Vote
Measure the environment
Calculating comfort
• Using the responses of subjects in real
situations where adaptive changes can be
made the value of the ‘neutral’ temperature
Tneutral (at which a vote of ‘neutral’ would be
expected) can be calculated statistically.
• Its dependence on the mean operative
temperature can be deduced with some
precision if the database is adequate
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From Nicol, Humphreys and Roaf, Adaptive Thermal Comfort, principles & practice
Another approach is to find the number of
people voting comfortable at difference
temperature and calculating at what
temperature this reaches a maximum
Comfort and mean temperature
• We can calculate the mean temperature for a
particular survey
• We can calculate the neutral temperature for
the people in the survey.
Survey results Tneutral and Toperative
19From Humphreys (data from 700+ building surveys)
95%
confidence
intervals
Red dots H/C
Open dots M/M
Blue dots F/R
Each dot is the
mean value for a
whole surrvey
This does not mean that
people will easily and
instantly make themselves
comfortable.
People will learn to make
themselves comfortable
over time.
Their ability to do so will
depend on the
opportunities which the
building they inhabit gives
them to do so.
Comfort and outdoor temperature
• Also of interest is that way in which comfort
temperature changes with outdoor
temperature
• This will allow designers to take account of the
climate and suggest what allowances to make
• Differences are found in the response of
people in buildings with no heating or cooling
and those with mechanical control
Comfort and outdoor temperature
22From Humphreys et al, 2010 from 700+ Buildings surveys
Neutral temperatures in Free-
running buildings
For any outdoor temperature
there will be a range of possible
indoor comfort temperatures
Comfort and outdoor temperature
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Neutral temperatures in heated and cooled buildings
where indoor temperature is decoupled from the
outdoor temperature
European Standard EN15251
• This standard entitled Indoor Environmental
Input Parameters for Design and Assessment
of Energy Performance of Buildings Addressing
Indoor Air Quality, Thermal Environment,
Lighting and Acoustics was designed to set
limits for indoor conditions to ensure that the
Energy Performance of Buildings Directive did
not compromise the comfort of occupants in
the pursuit of energy reduction
Categories of building in EN15251
Category Applicability/level of
expectancy
Temperature
range in NV
buildings
I High: Buildings with high
expectancy for sensitive
occupants
± 2oK
II Normal: New buildings ± 3oK
III Acceptable: Existing buildings ± 4oK
IV Low expectancy only for short
periods
> 4oK
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FR comfort limits EN15251
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Tc = 0.33 Trm + 18.8
Palermo-Punta Raisa
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Tem
pera
ture
(oC
)
Daily mean outdoor temperature (To)
Running mean temperature (Trm)
Comfort temperature
ISO7730
summer
prEN 15251ASHRAE
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New CIBSE Guidance for NV buildings
in summer
• Temperatures should be limited to those
specified by EN15251. Category II is used
because it applies to normal new buildings for
which the guidance will most commonly used
• In addition three Criteria are specified (also
related to EN15251) which specify the
duration of exceedance of EN15251 limits and
its allowable extent
Defining free-running compliance
• The CIBSE recommendation is that new buildings,
major refurbishments and adaptation strategies
should conform to Category II in Standard BS
EN15251 (see previous slide and equation below)
for buildings in free running mode.
• For such buildings the maximum acceptable
temperature (Tmax) can be calculated from the
running mean of the outdoor temperature (Trm)
using the formula:
• Tmax = 0.33Trm + 21.8 (oC)
New CIBSE adviceBuilding
overheats when it
exceeds the
maximum
temperature
(Tmax) for a
category II
building in the
free-running
adaptive graph in
EN15251
Defining free-running compliance
• The criteria are all defined in terms of ΔT the
difference between the actual operative
temperature in the room at any time (Top) and
Tmax the limiting maximum acceptable
temperature. ΔT is calculated as
• ΔT = Top - Tmax (oK)
• ΔT is rounded to the nearest whole degree (i.e.
for ΔT between 0.5 and 1.5 the value used is 1K,
for 1.5 to 2.5 the value used is 2K and so on)
Criterion 1 Hours of Exceedence (He): sets a limit for the
number of hours that the operative temperature can
exceed the threshold comfort temperature (upper limit of
the range of comfort temperature) by one degree or more
during the occupied hours of a typical non-heating season
(1st May to the 30th September)
Definition:
The number of hours (He) during which ΔT is greater than
or equal to one degree (K) during the period May to
September inclusive shall not be more than 3% of
occupied hours.
If data are not available for the whole period (or if
occupancy is only for a part of the period) then 3% of
available hours should be used.
THE NEW CRITERIA
Background:
Provides useful information about the building’s thermal
characteristics and potential risk of overheating over the
range of weather conditions to which it will be subjected.
Courtesy of Paul TuohyESRU, University of Strathclyde
THE NEW CRITERIA
Criterion 2 – Daily Weighted Exceedence (We): deals with
the severity of overheating, which can be as important as
its frequency, the level of which is a function of both
temperature rise and its duration. This criterion sets a daily
limit for acceptability
Definition:
To allow for the severity of overheating the weighted
Exceedence (We) shall be less than or equal to 6 in any one
day.Where
We = Σhe x wf = (he0 x 0) + (he1 x 1) + (he2 x 2) + (he3 x 3)
the weighting factor wf = 0 if ΔT ≤ 0, otherwise wf = ΔT, and hey = time
in hours when wf = y
THE NEW CRITERIA
Background:
This criterion covers the severity of overheating, which is
arguably more important than its frequency, and sets a
daily limit of acceptability that is based on Method B –
‘Degree hours criteria’ in BS EN15251; 2007
Exemplar case study
Courtesy of Paul Tuohy
ESRU, University of Strathclyde
THE NEW CRITERIA
Background:
This criterion covers the severity of overheating, which is
arguably more important than its frequency, and sets a
daily limit of acceptability that is based on Method B –
‘Degree hours criteria’ in BS EN15251; 2007
Exemplar case study
Courtesy of Paul Tuohy
ESRU, University of Strathclyde
THE NEW CRITERIA
Criterion 3 - Upper Limit Temperature (Tupp): sets an
absolute maximum daily temperature for a room, beyond
which the level of overheating is unacceptable.
Definition:
To sets an absolute maximum value for the indoor
operative temperature the value of ∆T shall not exceed 4K.
Background:
The threshold or upper limit temperature is fairly self-
explanatory and sets a limit beyond which normal adaptive
actions will be insufficient to restore personal comfort and
the vast majority of occupants will complain of being ‘too
hot’. This criterion covers the extremes of hot weather
conditions and future climate scenarios.
THE NEW CRITERIA
Another way to display data
A straight scatter plot of Top against Trm can be
easier to read and relate to limits
Graph
from
TM36
28oC
25oC
15oC 25oC
Standards in A/C buildings
• In buildings with mechanical ventilation the comfort limits are set using Fanger’s Predicted Mean Vote (PMV) but this is usually presented as a temperature since most mechanical systems are controlled by temperature
• The limits for Category II with mechanical heating or cooling buildings are that the PMV should not exceed 0.5. It requires a knowledge of the metabolic heat and thermal insulation of the clothing worn
Categories of building in EN15251
Category Applicability/level of
expectancy
PMV range in
mechanically
cooled buildings
I High: Buildings with high
expectancy for sensitive
occupants
± 0.2
II Normal: New buildings ± 0.5
III Acceptable: Existing buildings ± 0.7
IV Low expectancy only for short
periods
> 0.7
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Buildings with mechanical cooling
Type and Assumed met Winter Summer
use of space (clo = 1.0) (clo = 0.5)
pmv> +0.5 +0.5
Residential (sedentary) 1.2 25.0 26.0
Residential (active) 1.5 25.0 Offices 1.2 24.0 26.0
Public spaces (auditoria, cafe etc) ~1.2 24.0 26.0
Classrooms 1.2 24.0 26.0
Kindergarten 1.4 22.5 25.5
Shops 1.6 22.0 25.0
Maximum temperatures for different types of indoor
space. Clothing is assumed to be 1.0 clo in winter and 0.5
clo in summer (after BSI, 2007).
Thanks for
your
attention !