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

[email protected]

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

12

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

13

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

15

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


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


23

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

25

FR comfort limits EN15251

26

Tc = 0.33 Trm + 18.8

Palermo-Punta Raisa

19.00

20.00

21.00

22.00

23.00

24.00

25.00

26.00

27.00

28.00

29.00

30.0001

/06/

2002

08/0

6/20

0215

/06/

2002

22/0

6/20

0229

/06/

2002

06/0

7/20

0213

/07/

2002

20/0

7/20

0227

/07/

2002

03/0

8/20

0210

/08/

2002

17/0

8/20

0224

/08/

2002

31/0

8/20

0207

/09/

2002

Tem

pera

ture

(oC

)

Daily mean outdoor temperature (To)

Running mean temperature (Trm)

Comfort temperature

ISO7730

summer

prEN 15251ASHRAE

55

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

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

40

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 !

tm52 the limits of thermal comfort cardiff

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