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There are two concerns with respect to human interaction to the thermal environment

-Thermal stress and strain-Thermal comfort.

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Excessive thermal stress means that the body has to work hard to avoid he core temperature moving outside narrow limits (37 + 2 C). If that happens we’re in a serious situation which leads to serious health effects and may be fatal. Our efforts to prevent this happening can also lead to adverse effects.

This is most likely to occur in extreme environments or, sometimes, in more moderate environments where particularly heavy work is being performed or clothing is worn which prevent metabolic heat escaping.

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Thermal comfort is most likely to be an issue in workplaces such as offices, but complaints or concerns can sometimes occur in manufacturing environments and other types of workplace

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It is also something that may need to be addressed in leisure facilities.

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The first thing to note about “thermal comfort” that differentiates it from thermal strain is that the body is not experiencing a level of stress that it can’t cope with. There isn’t a physiological problem and ill health will not occur due to excessive thermal strain.

But that encompasses a wide range of conditions. Will all of them be “comfortable”? Experience clearly shows that the answer to that is “no” !

So what is thermal comfort?

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This is the definition given in ISO 7730, (more about that later)

So thermal comfort is a psychological or psycho-social issue

It’s about people’s opinions, and you can’t keep everyone happy all of the time.

Studies have suggested that the very best you can do is achieve 95% satisfaction . So there will always be 1 person in 20 who is dissatisfied. Change things for him/or her, someone else will start to feel unhappy!

In practice you’re doing well if more than 80% are happy!

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But what we think is influenced by the environment and this is true of our perception

of thermal comfort. So if we want to understand what people consider to be a

“comfortable” state we need to understand how we interact with the thermal

environment.

Human beings create heat, so one thing we have to consider is how much heat we’re

generating That really depends on our activity level or, in an occupational setting, our

work rate. The more physically demanding our activity, the more heat we’re

producing.

Depending on circumstances we may lose heat to the environment, or we can gain it.

So there is heat exchange. There are a number of ways that this occurs.

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Convection is one of the main ways that heat transfer between people and the

environment occurs.

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Radiant heat can be a problem in some cases. Everything gives of radiant heat – the

amount depends on the surface temperature. In thermal comfort situations it is unusual

to find hot surfaces that give off significant radiant heat. However, solar radiation can

be a problem in some cases, particularly in buildings such as offices with lots of

windows.

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Evaporation is one of the main ways we lose heat.

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We gain heat by convection if our skin temperature is higher than air temperature.

However, in most situations where there are complaints about thermal comfort the air

temperature will be lower than skin temperature and we are more likely to be losing

heat by this means

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Radiant heat depends on the mean radiant temperature. This is the mean temperature

of all the surrounding surfaces including the floor, walls, ceiling and any objects

present.

Radiant heat exchange is also affected by solar radiation.

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Evaporative cooling depends on the air temperature and relative humidity

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These factors influence our heat balance. But there’s another one that interferes with

the exchange of heat between the body and the environment

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Clothing level has a significant influence on heat exchange. It interferes with

convective and radiant heat exchange and affects evaporative efficiency. Heavy

clothing can even influence metabolic rate (although that won’t normally be a

problem for thermal comfort related situations)

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So if we are to assess thermal comfort thoroughly we need to quantify these four environmental parameters

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And we also need to consider the “human factors” – metabolic rate and clothing.

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Draughts cause local cooling and are likely to lead to complaints

Radiation asymmetry may occur where there is a directional heat source and can often be associated with solar gain through windows

As hot air rises, in most buildings air temperature increases with distance above the floor. The extent of this difference depends very much on the type of heating employed. With radiators the temperature varies little with height, but with warm air heating a significant temperature gradient can result. If the gradient is sufficiently large, local warm discomfort can occur at the head and/or cold discomfort can occur at the feet, even though the body may be thermally neutral

Floor temperature is not normally an issue in most workplaces but needs to be considered in situations where people may have bare feet – e.g. Swimming pools, gyms, changing rooms and the like

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There are two basic approaches we can take to evaluating thermal comfort

-a Qualitative approach where we may not take any measurements but rely on other forms of evidence

- a Quantitative approach where we carry out measurements. (However, interpreting the results from the measurements presents some difficulties)

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A quantitative assessment involves talking to and interviewing people in the workplace and looking around for obvious clues, using experience to make a subjective judgement.

The thing to remember is that “comfort” is subjective – one person may complain about feeling warm while another may consider the same environment to be too cool. So the best way to find out what someone thinks is to ask them!

There are a number of questionnaires and checklists available from various sources which can help with this qualitative approach. Some examples follow (there are others, or you can even devise your own)

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This very simple checklist is available on the HSE website

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This is a 5 point scale devised by Fanger (more about him later)

The BOHS Technical Guide on the Thermal Environment has a couple of examples of questionnaires that can be used during a qualitative assessments.

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This one is fairly simple and mainly addresses “whole body” comfort

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This is a little more comprehensive. It allows evaluation of local discomfort.

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If we’re going to undertake a thorough quantitative assessment we’ll need to quantify 6 things.

The environmental parameters

-Air temperature-Mean radiant temperature-Relative humidity-Air velocity

And two “human” factors

-Work rate (which affects metabolic rate) and-Cloting level

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A standard thermometer measures air temperature.

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The traditional way of measuring relative humidity is to use a “whirling” or “sling” hygrometer. It has two thermometers – one of which has it’s bulb covered with a dampened muslin wick. The difference in the readings obtained is proportional to relative humidity which is determined using a psychometric chart or special slide rule.

Electronic humidity meters are available and may be less embarrassing to use! But they will need calibrating.

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Radiant heat can be assessed using a globe thermometer.Note that it does NOT give a direct reading of mean radiant temperature as it is affected by both radiant and convective heat exchange. The reading is known as the “globe temperature”. If the air temperature and velocity are also known, then it is possible to calculate the mean radiant temperature/

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This is a smaller version of the globe thermometer. Note that it is less accurate the larger version.

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The difficulty with measuring air velocity is that in most situations where we need to assess thermal comfort it is relatively low and omni-directional which makes measurement difficult.

The kata thermometer does not measure temperature but the time it takes to cool is determined and from this, if the air temperature is also known, the average air velocity can be estimated.

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Various electronic instruments, such as this, are available. Remember that they will need to be calibrated from time to time

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Metabolic rate can be estimated using standard tables found in ISO 7730 and other texts

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Like metabolic rate, clothing level can be estimated using standard tables found in ISO 7730 and other texts.

1 Clo represents the insulation provided by an “American business suit”

Now we have quantified six parameters what do they mean? We need to find a way of pulling them together so we can decide whether we have a problem and, if so, how big the problem is.

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The PMV / PPD index is widely use when assessing thermal comfort.

It was developed by Professor P O Fanger who conducted extensive research into thermal comfort using a large number of subjects both in Denmark and the United States

It forms the basis of the International Standard ISO 7730

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The first step is quantify the main factors that affect thermal comfort and then to determine the predicted mean vote (PMV) .The PMV is then used to determine the predicted percentage dissatisfied (PPD).

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This Fanger’s equation, develoed from his research

As you can see it is very complex and quite unsuitable for manual calculation. However, ISO 7730 contains a BASIC programme which can be used to obtain the PMV and PPD. There is also a series of charts, derived using the equation which are relatively easy to use.

BS EN ISO 7730A quantitative approach to evaluating thermal comfort that uses Fanger’s PMV PPDIndex

Also provides guidance for the design of a comfortable thermal environment with recommendations for both whole body and local thermal comfort conditions.

It provides a computer programme (in Basic) and tables, either of which can help you determine the PMV and PPD for a given situation

The tables do not cover all possible conditions but allow a reasonable estimate to be made

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When using the tables you need to determine the operative temperature.

At low velocities this is the mean of the air temperature (dry bulb) and mean radiant temperature (NOT the globe temperature). As the globe thermometer responds to both convective and radiant heat the globe temperature will be close to the operative temperature at low velocities.

What do you think?

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So, which is the best approach to use when people are complaining about feeling too hot or cold?

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Well, when investigating a problem its probably best to use a combined approach

In my view the first think to do is to talk to people to find out what they think and look around for obvious problems. Thermal comfort is subjective so if people are complaining there’s a problem even if using the PMV/PPD index predicts people will be generally happy. If they’re unhappy, something needs to be done.

In order to decide on solutions we need to find out why we have a problem. Quite often this can be determined by looking around and talking to people. However this is where carrying out measurements can help. They can help you to determine which of the 6 main factors are likely to be causing the problem

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http://www.hse.gov.uk/temperature/thermal/index.htm

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http://www.bohs.org/resources/res.aspx/Resource/filename/840/TG12.pdfhttp://www.bohs.org/resources/res.aspx/Resource/filename/1473/04_TG12_Addendum_to_2nd_Edition.pdf

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http://www.innova.dk/

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www.diamondenv.co.uk

http://diamondenv.wordpress.com/

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