What is PMV? What is PPD? Basics of Thermal Comfort

shayan pakravan
Reading time: 15 min

In the design of HVAC systems, the question arises as to what conditions the ambient air needs for a person to feel comfortable in it. To answer this question, the concepts of thermal comfort, PMV and PPD are defined. In other words, an attempt has been made to find out what percentage of people feel satisfied by checking the different parameters of the ambient air created by HVAC. In this article, we will review PMV and PPD and tell you about their application in HVAC design. You can overview some of the applications in HVAC CFD Simulations.

Contents

What is PMV exactly? PMV meaning refers to “Predicted Mean Vote,” which is the most widely used thermal comfort index developed by Professor Ole Fanger in the 1970s. Many people search for “what is a PMV” because it’s essential for designing comfortable indoor environments according to ASHRAE Standard 55 and ISO 7730. The PMV index helps engineers predict how occupants will perceive the thermal environment on a 7-point scale from cold (-3) to hot (+3).

What is Thermal Comfort?

Thermal comfort refers to the conditions in which a person is comfortable with the thermal conditions of the environment. Thermal comfort depends on various parameters such as physical activity of the person, humidity and air temperature. This feeling is measured by different statistical methods. One of these methods is PMV. The concept of thermal comfort is used to design different HVAC systems. In other words, a person who is too hot nor too cold feels thermally comfortable.

Thermal comfort parameters include several factors that directly affect how people perceive their environment. When discussing PMV thermal comfort, engineers must consider both personal factors (like clothing and activity level) and environmental conditions (such as air temperature and humidity). Understanding these PMV parameters helps create spaces where occupants don’t feel too hot or too cold, which is the essence of thermal comfort. Moreover, thermal comfort zones represent the range of environmental conditions within which most people feel comfortable.

Environmental Parameters Affecting Thermal Comfort

Four environmental parameters are effective in thermal comfort:

Air temperature
Mean radiant temperature
Air velocity
Relative humidity

PMV calculations require precise measurement of these four environmental parameters. The air temperature and mean radiant temperature are often combined to determine the operative temperature, which better represents what humans actually feel. Air velocity affects heat loss through convection, while relative humidity impacts evaporative cooling from the skin. For accurate thermal comfort assessment, all these parameters must be measured according to standards like ISO 7730 and ASHRAE Standard 55. Proper calibration of measurement tools is essential when evaluating PMV and PPD thermal comfort conditions.

Personal Parameters Affecting Thermal Comfort

Two individual parameters also affect thermal comfort:

Activity level
Clothing

PMV thermal comfort analysis must account for these personal factors because they significantly impact how people experience their environment. Metabolic rate (measured in met units) varies with activity level from 0.8 met when sleeping to over 4.0 met during intense physical exertion. Similarly, clothing insulation (measured in clo units) ranges from 0.1 clo for light summer clothing to over 1.0 clo for winter outfits. When calculating PMV, these values must be accurately estimated to determine whether occupants will feel comfortable or not. The Fanger thermal comfort model integrates these personal parameters with environmental conditions to predict overall satisfaction with the thermal environment.

What is The Predicted Mean Vote (PMV)?

PMV or Predicted Mean Value is one of the most widely used thermal comfort indices. It was developed in the 1970s by Danish scientist Ole Fanger. The PMV equation is based on the heat exchange between the human body and the surrounding environment. Using this equation, the PMV which is measurable on a scale from -3 to +3, classifies the thermal state of the body, from cold (-3) to hot (+3).

Many engineers search for “what is PMV” and “PMV meaning” because it’s so crucial for designing comfortable indoor spaces. The PMV index predicts the average thermal sensation of a large group of people exposed to the same environment. When PMV equals zero, thermal neutrality is achieved, meaning most people feel neither too warm nor too cold. The PMV equation combines all six thermal comfort parameters (four environmental and two personal) into a single value that predicts occupant satisfaction. According to ASHRAE Standard 55 and ISO 7730 thermal comfort guidelines, maintaining PMV between -0.5 and +0.5 generally creates acceptable conditions for most occupants.

In various conditions in terms of air conditioning, people’s opinions were asked about the feeling of comfort, and with the statistical results obtained, the PMV index was created, which represents a number from -3 to +3, as in the following table:

Table 1. The PMV scale range

PMV index	Thermal sensation
+3	Hot
+2	Warm
+1	Slightly warm
0	Neutral
-1	Slightly cool
-2	Cool
-3	Cold

Many standards are used to describe and calculate PMV. One of the most famous of them is ASHRAE Standard 55. ASHRAE is the abbreviation of the American Society of Heating, Refrigerating, and Air Conditioning Engineers. In this standard, the comfort zone or acceptable PMV range is defined as -0.5<PMV<+0.5. This means that most people feel comfortable in the mentioned range.

In the PMV index, -3 indicates a very cold environment and +3 indicates a very hot environment, and 100% of people feel uncomfortable in these two environments.

The PMV equation is:

PMV = [0.303e^(-0.036M) + 0.028]L

Where L is the thermal load on the body defined as the difference between internal heat production and heat loss to the actual environment for a person at comfort skin temperature and evaporative heat loss by sweating at the actual activity level.

The PMV equation looks complex but essentially calculates the thermal comfort balance between heat produced by the body and heat lost to the environment. Engineers use this PMV formula to predict how people will respond to different indoor conditions. While software and online PMV PPD calculators now make these calculations easier, understanding the underlying principles remains important. The thermal load variable “L” in the PMV formula represents the difference between heat production and heat loss, which determines whether someone feels too warm, too cold, or just right. For accurate thermal comfort assessment, all six parameters must be measured properly and entered into the PMV equation.

What is The Predicted Percentage of Dissatisfied (PPD)?

PPD describes how many percent of people feel thermally dissatisfied with each PMV. For example, at PMV=0, 5% of people feel uncomfortable. The equation between PPD and PMV is as follows:

$PPD=100-95\times exp(-0.03353\times PMV^4-0.2179\times PMV^2)$

The result of the above equation is shown in the plot below. It is clear that the thermal conditions of the environment can never be such that 100% of people feel satisfied. PPD thermal comfort measurements always show that even in ideal conditions (when PMV equals zero), approximately 5% of people will still be dissatisfied. This highlights an important limitation of the PMV and PPD approach—it’s impossible to please everyone simultaneously. The PPD percentage increases rapidly as PMV moves away from zero in either direction. For instance, a PMV of ±0.5 corresponds to about 10% dissatisfied occupants, while a PMV of ±1.0 results in approximately 25% dissatisfaction. According to thermal comfort standards like ASHRAE Standard 55 and ISO 7730, acceptable thermal environments should maintain PPD below 10% (corresponding to PMV between -0.5 and +0.5).

PMV and PPD Relation

There is a relationship between PMV and PPD indices. The more the PMV moves away from neutral (PMV = 0), the higher the PPD becomes.

The graph below shows the relationship between PMV and PPD. This curved relationship shows why maintaining PMV close to zero is so important for occupant satisfaction. When designing HVAC systems, engineers use this PMV PPD relationship to predict how many occupants might feel uncomfortable under various conditions. The formula reveals that even at perfect neutrality (PMV = 0), at least 5% of people will be dissatisfied with the thermal environment. This minimum PPD thermal comfort threshold explains why unanimous satisfaction is impossible to achieve in shared spaces. The PMV and PPD relationship is fundamental to modern thermal comfort standards and guides the design of building systems worldwide.

The relationship between PMV and PPD is shown in this plot. The more we move away from PMV=0, the more dissatisfied people are with the thermal conditions of the environment.

Acceptable Thermal Comfort Limits

According to ASHRAE Standard 55, the acceptable thermal comfort limit is when PMV is between -0.5 and +0.5. According to this standard, if PMV is within this range, the vast majority of people (more than 90%) will feel comfortable in that environment.

These thermal comfort limits established by ASHRAE Standard 55 and ISO 7730 provide practical guidance for HVAC designers. Maintaining PMV between -0.5 and +0.5 ensures a PPD below 10%, meaning more than 90% of occupants should find the thermal environment acceptable. However, different building types might have different target values—for example, hospitals might aim for narrower PMV ranges than warehouses. Seasonal variations in clothing also affect acceptable thermal comfort ranges, with slightly warmer temperatures preferred in summer compared to winter. Modern building management systems often monitor PMV continuously to maintain optimal thermal comfort conditions while minimizing energy consumption.

Calculation of PMV and PPD

Calculating PMV and PPD requires various data and is complex. It is better to use a PMV Calculator. Here, we describe the method of calculating PMV and PDD based on ASHRAE 55. First, we introduce the parameters needed to solve the problem.

Calculate Clothing Area Factor (FCL): This factor can be calculated using the following equations and Table 2. Note that clo is a unit of measurement used to express the thermal insulation provided by garments and clothing ensembles and 1 clo is equal to 0.155 m²·K/W.

$\mathrm{if} \ 0.155 \times I_{cl} < 0.078 \ \mathrm{then} \ FCL=1+1.29\times0.155\times I_{cl}$

$\mathrm{if} \ 0.155 \times I_{cl} \geq 0.078 \ \mathrm{then} \ FCL=1.05+0.645 \times 0.155\times I_{cl}$

Table 2. Clothing insulation (I_cl) values for typical ensembles, adopted from ASHRAE 55

Clothing Description	Garments Included	I_cl [clo]
Trousers	Trousers ,short -sleeve shirt	0.57
	Trousers ,long -sleeve shirt	0.61
	#2plus suit jacket	0.96
	#2plus suit jacket ,vest ,T -shirt	1.14
	#2plus long -sleeve sweater ,T -shirt	1.01
	#5plus suit jacket ,long underwear bottoms	1.30
Skirts /Dresses	Knee -length skirt ,short -sleeve shirt (sandals )	0.54
	Knee -length skirt ,long -sleeve shirt ,full slip	0.67
	Knee -length skirt ,long -sleeve shirt ,half slip ,long -sleeve sweater	1.10
	Knee -length skirt ,long -sleeve shirt ,half slip ,suit jacket	1.04
	Ankle -length skirt ,long -sleeve shirt ,suit jacket	1.10
Shorts	Walking shorts ,short -sleeve shirt	0.36
Overalls /Coveralls	Long -sleeve coveralls ,T -shirt	0.72
	Overalls ,long -sleeve shirt ,T -shirt	0.89
	Insulated coveralls ,long -sleeve thermal underwear tops and bottoms	1.37
Athletic	Sweat pants ,long -sleeve sweatshirt	0.74
Sleepwear	Long -sleeve pajama tops ,long pajama trousers ,short 3/4length robe (slippers ,no socks )	0.96

Metabolic Rate (M): It represents the rate of body heat production per unit of time. Use Table 3 to calculate the metabolic rate.

Table 3. Metabolic Rates for Typical Tasks, adopted from ASHRAE 55

Resting	Met Units	W/m²	Btu/h.ft²
Sleeping	0.7	40	13
Reclining	0.8	45	15
Seated, quiet	1	60	18
Standing, relaxed	1.2	70	22
Walking (on level surface)
0.9 m/s, 3.2 km/h, 2.0 mph	2	115	37
1.2 m/s, 4.3 km/h, 2.7 mph	2.6	150	48
1.8 m/s, 6.8 km/h, 4.2 mph	3.8	220	70
Office Activities
Reading, seated	1	55	18
Writing	1	60	18
Typing	1.1	65	20
Filing, seated	1.2	70	22
Filing, standing	1.4	80	26
Walking about	1.7	100	31
Lifting/packing	2.1	120	39
Driving/Flying
Automobile	1.0-2.0	60-115	18-37
Aircraft, routine	1.2	70	22
Aircraft, instrument landing	1.8	105	33
Aircraft, combat	2.4	140	44
Heavy vehicle	3.2	185	59
Miscellaneous Occupational Activities
Cooking	1.6-2.0	95-115	29-37
House cleaning	2.0-3.4	115-200	37-63
Seated, heavy limb movement	2.2	130	41
Machine work
sawing (table saw)	1.8	105	33
light (electrical industry)	2.0-2.4	115-140	37-44
heavy	4	235	74
Handling 50 kg (100 lb) bags	4	235	74
Pick and shovel work	4.0-4.8	235-280	74-88
Miscellaneous Leisure Activities
Dancing, social	2.4-4.4	140-255	44-81
Calisthenics/exercise	3.0-4.0	175-235	55-74
Tennis, single	3.6-4.0	210-270	66-74
Basketball	5.0-7.6	290-440	90-140
Wrestling, competitive	7.0-8.7	410-505	130-160

Air Temperature (TAA): Ambient air temperature is required in Kelvin.
Mean Radiant Temperature (TRA): is defined as the uniform temperature of an imaginary black enclosure in which an occupant would exchange the same amount of heat by radiation as in the actual non-uniform environment. it can be measured using instruments such as a globe thermometer. Its unit should be Kelvin in our calculation.
Relative Air Velocity (VEL): It is the relative velocity of air compared to the velocity of people.
External Work (W): It is called the work done by a person that results in the release of mechanical energy.It is usually zero.
Relative Humidity (RH): This parameter indicates the ratio of air humidity to the maximum humidity it can have.
Water Vapor Pressure (PA): The partial pressure exerted by water vapor in the air. Note that:

$v\mathrm{if} \ PA=0 \ \mathrm{then} \ PA=RH\times 10 \times exp(16.6536-4030.183/(TAA-38))$

So far, all the parameters needed to solve the problem have been introduced. Next is the internal heat production in the human body:

$MW=M-W$

In the next step, calculate surface temperature of clothing by iteration, first guess for surface temperature of clothing is:

$TCLA = TAA + (308.5-TAA) / (3.5\times(6.45\times I_{cl} +.1))$

Then do the following calculations:

$P1 = I_{cl} \ \times FCL \ ; \ P2 = P1\times 3.96 \ ; \ P3=P1\times100 \ ; \ P4=P1\times TAA$

$P5 = 308.7-0.028 \times MW+P2\times (TRA/100)^4$

$XN=TCLA/100 \ ; \ XF = XN$

$XF = (XF+XN) / 2$

Heat transfer coefficient by natural convection is:

$HCN=2.38 \times ABS(100\times XF-TAA)^{0.25}$

$\mathrm{if} \ HCF>HCN \ \mathrm{then} \ HC=HCF \ \mathrm{else} \ HC=HCN$

$XN=(P5+P4\times HC-P2\times XF^4) / (100+P3\times HC)$

Choose a stop criteria in iteration and if ABS(XN-XF) is more than that, repeat all the calculations from the following equations:

$XF = (XF+XN) / 2$

If ABS(XN-XF) is less than the stop criteria in iteration then:

$TCL=100\times XN-273$

In the next step, heat loss components will be calculated. Heat loss diffusion through skin is:

$HL1=3.05\times 0.001 \times (5733-6.99\times MW-PA)$

Heat loss by sweating is:

$\mathrm{if} \ MW>58.15 \ \mathrm{then} \ HL2=0.42\times(MW-58.15) \ \mathrm{else} \ HL2=0$

Latent respiration heat loss is:

$HL3=1.7\times 0.00001\times M\times (5867-PA)$

Dry respiration heat loss is:

$HL4=0.0014\times M \times (34-TA)$

Heat loss by radiation is:

$HL5=3.96\times F_{cl}\times (XN^4-(TRA/100)^4)$

Heat loss by convection is:

$HL6 = F_{cl}\times HC\times (TCL-TA)$

In the next step, we will do the final calculations related to PMV and PDD. Thermal sensation transfer coefficient is:

$TS = 0.303\times exp(-0.036\times M) +0.028$

The predicted mean vote is:

$PMV=TS\times (MW-HL1-HL2-HL3-HL4-HL5-HL6)$

The predicted percentage of dissatisfied is:

$PPD=100-95\times exp(-0.03353\times PMV^4-0.2179\times PMV^2)$

Calculation of PMV and PPD in ANSYS Fluent

One of the most accurate methods for calculating ambient air parameters and evaluating HVAC performance is using CFD simulations by ANSYS Fluent. Using simulation results, PMV and PPD can be calculated in different environments in different HVAC functions. 3 methods are possible to calculate PMV and PPD in Fluent.

User-Defined Functions (UDF)

In this method, problem parameters such as air temperature are obtained by simulation, and the user calculates PMV and PPD by writing functions in the form of UDF using these parameters and the equations in this article. It should be noted that writing UDF is a complex task and requires expertise. UDF functions are written in the C programming language and provide full access to Fluent’s internal data structures.

The window in ANSYS Fluent software for writing UDF.

Custom Field Function

In this method, unlike UDF, the definition of functions is much simpler, but it is limited to the use of simple functions and more suitable for post-processing. This method is also used to calculate PMV and PPD in a simpler way than the equations of this article.

Custom field function window in ANSYS Fluent

Expression

Expressions allow defining simple mathematical relationships between existing variables. This method is the easiest way to define calculations and does not require programming knowledge.

Expression window in ANSYS Fluent

Conclusion

We have seen that with the help of PMV and PPD, the conditions that the HVAC system needs to provide can be calculated. As seen in the PMV calculations, many parameters are effective in the pleasantness of the thermal conditions of the environment.

After you have calculated the desired PMV and PPD for the environment and obtained each parameter, then you need to see if your HVAC is capable of providing those conditions and how it provides those conditions. A convenient method for HVAC evaluation is to use CFD simulations with ANSYS Fluent.

With these simulations, air temperature, air flow, and humidity levels in the entire environment can be predicted, defects are found, and then actions can be taken to fix them. Such projects have been carried out so far by CFDLAND experts, Like the project in the picture above or the projects in CFD Project. You can order HVAC CFD simulation projects with ANSYS Fluent to CFDLAND. Be sure of the quality and speed of our work.

Related Blogs

To expand your understanding of thermal comfort factors and their practical applications, explore these related resources:

Data Center Cooling Systems – Discover how PMV and PPD metrics guide the design of sophisticated cooling solutions for data centers, where maintaining optimal thermal conditions is essential for both equipment performance and human comfort in technical workspaces.
Natural Convection: Understanding Heat Transfer in Fluid Systems – Learn how passive heat transfer mechanisms influence thermal comfort in buildings and how these principles can be leveraged in energy-efficient HVAC design to achieve better PMV values with lower energy consumption.
A Thorough Study Over Spray Cooling Systems – Explore innovative evaporative cooling technologies that can precisely control both temperature and humidity—two critical parameters in the thermal comfort equation that directly impact PMV and PPD calculations.
Introduction to DPM Droplet Evaporation Simulation – Understand how advanced computational modeling can predict evaporation effects in climate control systems, enabling more precise management of the humidity parameters that significantly influence perceived thermal comfort.