Heating Systems: Climate and Indoor Environment

Climatic Conditions and Elements

Knowledge of climatic conditions and elements is essential for the precise design of heating systems and for their efficient operation, particularly concerning regulation. The primary elements tracked for these purposes include air temperature, wind, solar radiation, outdoor air humidity, and atmospheric pressure.

Air temperature is monitored across various time intervals to manage operation and regulate thermal output. Key metrics used for evaluation include the following:

Average Daily Outdoor Air Temperature ($\theta_{e,d}$): This value is calculated as the sum of temperatures measured in the shade at 7:00, 14:00, and 21:00. The evening measurement at 21:00 is considered the prevailing temperature and is weighted double in the calculation:

$\theta_{e,d} = \frac{\theta_{e,0} + \theta_{e,14} + 2 \times \theta_{e,21}}{4} \, [^{\circ}\text{C}]$

This metric determines the start and end of the heating season. The standard heating period begins on September 1st and ends on May 31st of the following year. Heating is initiated within this period if the average daily outdoor air temperature falls below $13 \, ^{\circ}\text{C}$ for two consecutive days. This same logic applied to the suspension or termination of heating.

Average Monthly Outdoor Air Temperature: This data serves as the basis for calculating the heat requirement for heating residential buildings throughout the year. It is determined by summing the average daily temperatures for a specific month and dividing that sum by the number of days in that month.

Annual Temperature Profile: This is used to determine the overall length of the heating season and the average temperature maintained throughout that period.

Design Outdoor Temperature ($\theta_e$): This is the calculated design temperature of the outdoor air during the winter season. It is used to determine the heating power requirement of the heating equipment. It is calculated based on the temperature zone and the altitude of the building's location using the following formula:

$\theta_{e} = \theta_{e,100} + \Delta \theta_{e,0} \times \frac{h - 100}{100} \, [^{\circ}\text{C}]$

Where:

• $h$ is the altitude of the building location in meters above sea level (m n. m.), with the $±0,0$ reference point of the building usually at the level of the 1st floor (1. NP).
• $\Delta \theta_{e,0}$ is the basic temperature gradient above $100 \, \text{m n. m.}$ as defined in CSN (Czech National Standard) tables.
• This value can also be determined according to data from a local meteorological station or specific investor requirements.

Wind, Solar Radiation, and Other Elements

Wind significantly influences the natural ventilation of rooms, a process known as infiltration. The dynamic pressure of the wind causes air to penetrate through gaps in windows and doors. This occurs because the flow of wind around a building creates a pressure differential between the windward (leeward) and leeward sides, leading to increased heat losses. Wind loads are categorized by geography:

• Normal Load: Examples include City of České Budějovice, Prague, Plzeň, and Hradec Králové.
• Increased Load: Examples include Pelhřimov, Prachatice, Havlíčkův Brod, and Karlovy Vary.

Solar radiation impacts the correct design of heating system regulation. In the spring and autumn, it provides beneficial heat gains from sunlight. In the summer, solar gains can be significant, particularly through glazed portions of the building facade.

Outdoor air humidity and atmospheric pressure do not have a fundamental impact on calculating the overall size of heating equipment, but they remain relevant. Air humidity can deteriorate the thermo-technical properties of building structures. Atmospheric pressure is used specifically for calculating the natural draught of chimneys.

Thermal Comfort and Factors

Thermal comfort is defined as a state of mind expressing satisfaction with the thermal environment. It is a subjective feeling perceived by a person in a given environment, where they feel neither too cold nor too hot. The factors influencing thermal comfort are categorized as subjective and objective.

Main Factors:

• Air temperature.
• Temperature of surrounding surfaces.
• Relative air humidity.
• Air movement.
• Air pressure.
• Clothing.
• Type of activity.

Secondary Factors:

• Food intake.
• Physical condition.
• Age.
• Gender.
• Constitution.

Minor Subordinate Factors:

• Air composition and electrical charge in the air.
• Acoustic and optical influences.
• Psycho-sociological factors.

It is important to note that in any given environment, there are always at least $5 \,\%$ of people who will be dissatisfied.

The Thermal Comfort Equation and Heat Production

The status of thermal comfort can be expressed via the following equation:

$M - W = C_{res} + E_{res} + K + C + R + E + S$

Where:

• $M$ = metabolic heat.
• $W$ = mechanical and total energy expenditure.
• $C_{res}$ = heat exchange by convection in the respiratory tract.
• $E_{res}$ = heat exchange by evaporation in the respiratory tract.
• $K$ = heat exchange by conduction on the skin.
• $C$ = heat exchange by convection on the skin.
• $R$ = heat exchange by radiation on the skin.
• $E$ = heat exchange by evaporation on the skin.
• $S$ = accumulated heat in the body.

Human thermal production arises from the release of metabolic heat during metabolism. This heat produced by the human body must be transferred to the surrounding environment, primarily through the convection of surrounding air, radiation into the space, and the evaporation of sweat. Every person produces a specific thermal output depending on their activity, body type, and food intake. For example, a person produces approximately $80 \, \text{W}$ while sleeping and $300 \, \text{W}$ while walking. Thermal equilibrium is defined as the balance between heat production and heat intake from the surrounding environment.

Internal Environment of Buildings

The thermal state of the indoor environment is determined by several physical variables: air temperature ($\theta_{ai}$), mean radiant temperature ($\theta_r$), operative temperature ($\theta_o$) or resultant temperature ($\theta_v$), internal design air temperature ($\theta_i$), air flow velocity, and relative air humidity ($\phi$).

Air Temperature ($\theta_{ai}$): This refers to the air temperature in the interior without the influence of radiation from surrounding surfaces. This temperature is variable; the design goal is to ensure the difference in temperature in the vertical direction is as small as possible.

Mean Radiant (Radiative) Temperature ($\theta_r$): This is a calculated value used to determine the operative temperature. It represents the imaginary uniform temperature of surrounding surfaces and expresses the radiative effect of the environment. It is measured using radiometers or a globe thermometer.

Operative Temperature ($\theta_o$): This variable expresses the interaction between air temperature ($\theta_{ai}$) and mean radiant temperature ($\theta_r$). It is used to evaluate the effect of the thermal environment on a person and to determine heat or cold stress according to government regulations. It also serves as the basis for calculating fluid replacement requirements in settings such as production plants.

Internal Design Air Temperature ($\theta_i$): This is the design internal temperature for the winter period as specified by CSN standards based on the purpose of the room. For permanently occupied residential buildings, standard values include:

• Living rooms, kitchens, WC: $20 \, ^{\circ}\text{C}$.
• Bathrooms: $24 \, ^{\circ}\text{C}$.
• Heated secondary rooms: $15 \, ^{\circ}\text{C}$.
• Heated staircases: $10 \, ^{\circ}\text{C}$.

Air Flow Velocity in the Occupied Zone ($w_i$): Air flow in a room is caused by either forced or natural ventilation. Excessive air movement produces a draft sensation. The target permissible air velocity is approximately $w = 0.1 \, \text{m/s}$ for room temperatures between $18 \, ^{\circ}\text{C}$ and $22 \, ^{\circ}\text{C}$. At higher temperatures ($26 \, ^{\circ}\text{C}$ to $28 \, ^{\circ}\text{C}$), a higher air flow velocity is acceptable, up to a maximum of $w = 0.5 \, \text{m/s}$.

Relative Air Humidity ($\phi$): This indicates the saturation of air with water vapor. For humans, an acceptable relative humidity range is between $30 \, \%$ and $70 \, \%$. In calculations, a relative humidity of $50 \, \%$ is usually assumed under normal conditions. The amount of humidity depends on the operation (e.g., winter gardens, swimming pools, or drying rooms). For example, a person produces $100 \, \text{g/h}$ of moisture during activity and $50 \, \text{g/h}$ during sleep.

Internal Surface Temperature Factor ($f_{Rsi}$): This is a dimensionless number that indicates the surface temperature as a function of the interior and exterior temperatures. It has replaced the use of the dew point temperature, which is the temperature at which air becomes saturated with water vapor. The formula is:

$f_{Rsi} = \frac{\theta_{si} - \theta_e}{\theta_{ai} - \theta_e}$

Where $\theta_{si}$ is the internal surface temperature.