Building Services Engineering - Heating Systems and Heat Transfer

Vocabulary practice flashcards covering thermal comfort, heat transfer mechanisms, U-value and thermal resistance calculations, thermal capacity, heating system classifications, and low-carbon technologies from Lecture #2.

Thermal Comfort

According to BS EU ISO 7730, that state of mind which expresses satisfaction with the thermal environment.

Six Primary Factors of Thermal Comfort

The personal and environmental variables governing thermal comfort: air temperature, air velocity, mean radiant temperature, humidity, clothing level, and activity level.

Modes of Heat Transfer

The three distinct mechanisms by which thermal energy moves: conduction (through solids), convection (through moving fluids), and radiation (via electromagnetic waves).

Conduction

Heat transfer occurring directly through a solid material or between solids in physical contact, governed by the relationship $Q = \frac{k A (T_1 - T_2)}{\Delta x}$.

Convection

The transfer of heat through the physical movement of fluids (liquids or gases), which can occur naturally (free convection) or mechanically (forced convection).

Thermal Radiation

The transfer of heat by electromagnetic waves emitted from warm surfaces, heating objects and individuals directly without requiring a material medium.

Thermal Conductivity ($k$)

The heat flow in Watts across a homogeneous material having a thickness of $1\,m$ and a surface area of $1\,m^2$ for a temperature difference of $1\,K$ (or $1\,^\circ C$), measured in $\text{W}/(m\cdot K)$.

Thermal Resistance ($R$)

A material's opposition to the flow of heat, defined for a homogeneous layer as thickness divided by thermal conductivity ($R = \frac{L}{k}$), with units of $m^2\cdot K/W$.

Thermal Transmittance (U-value)

The reciprocal of total thermal resistance ($U = \frac{1}{R_t}$) describing the rate of heat transfer across a building fabric element per unit area per unit temperature difference, measured in $\text{W}/(m^2\cdot K)$.

Surface Resistances ($R_{si}$ and $R_{so}$)

The thermal resistances provided by the thin boundary layers of still air clinging to the internal ($R_{si}$) and external ($R_{so}$) surfaces of a building envelope element.

Total Thermal Resistance of a Multi-Layer Wall

The sum of the thermal resistances of all constituent material layers and boundary air films in series: $R_{\text{total}} = R_{si} + \sum \frac{L_j}{k_j} + R_{so}$.

Thermal Capacity ($C_{th}$)

A material's ability to store thermal energy as its temperature rises, calculated as $C_{th} = V \cdot \rho \cdot c_p$ (Volume $\times$ Density $\times$ Specific Heat Capacity), expressed in $J/K$ or $MJ/K$.

Thermal Stability

The tendency of a building's internal temperature to resist rapid fluctuations; high thermal capacity produces high thermal stability, whereas low thermal capacity causes low stability.

Lightweight Buildings

Buildings constructed with low thermal mass materials (such as timber or metal cladding) that warm up quickly and respond rapidly to heating controls, but are prone to summer overheating.

Heavyweight Buildings

Buildings constructed with high thermal mass materials (such as concrete or dense masonry) that require longer pre-heating times, cool down slowly, and attenuate indoor temperature extremes.

Thermal Bridge

A localised area within a building construction element having significantly higher heat transmission than surrounding materials, creating accelerated heat loss and cold spots prone to condensation and mould.

Steady-State Total Heat Loss

The combined rate of heat loss through the building envelope by fabric conduction and ventilation/infiltration: $Q_{\text{total}} = \sum (A \cdot U) \cdot \Delta T + \frac{1}{3} \cdot N \cdot V \cdot \Delta T$.

Air Change Rate ($N$)

The frequency with which the complete volume of air inside a room or building is replaced with fresh or outdoor air within one hour, expressed in air changes per hour ($\text{ach}$).

Low Temperature Hot Water (LTHW)

A category of hydronic heating system operating at a flow temperature of $< 90\,^\circ C$ with a minimum operating pressure of $1\,bar$.

Medium Temperature Hot Water (MTHW)

A category of hydronic heating system operating at temperatures between $90\,^\circ C$ and $120\,^\circ C$ with a minimum operating pressure of $3\,bar$.

High Temperature Hot Water (HTHW)

A category of hydronic heating system operating at temperatures greater than $120\,^\circ C$ with a minimum operating pressure of $5\,bar$.

Heat Pump

A low-carbon thermodynamic system that extracts ambient heat from air, ground, or water using an electrically driven compressor cycle, where $1\,kWh$ of electricity generates $4\,kWh$ of heat.

District Heating Network

A centralized thermal distribution system that generates heat at a shared plant (such as a boiler house or CHP unit) and circulates it to multiple buildings via insulated hot feed and return pipework.

Combined Heat and Power (CHP)

An integrated, energy-efficient technology that simultaneously produces electricity and recovers usable thermal energy from a single fuel input.

Evacuated Tube Collector

A solar thermal collector featuring parallel glass tubes under vacuum containing absorber fluid pipes, virtually eliminating conductive and convective heat losses to maximize solar heat collection.

Flat Plate Collector

A solar thermal device consisting of a glazed, insulated shallow enclosure containing a dark absorber plate with fluid channels to collect solar radiation for domestic hot water or heating.

Mass Flow Rate of Heat Transfer Medium ($\dot{m}$)

The rate of water flow needed to transport a given thermal heat load $\Phi_w$, determined by $\dot{m} = \frac{\Phi_w}{c_p \cdot (t_f - t_r)}$, where $t_f$ and $t_r$ are flow and return temperatures.

Emitter Mean Water Temperature ($t_m$)

The average temperature of the heat-carrying fluid inside an emitter, defined as $t_m = \frac{t_f + t_r}{2}$, used to determine emitter heat output relative to room air temperature ($t_{ai}$).

Low and Zero Carbon (LZC) Technologies

Energy supply technologies—including heat pumps, solar thermal systems, biomass boilers, and CHP—that generate heating, cooling, or power with minimal or zero net $\text{CO}_2$ emissions.

Limiting U-value

The statutory maximum thermal transmittance specified by Building Regulations Part L for building elements in new dwellings (such as $0.26\,\text{W}/(m^2\cdot K)$ for walls and $0.16\,\text{W}/(m^2\cdot K)$ for roofs).