Principles and Heating Systems

What the systems are for

Building services exist to maintain conditions in which people can work comfortably and safely, and in which processes and equipment can operate. For occupants that means controlling temperature, humidity, air movement and air quality within ranges that most people find acceptable — and it is worth noting at the outset that no single condition satisfies everyone, because thermal comfort varies with clothing, activity, age, health and individual physiology.

The comfort factors

• Air temperature — the most discussed and not the only factor.
• Radiant temperature — the effect of surrounding surfaces. A person near a cold window feels cold even where the air temperature is correct, which explains a large proportion of perimeter complaints.
• Air movement — necessary for ventilation, and perceived as a draught when too high or too cold at the wrong location.
• Humidity — very low humidity causes dryness and static; very high impairs cooling by evaporation and encourages mould.
• Clothing and activity, which the building cannot control.

Heat transfer and building heat loss

Heat moves by conduction through solid materials, convection through air movement, and radiation between surfaces. A building loses heat through the fabric — walls, roof, floor, glazing — and through air leakage and ventilation. It gains heat from solar radiation, occupants, lighting and equipment. Those internal gains are substantial in modern offices, which is why well-insulated buildings full of people and equipment frequently need cooling even in cold weather.

Heating systems

• Boilers — gas, oil or biomass, heating water circulated to emitters. Condensing boilers recover heat from flue gases and are efficient only when the return water temperature is low enough to allow condensation, which is why a condensing boiler in a system designed for high flow temperatures does not deliver the expected efficiency.
• Heat pumps — moving heat from air, ground or water rather than generating it, delivering several units of heat per unit of electricity. Increasingly the default in new and refurbished buildings, and they perform best with low-temperature emitters such as underfloor systems or oversized radiators.
• Electric heating — simple, fully efficient at the point of use and expensive to run relative to alternatives.
• District heating — heat delivered from a central plant via a network.
• Emitters — radiators, underfloor circuits, fan coil units, radiant panels and warm air systems.

Distribution

Heat is distributed by circulating water through pipework, driven by pumps and controlled by valves. Common problems are air in the system, which prevents circulation; sludge and corrosion products, which block emitters and damage pumps; poor hydraulic balance, so that some parts of a building overheat while others never warm; and failed control valves stuck open or closed. Water treatment and periodic system cleaning are not optional extras — a neglected system loses efficiency progressively and fails early.