Heating Services in Buildings

David E. Watkins

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119971665
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Bau- & Architekturtechnik

Water based heating systems are efficient, flexible, versatile and offer many advantages over other heating systems. These advantages (fast response, good controllability, efficient zonal heating and largely silent operation) all require that initial design, installation, commissioning and maintenance be carried out to a high standard by competent engineers.

Heating Services in Buildings provides the reader with a detailed and thorough understanding of the principles and elements of heating buildings using modern water based heating systems. A key theme of the book is that there is little difference, in the approach to the design and engineering, between domestic and commercial installations. The author's detailed but highly practical approach to the subject ensures there is sufficient information for students from both a craft background and those with more academic backgrounds to understand the material. This approach is complemented by straightforward, easy-to-use diagrams.

Heating Services in Buildings supports a range of educational courses, including degree level building services engineering; NVQ Level 4 Higher Professional Diploma in Building Services Engineering; City & Guilds supplementary heating course and the Heating Design and Installation Course accredited by the European Registration Scheme (ERS).

1

Introduction to Heating Services

The broad term ‘central heating’ is used to describe many types and forms of heating, and some usage is totally misleading and inaccurate, through ignorance of the subject. This chapter is a basic introduction to the mechanics of central heating, which is discussed in greater detail in the following chapters.

If we examine the term, it implies a system where heat is produced from a central source and distributed around the whole building. The method of heat generation and distribution may vary with the type of heating system employed.

Central heating is sometimes referred to as space heating. To be understood fully, this must be described by its type or system arrangement, and may be categorised as being either full, part or background heating.

Full central heating may be defined as being a system of heating from a central source where all the normally habitable or used rooms/spaces are heated to achieve guaranteed temperatures under certain conditions. By today’s standards, all heating systems installed in residential dwellings and most commercial buildings should conform to this category, unless there are acceptable reasons for not doing so.

Partial central heating is the term applied where only part of the building is to be heated, but even then the rooms or spaces that are heated should still have guaranteed temperatures under stated conditions. This form of central heating would be a rare occurrence for a residential dwelling but not so uncommon for some commercial buildings, especially where part of the building complex is not normally occupied.

The term ‘background heating’ is used to describe a form of central heating whereby lower than normal or standard recommended temperatures are aimed at for the type of building involved. The term is sometimes used to refer to heating systems installed in buildings where the room temperatures are not guaranteed. This form of heating is unacceptable by today’s standards on both environmental and efficiency grounds.

It should be noted that, unless otherwise specified, full central heating should normally be designed to current regulations and standards and installed in a professional manner. In some instances, usually due to a specific use or financial reasons, the client may only require or specify partial heating to be installed, sometimes with the request that safeguards are included to allow the system to be extended at a later date to achieve full central heating.

Background heating, where lower than normal or recommended temperatures are aimed at, should only be used when specifically requested by the client for some reason. Even then, agreed temperatures should be incorporated into the design and guaranteed before any installation work commences. Under no circumstances should any heating system be installed without first agreeing specific room temperatures to be achieved when certain conditions exist. These conditions are discussed in Chapter 2.

Having understood the extent of the heating system and its classification, be it full, part or background heating, heating systems may be further divided under the headings of ‘wet’ or ‘dry’ systems. The terms wet or dry refer to the medium used to convey the heat from its source of generation to its point of use. Wet systems may be further classified by the piping circulation arrangement, with dry systems being divided into warm air and electric heating.

Figure 1.1 indicates the broad classifications of heating systems.

Figure 1.1 Heating system categories

Heating systems can be sub-divided even further, but this will be explained in Chapter 21.

Wet Heating Systems

All wet types of heating systems employ a liquid as a medium to convey the heat from its source of generation. It is then distributed around the system to each heat emitter, where it transfers part of that heat through the heating surface of the heat emitters. Finally, the liquid is returned to the source of generation for the process to cycle continuously. The source of heat is commonly referred to as a boiler.

In all domestic heating systems, and most heating systems for other types of buildings, water is chosen as the medium for conveying the heat due to its low cost and being readily available. However, water does have the disadvantages of a low boiling point and high freezing point; it can also be corrosive to metallic materials and has a limited heat carrying capacity. The corrosive nature of the water can be reduced by water treatment, which is discussed later in this volume.

The temperature limitations and heat carrying capacity of water will have to be accepted unless we change the atmospheric conditions of the system, or we can change the liquid. Liquids known as ‘thermal fluids’ are available and have been used successfully on larger commercial type heating installations. They possess different properties to water, such as being less aggressive to common materials, having higher boiling points and lower freezing points, a greater heat carrying capacity than water and, in some cases, a lower viscosity. The merits of thermal fluids are much superior to those of water but are generally discounted for all domestic heating systems owing to their higher capital cost and not being readily available. They are also rarely used on larger commercial systems for the same reasons, but when conditions are right they can be considered attractive. The difficulty of availability can cause problems when replacement fluid is required immediately, following any emergency maintenance work. Thermal fluids have been used for domestic applications on limited occasions in countries that experience much lower temperatures than in the UK, as the lower freezing point of the fluid can be an important advantage when sub-zero ambient temperatures are experienced for prolonged periods with the heating system in a non-operating mode. They have also been employed as the heat carrying medium for some solar heating systems.

The purpose of the water used in heating systems differs from that used in domestic hot and cold water installations. In those systems, water is the end product or consumable item and after it has been used, it is discharged to waste. The water employed in a heating system is a non-consumable substance. It is the medium used to carry the heat required and, after it has transferred some of the heat, it is returned to the boiler to be re-used over and over again.

Dry Heating Systems (Warm Air)

Warm-air dry-type heating systems differ from wet-type heating systems insofar as the fluid employed is not only the medium used to convey the heat, but is also the end product. As the name implies, air is the fluid used to carry the heat from its source of generation, a warm air heater. It is then distributed, usually through a network of ducting, where it is arranged to enter directly into the room under controlled conditions to displace the cooler air. Finally, a mixture of the two is partly returned to the warm air heater for the process to be repeated.

Warm-air heating systems are generally disliked by many occupants of dwellings that have such systems installed, but this is usually because the systems are either not designed correctly, not installed correctly or are, in many cases, incomplete. This is mainly down to ignorance of the fundamental principles of warm air heating, which, if given the respect deserved, can be a very good form of heating. This work exclusively concentrates on wet-type heating systems since it is aimed at students and engineers in the plumbing industry.

Dry Heating Systems (Electricity)

Electrical heating systems may technically be classified as dry systems, but they do not employ a medium as they generate their heat at the point of use. For this reason, electrical heating systems are not included in this book, with the exception of heating systems that use electricity as the source of power to heat the water. Here they are classified as being wet or hydronic heating systems.

Supplementary Heating

This is a term applied to describe heating appliances, either fixed or portable, that are used to supplement the central heating system – either during extreme cold spells when the outside air temperature falls well below the base design temperature, or during the heating-off season in spring or autumn, when the outside temperature drops to below that considered comfortable.

Examples of such heating appliances include:

• Radiant electric fires, portable and fixed
• Oil filled radiators
• Oil room heaters
• LPG room heaters
• Gas fires
• Open solid fuel fires.

The list is not intended to be exhaustive, but meant to serve as a general representative selection of supplementary heating appliances.

2

Wet Heating Systems

Wet heating systems, commonly referred to as hydronic heating systems because they use a liquid as a medium, nearly always employ water as the medium to convey the heat from its source of generation, a boiler. This is rather a misnomer, as a boiler must be designed to avoid boiling the water, but is probably a leftover term from the days of raising steam. The heated water is circulated around the system, transferring part of its heat, and returns back to the boiler for the process to be repeated.

The water is fed into the heating system via a fixed piped connection to either a feed and expansion cistern, or a direct connection, as in the case of a sealed heating system. The water is allowed to enter the heating system slowly, thus avoiding creating turbulence, to fill it with all air expelled through the open vent, or by releasing it using manually operated air vents or automatic air release vents.

Water has many advantages as a heat carrying medium when used in hydronic heating systems; not least its plentiful availability. For this reason water is almost exclusively used for domestic heating systems.

Hydronic heating systems are classified by the following basic principles:

• Temperature of medium
• Pressure of system
• Circulation method of medium
• Piping arrangement for distribution.

The classifications are to a certain extent inter-related, as the selection of one of the basic operating principles has an influence on the selection of the others, which is explained in the following discussion.

TEMPERATURE AND PRESSURE

The classification of hydronic heating systems by the temperature of the circulating water exiting the boiler is closely related to the operating pressure of the system, and the two must be considered together. This is because pressure is required to maintain the water in a liquid form at high temperatures: as water will boil and convert to steam at 100°C at atmospheric pressure when measured at sea level, any increase in that pressure will have a corresponding increase in the boiling temperature of water. Likewise, any decrease in pressure below atmospheric pressure will have the effect of allowing water to boil at temperatures lower than 100°C.

Table 2.1 gives the temperature/pressure classification commonly used in the UK. The minimum pressures listed are those required to prevent the water from evaporating but should not be confused with their vapour saturation pressures, which are lower.

Table 2.1 Hydronic design ...