Principles of Domestic Heating and Heat Distribution

Hot water supply
Hot water for domestic purposes is usually taken from a hot water tank or cylinder. The heat source is usually in the form of a solid fuel, oil or gas boiler. Other water heating alternatives include a back boiler to an open fire or an electric immersion heater fitted to the hot water cylinder.  

A hot water supply system must be capable of producing sufficient hot water to meet all demands by the household, it should be economical to run and easy to install and maintain.

There are two main systems of hot water supply called the direct and the indirect systems.

Direct hot water system
In this system cold water is heated in the water jacket in the boiler. The heated water rises through convection currents and is replaced by cold water coming from the bottom of the hot water cylinder. A circulation is thereby set-up.

Hot water is drawn off as required for domestic use and is in turn replaced by cold water from the cold water storage tank in the attic. The hot water drawn-off is taken from the top of the hot water cylinder where the hottest water is stored. An expansion pipe connects to a horizontal pipe at the top of the hot water cylinder and runs vertically from the hot water distribution pipe to the cold water storage tank.

In the direct hot water system the water that is heated in the boiler and subsequently stored in the cylinder is the same water that is drawn-off for domestic use.

This makes this system unsuitable for supplying a central heating circuit.

The direct hot water system is also not suitable for hard water areas due to lime scale deposits which may eventually block the pipework. Lime scale deposits occur when water is heated to temperatures ranging from 50 to 70, and this is the typical temperature range for domestic hot water.

This system is seldom used due to the disadvantages stated.

Indirect hot water system
In this system hot water circulates between the boiler and the hot water cylinder. However, the cylinder is an indirect cylinder, which contains a coil.

This means that the water that is circulating between the boiler and the indirect cylinder is travelling in a closed circuit and it does not mix with the stored water in the cylinder.

Its sole purpose of travelling through the cylinder, in the coil, is to raise the temperature of the stored water.

The water that is heated in the boiler and which travels through the cylinder is referred to as the 'primary' circuit.

The 'secondary' circuit refers to the stored water in the hot water indirect cylinder which is used for domestic draw-off and replaced with water from the cold water storage tank in the attic.

The 'secondary' circuit refers to the stored water in the hot water indirect cylinder which is used for domestic draw-off and replaced with water from the cold water storage tank in the attic.

In the primary circuit it is the same water that is circulating continuously, apart from any water that needs to be replaced due to expansion. The advantages of the primary circuit are:

1. No lime scale build-up (furring) as fresh cold water is not constantly being introduced.
2. A central heating loop can be connected to the primary circuit, which isn't possible for the direct hot water system.
A disadvantage of this system is that extra piping is required and a second (smaller) feed cistern is required for the primary circuit.

Cisterns
Cold water storage cisterns are usually made of plastic, although galvanised mild steel cisterns were common in the past. The standard size for cold water cisterns is 228 litres. This is deemed adequate to serve both the cold and hot water supplies and provide a reserve in the event of mains failure. The standard size for the hot water storage cylinder is 114 litres.

The standard size for the hot water storage cylinder is 114 litres. This is normally situated in the 'hot-press' (airing cupboard) and is made from copper. It should be lagged to conserve energy.  Pipework is usually copper, although plastic pipes are gaining in popularity. The service pipe from the mains is usually polythene.

Ball valves are fitted on all cold water storage cisterns to prevent overflow. A ball valve floats in the cistern and automatically closes when the water reaches a certain level.

Heating Systems and Layout

The pipework supplying water to radiators may be small-bore (12, 15 or 22 mm diameter) or microbore (6, 8 or 10 mm diameter). It is important that the correct size of pipe is used to supply given sized radiators with the correct amount of heat. In determining the correct size of radiator to use consideration should be given to the amount of heat/energy required to combat heat losses in a specific room and the temperature at which the radiator will be required to operate at.

The two main methods of supplying hot water to radiators are:

1. The one-pipe or single-pipe system and
2. The two-pipe system.

The one-pipe system
Hot water from the boiler is fed to each radiator in turn with the cooler water from each radiator being fed back to the same pipe. As a result the temperature of the water is gradually getting lower as it enters each successive radiator. This makes the control of the distribution of heat difficult. The one pipe system is the easiest and cheapest to install, but control of individual radiators is virtually ineffective.

The two-pipe system
Hot water from the boiler is fed to each radiator by one pipe and the cooler water coming from each radiator is fed to a second pipe and returned to the boiler for re-heating. This is the more common heating system used. Depending on the length of the circuit, each radiator receives water at approximately the same temperature. This allows greater control of individual radiators, and also provides for a faster warm-up time.
The majority of small-bore heating systems used in dwellings are operated in association with an indirect hot water supply system. Connections can be made to the primary flow and return circuits or separate connections can be made to the boiler unit.

Controls
Controls are fitted to heating systems in buildings in order to:

• provide a comfortable environment
• provide it at specified times and do both economically by avoiding unnecessary use of energy.

This is a matter of making sure that the building is never warmer than necessary, and that it is not kept warm when not required.  The main types of control are concerned with adjusting the flow of water through the radiator as the temperature of the radiator is directly linked to the flow.

Typical controls in use are:

• The circulating pump. This is positioned on the return pipe beside the boiler. It pumps the hot water in the circuit to all radiators.

• Radiator valves. Each radiator has two valves: a wheel valve, which can be adjusted by hand to open or close the supply of hot water.

• Water to the radiator, and a lock shield valve, which is fitted on the return side of the radiator to balance the amount of heat in each radiator.

• Thermostatic valves. These allow control of individual room temperature

• Air vent/cock. This is located at the top of the radiator to allow trapped air to be vented.

• The trapped air prevents water from entering the radiator, meaning that it doesn't heat

• Gate valves. These allow a section of the system to be shut off, without the need to drain the complete system. For example, one should be placed at each side of the pump to allow the pump to be removed without draining the system.

• Room thermostat.  This is set to the desired room temperature and it electrically controls the circulating pump
• Boiler thermostat. This controls the temperature of the water in the boiler.

 • Time switch. Usually located near the boiler, it controls when the whole system comes on and for how long.

Next topic: Solar energy