The Basic Principle of Heating

The basic principle of heating is very straightforward. A certain quantity of water is heated at a boiler. The water is circulated in pipes to give up its heat through radiators or convectors and is returned to the boiler to be reheated.
In early systems circulation was by natural convection. When water is heated it becomes lighter and will rise naturally from the boiler. When water enters radiators it gives up its heat to the air via their large surface areas. As colder water is heavier it falls from the top connection on the radiator to the bottom opposite connection and eventually back to the boiler. This is known as gravity circulation. The top circuit connection at the boiler is called the flow and the bottom connection is the return.
The basic system as illustrated presents some practical difficulties. How do we get the water into the system and what happens when the water is heated and expands?
Feed and expansion cistern
The traditional method of dealing with filling and expansion is by the feed and expansion cistern. This is often called the feed and expansion tank, the header tank or the make-up tank, although strictly a tank is a closed vessel. The pipe connecting it to the system is called the feed and expan- sion pipe.
The cistern is supplied with water from the mains through a ball-valve. When the water reaches a pre-determined level, the valve is closed. The cistern must be the highest point of the system so that all parts can be filled from it.
When the water is heated, expansion can take place up the feed and expansion pipe and into the cistern. Because of this, the ball-valve is set to close when the cistern is only about one-third full, leaving a space to accept the expansion volume. If this space were not allowed, every time the water was heated it would overflow.
Providing a feed pipe enables us to accept expansion and to fill the system but it does not solve another problem. The system is full of air to begin with and the water cannot replace it if the air cannot escape. The answer is to provide another pipe through which the air can rise. This is known as the open vent or overflow pipe but, because of its very important other duty, it is best described as the open safety vent.
Other parts of the system which could trap air also need a venting facility. This is particularly true of radiators, which are supplied with air vents to be opened and closed by a vent key.
Safety
In the early boilers running on solid fuel, there was little control over the burning rate and occasionally the water boiled. This presented no danger because the open safety vent allowed the steam to escape. Modern boilers have thermostats which shut off the fuel supply in the case of oil and gas, or operate a damper or fan with solid fuel. With solid fuel there is always a delay and with other fuels there is always the possibility of a thermostat failing. It is vital therefore that the safety vent pipe offers an open and continuously-rising escape route for boiling water. There should be no valves, pumps or controls on this pipe.
If the boiling water vents safely, it must be renewed or the system will become overheated — and a boiled-dry boiler is more expensive to replace than a saucepan! It is therefore just as important that the feed and expansion pipe is also open to allow the replacement water to enter the system. In book after book, I find the pump is shown on this pipe and often other control valves; this should not be done. Circulation should never be restricted around the feed and expansion circuit.
Combined feed and vent
Sometimes what is called a combined feed and vent is fitted. Notice that I do not refer to it as a combined feed and safety vent. This is because it isn’t safe. If the water boils, it can vent through the vent pipe but replacement water cannot reach the boiler; it can’t be going down while the boiling water is coming up.

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