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Central Heating Systems CH01

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Mark Hyde

on 30 October 2018

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Transcript of Central Heating Systems CH01

Central Heating Systems
Circulation of the water can be either by gravity circulation to the heat exchanger in the hot water cylinder and pumped to the heat emitters or by means of a fully pumped system where both the hot water heat exchanger and heat emitters are heated using a circulating pump. Fully pumped systems have the advantage that system resistance created by the pipework, fittings and heat emitters can be overcome more easily and this enables the system to heat up more quickly, giving the occupants a more controllable system.
Re Cap-
1. What are the two basic categories of central heating system?

2. Which type of system, open-vented or sealed, has a higher flow temperature?

3. List two ways in which water can be circulated in an open-vented system.

4. What is meant by the term 'thermal comfort'?

5. If a system cannot provide comfortable temperatures when the external temperature is below -1 degree celsius, what is this system defined as?
Try to work out from memory what sort of heating system you have in your house...

Is it open vented, gravity fed, pumped, sealed and so on.
The water in low pressure open-vented heating systems is kept below 100 degrees celsius. For existing systems the flow water from the boiler is usually about 80 degrees celsius and the return water temperature is usually 12-15 degrees celsius lower.
Central Heating Systems
Domestic central heating systems fall into two categories. These are based on the way the system is filled with water and the pressure at which it operates. The two categories are:

* Low pressure, open-vented central heating system. This type of system is fed from a F&E cistern in the roof space. These can be both modern fully pumped systems and older style gravity hot water heating installations.

* Sealed, pressurised central heating system. This type of heating system is fed directly from the cold mains water supply and incorporates an expansion valve to take the water that has expanded due to being heated. These are generally more modern fully pumped systems or combination boiler systems.
Sealed heating systems operate at a higher pressure, with modern systems incorporating condensing boilers operating at a slightly higher temperature of 82 degrees celsius for the flow and with a return temperature of 20 degrees celsius lower. This increases the heat flow rate and therefore the efficiency of the system.

For both systems the temperature difference of the flow and return represents the amount of heat output to the heated areas.
Thermal Comfort
Thermal comfort is achieved when a desirable heat balance between the body and the surroundings is met. How we achieve this balance is down to the design of the central heating system and the way it is installed. There are three definitions of central heating systems-

Full central heating- A system where heat losses for the whole system have been calculated to an agreed comfortable temperature.

Selective central heating- A system where some of the rooms, such as the bathroom or the bedroom are heated but the rest of the dwelling is not calculated.

Background central heating- A system where smaller boilers or heat emitters are installed at low cost and only provide enough heat to take the chill out of rooms at a lower temperature. If a system cannot achieve comfort levels when the external temperature is -1 degree celsius then it is classified as background heating.
Low pressure, open-vented heating systems
Sealed System
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