Architectural Technology - Gas heating

No heating system is 100% efficient. Levels of efficiency vary widely. A badly maintained oil burner, for example, may be only 50-60% efficient while gas heating efficiency can be as high as 80-95%. The difference in efficiency levels results from the amount of heat being exhausted into the atmosphere through the flue rather than being recycled through the system.

History of gas production
The ancient Chinese were the first to use gas for lighting. They piped natural gas from the ground through bamboo tubes. But it was many centuries before the use of gas became widespread.

Modern gas production began in the late 1700s. The first successful coal-gas project was a system installed in 1765 in Britain, to light the offices of a coal mine. Once a gas supply was available for lighting, it was not long before it was put to other uses. The first gas cookers were introduced in 1840, and gas fires appeared in 1855. At present, most consumers are supplied with natural gas, but coal gas is used in some regions and gas is also obtained from oil.

Natural gas
Many countries have valuable deposits of natural gas. Gas from beneath the North Sea is now the main source of gas for Britain and other countries in northern Europe.

Natural gas consists mainly of methane, together with ethane and small amounts of propane, butane and nitrogen. It has a much higher calorific value than coal gas. This means that, when burnt, a given amount of it produces much more heat than the same amount of coal gas.

The demand for natural gas in Ireland is growing at an unprecedented rate. A survey conducted by Bord Gáis in 1999 highlighted that indigenous supplies are no longer adequate to supply the Irish market. It states that supplies from Kinsale Head are depleting and while the Corrib field is commercially viable it will not be available in time to avert a shortfall. The Irish market relies heavily on imported gas supplies and this is expected to continue until 2025.

Installation guidelines

General
Gas pipework is installed in a dwelling in order to convey gas in a safe manner from the meter to the various appliances, which may be installed both inside and outside the dwelling. When installing gas pipework the following guidelines should be adhered to.
• All gas mains are to be installed by the gas company or their appointed contractor.
• All gas main trenches shall be excavated to a depth to allow a minimum cover for the gas main of at least 750 mm.
• All gas main trenches shall be excavated to the width of the outside diameter of the gas pipe plus 300 mm. [image p2]
• Where it is necessary to cross or run close to any other service, a minimum clearance of 250 mm is recommended.
• Marker tape should be placed over all gas mains and services.

Meters
Meters are normally fitted in purpose designed cabinets external to the building. An isolation valve should be fitted to each meter. Where more than one meter is fitted in a cabinet each meter must be labelled clearly showing which dwelling is supplied by it.  

Piping materials
Piping materials should be selected by considering mechanical strength, appearance, corrosion potential and cost. Copper tubing is normally used for residential gas service piping.

External pipe runs in vulnerable positions should be of corrosion protected steel to provide against physical damage.

Where the piping is to be laid on a solid floor slab, the finished floor screed level must allow 25mm (minimum) cover over the installed pipe.
  
Pipes may be chased into a wall surface and covered with a plaster screed, or surface mounted and concealed in a purpose made duct.

Pipes passing across cavities, including cavity walls, should take the shortest route and be sleeved through the cavity.

In timber framed construction and dry lined walls, purpose designed protective metal channels must be provided.

Safety Considerations
Any person installing natural gas must be competent to do so in accordance with I.S. 813 Domestic Gas Installations. It is necessary to provide an appliance valve as close as possible to each appliance supplied with gas.

 
 
Purging natural gas installations
Every installation must be purged of air or air/gas mixture whenever a gas supply is made available for the first time or when an existing system has been shutdown and is being re-commissioned.

This is necessary because a gas/air mixture in the meter or pipework is potentially explosive. The installation and appliances must only contain a 100% gas concentration. While an appliance may initially light and burn correctly, if there is a pocket of air in the internal installation, the appliance flame will extinguish as the air reaches the appliance burner

Electrical cross bonding
Electrical cross bonding is required to create a zone in which voltage differences and therefore hazards from electrical shocks or sparks are minimised. This is achieved by connecting separate conductive components together with earthing cable.

This is necessary in case an electrical problem occurs on wiring or an appliance. It is possible for stray electrical currents to be transmitted to and through the gas pipework.

Emergency procedures
In the event of damage to a gas main stop work immediately and take the following precautionary measures:

• Report all damage even if there is no smell of gas.
• Shut down all working plant in the area of damage.
• Keep people away from the affected area.
• Prevent all sources of ignition (e.g. smoking, naked flames etc.).
• Do not use mobile phones near the gas leak.
• Do not try to repair the damage.
• Contact the gas company. Bord Gáis emergency service number is 1850 205050
• Provide assistance to the gas emergency crew as necessary.

Natural gas is a highly flammable gas, which is transported through a network of polythene and steel pipes at pressures ranging from 70 Bar to 20 mBar. Damage to a gas main can result in the escape of large volumes of gas into the atmosphere in an uncontrolled manner. Even if there is no smell of gas the damage should be reported regardless of how minor the damage may appear to the naked eye.

Gas heating systems
Gas-fired heating systems generate heat in either a furnace or a boiler. A furnace heats air that is blown through air ducts and delivered to rooms through grills. A boiler heats water or steam that circulates through pipes to radiators. Hot-water heating systems are more common than steam systems.  

Inside the furnace or boiler, gas is released into a combustion chamber where it is mixed with air and burned. The combustion products are vented out of the home through a flue pipe. The flames heat a metal box called a heat exchanger. In a furnace, air is heated in the heat exchanger, while in a boiler, water is heated in the heat exchanger.

For hot- water systems, the water is heated to between 180 and 200 F; in steam systems, the water is boiled, creating steam. A commonly used method for improving the efficiency of conventional gas systems is adding heat exchangers to extract more heat from the exhaust before it leaves the home.

These combustion by-products must be fairly hot so they will rise out of the system, but the hotter they are, the more heat they waste to the environment that could have been used in the home. Stack temperatures as high as 400°F are not uncommon, which means there is a great opportunity to save energy and money.

Heating system controls regulate when the various components of the heating system turn on and off. The most familiar control is the room thermostat, which activates the system to keep you comfortable. But there are other controls in a heating system, including aquastats, valves, vents, fan thermostats, and dampers.