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Gas-fired systems

If you are given an option and the fixed costs are comparable, a gas burner is preferred to an oil burner. It does not require annual maintenance and is less costly to install. Gas burners have a cleaner, quieter operation, and the supply of gas is not dependent on the weather. In addition, replacement costs are lower.

In this system, gas is supplied at low pressure to the burner through an automatic gas valve. The valve in turn is controlled by the thermostat or aquastat (if the system generates domestic hot water). At the burner, the gas that is mixed with air is ignited by a pilot light. Associated with the pilot light is a safety control, a thermocouple. The thermocouple closes the gas valve when the pilot light goes out. Basically, the thermocouple converts heat (from the pilot light) into a small electric current that keeps an electrically operated valve in the main gas line open. When the pilot light goes out, the current ceases, and the valve closes. If the thermocouple is faulty, the pilot will not light. Rather than have a pilot light that is constantly lit, many new boilers use an electronic ignition to fire up the burner.

Some gas-fired systems are equipped with a self-energizing control unit (Powerpile) that enables them to operate in the event of an electrical power outage. This is a particularly desirable feature, especially for those areas prone to power failures. The control unit basically consists of a special pilot-thermocouple assembly, gas valve, and thermostat. It can be used on steam, hot-water, and warm-air systems. With a self-energizing control unit during a power failure, a steam system will operate normally and a forced-hot-water or warm-air system will function as a gravity system.

As with oil burners, gas burners require a small but steady draft. However, rather than using a draft regulator, gas-burning equipment uses a draft diverter-hood. (See FIG. 16-6.) The hood is usually located on the exhaust stack above a boiler and is often built into the sheet-metal casing of a furnace. A draft-diverter hood is completely open on the underside and thus prevents air currents in the chimney (resulting from downdrafts) from blowing out the pilot-light flame.

All gas-burning equipment should have the American Gas Association’s seal of approval. This indicates that a similar unit has been tested for safety. Look for the seal as you inspect the equipment. It is usually located on a plate mounted on the front of the boiler or furnace.

What type of pipe is used to supply gas to the furnace or boiler? It should be a rigid black iron pipe rather than a flexible pipe or a copper pipe. Many utility companies buy their gas from the cheapest supplier, and therefore, the gas can contain impurities that will react with copper. Although most municipalities allow flexible gas pipe for movable appliances such as clothes dryers and stoves, they generally require a rigid gas-supply pipe for stationary equipment such as a boiler, furnace, or domestic water heater. If the gas-supply pipe to the heating system is other than a black iron pipe, check with your local utility company for their requirements.

Check the gas meter to see if it is adequately sized to supply gas to the heating system and domestic water heater during peak periods. An inadequately sized gas meter might be found in a house that originally had a gas-fired water heater and an oil-fired heating system. Sometimes when the oil-fired system is converted to or replaced by a gas-fired system, the installer does not notify the utility company to replace the meter, which was sized to supply gas for only the water heater and kitchen stove.

A gas meter is rated in cubic feet per hour, and one cubic foot of natural gas is approximately equal to 1,000 Btu. Therefore, a meter rated at 250 cubic feet per hour can supply gas that is approximately equivalent to 250,000 Btu per hour. You can check to see if the gas meter is adequately sized by adding up the input Btu/hour requirements for the boiler or furnace and the water heater, and comparing the total with the meter. The input Btu will generally be found on the data plate that is mounted on the outer casing of the appliance. (The gas requirement for the kitchen stove is considered negligible for this comparison). The Btu capacity of the meter should be equal to or greater than the sum of the input Btu requirements for the heating system and water heater. If it isn’t, make a note on your worksheet to notify the utility company.

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