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8. Gas Water Heaters and Other Equipment

8. Gas Water Heaters and Other Equipment

If natural gas is being supplied to your home for space heating purposes, you also have the option to use gas for other household activities.

Most Canadian homes heated with natural gas also use gas for their domestic hot water supply. Domestic water heaters are the second largest individual users of energy in most Canadian houses, after the space heating system. Depending upon the house type and on the number and lifestyles of the inhabitants, hot water consumption may account for more than 20 percent of total annual energy consumption.

When compared with electricity, one of the principal advantages of a gas-fired water heater is its cheaper operating cost. The overall efficiency of a gas-fired water heater is measured by the energy factor, which takes into account standby losses, combustion system efficiency and recovery efficiency. Most direct heat loss from water heaters is made up of losses by air and heat flow up the flue, both when the burner is firing and when it is not; by heat conducted through the tank walls and base; and by hot water convection losses through the hot and cold water feed pipes.

Water Heater Technologies

CONVENTIONAL GAS WATER HEATERS

Gas-fired residential water heaters typically consist of a steel cylinder storage tank. Capacities of 30, 40 and 50 gallons (114, 151 and 189 litres, respectively) are most common. They also have one or two inches (2.5 to 5.0 cm) of insulation placed between the tank lining and the outer jacket, a cold water supply inlet and a hot water outlet pipe, a draft hood, and a flue, as shown in Figure 14. The gas burner is located inside a combustion chamber at the bottom of the storage tank and has a continuously burning pilot light that is used to ignite the main burner. Air for combustion is brought in through air openings located at the bottom of the combustion chamber.

Figure 14 Conventional gas-fired water heater

A combined combustion thermostat and gas valve unit controls both the temperature of the water in the tank and the gas flow. The flue passes vertically through the centre of the tank cylinder to the outside. Its main job is to conduct the combustion products from the combustion chamber to the vent system. Approximately one-half to two-thirds of the standby losses are through the flue. Overall seasonal efficiencies are around 55 to 60 percent.

INCREASED EFFICIENCY GAS WATER HEATERS

If you are in the market for a new gas water heater, increased tank insulation and heat traps are options that are available as part of the original equipment. There are also gas-fired boilers on the market that provide a continuous supply of domestic hot water. The boiler circulates cold water through a finned copper coil immersed in the boiler water. It is set to maintain the supply of hot water during the off-heating season. New designs for water heaters, such as the following, offer increased efficiency and performance.

Power-Vented Gas Water Heaters

To make water heaters compatible with the new standard- and high-efficiency furnaces, manufacturers have developed new free-standing gas-fired water heaters with induced draft fans that can push the exhaust gases either up the chimney vent or out the side wall of the house. Most of these units retain the draft hood with its dilution air requirement and have a continuous pilot light. Overall efficiencies are not much different from those of conventional water heaters.

Direct-Vent Gas Water Heaters

Direct-vent water heaters, also referred to as "sealed combustion" water heaters, draw combustion air from outside the building, rather than from the room, directly into the combustion chamber. Exhaust gases are vented, with the aid of a blower, to the outside. Efficiency is improved by reducing off-cycle losses. A direct-vent water heater offers energy savings of around 20 percent.

High-Efficiency Condensing Gas Water Heaters

Gas water heaters that are more efficient than before are appearing on the Canadian market. An additional heat exchanger uses the incoming cold water to cool the heat-exchange surface areas to the condensation point of the flue gases. The condensate is either collected for a later neutralizing treatment or sent down the floor drain to the sewer system. Corrosion-resistant materials must be used for the condensing part of the heat-exchange surface. These materials are more expensive than those used in conventional water heaters. Because flue gases are cooled, they can be vented through a side-wall plastic PVC or ABS vent, which is a cheaper option than a central corrosion-resistant vent. Such a unit has the potential for efficiencies above 90 percent.

Note that gas water heaters are covered by federal and provincial/territorial energy efficiency standards.

Options for improving the efficiency of the domestic hot water system by selecting and properly installing more efficient equipment are discussed below. In the past, tap water was usually set at 60°C (140°F). Today, primarily due to fears of scalding small children, the set temperature is often somewhat lower.

REDUCING ENERGY LOSSES

There are three basic types of gas-fired tap water heating systems: conventional water heaters that heat the water directly in a tank; instantaneous heaters without a tank that heat the water only when it is being used; and systems that heat the water in conjunction with another energy use, usually for space heating. For the latter, it can be in the form of a "tankless coil" inside the boiler or a storage tank tied to the boiler through an efficient water-to-water heat exchanger.

The operating efficiency of a domestic hot water system can be improved significantly by carefully designing the system. Selecting equipment that generates the hot water more efficiently reduces stack and standby losses. Modifying an existing system, including piping modifications, can also reduce some of the standby losses.

REDUCING STANDBY LOSSES

The term "standby loss" refers to heat lost from the water in a domestic water heater and its distribution system to the surrounding air. It is a function of the temperature difference between the water and the surrounding air, the surface area of the tank, and the amount of insulation encasing the tank.

You should consider the following options to reduce standby losses.

  • Insulate the tank with an approved insulating blanket. It is extremely important not to insulate over any controls or obstruct the vent connections or combustion air openings. Furthermore, the insulation should not come in contact with the vent connector.
  • Install a heat trap above the water heater. A heat trap is a simple piping arrangement that prevents hot water from rising in the pipes, thereby minimizing the potential for this loss.
  • Insulate the hot water pipes to reduce heat loss from the pipes themselves. Pipe insulation is available in a variety of materials and thicknesses, with easy application to most hot water pipes. Use insulation with an RSI (insulation value) of at least 0.35 (R-2) over as much of the pipe as you can easily access.

Before carrying out any of the steps listed above, check with your local installer or gas utility to ensure that you will not compromise the safety or operation of the appliance.

If you have chosen natural gas as your home heating fuel, you should consider other uses for the natural gas that is piped into your home. Examples include switching to a gas range in the kitchen or to a gas clothes dryer. Even though you may need to spend more money initially for these appliances, these changes will probably save you money in the long run because gas appliances cost less to run than their electric counterparts.

Many people enjoy using gas barbecues in the summer months. In a relatively recent development, a natural gas line can be brought to the backyard, where quick-connect fittings allow you to connect the gas line directly to a gas barbecue. A new barbecue running on natural gas costs about $20 to $40 more to purchase than a conventional propane barbecue. Hookup charges will vary by region. The costs of natural gas for a barbecue will be considerably less than the cost of propane refills for your tank. However, it is not possible to retrofit your old propane barbecue to run on natural gas.

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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency

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