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7. Domestic Water Heaters

7. Domestic Water Heaters

An increasing number of Canadian homes that are heated with oil also use oil 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 the number and lifestyles of the inhabitants, hot water consumption may account for more than 20 percent of total annual energy consumption. One of the principal advantages of an oil-fired water heater is its quick recovery (due to higher combustion temperatures) – twice as fast as natural gas and five times as fast as electricity, based on the heat output of typical residential water heaters.

Free-standing oil-fired water heaters (see Figure 11 on page 55) now use burners with flame retention heads and other modifications that improve efficiency. They can be connected to an existing chimney or, in some cases, can be side-wall vented, if approved for it.

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.

This chapter examines the options for improving the efficiency of the domestic hot water system by selecting and properly installing more efficient equipment. In the past, tap water was usually set to 60°C (140°F). Nowadays, the temperature is set lower to prevent scalding young children.

Reducing Energy Losses

There are two basic types of oil-fired tap water heating systems: conventional water heaters that heat the water directly in a tank and systems that heat the water in conjunction with another energy use, usually 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 designing the system carefully and selecting equipment that generates hot water more efficiently and reduces stack and standby losses. Modifying an existing system, including piping modifications, can also reduce some of the standby losses. Comparable to the AFUE of furnaces, the energy factor measures the seasonal performance of water heaters – the higher the number, the better the efficiency.

Figure 11 Oil-fired water heater

 

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:

  • Install a heat trap above the water heater. A heat trap is a simple piping arrangement that prevents hot water from rising up in the pipes, thereby minimizing the potential for heat loss.

  • Insulate hot water pipes to reduce the 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 (insulating value) of at least 0.35 (R-2) over as much of the pipe as you can easily access.

    It is extremely important not to insulate over any controls or obstruct the vent connections or combustion air openings. The insulation should not come in contact with the vent connector.

  • Place the water heater over a layer of rigid thermal insulation to reduce heat loss through the bottom of the tank. This is particularly applicable to electric water heaters and external storage tanks for integrated space and water heating systems.

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

Integrated Space-Water Heating Systems

Improvements to the building envelopes of homes have reduced the space heating load to the point where, in highly energy-efficient homes, it is sometimes difficult to justify the expense of a high efficiency furnace solely to satisfy the heating load. To take advantage of the efficiency potential of the new technologies, it may make sense to combine space heating with other functions, particularly water heating. Domestic hot water loads have remained fairly constant and have even increased over time, making it a good idea to put more effort into improving the efficiency of the hot water generator. Therefore, it would be natural to combine space and water heating systems.

Figure 12 Schematic of an efficient oil-fired integrated space-water heating system

 

Combining the functions of space and water heating in one unit can lead to capital cost reductions for the equipment and, potentially, to increased efficiencies of operation. A schematic of such a system is shown in Figure 12.

The efficient integrated oil system couples a mid-efficiency, low-thermal mass boiler fired with a high-static burner to a well-insulated water storage tank, using an efficient water-to-water heat exchanger. When the house thermostat calls for heat, the boiler supplies heat to the house, either directly into a hydronic system or through a fancoil into a forced-air distribution system. When the house thermostat demand is satisfied, the boiler, instead of shutting off, continues to run, but dumps the heat across the heat exchanger into the tap water storage tank.

There are also oil-fired boilers on the market that provide a continuous supply of domestic hot water by circulating cold water through a finned copper coil immersed directly in the boiler water. This system is known as a tankless coil. The boiler must be kept hot even during the summer to give an adequate supply of tap water.

In the past, these systems were extremely inefficient and were usually grossly oversized for the house heat demands. Today, the efficiencies of some new tankless coil boilers have been improved by using a low-mass boiler with the coil and a well-insulated external storage tank, coming closer to the system described in the previous paragraph.

Another arrangement uses a conventional oil-fired water heater as the basic energy generator, with heated water being supplied to the house through a fan coil. Although this system has some advantages in terms of lower initial capital costs, its efficiency may not be as high as the systems described above.

Integrated systems are now being developed that offer promise for further improvement. If you are considering upgrading or replacing your heating system, you may want to think about installing an integrated space and water heating furnace or boiler. Your energy consumption for space and water heating may be higher if these services are provided by two separate units than if provided by a single integrated unit. In other words, integrated units may offer energy savings while still providing the same space heat and hot water. Mid-efficiency units are available and offer seasonal efficiencies of 80 to 95 percent for both space and hot water heating.

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

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