Electric Boilers: General Information

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Electric Boiler: General Information

Equipment for Electric Hydronic Systems

DESIGN AND OPERATION

Electric hot water or hydronic systems deliver heat to a house by means of hot water. The three main components of such a system are:

1.	a boiler to heat the water;
2.	heating equipment – generally baseboard heaters or radiators – in most rooms, often installed against an outside wall; and
3.	a pump to circulate water from the boiler to the radiators and ensure that it flows back through the pipes.

Figure 5: Central boiler for an electric hydronic system

The boiler in an electric hot water heating system is compact. Its heating elements are immersed directly in the water (as in an electric kettle). Where space is limited, the boiler can be installed on a basement wall, in a closet, under a kitchen cabinet–it can even be hung from basement ceiling joists.

If you are replacing a boiler in an existing hydronic system with a new electric boiler, you can probably use the existing heat distribution pipes.

MAXIMIZING EFFECTIVENESS OF HYDRONIC SYSTEMS

As with forced-air furnaces, there are several ways to improve the performance of hydronic heating systems.

Improving Heat Distribution

Old-fashioned gravity systems that circulate the water by natural convection are much less efficient than systems that use pumps. Slow hot water circulation causes home temperatures to fluctuate noticeably, and it takes a long time to restore temperatures after a night-time setback. Also, a gravity system cannot circulate hot water to radiators or baseboard heaters in basement living areas, where they would be below the level of the boiler. All of these problems can be overcome by adding a circulating pump, and replacing the open expansion tank in the attic with a sealed and pressurized expansion tank near the boiler. If you have a gravity system, consult your heating and plumbing contractor about the possibility of improving it.

Balancing the Heat

Balancing the heat delivered to different areas of the house is as important with hydronic heating as it is with a forced-air system. Radiators are often fitted with simple manual valves that can be used to control the amount of water flowing through them. Such valves can be used to vary the heat delivered to different rooms in the same way that balancing dampers are used in a forced-air system.

One device that can vary the heat output automatically is a thermostatic valve (Figure 6) that can be set to control the temperature in any room. However, this will not work on radiators or baseboard heaters installed on what is called a "series loop" system. In a "series loop", the water must pass through all the heating units on its way back to the boiler. If there is more than one loop in the system, some balancing of the heat output can be achieved by adjusting the valves that control the water flow through each loop. The same type of baseboard radiators are equipped with built-in air dampers which allow heat output to be regulated to some extent.

Figure 6: Thermostatic radiator valve

Thermostatic radiator valve

Conventional hydronic systems usually have the boiler temperature set at 82^oC (180^oF). It is possible to reduce energy consumption in a number of hydronic systems by means of a regulator valve that causes the temperature of the water circulating in the system to vary in relation to the temperature outside. As it becomes warmer outside, the temperature of the water is reduced.

Automatic Setback Thermostat

The easiest way to save heating dollars is to lower the temperature setting of your home, when possible. An automatic setback thermostat will adjust your home's temperature automatically. These thermostats have a mechanical or electronic timer that allows you to preset household temperatures for specific periods of the day and night. As a general rule, you will save two per cent on your heating bill for every 1^oC you turn down the thermostat at night.

The thermostat can be programmed to reduce the temperature an hour before you go to bed and to raise it again before you get up in the morning. You could also reduce the temperature during the day when the house is unoccupied, and raise it shortly before you return. For example, you could have the temperature set at 17^oC (63^oF) when you are sleeping or not at home, and at 20^oC (68^oF) when you are awake.

Experiment with the unit until you find the most comfortable and economical routine for you and your family.

If you have a hydronic system, you can also reduce energy usage through zone control. With this system, thermostat-controlled valves on each radiator permit the control of individual room temperatures. A heating and plumbing contractor can provide more information about zone control and can install all the required equipment when the heating system is installed. Zone controls are also available for forced-air systems, usually with dampers in main heating ducts controlled by separate thermostats located in various parts of the house.
NOTE: For all-electric heat pump systems, setback thermostats are generally not recommended.

Improved thermostats

Greatly improved electronic thermostats are now available on the market. They are very sensitive and help reduce temperature fluctuations to less than 0.5-1C^o, whereas fluctuations usually range on an average from 1.5-2 C^o. They ensure that the furnace or electric baseboard heater starts up as close as possible to the set temperatures. The energy savings generated by these devices vary according to the model.

One model used with baseboard electric heaters will switch the heater on and off to maintain ambient temperature within +/- 0.5°C of the set point. It could save around 3% on energy use while improving comfort considerably. This model, however, is not recommended for fuel fired furnaces or wherever short cycling is not desirable.

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