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Tankless Water Heaters
Operation
As the name suggests, tankless water heaters have no tank, and therefore no storage capacity at all. Tankless heaters include a burner, heat exchanger, venting system, and the necessary controls. When the faucets and fixtures in the home are sitting unused, the water heater is dormant. When a hot water faucet is opened, the tankless heater detects the water flow and ignites the burners. These powerful burners quickly heat the water inside the thin, small diameter heat exchanger. As hot water is drawn out, fresh cold water is drawn in and the cycle continues. A significant advantage of this system is that you can’t empty all of the hot water out of the tank because there is no tank – just continuous hot water.
Small Size
Without the tank, water heaters of this type are much smaller, and are usually wall-mounted.
Fuel and Venting
Most tankless water heaters are fuelled by natural gas or propane and are sidewall-vented.
Efficiency
Tankless water heaters are often much more efficient than conventional water heaters, using direct venting, modulating burners, and/or condensing combustion systems.
Mixing Valve
Most systems include a tempering valve and a means of setting a maximum water temperature to avoid scalding.
Remote Control
Some tankless systems include a remote control, which can be used to monitor the performance of the system, display error codes or change the desired water temperature.
Other Uses
Tankless water heaters may also be used to heat the home, either as part of a forced air combination system, or a radiant heating system.
Maintenance
Problems: Conventional water heaters, due to their simplicity, are relatively inexpensive, while tankless water heaters are more expensive to purchase. Their complexity also means that maintenance and repairs can be more expensive as well. Dedicated isolating valves make draining or other regular maintenance simpler.
Fuel Supply
The water heater must have a continuous fuel supply. Gas piping should be steel, not aluminum, copper or brass. Any gas odor is a serious problem. Malfunctioning burners, sensors or control will cause poor operation or may result in the system not working at all.
Scale Build-up
The small-diameter of the heat exchanger means that these units are susceptible to clogging with scale, especially in areas with hard water. In hard water areas, annual descaling is recommended.
Dirty Filter
If the water heater is equipped with a water filter, this should be checked and cleaned monthly or performance will suffer.
Maximum Flow Rate
The hot water flow rate is not only dependant on the heating capacity of the water heater and the output water temperature, but will also depend on the temperature of the incoming water. Homes in northern climates draw water from colder water supplies, and since it takes longer to heat up colder water, tankless water heaters installed in these homes will have lower hot water flow rates.
Minimum Flow Rate
The burners are triggered by sensors that detect the flow of water. If the flow of water is too low, typically less than ½ gallon per minute, the burners will not turn on and hot water will not be delivered. Water-saving fixtures may not allow enough flow to turn the water heater on, especially when the water heater has a high minimum flow rate.
Relief Valve
The temperature/pressure relief valve lets water escape if the temperature or pressure is too high. This valve should be connected to a tube which discharges 6 to 12 inches above floor level so hot water won’t be sprayed on to anyone nearby. Some codes require that the tube discharge outside the building. The tube should be as large as the tank fitting and the tube end should never be threaded, capped or plugged. The tube should be able to withstand 250°F temperatures, should have no shut-off valve, and should be as short and as straight as possible.
Indirect Water Heaters
Indirect heaters can be used with hot water or steam boilers. Cold water is heated by passing through a coil in the boiler or a heat exchanger attached to the boiler. These units, which are often undersized, usually have no storage tank. The house boiler must be run year round. This wastes energy and reduces the boiler life. Since the boiler water temperature varies, a mixing valve (manual or automatic) should be provided. The coils frequently scale up, reducing hot water pressure. Replacement with a tank system is usually recommended.
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