Tankless or demand water heaters eliminate a longstanding downside of conventional storage tank models: the tank. Between uses, a storage tank of hot water sits idle, gradually losing heat energy. Although tank design and insulation efficiency have improved, some heat loss is inevitable as time elapses before the next person opens up a hot water tap somewhere in the house. If the tank sits long enough, it will eventually drop below the thermostat setting and the burner will ignite to reheat the water. Hot water that comes out of the tap and is used only once may have been reheated multiple times before eventual usage.
Tankless heaters incorporate a high-temperature gas or electric burner to flash-heat the flow of water as it passes through the heater on the way to the open tap. No water is stored. A properly-sized tankless heater should be able to heat as much water on-demand as normal household usage requires. Theoretically, the house should never run out of hot water. Because energy is not wasted heating and reheating water in a storage tank, tankless units typically offer energy savings between 30 and 50 percent over a conventional storage tank heater. Some of the other advantages of a tankless water heater replacement include:
- Less water waste as hot water is produced instantly without waiting.
- Compact size and space-saving design.
- Longer expected service life — up to 20 years.
Tankless units come with their own unique set of drawbacks, as well, however:
If not carefully sized, multiple simultaneous demands from several sources may outstrip the tankless unit's capacity, resulting in output that is below the desired temperature.
In households where water pressure varies as cold water taps are opened or toilets are flushed, the output water temperature of a tankless heater may fluctuate as well.
Tankless heaters are generally more effective when centrally located in the house, as close to all hot water faucets as possible. Gas tankless heaters require a flue for venting so alterations to the house may be required to accommodate installation of the vent pipe in that location.
Electric tankless heaters draw a large amount of amperage. Some household wiring systems may not be up to the electrical load and may require upgrading to supply sufficient power.
Tankless heaters require flushing and cleaning with special solutions to dissolve internal mineral deposits twice a year. These deposits degrade the efficiency of the tankless heat exchange process.
Tankless heaters are sized differently than storage tank models. Contractors utilize software designed for tankless sizing that integrate many variables into the calculation. Basically, sizing a tankless water heater replacement involves computing the total gallons of hot water you're likely to use at any given time. This requires calculating the flow rates in gallons per minute of all hot water outlets such as faucets, showerheads, laundry, etc. Data about the average temperature of municipal cold water entering the home is also required to compute the temperature rise necessary to produce the desired water temperature output of the heater. When all these factors are entered into the software, your plumbing contractor can arrive at an accurate sizing determination and recommend a tankless water heater replacement with adequate capacity.