Tankless, also known as on-demand, Instantaneous, or continuous, water heaters use a high-fire burner to quickly raise the temperature of water passing through the copper heat exchanger. It heats the water as there is a demand for hot water. When the tankless heater senses the demand for hot water, the burner goes through an ignition sequence, ignites, and begins to heat the flowing water. The modulating burner adjusts its intensity to match the demand, the incoming water temperature, and the thermostat set point. This operational sequence requires a water flow sensor, multiple temperature sensors, a modulating value, a variable speed combustion blower, and the electronics to control the process.
The tankless technology is not a new technology, having been in existence for about 30 years in other parts of the world. However, in Europe and Asia, these units were designed for more compact homes where usages are less demanding.
Tankless water heaters can be electric, using electric elements to heat the water, or gas-fired, using a gas burner to heat the water. Since there is no storage tank for hot water, the tankless technology does not incur standby heat loss. As hot water need diminishes, the burner reduces output to match the lower demand.
Tankless water heaters are rated by their output of gallons per minute (gpm) at a 45º temperature change. If the flow demand is within the capacity of the tankless water heater, it will deliver endless hot water. However, if the flow demand exceeds the heating capacity, the water flow will be restricted so that the water delivered is at the set point temperature. The water flows through the heat exchanger (usually through copper tubing) with a normal rotation of 3 – 5 passes.
When considering electric tankless, take into account voltage, amperage, and circuit breaker requirements. If the preference for fuel type is gas, then identify whether your have access to Natural Gas or Propane. In many cases, the gas line and venting requirements exceed what you may currently have with a tank type water heater.
The capacity of a tankless heater will influence the performance. If a tankless heater has an 8 gpm capacity, it can heat 8 gallons per minute to a temperature rise of 45ºF. If the heater set point is 120º and the cold water input is 75º, then 8 gpm of 120º water can be produced. However, if the inlet cold water is only 35º then the heater is able to heat 8 gpm to only 80º. In this case, in order to maintain 120º hot water, the heater will reduce the flow from 8 gpm to 4.24 gpm. So the amount of “endless” hot water is influenced by the size of the unit.
Also look for the energy factor (EF) when evaluating tankless and tank type water heaters. Efficiency is the amount of energy output as compared to energy input. The better the efficiency, the better the performance. For example, if 100% of the heat from a water heater transferred directly into the water, then the unit would be 100% efficient. For an 80% efficiency rating, 80% of the heat from the tankless water heater goes directly into the water, and 20% escapes and is released into the outside air through the vent. Be sure to inquire with your plumber the various tank and tankless options and their efficiency rating. There are several tank water heaters available with high efficiency ratings that match or exceed those of a tankless heater.
If you are considering tankless, keep in mind that typically gas-fired tankless water heaters out-perform electric tankless via higher flow rates. The primary factor for fuel type is which source is available to your home.