Tankless water heaters Advantages and Disadvantages

Advantages

Tankless water heaters provide many advantages

Long term energy savings:

Though a tankless water heater typically costs more initially, it usually costs less to operate because of lower energy use—since it only heats water when required instead of continuously maintaining a tank of heated water. Even homes or buildings with high demand for hot water may realize some level of savings. If instant hot water at taps at limited hours is a priority, a recirculation system can be accommodated by using an aquastat and timer to decrease the added heat loss from the recirculation system. If the storage tank of an electric heater is highly insulated, so that the outer surface of the tank is only slightly warmer than the ambient air, the savings with a tankless heater is less.

Savings in water use:

Users in remote points in the building do not have to run the hot water as long, waiting for it to get to the faucet.

Unlimited hot water:

Though flow rate determines the amount of hot water the heater can produce, it can deliver it at that flow rate indefinitely. However, this can also be an ecological disadvantage, as running out of hot water limits use, but a tankless heater provides no such limit.

Less physical space:

Most tankless water heaters can be mounted on a wall or internally in a building's structure. This means less physical space must be dedicated to heating water. Even systems that can't be mounted on walls take up less space than a tank-type water heater.

Reduced risk of water damage:

No stored water means there is no risk of water damage from a tank failure or rupture, though pipe or fitting failure remains possible.

Temperature compensation:

A temperature-compensating valve tends to eliminate the issue where the temperature and pressure from tankless heaters decrease during continuous use. Most new generation tankless water heaters stabilize water pressure and temperature by a bypass valve and a mixing valve incorporated in the unit. Modern tankless are not inversely proportional, because they regulate the amount of water they heat and discharge, and therefore stabilize water temperature by using a flow control valve. Temperature change, not flow speed, is the issue the water heater must address. The wider the temperature rise, the less flow from the unit—the smaller the temperature rise, the greater the flow. The flow control valve, in conjunction with thermistors, maintains a stable temperature throughout the use of the unit.

Safety:

Tankless water heaters precisely control water temperature, which means dangerous temperature levels and spikes are less likely.[9] An additional safety advantage stems from reduced exposure to dissolved toxic metals, which tend to occur at higher concentrations in hot water which has resided in a conventional water heater tank for significant periods of time.

Disadvantages

Tankless water heaters also have some disadvantages

Startup costs:

Beyond the 2x-4x larger initial purchase price (as compared to a tanked water heater), installing a tankless system comes at an increased cost, particularly in retrofit applications. They tend to be particularly expensive in areas such as the US where they are not dominant, compared to the established tank design. If a storage water heater is replaced with a tankless one, the installer may have to increase the size of the electrical wiring or gas pipeline to handle the load, and replace the existing vent pipe—possibly adding expense to the retrofit. Many tankless units have fully modulating gas valves that range from as low as 10,000 to over 1,000,000 BTUs[clarification needed]. Most electrical tankless heaters require AWG 10 or 8 wire, corresponding to 5.5 or 8.5 mm2 for typical POU (point of use) heaters at North American voltages. Larger whole-house electric units may require up to AWG 2 wire. In gas appliances, both pressure and volume requirements must be met for optimum operation.

Start-up delay:

There may be a longer wait for hot water. A tankless water heater only heats water on demand, so idle water in the piping starts at room temperature. Thus, there may be a more apparent "flow delay" for hot water to reach a distant faucet (in non-point-of-use systems). Many models sold in the UK have introduced a small heat store within the combination boiler to address this issue. This "keep hot" facility considerably improves the standard of hot water service, which some people otherwise find unacceptably poor with a combination boiler, but it uses considerably more fuel especially in summer.

Intermittent-use:

There is a short delay (1–3 seconds) between when the water begins to flow and when the heater's flow detector activates the heating elements or gas burner. In the case of continuous-use applications (showers, baths, washing machines) this is not an issue as the heater never stops heating. However, for intermittent-use applications (i.e., turning off/on a hot water faucet at a sink) this can result in initially hot water, followed by a small amount of cold water as the heater re-activates, followed again by hot water. This is particularly an issue if hot water pipes are poorly insulated. The user experience is that after initially getting hot water flowing, the user turns off the valve and then a short time later turns the valve back on again. Hot water starts flowing once again at the valve from the hot water already in the piping, but at the same time, some heaters must let some amount of cold water into the piping during the reactivation time. Some time later (depending on the length of piping from the tank to the valve) this cold section of water arrives at the sink, followed shortly thereafter by hot water again. The initial thought of the user can be that the heater is failing intermittently.

Recirculation systems:

Since a tankless water heater is inactive when hot water is not being used, they are incompatible with passive (convection-based) hot water recirculation systems. They may be incompatible with active hot water recirculation systems and certainly use more energy to constantly heat water within the piping, defeating one of a tankless water heater's primary advantages. On-demand recirculating pumps are often used to minimize hot water wait times from tankless water heaters and save water being wasted down the drain. On-demand recirculating pumps are activated by push-button or other sensor. A water contacting temperature probe installed at the hot water usage point signals the pump to stop. Single-cycle pumping events only occur when hot water is needed, thereby preventing the energy waste associated with constantly heating water within piping.

Achieving cooler temperatures:

Tankless water heaters often have minimum flow requirements before the heater is activated, and this can result in a gap between the cold water temperature, and the coolest warm water temperature that can be achieved with a hot and cold water mix.

Maintaining constant shower temperature:

Similarly, unlike with a tank heater, the hot water temperature from a non-modulated tankless heater is inversely proportional to the rate of the water flow—the faster the flow, the less time the water spends in the heating element being heated. Mixing hot and cold water to the "right" temperature from a single-lever faucet (say, when taking a shower) takes some practice. Also, when adjusting the mixture in mid-shower, the change in temperature initially reacts as a tanked heater does, but this also changes the flow rate of hot water. Therefore, some finite time later the temperature changes again very slightly and requires readjustment. This is typically not noticeable in non-shower applications.

Operation with low supply pressure:

Tankless systems are reliant on the water pressure that is delivered to the property. In other words, if a tankless system is used to deliver water to a shower or water faucet, the pressure is the same as the pressure delivered to the property and cannot be increased, whereas in tanked systems the tanks can be positioned above the water outlets (in the loft/attic space for example) so the force of gravity can assist in delivering the water, and pumps can be added into the system to increase pressure. Power showers, for example, cannot be used with tankless systems because the tankless systems cannot deliver the hot water at a fast enough flow rate required by the pump.

Time-of-use metering and peak electrical loads:

Tankless electric heaters, if installed in a large percentage of homes within an area, can create demand management problems for electrical utilities. Because these are high-current devices, and hot water use tends to peak at certain times of the day, their use can cause short spikes in electricity demand, including during the daily peak electrical load periods, which increases utility operating costs. For households using time-of-use metering (where electricity costs more during peak periods such as daytime, and is cheaper at night), a tankless electric heater may actually increase operating costs if the hot water is used during peak times.[citation needed] Instantaneous-type heaters are also problematic if they are connected to district heating systems, as they raise peak demands, and most utilities prefer all buildings to have hot water storage.

Power outage:

In case of a power outage, tankless heaters cannot supply hot water, unlike tank based heaters which can supply the hot water stored in the tank.

LED Light Strobe effect:

Most residential demand water heaters act by modulating the heating elements to match the flow rate. This is required to prevent overheating in the heating chamber. The resulting modulation of power being used has been known to cause "fluttering" in LED fixtures.[citation needed] Ordinary incandescent lamps are not similarly affected since the temperature of a tungsten element does not react to high-frequency modulations.