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== Tankless heaters ==

Tankless water heaters, also called instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters, are also available and gaining in popularity. These water heaters instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil.



Tankless heaters are often installed throughout a household at more than one point-of-use (POU), far from the central water heater, or larger models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a continuous flow of hot water and energy savings (as compared to a limited flow of continuously heating hot water from conventional tank water heaters).



=== How tankless water heaters work ===



When there is a demand for hot water (e.g. a hot water tap is opened for a sink, shower, tub, or washing machine) the tankless water heater's water flow turbine senses the flow and starts the heating process. The water flow turbine sends a signal to the control board which looks at multiple factors: incoming water temperature, desired water temperature as set on the temperature controller, and the calculated difference between the two temperatures. Depending on the calculated incoming and desired water temperatures, the gas or electric flow into the burner assembly is modulated and the electronic ignition sequence begins. Water is heated to the desired temperature as it circulates through the copper heat exchanger providing continuous hot water. When the hot water tap is turned off, the tankless water heater shuts down and is placed in a standby mode pending the next call for hot water.



=== Combination boilers ===

'''Combination''' or '''combi boilers''', combine the central heating (CH) with (tankless) domestic hot water (DHW) in one box. They are not merely infinitely continuous water heaters having the ability to heat a hydronic heating system in a large house. When DHW is run off, the combi stops pumping water to the hydronic circuit and diverts all the boiler's power to instantly heating DHW. Some combis have small internal water storage vessels combining the energy of the stored water and the gas or oil burner to give faster DHW at the taps or increase the DHW flowrate.



Combi boilers are rated by the DHW flowrate. The kW ratings for domestic units are 24 kW to 54 kW, giving approximate flowrates of {{convert|9|to|23|l|usgal}} per minute. There are larger commercial units available. High flowrate models will simultaneously supply two showers.



A further advantage is that more than one combi unit may be used to supply separate heating zones, giving greater time and temperature control, and multiple bathrooms. An example is one combi supplying the downstairs heating system and another the upstairs. One unit may supply one bathroom and one another. Having two units gives backup in case one combi is down, provided the 2 systems are connected with valves that are normally closed.



Installation cost is significantly lower and less space is required as water tanks and associated pipes and controls are not required.



Combi boilers are highly popular in Europe, where in some countries market share is 70%.



Combination boilers have disadvantages. The water flow rate is likely to be less good than from a storage cylinder, particularly in winter. The power rating needs to be matched to heating requirements; heating water ‘on demand’ improves energy efficiency but limits the volume of water available at any moment. The water supply pressure must not be too low. A combination boiler has more moving parts that can break down, so can be less reliable than a tank system.. A combination boiler, which is heating your home and hot water directly (rather than a hot water tank) has more moving parts that can break down.



=== Electric shower head ===

As the name implies, an [[electric heating]] element is incorporated into such shower head to heat the water as it flows. Invented in Brazil in about 30s and widely used since the 40s, the electric shower is a home appliance very commonly used in South American countries due to the higher costs with gas canalization. At one time, an electric shower cost less than a hair dryer. Electric Showers work like a coffee maker, but with a larger water flow. When the water flows inside, the pressure inflates a diaphragm which closes the electrical contacts of the heater coil with the live contacts, turning on the device. Once the water is stopped, the device turns off automatically. An ordinary electric shower used to have three heat settings: low (2.5 kW), high (5.5 kW) or cold (0 W) to use when a central heater system is available or in hot seasons.



The power consumption of electric showers in the maximum heating level is about 5.5 kW for 120 V and 7.5 kW for 220 V. The lower costs with electric showers compared to the higher costs with boilers is due to the time of use: an electric shower uses energy only during the bath, while a boiler works many times a day to keep a larger quantity of water hot for use throughout the day. So electric showers can save energy compared to gas central heaters. A 20 minute bath by an electric shower can cost about US$0.10, but the same bath using water from a gas heater can cost three times as much. This difference can be larger where the electricity is cheaper than the gas supply or in tropical countries where the maximum power consupmtion is required only during the cold seasons.



There is a wide range of electric showers all with various amounts of heating controls. The heating element of an electric shower is made from a coil made of nickel or an alloy of nickel and chromium or can even be made of sheathed heater element, like the ones used in oil heaters, radiators or irons - they provide more safety as there is insulation between the electric parts and the water. Due to electrical safety Standards, modern electric showers are made of plastic instead of the metallic casings like in the past. As an electrical appliance which works with higher electrical currents than a washer or a dryer machine, the installation of electric showers needs careful planning and must be made directly from the electrical distribution box, with exclusive 6 mm wires, electric connectors for 50 A and a ground system. A poorly installed system with old aluminium wires or bad connections are very dangerous as the wires can overheat. Some changes in the electrical public distribution were important before a wide use of electric showers at first. Electric public transformers with higher KVA capacity are required due to the increase of the electrical demand. In Countries where almost all houses use electric showers like Brazil, an ordinary street transformer per square have 112.5 to 150 kVA of capacity and buildings must have their own transformers to support the electrical domestic demand without overloads in the electric distribution.



=== Various types and their advantages ===

Point-of-use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because no hot water is left in the pipes after the water is shut off. However, point-of-use tankless water heaters are usually used in combination with a central water heater since they are usually limited to under 6 litres/minute (1.5 U.S. gallons/minute), as the expense of buying a heater for every [[kitchen]], [[laundry room]], [[bathroom]], or [[sink]] can outweigh the money saved in water and energy bills. In addition, [[point of use]] water heaters until recently were almost always electrical, and [[electricity]] is often substantially more expensive than [[natural gas]] or [[propane]].



Tankless heaters can ideally be somewhat more efficient than storage water heaters. In both kinds of installation (centralized and POU) the absence of a tank saves [[energy]] as conventional water heaters have to reheat the water in the tank as it cools off, called standby loss. There is a misconception that the energy lost by a tanked heater stored inside a home merely helps to heat the home. This is true of an electric unit, but for a gas unit some of this wasted energy leaves through the exhaust vent. However, if the building needs to be cooled to maintain normal temperatures this results in a loss in efficiency. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the [[faucet]] and the water heater. This water waste can be avoided if a recirculating pump is installed, but at the cost of electricity to run the pump and wasted energy to heat the water circulation through the pipes.



Tankless water heaters can be divided into two categories: "full on/full off" and "modulated". Full on/full off units do not have a variable [[Power (physics)|power]] output level; the unit is either fully on or completely off. Modulated tankless water heaters base the heat output on the flow of water running through the unit. This is usually done through the use of a flow sensor, modulating gas valve, inlet water temperature sensor and an outlet water temperature sensor-choke valve and means that the occupants should receive the same output [[temperature]] of water at differing velocities, usually within a close range of ±2 °C.



The high-efficiency condensing combination boiler provides both space heating and water heating, an increasingly popular choice in UK houses. In fact, combination boilers now account for over half of all the new domestic boilers installed in Britain.



Under current North American conditions, the most cost effective configuration from an operating viewpoint is usually to use a central tankless water heater for most of the house, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house, and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When consumers are considering a whole house gas tankless unit, they are advised to look at how the unit functions when raising the water temperature by about 42 °C (75–77 °F). Thus, if they live in a cold weather climate, they are advised to look at the unit's capacity with 3-10 °C (38–50 °F) inlet water temperatures, and find a size that produces approximately 15 litres/minute (4 gpm) even in winter if they have a typical-sized house and desire what is called a 2-appliance heater. This same unit may produce 25-30 litres/minute (6.3–6.9 gpm) in summer with higher inlet temperatures, but there is greater interest in year round production and usability.



=== Advantages ===

There are certain advantages to tankless water heaters :



* '''Long term energy savings:''' Although a tankless water heater might cost more initially it may result in both energy and cost savings in the long term. As water is heated only when it is needed, there is no storage of hot water. With a tank, water is kept warm all day even if it never gets used and heat loss through the tank walls will result in a continual energy drain. Even in homes or buildings with a high demand for hot water, a tankless water heater may provide some level of savings. In a typical home these savings are quite substantial. If instant hot water at the taps at limited hours is a priority, a recirculation system similar to those in the tank-type systems can be accommodated by using an aquastat and timer in order to decrease the added heat loss from the recirculation system. It has to be said though that if the storage tank is highly-insulated—a few tanks are available with excellent levels such as 100 mm or more polyurethane foam—the savings become minimal. For one consumer-grade electric storage water heater, the surface temperature was less than 1 °C higher than the air temperature.

* '''Unlimited hot water:''' As water is heated while passing through the system an unlimited supply of hot water is available with a tankless water heater. Although flow rate will determine the amount of hot water that can be generated at one time it can be generated indefinitely. However, this can also be a disadvantage as running out of hot water self-limits use while a tankless heater has no such limit.

* '''Less physical space:''' Most tankless water heaters can be mounted on a wall or even internally in a building's structure. This means less physical space has to 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, although the risk of water damage from a pipe or fitting failure remains. Improper piping in either the hot or cold water lines to the tankless water heater can result in water damage though.

* '''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, like the Takagi TK3, TK3 PRO, TM32, and the TM50 stabilize water pressure and temperature by a bypass valve and a mixing valve which is incorporated in the unit. Modern Tankless are not inversely proportional, because they will regulate the amount of water that is created and discharged, therefore stabilizing water temperature by utilizing a flow control valve. Flow speed is not the issue, but change in temperature is the important issue to address. The wider the temperature rise, the less flow you receive from the unit. The smaller the temperature rise, the more flow you receive. The flow control valve in conjunction with thermistors, maintains a stable temperature throughout the use of the unit.

* '''Safety''' Tankless Water Heaters can precisely control the temperature of the treated water, which means dangerous temperature levels and spikes are no longer a problem.



=== Disadvantages ===

Tankless heaters also have several disadvantages:

* '''Start-up delay:''' There is a longer wait to obtain hot water. A tankless water heater only heats water upon demand, which is one of its chief advantages, so all idle water in the piping starts at room temperature. Thus there is 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 combi. 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 combi., but it uses considerably more fuel especially in summer.

* '''Intermittent-use:''' There is a short delay (1–3 seconds) between the time when the water begins flowing and when the heater's flow detector activates the heating elements or gas burner. In the case of continuous-use applications (showers, baths, washing machine) this is not an issue. However, for intermittent-use applications (for example when a hot water faucet is turned on and off repeatedly at a sink) this can result in periods of hot water, followed by some small amount of cold water as the heater activates, followed quickly by hot water again. The period between hot/cold/hot is the amount of water which has flowed though the heater before becoming active. This cold section of water takes some amount of time to reach the faucet and is dependent on the length of piping.

* '''Installation cost:''' 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 being replaced with a tankless one, the size of the [[electrical wire|electrical wiring]] or gas pipeline may have to be increased to handle the load and the existing vent pipe may have to be replaced, possibly adding expense to the retrofit installation. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 [[BTU]]s. For electrical installations (non-gas), [[American wire gauge|AWG]] 10 or 8 wire, corresponding to 10 or 6&nbsp;mm<sup>2</sup>, is required for most POU (point of use) heaters at North American voltages. Larger whole house electric units may require up to [[American wire gauge|AWG]] 2 wire. In gas appliances, both pressure and volume requirements must be met for optimum operation.

* '''Heat source flexibility''' Tankless heaters are ''sometimes'' limited to a choice between problematic energy sources: gas and electricity. This ''sometimes'' makes it difficult to include other heat sources, ''sometimes'' including certain renewable energy options. One exception is [[solar water heating]], which can be used in conjuncion with tankless water heaters. However, tank-type systems have a much wider choice of heat sources available, such as [[district heating]], [[central heating]], [[geothermal heating]], [[micro CHP]] and [[ground-coupled heat exchanger]]s.

* '''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 will 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 will initially react as a tanked heater does, but this also will change the flow rate of hot water. Therefore some finite time later the temperature will change again very slightly and require 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. 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.