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 most 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 output level; the unit is either on or 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[5] 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.[6]
In certain parts of South America as well as Costa Rica and Puerto Rico, a point of use style water heater commonly referred to as the "Electric Shower Head" is used in many residential and some commercial installations. As the name implies, an electric heating element is incorporated into such shower heads to heat the water. However, many of these units are often poorly installed, often with exposed wiring in wet locations.
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.
There are certain advantages to tankless water heaters :
- Long term energy savings: Although a tankless water heater might cost more initially it will result in both energy and cost savings in the long term. As water is heated only when it is needed there is no wasteful heating of water. With a tank water will be kept hot 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 will 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.
- 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.
- 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.
- No risk of water damage: No stored water means there is no risk of water damage from a tank failure or rupture. Improper piping in either the hot or cold water lines to the tankless water heater can result in water damage though.[citation needed]
- 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 mxing 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 delta T 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.
Tankless heaters also have several disadvantages:
- Installation cost: Installing a tankless system comes at an increased cost, particularly in retro-fit 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 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 BTUs. For electrical installations (non-gas), AWG 10 or 8 wire, corresponding to 10 or 6 mm², is required for most 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.
- Heat source flexibility Tankless heaters are limited to a choice between expensive and CO2 problematic energy sources: gas and electricity. This makes it impossible to include other heat sources, including renewable energy. Tank-type systems have a much wider choice of heat sources available, such as district heating, central heating, solar heating, geothermal heating, micro CHP and ground-coupled heat exchangers.
- Start-up delay: There is a longer wait to obtain hot water. A tankless water heater only heats water upon demand, 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.
- Intermittent use: There is a short delay 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) this can result in periods of hot water, then 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.
- 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.
- 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 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 it 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-amperage 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.
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