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.
Small 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).
Advantages
There are certainly 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.)
- 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 compensationA 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 Navien 7, Noritz, and Takagi TK3 tankless water heaters, all stabilize water pressure and temperature by a bypass valve and a mxing valve which is incorporated in the units.
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 flexibilityTankless heaters can be powered by gas, propane, and electricity. 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. Many models sold in the UK have introduced a small heat store within the combi. to address this problem. 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 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. 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.
Tankless or wall mounted water heaters are great for some people/ families and not for others.
Their are many things to consider! Talk to a trustworthy plumber near you.
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