Tankless Water Heaters
Tankless water heaters are also known as instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters. These units are gaining in popularity due to their efficiency, reliability and safety. These water heaters heat the water as it flows through the unit. No water is retained internally to keep hot for the next use. Tankless water heaters are often installed throughout a household at the point-of-use (POU) far from the central water heater. Alternatively, a larger model can 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 workWhen there is a demand for hot water (such as a hot water tap is opened) the tankless water heater senses the flow and starts the heating process. The water flow sensor sends a signal to the control board which looks at the following factors: incoming water temperature, desired water temperature, and the difference between the two temperatures. Depending on the incoming and desired water temperatures the heating element is heated to an appropriate level to raise the in coming water temperature to the desired temperature. 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. Tankless water heaters can more efficient than storage water heaters. In both centralized and point-of-use installations 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. 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 fully on or completely off. Modulated tankless water heaters base the heat output on the flow of water running through the unit and means that the occupants should receive the same output temperature of water at differing flow rates, typically within a close range of roughly 2°C. 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. In the beginning only electric tankless water heaters were available and are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When you are considering a whole house gas tankless unit, it is are advised to look at how the unit functions when raising the water temperature by about 75–77 °F (42 °C). If you live in a cold weather climate, you are advised to look at the unit's capacity with 38–50 °F (3-10°C) inlet water temperatures, and find a size that produces approximately 4 gallons per minute (gpm) even in winter.
AdvantagesHere are a few advantages to going tankless:
- 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 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 use. In a typical home the resulting savings are quite substantial. We would be remiss is we didn't point out that if the storage tank is highly-insulated the savings become reduced.
- 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 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 which is incorporated in the unit. 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.
DisadvantagesLike anything in life there are also disadvantages to the tankless way:
- 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).
- 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 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 sometimes limited to a choice between CO2 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 conjunction 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 exchangers.
- 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.
Before buying a Tankless Water HeaterBefore buying a Tankless Water Heater, consider the following:
- Type of Heat Source
- Incoming Water Temperature
You will first have to decide how you are going to power your a Tankless Water Heater. You will need to choose whether you will want an electric, a natural gas or a propane powered water heater. For an electric water heater you will need to consider the following:2. Location
Consult with a qualified, licensed electrician to answer the questions listed above. For a gas-fired tankless water heater you have the following things to consider: the gas-type, supply line and venting requirements. You will first need to identify whether your gas type is Natural Gas or Propane. Next you must examine your current gas line to ensure that it will meet the requirements of your new gas-fired tankless water heater. The requirements of the new heater may exceed that of your existing tank-style heater. Then you will need to determine the venting requirements for your specific installation. There are a few important things to keep in mind when purchasing the gas venting accessories for your gas-fired tankless water heater.
- Voltage - What voltage is your service? Units are available to accommodate 110V, 120V, 208V, 220V, 240V, and 277V.
- Amperage - How much current capacity do you have? You must make sure your electrical service can provide the current necessary to run the water heater.
- Circuit Breaker - Can your current circuit breaker accommodate the new circuit for your water heater?
- Be sure that you purchase Category III stainless steel (UL1738 certified) venting for your gas-fired tankless water heater. "Type B" venting accessories are not acceptable. Also, be sure to check local building code to ensure that your specific needs will be completely met.
- Additionally, many Tankless Water Heater manufacturers offer gas venting "kits". It is recommended that you evaluate the needs of your specific installation to ensure that they will be getting all of the necessary gas venting accessories. Depending on where you will be installing the tankless water heater, a pre-made kit will probably not meet your needs. Ensure that you measure out the vent route and consider where the discharge will go through the wall or ceiling, consider the necessary clearances, and consider ample access to air for combustion, then buy the appropriate gas venting pieces.
Where you will be installing the unit needs to be considered. You want to make sure the unit will fit in its intended location. Can you get the electrical or gas supply required to the installation location or will work need to be done to accommodate the new heater.3. Demand
It is important to recognize the number of fixtures that will require hot water. Each fixture will have its own demands. The chart below illustrates the typical flow rates (demand) for some standard fixtures:4. Incoming Water TemperatureCommon Flow Rates in gpm
The flow rate is especially important, since tankless water heaters will generate a temperature rise based on the flow rate demanded. For example, consider a unit that will raise the water temperature by 50°F at 1.5 gpm, 30°F at 2.25 gpm and 25°F at 3.0 gpm above the incoming water temperature. Whereas a larger unit can raise the water temperature by 90°F at 1.5 gpm, 90°F at 2.25 gpm, and 80°F at 3.0 gpm, above the incoming water temperature. This means that if you are using a 1.5 gpm shower and a 1.5 gpm kitchen sink simultaneously, a total demand of 3.0 gpm, the first example will raise the temperature 25°F, whereas the second will raise the temperature 80°F.
Type of Fixture Lavatory Bathtub Shower Kitchen Sink Pastry Sink Laundry Sink Dish-washer Flow Rates 0.5 2.0 – 4.0 1.5 – 3.0 1.0 – 1.5 1.5 – 2.5 2.5 – 3.0 1.0 – 3.0
You must consider your incoming water temperature. If you live in a cold climate, like Vermont, your incoming water temperature will likely be much lower than if you live in a warm climate, like Texas. Your best bet is to find out how much temperature rise you will need in order for your hot water to reach the desired heat. If the incoming water temperature for your shower is 65°F, you are using a 2.0 gpm shower, and you want to raise that temperature to 115°F, you will want to look for a tankless water heater that will provide at least a 50°F temperature rise at 2.0 gpm (115°F - 65°F = 50°F). However, if you anticipate additional simultaneous demand, such as the hot water from a sink being used while someone is showering, you will need to add the sink's gpm to the shower's gpm in order to determine your overall gpm demand and then find the temperature rise necessary to meet your overall needs.5. Application
You may have a specific application in mind for your tankless water heater. Here are a few examples: A single point application is one where only one fixture will require an electric tankless water heater. You need to supply two fixtures, like two sinks, in close proximity. You need a booster for temperature loss from long pipe runs, dishwashers and sanitation. Thermostatic units are also good for applications where precise temperature control is essential; such as schools, hospitals and laboratories. You want to supply a residential shower, and entire bathroom, a smaller houses, a condo, a summer cabin or an apartment. You need to accommodate an industrial booster, a higher flow rate application such as a wash down station or a higher flow rate accurate temperature control application such as a photo lab. You want to supply your whole house. Larger whole house units are designed to serve an entire house, a large apartment, a condo or a cabin where multiple points of use will exist.