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Hot Water Tank

Hot Water Tank

Tankless water heaters—also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters—are gaining in popularity. These high-power 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 may be installed throughout a household at more than one point-of-use (POU), far from a central water heater, or larger centralized 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 plentiful continuous flow of hot water (as compared to a limited flow of continuously heated hot water from conventional tank water heaters), and potential energy savings under some conditions.

How tankless water heaters work
When there is demand for hot water (someone opens a hot water tap), the tankless water heater's water flow turbine senses the flow and starts heating the water as it passes through. The control board that monitors the water flow turbine looks at multiple factors: incoming water temperature, desired water temperature as set on the temperature controller, and the calculated difference between the two. Depending on the calculated incoming and desired water temperatures, the controller modulates the gas or electric flow into the burner assembly. The heater brings water 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 off and returns to standby mode until the next call for hot water. Your home inspector can review the function and advantages as well as how to install a tankless on demand hot water system during your home or commercial inspection.

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 9 to 23 litres (2.4 to 6.1 US gal) 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 usually less than from a storage cylinder, particularly in winter. The power rating must match 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 and it is recommended that a minimum of 3/4 inch water supply is available. Some have proposed that a flow regulator valve can control the amount of water used. Additionally, a combination boiler has more moving parts that can break down, so can be less reliable than a tank system.

Point-of-use (POU) tankless types

Point of Use (POU) tankless water heaters are located immediately where the water is being used, so the water is almost instantly hot, which reduces water wastage. POU tankless heaters also can save more energy than centrally installed tankless water heaters, because no hot water is left in lengthy supply pipes after the flow is shut off. However, POU tankless water heaters are often installed in combination with a central water heater, since the former type have usually been limited to under 6 litres/minute (1.5 US gallons/minute), which is sufficient for only light usage. In many situations, the initial expense of buying and installing a separate POU heater for every kitchen, laundry room, bathroom, and sink can outweigh the money saved in water and energy bills. In the US, POU water heaters until recently were almost always electrical and electricity is often substantially more expensive than natural gas or propane (when the latter fuels are available).
In recent years, higher-capacity tankless heaters have become more widely available, but their feasibility may still be limited by the infrastructure's ability to furnish energy (maximum electrical amperage or gas flow rate) fast enough to meet peak hot water demand. In the past, tank-type water heaters have been used to compensate for lower energy delivery capacities, and they are still useful when the energy infrastructure may have a limited capacity, often reflected in peak demand energy surcharges.
In theory, tankless heaters can always be somewhat more efficient than storage tank water heaters. In both kinds of installation (centralized and POU), the absence of a tank saves energy compared to conventional tank-type water heaters, which have to reheat the water in the tank as it cools off while waiting for use (this is called "standby loss"). In some installations, the energy lost by a tanked heater located inside a building merely helps to heat the occupied space. This is true for an electric unit, but for a gas unit some of this lost energy leaves through the exhaust vent. However, if at any time the building must be cooled to maintain comfortable temperatures, the heat lost from a hot water tank located in the conditioned space must be removed by the air conditioning system, thus requiring larger cooling capacity and energy usage.
With a central water heater of any type, any cold water standing in the pipes between the heater and the point-of-use is dumped down the drain as hot water travels from the heater. This water wastage can be avoided if a recirculation pump is installed, but at the cost of the energy to run the pump, plus the energy to reheat the water recirculated through the pipes. Some recirculating systems reduce standby loss by operating only at select times—turning off late at night, for example. This saves energy at the expense of greater system complexity.

Tankless heater controls

Electric point-of-use (POU) tankless water heater, wall-mounted under a sink (Germany)
Tankless water heaters can be further divided into two categories according to their heating capability: "full on/full off" versus "modulated". Full on/full off units do not have a variable power output level; the unit is either fully on or completely off. This can cause an annoying and possibly hazardous variation of hot water temperature, as the flow of water through the heater varies. Modulated tankless water heaters change their heat output in response to the flow rate of water running through the unit. This is usually done by using a flow sensor, a modulating gas valve, an inlet water temperature sensor, and an outlet water temperature sensor-choke valve. A properly-configured modulating heater can supply the same output temperature of water at differing water flow rates within their rated capacity, usually maintaining a close range of ±2°C.
A high-efficiency condensing combination boiler provides both space heating and water heating, and is 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 often is to install a central (tank-type or tankless) water heater for most of the house, and to install a POU tankless water heater at any distant faucets or bathrooms. However, the most economic design may vary according to the relative electricity, gas and water prices in the locality, the layout of the building, and how much (and when) hot water is used. Only electric tankless water heaters were widely available for many years. and they are still used for low-initial-cost POU heaters, but natural gas and propane POU heaters have now become available for consideration.
In British Columbia on demand natural gas heaters are becoming more and more common and AAA Commercial and Residential Inspections has tested out these systems in their own homes with great success. In particular, on demand condensing units with 94% efficiency have tested long-term with greatly reduced hot water heating costs. In addition, there is currently a $500 rebate from Fortis Gas until June 2013.


Tankless advantages

Tankless water heaters provide many advantages:

Mobile water heater in a train car (Russia)
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.
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.[1

Tankless disadvantages

Gas tankless water heater (Taiwan)
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 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. Most electrical installations 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 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. In tests conducted in British Columbia by AAA Commercial and Residential Inspections in their own homes this was not considered a significant problem.

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. Again, in actual tests conducted in British Columbia by AAA Commercial and Residential Inspections in their own homes this was not considered a significant problem.

Heat source flexibility Tankless heaters use either gas and electricity for heat. Their design may not conveniently allow other heat sources, including certain renewable energy options. One exception is solar hot water heating which can be used in conjuncion with tankless water heaters. Tank-type systems can use many heat sources such as geothermal heating.
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. However, the newer tankless systems available appear to have alleviated this issue. Talk to your home inspector about this is more detail.

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. However, the tankless units available to Canadian consumer now have super accurate digital temperature controls and this does not appear to be an issue. Talk to your house inspector about this for more detail.

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. Generally water must be supplied to a centralized tankless set-up with a 3/4 inch supply line.

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. However for the typical home in British Columbia with a natural gas fired tankless unit this should not be an issue. Your home inspector can review this with you in detail.

Credit: Wikipedia with additional comments from Jonathan Newman, AAA Residential and Commercial Inspections, home and commercial inspectors