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TANKLESS Tankless Water Heater Product Guide Tankless Water Heater Applications Tankless Water Heater Electrical Guide Tankless Water Heater Installation Tankless Water Heater Cost Comparisons Tankless Water Heaters Service Guide
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THE CURRENT DEFIENCIES OF OTHER TANKLESS WATER HEATERS Temperature Control Improves Slightly Redesign efforts in the ‘70’s introduced new technology for tankless water heaters that attempted to better control the power to the heating elements for electric tankless water heaters. These controls used temperature comparisons for determining the required power level, instead of the old and slow thermostat. The heating elements would be turned "on" or "off" depending upon the difference in the hot water temperature measured at different locations within the water heater, using the outlet temperature as the principal controlling reference-temperature. The purpose of these control schemes was to provide a more rapid response to turning on, turning off or varying the power input to the heating elements. These control schemes were essentially based on the same type control theory that is utilized in electric ovens. As the temperature increases, the power is reduced or cycled in order to attempt to maintain a fixed water temperature. This type control works well when the flow rate is constant and the speed of heating is relatively slow i.e. the single fixture tankless water heater heats water at a rate slow enough, by design, to avoid over-temperature and then maintained at a steady flow rate. The whole house tankless water heaters needed a great deal more. The continuing current deficiencies experienced in most electric tankless heaters today are outlined in the subsequent passages. Inadequate Control Technology–The Dynamics of a Whole House Applications Require Tankless Water Heaters with Highly Responsive, Power and Temperature Control Water heaters or ovens typically use commodity type temperature sensors. These sensors for temperature measurement do not provide an immediate indication of the full change in hot water temperature, but rather take seconds to provide a reliable measurement of the total temperature change. This means that the control is always sensing the full temperature change seconds after it has been experienced. Tankless water heater control systems respond to hot water temperature change, i.e., water temperature dropping, by adding additional power. In other high wattage tankless water heaters, when power is applied the water is overheated before the control senses it. This occurs because of the inherent delay in sensing the change in water temperature. When the temperature sensor senses overheating, the reverse occurs with the control turning off the power and again for too long. The water cools below the desired temperature before the sensors indicate the change to the control system. These conditions will continue with the hot water temperature continuing to oscillate, first in large temperature swings, then reducing to ever smaller swings, assuming the water flow has now stabilized, until the temperature stabilizes at the desired water temperature. This settling out process takes, in best-case scenarios, about one minute with the older tankless water heater control technology and is characteristic in what we experienced in classic "proportional integral control", (PID). These temperature fluctuation conditions re-occur when hot water flow rates are just modestly changed. One can imagine what happens in the scenario described. When one has set the shower temperature, and then someone else turns a hot water faucet on or off, –it’s not a pleasant experience. The problem that exists in the reality of the application is very basic. When someone is showering, using the older tankless water heater technology, there is nothing smooth about the temperature changes resulting from changes in water flow. As mentioned earlier, tankless heaters of the past have worked best when they have a constant flow of water passing through them. Water heating for domestic use, however, is relatively dynamic. Rarely can one depend on a fixed or constant rate of water flow. For example when most of us prepare to take a shower, we turn the hot water on full blast, to speed up the delivery of hot water to the fixture. Then, as the hot water reaches the fixture, we rapidly reduce the hot water flow and adjust the hot to cold-water ratio, in order to establish a comfortable steady-state temperature. While we are in the shower, it’s not uncommon for someone else to turn on hot water for other use. This results in a higher flow and greater hot water requirement. From a fixed temperature source, like the tank type water heater, the temperature of the water delivered to the faucet declines slowly during use. There are, however, no rapid fluctuations in temperature except from a pressure drop resulting in flow changes in the ratio of either the hot or cold water of the tempered water supply. With the use of a storage-tank water heater, one can quickly adjust the fixture to smoothly compensate for temperature changes resulting from flow changes. Again this is because, with a tank hot water heater, we are using a supply of hot water with a fixed temperature, albeit not constant and not trying to control its instantaneous heating. Without going into a dissertation of advanced PID control theory, it is sufficient to note that, when applied to the dynamics of the applications for the domestic water heater, the classic "PID" control schemes, most commonly used, inherently produce the same unacceptable results. One might argue that with the use of flow measurement devices and faster response temperature sensors, these problems could be relatively easily overcome. True, but for two problems. The first is that the typical flow measurement devices, which provide immediate indication of changes in flow rate, such as turbines, are installed in the water flow of the heater. As anyone knows, water is a very aggressive environment leading to reliability and maintenance issues from the use of such devices and hot water is more agressive. The second and equally important issue is cost. These type of devices add unacceptable expense to the tankless water heater which is already cost sensitive. The same cost issue exists with rapid response temperature sensors. Finally, the classic methods of control used in "PID" are simply not adequate, in such dynamic applications, to maintain the desired temperature control. High Mineral Deposit Accumulation Caused by Inherent Over-boiling in Older Tankless Water Heater Technology. Further complicating the difficulty of producing an acceptable tankless electric water heater are the scaling and minerals issues. All typical electric tankless water heaters used today contain and control one or more heating elements. The elements of these water heaters produce a high heat output within a heating chamber(s) which only contain a very small quantity of stored water. In the case of the single element "point of use" tankless water heater, the one element is always turned on to a preset full power level. Tankless water heaters with multiple heating elements turn on at least one of the elements to full power, regardless of the control schemes ability to modulate the level of power. The individual wattage (heating capacity) of the single element will require at least the first element to turn on full, just to heat the minimum flow of hot water. As the flow increases, additional elements are turned on, partially or fully, as required to heat the water. When the flow of hot water stops, the electric tankless water heater is turned off. In most designs, there is a very small amount of water, partially heated, with which to absorb the remaining heat (called latent heat) from the heating element. This latent heat saturates the water, resulting in the hot water, within the heating chamber being overheated, (over-boiled at shut down), to temperatures reaching 160° - 180° F. These high hot water temperatures cause the minerals in the water to be constantly boiled out. These deposits are then discharged, with new flow, into the showerheads or appliance screens, clogging them up, and creating a maintenance nightmare. Considering the estimated 40 or more times a day that a family turns on and off a hot water fixture, the cumulative amount of mineral residue becomes significant.
These very conditions are exacerbated by the existing design and control schemes utilized by tankless water heater technology, other than the SEISCO. Small "point of use" tankless water heaters generally are quite susceptible to overboiling as they have less than a pint of hot water contained in the heating chamber. Larger similar tankless water heaters with multiple elements have similar small volumes of water within each individual heating chamber. In order to develop a tankless water heaters, which have sufficient power to heat flowing water, to satisfy the normal hot water requirements for a household, one needs sufficient heating capacity (power). Therefore, this inherent "over-boil" design issue is further complicated as one increases the wattage (heating capacity) of the elements to provide sufficient power for whole house use but not the hot water volume. Power Quality Problems from Electric Tankless Water Heaters When one is controlling 28,000 watts of energy and modulating power from multiple heating elements with very high wattage, it is no wonder that power quality issues will result. Modulating even two heating elements of 5,000 watts or a total 10 kW can have the same effect on lighting as if you were standing at your air-conditioning thermostat flipping the switch on and off rapidly. Except for the SEISCO, no other high wattage electric tankless water heaters have addressed this problem successfully, Unreliable, High-Maintenance, Difficult to Service Non-Modular Tankless Water Heaters---The Throw Away!
The very problems discussed above have given rise to the characteristically high maintenance requirements for the typical electric tankless water heaters. High temperature over-boiling results in excessive scaling and mineral deposit accumulation in the heating chamber and on the elements. As a result of the type of sequential power control scheme that is used, multi-chamber heaters work the first two elements disproportionally more than the other remaining elements. Both these conditions cause early failures of the heating elements. In addition, as previously discussed, most typical electric tankless water heaters use mechanical flow switches, which are exposed to the water. These flow switches are susceptible to failure in a relatively short period of time.
The heating elements in a very large percentage of these tankless water heaters are not the standard "screw-in" type of water heating elements. Often, the consumer must buy a special element and sometimes even replace the entire heating chamber in order to repair his water heater. This is particularly true with most point of use type electric tankless water heaters. Finally, some electric tankless water heaters can be activated when there is no water in the water heater itself. Such activation will destroy the heating elements and can seriously damage the water heater itself. In general then, most problems, for other existing electric tankless water heater technology result from early failure of the water heater elements, and excessive mineral deposits which in part are purged from the water heater accumulating in faucet aerators, and appliance screens. The users of such systems either have to install filters downstream of the water heater or constantly clean their aerators and appliance screens. |
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