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www.fueloilnews.com | FUEL OIL NEWS | AUGUST 2016 43 not only to supply the 'quantity' of water required, but also to 'quantify' the amount of hot water at the desired temperature. Stacking is common in all hot water heaters including an internal or external tankless. Stacking occurs whenever a hot water heater develops temperature and the domestic water is not being drawn. This will normally be caused by many quick or short draws of hot water. Most con- trols and heaters in use are not designed to accommodate this. It is important to iden- tify and respect this factor since it will lead to higher temperatures during an initial draw from the heater. One of the areas we have noted is the maximum temperature allowed under the law, but does this take in the initial stacking spike? Boiler-powered heaters are a major con- cern. These units may produce very high domestic water temperatures since the boiler may be required to operate at temperatures much higher than hot water demands. A hot water heating system designed to produce and supply 180° F water to the radiation will also supply 180° F water to the heat exchanger in use. Steam boilers can and may produce water/vapor temperatures on the heat exchanger in use as high as 240° F dur- ing peak winter operation. Direct-fired hot water heaters are not the perfect solution, since operating con- trols are also subject to stacking, and the controls can be easily changed by any- one, including the end user. In fact, many states dictate that a tempering or mixing valve be used on any DHW producer. Simple and inexpensive methods can be used to prevent scalding and in the following sections, we will offer you time- proven and industry-accepted methods. TANKLESS HEATERS With tankless heaters the following is advised: Install a good quality mixing or anti- scald valve and trap the valve 8" to 12" below the heater. See Figure 1. Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger coil. When an external tankless is used with a circulator, always use a flow-check valve to eliminate gravity flow through the tankless during heat cycles. See Figure 2. NEVER restrict the flow of cold water to a mixing valve. TANKLESS & STORAGE TANK Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger storage tank (booster) and a larger coil. Operate the tank aquastat at a lower temperature. Create a heat trap or install a flow- check valve in the supply line from the tankless to the storage tank. See Figure 3. Install a venturi or monoflo tee, Figure 4, to create an even temperature exiting the tank, Figure 5. INDIRECT HEATERS To try to reduce the possibility of scald- ing with indirects the following steps are recommended: Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger indirect tank, or at least an increase in the coil size. Operate the indirect at a lower tem- perature. Create a heat trap or install a flow- check valve in the supply line from the boiler to the indirect. Install a venturi or 'monoflo' tee to create an even temperature exiting the tank. DIRECT-FIRED WATER HEATERS Even with direct-fired heaters. where there is a reduced risk of scalding since the operating control is not affected by the presence of higher temperature boiler water, precautions should be taken to reduce risk of scalding. Here are some recommendations: Operate the heater at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger water heater. Lower the nozzle size on oil, and the gas flowrate on gas, if possible. If a longer burner run can be created, it will reduce the effects of stacking and Figure 1 Figure 2