DHW production for the new millennium

For as long as I can remember, domestic hot water (DHW) production in the U.S. has mainly been handled by a tank-style device. Commonly, the tank-style heater is directly fired with a fossil fuel, or heated with an electric resistance element. If the home or building has a boiler, an indirect-style tank can be utilized. You may, on occasion, come across a wood-fired DHW tank.

Why a tank? One advantage of a tank-type heater is an abundance of DHW always at the ready. Additionally, tank construction is fairly straightforward and easy to manufacture in large quantities. Tank-style heaters account for millions of sales for the new and replacement market. Tanks come in a large variety of sizes, including stubby versions and under-the-counter options. For these reasons, and the affordable price, the tank-style will continue to be a top choice for plumbers and consumers.

Tank-style water heaters are simple to install, easy to service and are expected to last 10 years or more. Energy standards are mandating higher efficiencies, so better burner and control technology, as well as an increased insulation thickness, are now standard equipment. Tanks may be glass-lined, stainless steel, cement or stone-lined; and years ago copper tanks were built. Plastic, or composites are also being used to fabricate tanks.

Tanks can be duel-fueled also. A solar tank will often have a heat exchanger, internal or external, and also an electric element for back up. Hybrid versions of solar tanks can have a gas or LP-fired burner instead of the electric. Tank-style water heaters can be vented into a conventional flue, sidewall-vented or power-vented.  This allows the fuel-fired tank to be placed almost anywhere in the buildings, within code compliance, of course.

Yet another option is a dual-coil, indirectly fired tank. One coil input could be solar, and the second, upper coil could be fed from a boiler, wood-fired appliance or heat pump, for instance.

As we become more of a global economy, we will start to be introduced to other options for DHW production. This is driven primarily by the higher energy costs in countries around the globe. Home and building owners will give up some of the dump volume for a smaller, tankless heater, that provides a continuous supply of DHW with little, if any, storage. An additional feature is a smaller footprint and choices of venting options.

One of the earliest tankless heaters I remember seeing was the Paloma brand. The history of this company dates to 1911, and LP heaters became readily available around 1954 in Japan. Around 1973, Paloma Industries was established in California to service the U.S. market. They are still in production today, along with dozens of other great brands.  

Properly sized, installed and maintained, a tankless-style water heater can be an excellent choice. Owners need to be made aware of the difference in use and some of the limitations. Service becomes important in hard water conditions to assure adequate DHW production and equipment life. Over the years, the tankless heaters have been evolving. Thanks to microprocessor controls and condensing technology, the tankless units have become more user-friendly and efficient. Some of the inconveniences have been engineered out, and small storage tanks have been added to prevent the “cold sandwich” effect, that is, when water alternates between hot, then cold, then hot. Tankless sales have been growing, and the public is becoming more comfortable with the tankless lifestyle.

Here are a few other technologies to consider: heeat pump water heaters are available in several styles. This is a tank-style heater with a heat pump as the energy converter. The intent is to use the efficiency gain provided by the heat pump cycle, as opposed to straight electric resistance. Pay attention to the intended application in regards to locating the equipment in the building and recovery ratings. The heat pump can be integrated, or a heat pump module added, to a tank of your choice. 

If you have a heat pump for heating and cooling your home or building, you may have the option to add a desuperheater heat exchanger. When the heat pump is in cooling mode, the heat removed from the space can be transferred into the DHW storage. Large commercial kitchens refrigeration units often use desuperheaters to transfer energy into their hot water storage tank that would be normally be dumped outside or into the building space.

Another concept that is coming back is Photovoltaic Thermal (PVT). In these systems, the output from a PV module is directed to a resistance element in a tank. Typically, a DC element is used to direct connect to the module. One selling feature is that a directly wired PV to tank system may not require permitting to install, if it is not connected to the grid. Certainly, an electrician should be involved for proper wiring and code compliance. With PV module prices still dropping, this may “pencil out” in some areas with high kWh rates.

With large utilities starting to reconsider grid-tie payback, another clever product just hitting the market allows excessive PV output to be turned into thermal energy with a resistance element in a tank or pump module. If the utility refuses to pay any or a reasonable rate for your excess, a control monitors the point of excess generation and diverts it to the element in the tank or pump module. This too could be used for DHW production, or a hydronic system assist. I am a huge fan of a solar component SDHW solar domestic hot water; now you have choices for thermal or electric SDHW.

So, there you have some of the options. Determine which one (or several?) best fits you or your customer’s needs, plumbers — protecting the health and keeping the nation in hot water. 

Bob “Hot Rod” Rohr has been a plumbing, radiant heat and solar contractor and installer for 30 years. Rohr is a longtime RPA member and Plumbing Engineer and PHC News columnist. Bob joined Caleffi North America as manager of training and education. He can be reached at hotrodradiant@mac.com.

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