Let’s look at some of the pros and cons of heat pump water heaters.

First, some advantages…

• As mentioned in the previous article about heat pump water heaters, fuel efficiency: these units have lower operating costs, using only 33-50% of the electricity that other electric water heaters use.
• If you live in a warmer climate and use electricity to heat water, a heat pump may be your best choice for efficient energy use.
• Cool exhaust air can be exhausted into the room–possibly helping to cool the room in summer–or to the outdoors.
• The life expectancy is 20 years. Of course, as with most appliances, proper installation and maintenance can extend the life expectancy significantly, and can reduce losses in efficiency.

Now, some disadvantages…

• Heat pump water heaters are most effective in mild climates—a 40-90-degree temperature range year-round is needed for effective use; they will not function efficiently in a cold space.
• They have higher initial costs—purchase and installation–than conventional storage water heaters.
• If the heat pump is not in a warm area from which to pull heat, the recovery rate can be modest, variable, and relatively low.
• This makes use of a storage system, so there will be stand-by heat losses.
• If the water pump is installed in a heated room, it will steal some of the heat, thus heating that room becomes less efficient.
• These are rather complex to install, especially if the heat for the unit is brought in from a remote site or if a separate tank is used. It is probably best to hire a contractor.
• Heat pump water heaters are not widely available, possibly because of lack of demand due to initial cost. Also, lack of contractor and consumer awareness of this option has led a number of manufacturers to discontinue their lines of heat pump water heaters. (Sounds like a marketing problem!)

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The most commonly known use of heat pumps is to heat and cool homes. But they can also be used to heat water, either in combination with a home heating/cooling system or as a stand-alone water-heating system. Because this type of water heater does not generate heat, but instead moves it from another source, it is more efficient than most other types of water heaters, using 33-50% of the energy required for a conventional electric storage tank water heater.

A stand-alone heat pump water heating system can be purchased in two different ways: either with a built-in water storage tank (an integrated unit) or as an add-on to an existing water heater tank. The heat-pump, which has a life expectancy of up to 20 years, may outlast the tank, so the add-on may be the wiser choice, if it is an option.

Heat pump water heaters, for which electricity is the only fuel choice, use a compressor and a refrigerant fluid to transfer heat from air in the general vicinity of the heat pump to heat the water. They work better with warmer air, since they don’t have to work as hard to extract heat from the air source. Some better models can bring in warm air from an attic or from outdoors. For highest operating efficiency, they should be installed in a high-heat room, such as a furnace room. There are some geothermal models that take heat from the ground in winter and from indoor air in the summer. According to another source, this type is good for most locations, but an air heat pump is good only for warm climates with winters that are quite mild.

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Stand-by heat-losses associated with conventional storage water heaters are eliminated with demand (or instantaneous) water heaters, which heat the water as it is needed.

With a demand water heater, when a hot-water tap is turned on and water begins to flow, a heat exchanger is activated. The water runs into the water heater unit, passing through the heat exchanger, where it is heated by an electric element or a gas or propane burner, and is then sent to the tap.

This type of water heater generally has a higher purchase price than a conventional storage water heater, but lower operating and energy costs which could offset the purchase price¹.

Demand hot water heaters perform better when you take steps to use water efficiently, such as installing modern low-flow faucets and showerheads. Also, hot water locations in your home should be close together, and close to the demand water heater; if they are spread out, multiple heaters may be called for, thus raising initial costs.

For a slightly different slant, rather than being your primary source of hot water, a demand water heater can be used as a booster for washing machines, dishwashers, solar hot water systems, or wood-fired hot water systems².

We’ll learn more in future posts about advantages and disadvantages of demand water heaters.

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This is the most common type of residential water heater used in the United States today. It provides a ready tankful—20 to 80 gallons, normally—of hot water, which is drawn from the top of the tank when a hot water tap is turned on. Cold water enters the bottom of the tank to replace the hot water that has been drawn off, so that the tank is always full (although not always of hot water!)

Fuel sources for this type of water heater include natural gas, propane, fuel oil, electricity, and solar energy.

Some energy is necessarily wasted with this type of water heater since the water in the tank is constantly heated, even when there is no hot-water demand. This is known as stand-by heat loss. Newer models have less stand-by heat loss and, therefore, lower operating costs because they are better insulated than older tanks. If you are shopping for a new storage water heater, look for one with an R-Value (thermal resistance value) of not less than R-12. Some have an R-Value as high as R-25.The better-insulated tanks have a higher purchase price, but the price can be quickly recouped, depending on your specific use-patterns and energy costs. To help determine your most energy-/cost-efficient choice, check the Energy Guide label (which is required on storage water heaters) for either the annual estimated system-operating cost or for energy efficiency ratings.

If you currently have an older tank and want to increase energy efficiency, you can add an insulating blanket to the outside of the tank to reduce heat loss. The blanket must be the properly sized for your specific tank and not block safety or drainage valves, controls, or airflow. Adding an unnecessary insulation blanket, particularly in very humid locales, can cause condensation problems, possibly leading to mold or rust.

According to one source, (which I believe is not from the United States, so I’m not sure this is an option in the U.S.), efficiency can be increased if you have an intelligent electrical power distribution, heating the water when the power demand is low and turning off when it is high. “This could be implemented by allowing the power supplier to send load-shedding requests, or by the use of real-time energy pricing.”

Keep in mind that smaller tanks have less surface area, and therefore, less heat-loss, which can be 10-15 percent of the water heater’s operating cost, although, as indicated above, the more insulation, the less the heat loss.

Also, if the storage water heater is located in the home’s heated/cooled space, heat loss from the water heater can actually contribute to heating the home. However, in the months when the home is cooled, it also contributes heat, thus creating a larger cooling load. Often, this type of water heater is located in non-heated/non-cooled areas of the home, and therefore does not affect the temperature inside the home, but instead simply loses the heat to the outside air.

Also keep in mind that the space you have—where the water heater will actually be installed–may limit your choices. However, storage tanks come in a number of sizes and proportions, thus accommodating a fairly wide variety of space considerations.

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