We previously learned about some of the advantages of demand water heaters. Now let’s look at some disadvantages.

Disadvantages: a limited flow rate, which can cause difficulty keeping up with large demand; initial cost is a bit more than storage tank water heaters.

The typical flow rate with a demand water heater is two to five gallons per minute. If your household does not use hot water at more than one location at a time, this might be sufficient, but if someone is going to take a shower while the dishwasher is running, the water heater is not going to be able to keep up.

Gas tends to have a better flow rate than electric, however there is a constant, albeit small, energy loss with gas because of the energy consumed by the pilot light.

One way to combat this flow rate problem is to install demand water heaters in parallel sequence for simultaneous hot water demands. Or multiple demand hot water heaters can be installed.

At best, some manufacturers claim their product can perform as well as a 40-gallon storage tank water heater.

As stated in a previous article, the initial cost of a demand water heater is higher than that of a conventional storage tank type, but the future savings on monthly utility bills could make up for that.

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Like all water heaters, demand water heaters have advantages and disadvantages. For now, we’ll consider some of the advantages.

Advantages: low standby losses, as stated above; low operation costs–a possible 10-50% decrease in energy consumption for heating water; hot water is always available; long life expectancy; good for do-it-yourselfers; no tank to maintain or leak in the future.

Concerning energy savings, for homes that daily use no more than approximately 40 gallons of hot water, demand water heaters can be 24-34% more energy efficient than conventional storage tank water heaters; for 86+ gallons-a-day households, 8-14% more energy efficient; up to 27-50% more energy efficient if a demand water heater is installed at each hot water outlet.

Hot water is always available as long as your water supply lasts and the energy to heat it is available, though the low flow rate may take away some of the appeal of this. This will be discussed in a later article on the disadvantages of demand water heaters.

The life expectancy of a demand water heater is 20+ years, as compared to 10-15 years for a conventional storage tank water heater. These 20 years can be extended for many more years by having easily replaceable parts and by periodic maintenance, which also helps maintain efficiency.

Although, as one would expect, it is not suggested, it is possible for the do-it-yourselfer to install a demand water heater. Of course, the manufacturer should be consulted and should be able to provide you with installation and instruction manuals. (If you are considering a gas unit, you will need to consult with an experienced contractor to find out if your gas supply is sufficient and proper venting is possible.) And don’t forget to check with your local government concerning a permit, if needed, and local water heater installation codes.
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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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