Two types of hot water heaters are commonly used in homes today: direct vent and power-assisted vent. However, neither is the most efficient way to heat water. Direct-vent heaters use the house’s chimney to passively vent the system. The power-vent hot water heater uses an electric fan to vent through the side wall of a house, and therefore doesn’t require a chimney.
While a power-vent system is slightly more efficient (0.62-0.65 ratio of useful energy to total energy used vs. 0.55-0.62 for a direct-vent system), both systems still lose about 35 to 40 percent of every unit of gas used to heat the water. Also, the system’s storage tanks usually have only about one inch of insulation, so it doesn’t store hot water very well, either.
Even though the upfront cost is low, these standard water heaters can cost several hundred dollars per year to operate and will last about 12 to 15 years. So this is one area where it makes sense to upgrade your hot water heating system.
Two efficient options are available: on-demand (also known as tankless) hot water heaters and heat pump assisted heaters. Widely used in Western Europe, on-demand hot water heaters, as the name implies, heat water only as it is needed in the house rather than storing it in a tank. Thus, because there is no tank, they are sometimes advertised as providing unlimited hot water.
On-demand hot water systems burn gas with about 95 percent efficiency; therefore, they save a lot of energy compared to a traditional tank system. But equipment costs for an on-demand system are high, installation costs are high, yearly maintenance is required to prevent hard water deposits in the copper lines, and there are sometimes long wait times for hot water. Also, if you pay for municipal water, you will probably use more water, thus diluting the benefits of a more efficient system.
Next generation systems are ultra-efficient hot water tank systems. Internal heat exchangers boost the efficiency of a storage tank system to about 95 percent, the same as an on-demand system. While they do continuously heat a tank of water, the insulation value is usually better, so they store that energy more efficiently. Additionally, recovery time is so fast that you could downsize your tank with no noticeable difference in performance. Some tanks are glass-lined, which extends the life of the water heater and requires less maintenance.
There are new hybrid models entering the marketplace. These are on-demand systems with a small 5 or 10 gallon buffer tank attached. This effectively eliminates the performance issues with on-demand systems with less stand-by heat losses from the tank of water. Like traditional on-demand systems, these will need more frequent maintenance.
Tank systems can easily be installed to recirculate hot water. Recirculating hot water lines will lose energy to the home through the pipes, but it can save a lot of water by significantly reducing the amount of water going down the drain waiting for hot water. With a recirculating system, you would rarely have to wait more than 10 seconds for hot water at any fixture.
Recirculating hot water, in addition to being a nice comfort feature, can save thousands of gallons of water per person over the course of a year. Recirculating systems that continuously cycle water will use nearly twice the amount of energy to heat water, thus destroying some of the efficiency of the system. Using a pump timer or temperature control, occupancy sensor or switch in areas of hot water use, or a combination of these techniques, a recirculating line can make your home perform to the best of it’s ability.
A well-designed recirculating hot water system offers the best of all worlds – significantly less energy use to heat water and less than a pint of water wasted to get hot water at the end point of use.