Tankless Water Heaters
What Are the Options?
How to Make the Best Choice
What's on the Horizon?
Tankless water heaters, also known as instantaneous water heaters, provide hot water without using a storage tank. Like tank water heaters, tankless water heaters use either gas or electricity to operate. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water (see Figure 1). Tankless water heaters can be supplementary units placed at the point of use or can replace conventional tank water heaters.
 |
Figure 1: Simplified drawing of a tankless water heater Tankless water heaters heat water on a demand. A gas burner or electric element heating device is activated by the flow of water whenever a hot water valve is opened. Source: Home Energy Magazine |
Tankless water heaters can be very cost effective because unlike conventional water heaters, they do not have standby losses incurred by continuous use of energy to maintain water in a tank to a set temperature. They are also space savers and can be a good choice installed in a small home with one or two people or used where a faucet or shower is some distance from the current water heater. Tankless heaters are also well adapted for use as a heat booster for a dishwasher or a solar water heater.
Although they theoretically provide endless hot water, most tankless water heaters provide it more slowly than conventional tank water heaters. This limits the number of sources that can draw hot water at one time.
What Are the Options?
Gas or electric. Choose a gas water heater whenever possible, because the cost of gas is approximately one-third that of electricity on a per-Btu basis. Gas tankless systems also have wider applications because they produce hot water at higher flow rates. An electric tankless system is an appropriate choice only when gas is not an option and no space is available for a tank hot water heater. Efficient electric tank water heaters are available at moderate cost.
Efficiency. The term "energy factor" characterizes the efficiency of both tank and tankless water heaters. The energy factor is the portion of the energy going into the water heater that gets turned into usable hot water under average conditions. It takes into account heat loss through the walls of the tank, up the flue, and in combustion. The higher the energy factor, the more efficient the heater.
Because tankless water heaters don't have the losses associated with tanks, their energy factors are normally higher (although well-insulated, ultra-efficient tank heaters also have high energy factors). Energy factors for gas tankless water heaters range from around 0.69 to 0.84, compared with 0.55 for a conventional tank and 0.86 for an ultra-efficient tank heater. Conventional electric tank water heaters have an energy factor of about 0.87 compared with 0.91 for an ultra-efficient tank and 0.98 for electric tankless water heaters.
Standing pilot or electronic ignitions. Tankless water heaters with standing pilot lights waste energy, but they can be cost-effective in applications where water use is high–a beauty parlor, for example. Where water use is lower (as in a residence), use a tankless water heater with an electronic ignition.
Modulating or fixed energy input. Old-style models of tankless water heaters have a fixed energy output, so water temperatures vary inversely with flow rate. As the flow rate increases (more taps are turned on), the water becomes cooler. Conversely, as flow rate decreases the water becomes warmer. Newer models have modulating controls that increase or decrease energy input to maintain the selected outlet temperature despite varying flow rates and inlet temperatures. Use units with modulating controls when a steady temperature is required and also to prevent scalding in applications where people or animals may come in contact with the water. A modulating unit is unnecessary when temperature fluctuations are acceptable–in commercial laundries, for example.
Energy inputs. Electric heating element and gas requirements for tankless water heaters are much larger than for storage water heaters. A typical gas storage water heater has a gas input of 40,000 Btu/h. A gas tankless heater, though, may use up to 170,000 Btu/h and so may require larger gas lines and vents than conventional water heaters. Similarly, although a typical residential electric storage water heater draws at most 7,000 watts, a whole-house electric tankless heater can draw as much as 19,200 watts, and may require 8- or 10-AWG copper wiring. Specifications for tankless water heaters also include requirements for minimum flow rates to activate them (usually around 1/2 to 3/4 gpm), as well as minimum and maximum water pressure (usually 15 to 150 psi).
How to Make the Best Choice
Pick the right size. Choose an appropriate model based on peak demand, incoming water temperature, and desired outgoing water temperature. Most units advertised as "whole-house" will provide 2 to 3 gallons per minute (gpm) of hot water at a 70°F temperature rise. Choose the model of water heater closest to your flow rate and temperature rise needs.
To figure out your peak demand, list the number of hot water devices you expect to have open at one time, and add up their flow rates. (See Table 1). This is the desired flow rate for the demand of water. For example, assume that you want a tankless water heater to operate a shower and kitchen sink at the same time. Most low-flow showerheads require about 2 to 3 gpm–and most kitchen sinks about 1 to 15 gpm–of mixed hot and cold water. Because showers and kitchen sinks usually use partly cold water, assume your peak demand is 2 gpm for the shower and 1 gpm for the sink, for a total peak demand of 3 gpm. (See the 1999 ASHRAE HVAC Applications Handbook, page 48.9, for a list of flow rates for typical hot water uses. Or consult ASHRAE's Web page
| Table 1: Tankless water heater flow rate comparisons | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gas tankless water heaters provide hot water at higher flow rates than the electric tankless heaters. When choosing a tankless model, be sure that flow rates will meet your peak demand at the temperature rise you need. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Type of system | Temperature rise (gpm) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50°F | 75°F | 100°F | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Gas tankless systems | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Takagi T-K1 tankless | 5.3 | 3.5 | 2.4 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bosch Aquastar tankless | 3.2 | 2.5 | 1.5 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Electric tankless systems | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Eemax system | 2.4 | 1.75 | 1.35 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Sets | 2.5 | 1.65 | 1.25 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Source: E Source; Manufacturer specifications | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Calculate cost-effectiveness: Tankless water heaters can be cost-effective when used in high flow rate applications. Because there are so many variables involved in calculating cost-effectiveness, it's a good idea to do your own analysis.
Analysis of a hypothetical situation reveals that payback periods for tankless heaters are significantly longer in lower water use applications (68.8 gallons per day) than in high water use applications (250 gallons per day). In the example, tankless water heaters have paybacks ranging from approximately 5.5 to 11 years in low-water-use applications compared to approximately 3 to 7.5 years in high-water-use applications. See Table 2.
Table 2: Tankless water heaters: statistics, cost, and payback | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Company and product | Energy factor | Recovery efficiency | Initial cost | Life (years) | Annual cost for average household water usage (68.8 gallons/day) | Annual cost for high water usage (250 gallons/day) | Payback at low usage 68.8 gallons/day (years) | Payback at high usage 250 gallons/day (years) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| American Water Heater Company conventional tank water heater | 0.61 | 0.76 | $455.00 | 6 | $228.23 | $519.65 | – | – | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Polaris ultra-efficient tank water heater | 0.86 | 0.94 | $1,750.00 | 10 | $167.46 | $403.08 | 21.31 | 11.11 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Takagi T-K1 tankless water heater with electronic ignition | 0.84 | 0.84 | $1,050.00 | 20 | $175.78 | $439.44 | 11.34 | 7.42 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bosch Aquastar tankless water heater with standing pilot | 0.69 | 0.69 | $600.00 | 20 | $209.11 | $496.48 | 7.58 | 6.26 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bosch Aquastar tankless water heater with electronic ignition | 0.80 | 0.80 | $700.00 | 20 | $184.57 | $461.41 | 5.61 | 4.21 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Assumptions: 70 cents per therm; 50-gallon tank; 70°F rise. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Source: E Source | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Because the example here includes many assumptions that may be different than your real life situation, we recommend that you use your own figures to determine cost-effectiveness.
Tankless heaters may become even more attractive when replacement costs are considered, because tank water heaters must be replaced between 6 to 10 years and the tankless heaters last at least 20 years. Other important costs to consider include upgrading energy supply lines and the possibility of hiring an additional qualified technician with experience installing tankless water heaters.
What's on the Horizon?
It's a good bet that more models featuring electronic ignition will enter the tankless water heater marketplace. In addition, it seems likely that highly efficient condensing units will also be developed. Although more area will be required for the stainless-steel heat exchanger, they will probably be easier to install. Thus, a higher price may be balanced by greater energy efficiency and a simple venting system consisting of a polyvinyl chloride (PVC) pipe that passes through the side of a building.
