Gas-fired Tank Water Heaters
On a life-cycle basis, gas-fired tank water heaters are typically the least expensive means of heating water. One reason is that on a per-Btu basis, natural gas and propane are usually much less expensive than electricity. Another reason is that tank water heaters of 100 gallons (380 litres) or less are mass produced and are less expensive than most other types of water heaters. In some instances, other water heater types – such as electric heaters – are a better choice. But if gas is available and the water heater is in a location where a gas vent can be installed, a gas-fired tank water heater will likely be the least expensive to own and operate.
What are the options?
Residential heaters. Even though they are designed for the residential market, these water heaters can be appropriate for many small commercial facilities and even some large facilities. Residental water heaters are available with tank sizes up to 100 gallons (380 litres) and gas inputs up to 75,000 British thermal units per hour (Btu/h). Manufactured in large quantities, they are relatively inexpensive and widely available. The term "energy factor" is used to express the efficiency of residential heaters. It represents the amount of heat required to warm up a year's worth of hot water for a typical home, divided by the amount of energy input into the heater to warm that water. The higher the energy factor, the more efficient the heater. By varying the tank insulation, burner design, and a few other features, manufacturers make residential heaters available with energy factors ranging from 0.59 to 0.67. To determine the energy factor for a particular water heater, either obtain it from the manufacturer's literature or look it up in the Gas Appliance Manufacturers Association's Consumers' Directory of Certified Efficiency Ratings. With this directory, you can compare the efficiency of specific gas-fired water heaters on the market.
Commercial heaters. Two key differences between residential and commercial heaters are that commercial heaters are available with much higher gas input ratings (one million Btu/h or more) and larger storage tanks (up to 250 gallons or 950 litres). They also have larger pipe connections, more rugged controls and a few features only rarely found in residential heaters, such as flue dampers and electronic ignition. Unlike residential heaters, commercial heaters aren't rated for overall efficiency. Instead, they are rated for "thermal efficiency" – the portion of input gas energy that goes toward heating the water that is drawn from the tank. With few exceptions, commercial heaters are available with thermal efficiencies ranging from 78% to 80%. These heaters also must meet government standards for standby loss, which represents the portion of the stored energy lost when the burners are not operating. Standby losses are typically not published, but you can obtain them from the manufacturer.
Ultra-efficient heaters. With thermal efficiencies of 90% to 95%, these water heaters outperform the competition by far ( Figure 1). They use power burners and enhanced heat exchangers to force hot combustion gases into chambers and tubes that are submerged in the stored water. Ultra-efficient water heaters are vented with plastic pipes that go directly through an outside wall. Because they draw combustion air directly from outside, through one of those pipes, their combustion processes are sealed off from the occupied space. That makes them safer than other gas water heaters (which, on rare occasions, may leak harmful fumes).
 |
Figure 1: Polaris® water heater The Polaris® water heater features one of the highest thermal efficiencies: 95%. The high efficiency of this unit is achieved by the additional coiled heat-exchange surface, shown here. Source: American Water Heater Co. |
Tankless water heaters. Tankless water heaters, sized for both residential and commercial use, provide hot water without using a storage tank. They don't suffer standby losses, so they have much higher energy factors than conventional units. See Tankless Water Heaters for details.
How to make the best choice
Pick a size that's just right. Sometimes, plumbing contractors oversize water heaters so they can quickly specify a model they know will keep up with demand. That's bad news for customers who have to live with those water heaters because an oversized heater is less efficient and more expensive than an accurately sized one. To make the best selection, calculate the "peak one-hour draw," following the procedure from the American Society of Heating, Refrigerating and Air Conditioning Engineers' (ASHRAE) ASHRAE Handbook–HVAC Applications. This quantity represents the greatest amount of hot water likely to be required over the course of a single hour. Then find a water heater whose "first-hour rating" exceeds that. Alternatively, several manufacturers provide free software on their websites for sizing water heaters.
Compare the cost-effectiveness of heaters with different efficiency ratings. The actual operating cost for any water-heating application depends on how frequently the heater will operate and on the local cost of natural gas. These parameters vary greatly among applications, so we recommend that you estimate the annual operating costs for a few heaters, using a range of load assumptions, and compare them with their first costs. Use the calculator below to estimate the simple payback period for gas water heaters based on their efficiency and purchase price.
Cost-effectiveness calculator. This calculator determines the energy consumption and payback period for standard commercial and ultra-efficient commercial water heaters and compares them with consumption and payback for residential models. By necessity, this screening model makes simplifying assumptions, so use it for initial screening only. For more accurate performance predictions, conduct a more detailed analysis that includes additional factors, such as actual usage patterns, hot water loads and part-load performance of equipment.
Begin by entering the average daily water draw in gallons, local natural gas cost in U.S. dollars per therm and electricity cost in U.S. dollars per kilowatt-hour. For the residential model, enter the energy factor; for the commercial and high-efficiency models, enter the thermal efficiencies (as fractions, not percentages: 0.8, for example, rather than 80). These numbers are available from manufacturers' data sheets. For typical values, use 0.59 for the residential model energy factor, 0.80 for a standard commercial water heater and 0.95 for an ultra-efficient model. Finally, enter the purchase price for each unit and hit the "calculate" button.
If safety is an overriding issue, select an ultra-efficient heater. Every year, a few people are injured or killed when carbon monoxide and other products of combustion backdraft into occupied spaces. Ultra-efficient heaters use sealed-combustion technology so they cannot backdraft. Although backdrafting rarely occurs – and problems can be minimized by making sure that systems are properly sized and vented, ducts are leak-proof, and carbon monoxide monitors are installed – the peace of mind offered by ultra-efficient heaters may be worth it for you.
What's on the horizon?
Condensing tank-type water heaters, the highest-efficiency tank-type units available, have powerful burners that are well-suited for large-capacity applications. They also have high first costs. This technology may become more cost-effective for smaller-capacity applications as manufacturers develop ways to manufacture them from less-expensive components. One such new condensing product, called the Vertex, has a lower first cost but it is still relatively expensive. The U.S. Department of Energy and engineering consulting firm TIAX LLC collaborated with water heater manufacturer A.O. Smith on the development of the Vertex's technology. It is rated at 90% thermal efficiency, which is 18% to 36% higher than conventional gas tank-type water heaters.
Copyright © 2008 E Source Companies LLC
Last Modified: Jul 22, 2009
