What size are the water heaters in the houses you build? If you don’t know, you’re in the majority. Water heaters are inconspicuous basement dwellers—not metered like an electrical panel, and not given much thought unless they break, or your customer runs out of hot water. Yet, an appropriately sized water heater can actually cost less to buy than an oversized system.
Proper sizing saves money.
Sizing a water heater involves estimating the appropriate First Hour Rating (FHR) based on estimated hot water use. Many designers, builders, and contractors don’t know where to look for guidelines to calculate the appropriate size water heater by using FHR as a metric. As a result, contractors have become used to specifying a certain tank size by its volume (40 or 50 gallons is typical), or overestimate to play it safe (with a 70 gallon tank), regardless of the FHR. This results in needless oversizing, which will cost more. A smaller tank with a big burner may have the appropriate FHR and cost less to buy.
Estimating the appropriate FHR will help you select a water heater that will give homeowners enough hot water when they need it. A metric for determining water heater size, the FHR describes the number of gallons of hot water a water heater can provide in the first hour, which includes the full tank of hot water and the amount of cold water that can be heated as the hot water is used. A good estimate should consider how much water is needed during the most critical hour of the day. IBACOS’ research has shown that, on average, people use approximately 14 to 16 gallons of hot water for an eight-minute shower, depending on the temperature of the cold water and the temperature setting of the water heater. A normal sized bathtub requires somewhat less, about 11 to 12 gallons of hot water, while a large bathtub may require 37 to 40 gallons. Typical dishwashers use 10 gallons of hot water per cycle, in contrast to 6 gallons per cycle for a high-efficiency dishwasher. A typical “hot” load of laundry in a top-loading clothes washer uses approximately 40 gallons of hot water if the clothes are washed in hot water and rinsed in warm water. Horizontal-axis washers use 26 gallons for a typical “hot” load.
Guessing doesn’t pay off. Use guidelines to determine FHR.
There are two methods that can be used to help estimate FHR.
Gas Appliance Manufacturers Association (GAMA) Guidelines. The GAMA Guidelines are available at www.gamanet.org and contain a water heater selection method based on the FHR of water heaters. FHR and EF data on specific water heater models can also be found at this website.
HUD-FHA Guidelines. The HUD-FHA Table for Residential Water Heater Sizing is published in the ASPE Data Handbook and Chapter 48 of the 1999 ASHRAE Handbook of HVAC Applications. This table is included below. Note that because this table was created when showerheads had water flow rates of 5 to 8 gallons per minute, it may overestimate the size of water heaters.
A higher EF number is important for energy efficiency.
When selecting the right size water heater, paying attention to the EF (energy factor) rating will help you choose an efficient system. The EF is an efficiency rating used to compare one water heater to another. The higher the EF, the more efficient the system will be. The National Appliance Energy Conservation Act (NAECA) has guidelines that set a minimum EF based on the size of the water heater. Generally, the smaller the water heater, the higher the EF.
When specifying water heaters in the houses you build, taking shortcuts or making guesses won’t get you anywhere. Taking the time and effort to ensure the water heater is the right size can save you money. An additional benefit is that a properly sized system with a high EF can reduce hot water energy use, resulting in overall improved efficiency of the home.
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