How to Size Instant Hot Water Heaters
Instant hot, or on-demand, water heaters provide hot water with no reservoir tank. The water heaters must be sized correctly by the amount of hot water that is required for the household. The overall demand is the gallons per minute used by the water faucets, dishwashers and cloth washer in the home. All instant water heaters are rated by the water flow in gallons per minute (gpm) and the temperature rise. Most instant water heaters will raise the water temperature between 70 and 90 degrees F. Does this Spark an idea?
Instructions
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1
Make a list of all the hot water outlets in the home. This needs to include all faucets, dishwasher and clothes washer, if applicable.
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2
Estimate which of these faucets and devices will be running at the same time. In other words, will the dishwasher be operated during a shower or bath. Combine the items together of simultaneous usage.
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3
Calculate the flow rate for each faucet and device. Appliances will list the water consumption flow rates in the product specifications. A bathroom faucet will use between 0.5 gallons per minute (gpm) and 1.5 gpm depending on the type. A kitchen faucet will flow from 3.0 to 7.0 gpm. A typical shower will range between 1.0 and 2.0 gpm.
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4
Test the faucet flow rates by placing a one-gallon container under the faucet and time how long it takes to fill up. A one-gallon container that takes 30 seconds to fill has a water flow of 2.0 gpm. If the container takes two minutes to fill up that faucet will have a flow rate of 0.5 gpm.
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5
Total the overall amount of hot water flow rate from your home as found in Step 2. Use the data to properly size the instant water heater. Specify the amount of water temperature rise you will need as well. Some dishwashers that do not have an internal water heater may require hot water temperatures of 140 degrees F.
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Tips & Warnings
On average dishwashers range from 1.0 to 2.5 gpm and a clothes washer is from 1.5 to 3.0 gpm. You can reduce the overall flow rates from your water faucets by installing low-flow fixtures to all water outlets.
References
Resources
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