Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you’ll need for its application (whole house or a remote application, such as just a bathroom) in your home. It is important to note that you should never try and save money by undersizing your tankless water heater.
Determine the maximum number of devices you want to run and their total flow rate. Then, add up their flow rates (gallons per minute). This is the desired flow rate you’ll want for the demand water heater.
For example, let’s say you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons per minute and a shower head with a flow rate of 2.6 gallons per minute. The flow rate through the demand water heater would need to be at least 3.26 gallons per minute. To reduce flow rates, install low-flow water fixtures.
Determine required temperature rise. To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50°F (By using a low temperature assumption you ensure that you will not undersize your tankless unit. If you live in a warm climate your water temperature will likely be much higher.) For most uses, you’ll want your water heated to around 105–115°. In this example, you’d need a demand water heater that produces a temperature rise of 55°.
Sizing Example: An average shower will be between 104–106° and uses 2.6 gallons of water. Assuming your water temperature is 40° coming into your home, and you want to produce enough hot water to run 2 showers at the same time, what temperature rise would you need to produce to accomplish this?
Answer: You’ll need to raise the incoming water temperature from 40 degrees to 105. You’ll need to be able to heat at least 5.2 gallons of water. So you’ll need a tankless water heater that can produce at least a 60 degree rise in temperature at 5.2 gallons per minute.
How much hot water do you need at one time? Do you need to run 2 showers at the same time or maybe a shower and a couple sinks? The chart below shows the range of water usage range and average water temperatures for various fixtures. We suggest using 2.5 gpm for a shower and 1.0 gpm for a bathroom as a reference point in determining your total simultaneous water needs.
|Fixture||Average Flow GPM||Avg. Temp.|
For example, if you are running 2 showers at the same time, you will need 5 gallons of hot water per minute from the tankless water heater. If you were running a shower and the washing machine at the same time, you would need 4.5 gallons per minute from the water heater. In either of these situations, you will want to size the unit that meets or exceeds how much hot water you need at the same time.
Note: tankless water heaters are intended to heat potable (drinking) water only and the inlet water to a tankless unit should not be pre-heated.
Other Sizing Notes
Gas tankless water heaters are able to produce a larger temperature rise per gpm than electric models. Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70°F water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet.
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