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How to Calculate Tankless Water Heater Requirements - tankless water heater capacity sizing guidelines: here we explain how to calculate the size or capacity of tankless water heater you will need in a building. We name all of the factors that you should consider when buying a tankless or demand or point of use type water heater, such as incoming water temperature, desired output water temperature, and the total hot water flow rate in gallons per minute.
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You can calculate the approximate tankless water heater capacity required for any application as follows: Sketch of a point of use tankless water heater courtesy of Carson Dunlop Associates.
T1 = incoming water temperature
T2 = desired output hot water temperature at the plumbing fixture (say a shower)
GPM = anticipated hot water flow rate or usage rate in gallons per minute.
If your incoming water temperature to the building is 40 deg.F. and you want to provide hot water at a bath shower at 100 deg. F., and if your shower fixture runs at 3 gpm, then
T1 = 40 deg.F., T2 = 100 deg.F.
T2 - T1 = 60 deg.F. - that's the temperature rise you need.
Our photo (left) illustrates a Rheem EcoSense® tankless water heater. A wide range of sizes and capacities of demand type water heaters are available, so indeed it makes sense to look carefully at your requirements.
Your tankless water heater will need to be able to provide a 60 deg. temperature rise at 3 GPM.
Note 1: our example does not consider temperature losses in the piping between the water heater and the point of use nor the use of temperature limiting or anti-scald valves in the plumbing system, both of which reduce the actual hot water flow rate in gpm
While 100° may be a good shower temperature, this means that mixing valves at the shower will be delivering almost 100% hot water. This may make filling a bathtub slow or result in lower water pressure than anticipated. When we blend hot and cold water, we enjoy a higher flow rate and commensurately higher pressure at the shower.
Note 2: it's quickly apparent that if the hot water system is going to be asked to provide 100 deg.F. hot water to multiple fixtures simultaneously, the gpm heating rate of your tankless water heater is going to have to be big - and may be beyond the capacity of the equipment. In this case see our Multiple Ganged Tankless Water Heaters article.
Gas fired tankless water heaters can usually produce more hot water faster than an electric unit. Electric tankless water heaters are simplest to install and operate, requiring only wiring and water piping, no fuel piping, no venting or chimney.
If your building includes three or four bathrooms or even just two baths, and if the occupants are likely to want to run hot showers simultaneously at multiple fixtures, the performance of a single demand water heater or tankless water heater may be marginal unless the unit is quite large.
An Example of Sizing a tankless water heater: specifying hot water flow rate & temperature rise required
- Adapted & expanded from Tankless or Demand Type Water Heaters, & from Sizing a Water Heater, - U.S. DOE.
As we explain in detail at Capacities of Tankless Water Heaters, since there is no reservoir of hot water in a demand type water heating system, instead, tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate through the device. As DOE points out in their article, to size a demand water heater, you need to determine
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