Water Supply

Sizing Semi-Instantaneous Water Heaters

Step 1 - Determine the total fixture unit load

Determine the total fixture unit load for all the fixtures serviced by your water heater application using the Fixture Units Table. The following table shows an example for a college dormitory.

No. of FixturesType of Fix. Fix. UnitDemand Fix. Unit
150Private Lavatory.75113
120Private Shower1.5180
20Slop Basin2.550
8Clothes Washer2.016
Total Fixture Units:359

Step 2 - The Revised Hunter Curves

The original Hunter Curves were revised by ASHRAE (Figures 1 and 2) by grouping buildings based on their usage with type A, B, C and D. It is widely used in selecting semi-instantaneous water heaters.

Hunter Curve 1
Figure 1. Revised Hunter Curve
Hunter Curve 2
Figure 2. Revised Hunter Curve

Step 3 - Select the proper water heater/water temperature controller

Refer to the modified Hunter Curves in Figure 1 and Figure 2. Curve B represents dormitories. Enter the graph from the bottom at 359 fixture units and go up to curve B. Then move to the left horizontally to read approximately 3.8 L/s of hot water capacity required.

Note: Remember to add any constant flow capacities, as determined under "Important Note" below, to this 3.8 L/s gpm.

Important Note

Special consideration should be given to applications involving periodic use of gang showers, process equipment, laundry machine etc., as may occur in field houses, gymnasiums, factories, hospitals, etc. Because these applications could have all equipment on at the same time, their total hot water capacity should be determined and then added to the maximum hot water demand as read from the modified Hunter Curves. Use the following formula to determine total hot water capacity needed for these applications when final water temperatures are lower than that of the water heater.

(B-C)/(H-C) x (Total water flow from all gang shower heads in gpm) = Hot water needed (gpm)
B = Blended water temperature out of the fixture
H = Hot water temperature to the fixture
C = Cold water temperature to the fixture

Source: Armstrong International

Use Hot Water Mixing calculator to calculate the percentage of hot water for preparing the needed flow with blended water temperature.

On Screen Calculator by EngineerMate

This calculator is for multiple uses:

The calculator image is self-explanatory on its functions.

Water Heater Calculator
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