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Indirect Fired Water Heaters Installation and Operating Manual
Installation & Operating Manual
How to Properly Size Your Indirect Water Heater
Use the First Hour Rating (FHR) to properly size your
Indirect Water Heater. The First Hour Rating is the amount
of hot water in gallons the heater can supply per hour
(starting with a tank full of hot water), depending on tank
capacity, source of heat, and the size of the burner.
Estimate your peak hour demand as follows:
• Determine what time of day (morning, noon, evening) you
use the most hot water in your home. Keep in mind the
number of people living in your home.
• Use the worksheet below to estimate your maximum
usage of hot water during this one hour of the day -- this
is your peak hour demand. Note: The worksheet does not
estimate total daily hot water usage.
The worksheet example shows a total peak hour demand
of 165 gallons; therefore, this household would need a
water heater with a First Hour Rating of no less than 165
gallons.
Fixture Count Guide
To quickly estimate the minimum gallons of hot water
required based on a family size number of baths and hot
water appliances the First Hour Rating of the STS tank
must be equal to or exceed the total first hour
requirements.
Peak Hour Demand Worksheet
Description
Fill in the Blank
Example
40 Gallons for the first 2 people
___________________________
Gals.
40
Gals.
10 Gallons for each additional person
___________________________
Gals.
20
Gals.
20 Gallons for each bath after the first
(consecutive bath)
___________________________
Gals.
20
Gals.
10 Gallons if dishwasher is used
___________________________
Gals.
10
Gals.
20 Gallons if clothes washer is used
___________________________
Gals.
20
Gals.
TOTAL
___________________________
Gals.
110
Gals.
Heavy Usage Buffer
Use if family members take longer than average showers, etc.
x 1.5
x 1.5
First Hour Rating
___________________________
Gals.
165
Gals.
Proceed to pages 23 & 24 of this manual to locate your
Indirect Water Heater and Boiler in the sizing charts. Using
the First Hour Rating calculated in this worksheet, locate
your Indirect Water Heater and Boiler along with your First
Hour Rating. This will also determine the flow needed
between the Indirect Water Heater and Boiler.
