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CAUSES
Complaints regarding an insuffi cient supply of hot water are typically the result of
a water heater that cannot meet the demands of the residence (both people and
appliances). The demand for suffi cient hot water may also be exceeded if additional
people and/or appliances are added to the residence. Another factor that may reduce
the hot water output is mineral build-up. Reference Bulletin 13 to determine if mineral
build-up may be effecting your hot water output. The following test will help determine if
a water heater is supplying the intended amount of hot water.
TEST
Please read all the steps of the test prior to beginning. NOTE: This test is based on
140° F storage temperature and these amounts are approximate.
STEP 1
Do not use any hot water for two hours to ensure the accuracy of this test. Turn on the
cold water at full fl ow using a double handle faucet and time how long it takes to fi ll a
one-gallon jug. Calculate the Gallons Per Minute (GPM) fl ow rate as follows.
One-gallon Jug fi lled in:
STEP 2
Using a thermometer, turn on the hot water and time how long it takes for the water
temperature to drop 30 degrees.
Example: 10 seconds to fi ll the one-gallon jug equals 6 gallons per minute fl ow rate.
If the temperature drops 30 degrees in 6 minutes that equals 36 gallons of hot water.
This is normal for a 50 gallon water heater. The amount will be about 70% of the
capacity of the heater.
RESULTS
If the test performed above results in a 70% (+ /- 10 %) capacity, then the heater is
performing per specifi cations and a larger water heater should be considered in order
to meet the demand.
BULLETIN 35
NOT ENOUGH HOT WATER - GAS
5 seconds = 12 gallons per minute
10 seconds = 6 gallons per minute
15 seconds = 4 gallons per minute
20 seconds = 3 gallons per minute
30 seconds = 2 gallons per minute
Gallons Per Minute (GPM) Flow Rate
1.5
2
14 min.
10 min.
30 Gallon Tank
40 Gallon Tank
18 min.
14 min.
23 min.
17 min.
50 Gallon Tank
65 Gallon Tank
30 min.
22 min.
75 Gallon Tank
35 min.
26 min.
47 min.
35 min.
100 Gallon Tank
119 Gallon Tank
56 min.
42 min.
Minutes to drop 30° F
34
2.5
3
3.5
4
8 min.
7 min.
6 min.
5 min.
11 min.
9 min.
8 min.
7 min.
14 min.
12 min.
10 min.
9 min.
18 min.
15 min.
13 min.
11 min.
21 min.
17 min.
15 min.
13 min.
28 min.
23 min.
20 min.
17 min.
33 min.
28 min.
24 min.
21 min.
5
6
4 min.
3 min.
5 min.
4 min.
8 min.
6 min.
9 min.
8 min.
10 min.
8 min.
14 min.
11 min.
17 min.
14 min.
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