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Water Heater Size worksheet

Grades 6–7

Solve each problem and show your work. Round answers to 2 decimal places where needed.

1.
Problem 1.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 2
- Flow per shower head: 2 gpm
- Faucets running at the same time: 2
- Flow per faucet: 0.75 gpm
- Temperatures in: °F
- Incoming cold water temperature: 40 °F
- Hot water temperature wanted: 120 °F
Find: Flow rate needed
Answer
2.
Problem 2.
- Water heater type: Tank (storage)
- Showers in the busiest hour: 1
- Shaves (running water) in the busiest hour: 0
- Hand dishwashing or food prep in the busiest hour: 1
- Automatic dishwasher loads in the busiest hour: 0
- Top-loading clothes washer loads in the busiest hour: 0
- Front-loading clothes washer loads in the busiest hour: 1
Find: Minimum first hour rating
Answer
3.
Problem 3.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 2
- Flow per shower head: 1.5 gpm
- Faucets running at the same time: 1
- Flow per faucet: 0.5 gpm
- Temperatures in: °F
- Incoming cold water temperature: 40 °F
- Hot water temperature wanted: 125 °F
Find: Flow rate needed
Answer
4.
Problem 4.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 1
- Flow per shower head: 1.5 gpm
- Faucets running at the same time: 2
- Flow per faucet: 1 gpm
- Temperatures in: °F
- Incoming cold water temperature: 60 °F
- Hot water temperature wanted: 140 °F
Find: Flow rate needed
Answer
5.
Problem 5.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 2
- Flow per shower head: 2.5 gpm
- Faucets running at the same time: 1
- Flow per faucet: 1 gpm
- Temperatures in: °C
- Incoming cold water temperature: 5 °C
- Hot water temperature wanted: 55 °C
Find: Flow rate needed
Answer
6.
Problem 6.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 1
- Flow per shower head: 1.8 gpm
- Faucets running at the same time: 1
- Flow per faucet: 0.5 gpm
- Temperatures in: °F
- Incoming cold water temperature: 60 °F
- Hot water temperature wanted: 140 °F
Find: Flow rate needed
Answer
7.
Problem 7.
- Water heater type: Tank (storage)
- Showers in the busiest hour: 2
- Shaves (running water) in the busiest hour: 2
- Hand dishwashing or food prep in the busiest hour: 1
- Automatic dishwasher loads in the busiest hour: 1
- Top-loading clothes washer loads in the busiest hour: 0
- Front-loading clothes washer loads in the busiest hour: 1
Find: Minimum first hour rating
Answer
8.
Problem 8.
- Water heater type: Tank (storage)
- Showers in the busiest hour: 3
- Shaves (running water) in the busiest hour: 0
- Hand dishwashing or food prep in the busiest hour: 1
- Automatic dishwasher loads in the busiest hour: 0
- Top-loading clothes washer loads in the busiest hour: 0
- Front-loading clothes washer loads in the busiest hour: 1
Find: Minimum first hour rating
Answer
9.
Problem 9.
- Water heater type: Tankless (on demand)
- Showers running at the same time: 1
- Flow per shower head: 2 gpm
- Faucets running at the same time: 1
- Flow per faucet: 1.5 gpm
- Temperatures in: °C
- Incoming cold water temperature: 10 °C
- Hot water temperature wanted: 55 °C
Find: Flow rate needed
Answer
10.
Problem 10.
- Water heater type: Tank (storage)
- Showers in the busiest hour: 2
- Shaves (running water) in the busiest hour: 0
- Hand dishwashing or food prep in the busiest hour: 1
- Automatic dishwasher loads in the busiest hour: 3
- Top-loading clothes washer loads in the busiest hour: 0
- Front-loading clothes washer loads in the busiest hour: 1
Find: Minimum first hour rating
Answer

Answer key: Water Heater Size worksheet

1. Flow rate needed: 5.5 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $2 \times 2 + 2 \times 0.75 + 0$ $= 5.5 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $120 - 40$ $= 80 °F$
3. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 5.5 \times 80$ $= 220,000 BTU/h$
4. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{220,000}{3,412.14}$ $= 64.48 kW$
2. Minimum first hour rating: 38 gal
1. Showers: uses × 20 gallons each
  $1 \times 20 gal$ $= 20 gal$
2. Hand dishwashing or food prep: uses × 3 gallons each
  $1 \times 3 gal$ $= 3 gal$
3. Front-loading clothes washer loads: uses × 15 gallons each
  $1 \times 15 gal$ $= 15 gal$
4. Add it all up to get the peak hour demand, the hot water used in your busiest hour
  $20 + 3 + 15$ $= 38 gal$
5. Choose a tank whose first hour rating (FHR) is at least this much, so it can keep up
  $38 gal FHR or more$
3. Flow rate needed: 3.5 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $2 \times 1.5 + 1 \times 0.5 + 0$ $= 3.5 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $125 - 40$ $= 85 °F$
3. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 3.5 \times 85$ $= 148,750 BTU/h$
4. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{148,750}{3,412.14}$ $= 43.59 kW$
4. Flow rate needed: 3.5 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $1 \times 1.5 + 2 \times 1 + 0$ $= 3.5 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $140 - 60$ $= 80 °F$
3. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 3.5 \times 80$ $= 140,000 BTU/h$
4. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{140,000}{3,412.14}$ $= 41.03 kW$
5. Flow rate needed: 6 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $2 \times 2.5 + 1 \times 1 + 0$ $= 6 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $55 - 5$ $= 50 °C$
3. Convert the rise to °F, because a °F step is smaller: multiply by 1.8
  $50 \times 1.8$ $= 90 °F$
4. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 6 \times 90$ $= 270,000 BTU/h$
5. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{270,000}{3,412.14}$ $= 79.13 kW$
6. Flow rate needed: 2.3 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $1 \times 1.8 + 1 \times 0.5 + 0$ $= 2.3 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $140 - 60$ $= 80 °F$
3. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 2.3 \times 80$ $= 92,000 BTU/h$
4. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{92,000}{3,412.14}$ $= 26.96 kW$
7. Minimum first hour rating: 69 gal
1. Showers: uses × 20 gallons each
  $2 \times 20 gal$ $= 40 gal$
2. Shaves (running water): uses × 2 gallons each
  $2 \times 2 gal$ $= 4 gal$
3. Hand dishwashing or food prep: uses × 3 gallons each
  $1 \times 3 gal$ $= 3 gal$
4. Automatic dishwasher loads: uses × 7 gallons each
  $1 \times 7 gal$ $= 7 gal$
5. Front-loading clothes washer loads: uses × 15 gallons each
  $1 \times 15 gal$ $= 15 gal$
6. Add it all up to get the peak hour demand, the hot water used in your busiest hour
  $40 + 4 + 3 + 7 + 15$ $= 69 gal$
7. Choose a tank whose first hour rating (FHR) is at least this much, so it can keep up
  $69 gal FHR or more$
8. Minimum first hour rating: 78 gal
1. Showers: uses × 20 gallons each
  $3 \times 20 gal$ $= 60 gal$
2. Hand dishwashing or food prep: uses × 3 gallons each
  $1 \times 3 gal$ $= 3 gal$
3. Front-loading clothes washer loads: uses × 15 gallons each
  $1 \times 15 gal$ $= 15 gal$
4. Add it all up to get the peak hour demand, the hot water used in your busiest hour
  $60 + 3 + 15$ $= 78 gal$
5. Choose a tank whose first hour rating (FHR) is at least this much, so it can keep up
  $78 gal FHR or more$
9. Flow rate needed: 3.5 gpm
1. Add up the flow of everything running at once, since a tankless unit heats water as it flows
  $1 \times 2 + 1 \times 1.5 + 0$ $= 3.5 gpm$
2. Subtract the incoming water temperature from the temperature you want. This is the rise
  $55 - 10$ $= 45 °C$
3. Convert the rise to °F, because a °F step is smaller: multiply by 1.8
  $45 \times 1.8$ $= 81 °F$
4. Multiply 500 by the flow and the rise to get the heat needed. Each gpm warmed 1 °F takes about 500 BTU an hour
  $500 \times 3.5 \times 81$ $= 141,750 BTU/h$
5. For an electric unit, convert the heat to kilowatts of power (1 kWh = 3,412.14 BTU)
  $\frac{141,750}{3,412.14}$ $= 41.54 kW$
10. Minimum first hour rating: 79 gal
1. Showers: uses × 20 gallons each
  $2 \times 20 gal$ $= 40 gal$
2. Hand dishwashing or food prep: uses × 3 gallons each
  $1 \times 3 gal$ $= 3 gal$
3. Automatic dishwasher loads: uses × 7 gallons each
  $3 \times 7 gal$ $= 21 gal$
4. Front-loading clothes washer loads: uses × 15 gallons each
  $1 \times 15 gal$ $= 15 gal$
5. Add it all up to get the peak hour demand, the hot water used in your busiest hour
  $40 + 3 + 21 + 15$ $= 79 gal$
6. Choose a tank whose first hour rating (FHR) is at least this much, so it can keep up
  $79 gal FHR or more$