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If your water heater needs to be replaced, you will need to consider what size tank to purchase. This consideration is important because getting a tank that’s too small can mean running out of hot water when you need it and/or continuously running the tank to keep up with your family’s needs. On the other hand, choosing a tank that’s too big can lead to increased water and/or energy bills. A tank that’s too large also may not fit in the same location as your current tank.

To ensure you get the right size tank for your home, you should evaluate the following factors for tank-style heaters.

Your Household Size

More people living in a home means more water being used—for showers, dishes, and laundry. For a loose estimation of the tank capacity you need, you can factor 12 gallons of water per household resident (# of people x 12 gallons = approximate tank size needed). The breakdown based on this math is as follows:

  • 1-2 people need a 30-gallon water heater
  • 2-3 people need a 40-gallon water heater
  • 3-4 people need a 40–50-gallon water heater
  • 5+ people need a 50–80-gallon water heater

If you are purchasing a tank for a home that you do not plan to live in (i.e. income property), you may not know exactly how many people will live in the home. In this case, you can estimate the right tank size based on the number of rooms in the home—by adding 1 to the total number of rooms. For instance, in a four-bedroom house, estimate that 5 people will live in the house. Then, use the aforementioned guidelines to loosely determine the size tank you need.

Hot Water Usage Habits

Determining the tank size you need also requires examining your family’s water usage habits. The frequency at which you do certain activities and the time-of-day household members do certain activities can impact the rate that you use water. You may need a bigger tank if you and your family commonly:

  • Shower at the same time
  • Run multiple small loads of laundry each day instead of larger loads each week
  • Use water appliances at the same time (i.e. dishwasher, washing machine, etc.)
  • Do any of the aforementioned activities at the same time

To better estimate the size tank you need, you will need to compare the maximum amount of water used in your house in an hour against your water heater’s first-hour rating. Your peak usage should be lower than your first-hour rating or you may run out of hot water.

Determining Peak Water Usage | First-Hour Rating

The amount of hot water your tank can produce each hour (when the water tank is already fully heated and accounting for heat loss as unheated water refills the tank) is your tank’s first-hour rating (FHR). The FHR is different than your tank size; for instance, a 50-gallon water tank may have a 60-95-gallon FHR depending on whether it is a gas or electric tank. You can find the FHR of a tank on the EnergyGuide label on your water heater.

To estimate the maximum amount of water your household uses in an hour, determine what time of day more hot water is used and how much hot water is used. Average estimates for how much water is needed for certain tasks are as follows.

  • Showering or washing hair, 2 gallons per minute
  • Shaving, 0.5 gallons per minute
  • Hand-washing food or dishes, 2 gallons per minute
  • Using the laundry machine, 14 gallons per load
  • Running the dishwasher, 6 gallons per cycle

To determine your peak hour usage, add the total gallons being used during the busiest time of day. For instance, if every morning, you have 3 people showering, a load of laundry going, and 1 person shaving, you will need to add: (person 1’s minutes in the shower x 2) + (person 2’s minutes in the shower x 2) + (person 3’s minutes in the shower x 2) + (14) + (person 4’s shaving time x 0.5). If your maximum usage is lower than the tank’s FHR, then you’re good to go.

It is also important to note that these numbers may fluctuate if:

  • Members of your household take baths instead of showers. While a 10-minute shower only takes up 20 gallons of hot water, a morning bath utilizes a lot more water. Depending on its size, a full bathtub could hold around 40-140 gallons of water. This is an important consideration for families with young children.
  • You have a multi-head shower system. The amount of water you use per minute can increase if you use more than one shower head simultaneously.
  • You often have guests/family who live with you long-term. If your in-laws often visit for months in the summer or your child comes home from college in the summer, you don’t want to run out of hot water because you didn’t factor them into your usage estimates.

Contact Our Trusted Technicians

At Donmar Company Heating & Cooling, we have provided excellent service to over 19,500 customers and continue to grow. If you need help installing a gas or electric tank or deciding what size tank to purchase, our experienced professionals are equipped to help you. We also have experience with:

  • Water heater repairs and inspections
  • Recirculating system installations
  • Tankless water heater conversions
  • Boiler systems

For water heater installation or repair, reach out to our team online or at (703) 457-8676. Alleviate some of your stress by entrusting the work to capable professionals.