Large Commercial Hot Water Tanks Made of Copper NickelA correctly-sized hot water tank is vital for many businesses including hotels, apartments, and restaurants. A tank that’s too small won’t be able to deliver enough hot water during periods of peak demand, but a tank that’s too big will require you to pay for more hot water than you need.

This process is further complicated by the large number of variables that determine the ideal size of a business’s hot water tank, including the type of business, the number of customers, the number of hot out water outlets and the climate. Consequently, the size of commercial hot water tanks can vary greatly, from 50 gallons to over 11,000 gallons.

This article will provide the basic guidelines for sizing a commercial hot water tank, but you should always rely on an experienced professional when making your final decision.

Sizing Hot Water Tanks

The manufacturers of commercial water heaters typically sell the heating system and water tank as separate units. This approach allows customers to select these two components independently of each other, which is essential given the lack of a clear relationship between heating capacity and storage capacity in commercial heating systems. Your water tank must be able to handle both your peak demand and your maximum usage for the entire business day.

Assume, for example, that you have a restaurant that specializes in dinner. The restaurant operates at maximum capacity for three or four hours each day, with very little business outside that time frame. Since this business only uses hot water for a short period of time, it may be able to get away with using a water heater of relatively low power. However, it would need a large tank since there won’t be much time to heat additional water.

A popular hair salon might be at the other extreme on water requirements. This type of business uses water at a relatively high, but consistent rate throughout the business day. While it may not have the period of maximum usage that the restaurant does, it also doesn’t have a lull period that allows the water heater to heat more water. The salon in this example would be more likely to benefit from a powerful heater than a large storage tank.

Maximum Hot Water Usage Rate

Surgens Using Hot Water at a HospitalThe maximum rate at which a facility can use hot water is known as its dump load, which is an important factor in determining the best size for your water tank. You can determine your dump load by turning all hot water sources in your business to maximum and adjusting their temperature setting to the highest value. Measure the total rate of water usage to find your dump load, typically in gallons per hour (gph). You don’t necessarily need a water heater that can produce hot water faster than your dump load, since the water in the tank is already heated. The critical issue is the combination of your water heater and tank being able to keep up with peak demand.

Assume for this example that you have a dump load of 800 gph, but your water heater can only heat hot water at the rate of 400 gph. Assume further that your period of peak demand is 2.5 hours. Clearly, your water heater would be inadequate for a tankless system since you need 400 more gallons of hot water per hour than your heater can make. However, your heater might be sufficient if your tank is large enough to make up the difference.

Here’s how to calculate the tank you will need to sustain your dump load:


  • C is the capacity of the water tank in gallons
  • T is the time of your peak demand in hours
  • D is your dump load in gph
  • M is your water heater’s heating capacity in gallons per hour.

In this example, you will need a tank with a capacity of C=2.5(800-400)= 2.5(400)=1,000 gallons.

Maximum Daily Hot Water Usage

In practice, businesses rarely change from needing as much hot water as their system can produce to not needing any hot water. Even if your facility can keep up with your peak demand, you can still run out of hot water if your total requirement for the day is greater than the sum of the water heater’s daily production and the capacity of the tank.

You can calculate the smallest tank you will need to meet your daily requirements with the same formula C=T(D-H), except that T is the length of your business day in hours and D is the average usage rate of hot water in gph. Assume for this example that your business day is 12 hours long, you use an average of 600 gallons per hour, and your water heater produces 400 gallons per hour. Your water tank would need to be at least C=12(600-400)= 12(200)=2,400 gallons.

Realistic Hot Water Tank Calculations

Hot Water Tanks for Large CondosIn practice, professionals use a calculating tool to estimate a business’s hot water tank requirements. They will then adjust that estimate based their own experience. These tools use highly complex algorithms that take many characteristics of your business operations into account, including, but not limited to the following:

  1. Number of hot water fixtures
  2. Venting requirements
  3. Peak demand period
  4. Size limitations
  5. Business hours
  6. Climate

The venting requirements in a particular municipality can significantly reduce the largest water tank you can install. Many businesses also have physical restrictions that limit tank size. Climate can affect the optimum tank size by changing the water heater’s production rate.

Get a Quote for Commercial Hot Water Tanks

Commercial tanks are typically sold separately from water heaters because tank size often bears little relationship to heating capacity for businesses. Ideally, you want a tank that’s no larger than what is required to meet your business’s maximum usage requirements.

You can use your maximum usage rate and maximum daily usage of hot water to get a general idea of how big your tank will need to be, but a professional will ultimately be needed to make a recommendation based on a thorough evaluation of your facility.

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