All businesses use hot water, even if it’s just to heat the tap water in the bathroom. However, some businesses such as dormitories, hospitals, and processing plants require more hot water than others.

Surgens Using Hot Water at a HospitalSizing a water heating system requires the careful consideration of many factors, including the dump load, recovery rate, and usage factors. As we will explore in this article, a proper analysis will determine the optimal size of the water heater, and whether additional hot water storage is needed. This is important because if either of these is insufficient, there won’t be enough hot water available. On the other hand, an oversized system will waste money by heating more water than is needed, in addition to the initial cost of a heating system that’s too large.

Capacity of Water Heating Systems

One of the first considerations for sizing a hot water heating system is to determine the required dump load. The dump load is the maximum rate at which a location will use hot water. It is the demand that would be seen by turning on all of the hot water outlets at the same time and adjusting them to their highest temperature setting. The dump load is usually measured in gallons per hour (GPH).

A second parameter to consider is the water heating systems recovery rate. The recovery rate is a measure of capacity of the heating system, and is expressed in terms of the water flow, typically gallons per hour, it can produce at the hot water temperature setting required. In all cases the water heater’s recovery rate must be at least equal to the dump load. If the system’s recovery rate in less than the site’s dump load, then the system will not produce enough hot water to satisfy the peak demand.

Usage Factors that Impact the Sizing

Another important question concerns the usage profile of the hot water over the course of a day. For instance, does the dump load remain fairly constant throughout the day, or is it intermittent, with higher usage spikes followed by periods of lower demand for hot water? If the dump load is fairly constant, then the heating system’s recovery rate) should be sized to that. However, if the demand is intermittent, then a smaller heating unit, with additional hot water available from a storage tank incorporated into the system, would likely be the better way to go.

An example of this would be a restaurant that is only open for dinner. It only operates at maximum capacity for three or four hours each day, with very little activity outside of that time frame. Since it only uses hot water over a relatively short time span, it would likely be able to get away with using a smaller capacity in a system that drew off a tank holding hot water that was pre-heated during times of low demand (e.g., when the restaurant was closed).

Sizing a Hot Water Storage Tank

The manufacturers of water heating systems typically offer their water heaters and water storage tanks as separate components in a system. This approach allows consideration of the trade-offs between a larger heater / smaller tank scenario versus smaller heater / larger tank as it relates to space, weight, and cost.

To determine the appropriate tank size, let’s work through three examples. The equation to use is fairly straightforward:

C = T x (D – R)


  • C = the capacity of the water tank (in gallons)
  • T = the time span of the peak demand (in hours)
  • D = the dump load (in GPH)
  • R = the water heater’s recovery rate (in GPH)

Consider the case of a facility with a dump load of 5,000 gallons per hour, but for a duration of only 3 hours per day.

Example 1

For a water heater with a recovery rate of 2,000 GPH, the tank size required would need to be…

3 x (5,000 – 2,000) = 9,000 gallons

Example 2

Alternately, if a larger heater with a recovery rate of 4,000 GPH was used, the required tank size would need to be…

3 x (5,000 – 3,000) = 3,000 gallons

Example 3

Lastly, if the heater was sized for a recovery rate of 5,000 GPH

3 x (5,000 – 5,000) = 0 gallons

which means for a heater of that size no supplemental storage would be needed.

The last step would then be to evaluate the economics of the three (or more) options, as well as any space or building code considerations would apply in each case.


RECO USA custom builds water tanks and hot water heating systems for a wide variety of commercial businesses, manufacturers, and industrial facilities. We also keep standard tanks and systems on hand and offer quick-ship options for many of our commercial hot water heating systems.

Contact us to learn more.