Today’s shift in water heating systems toward lower flow fixtures, higher system temperatures and increased input-to-storage ratio has changed how plumbers, facility managers and building owners evaluate water heaters. They must consider the added complexity associated with these advancements and their impact on operation. Added to that is a facility’s constantly changing requirements based upon demand and seasonality.

Properly calculating the project scope and selecting an optimally sized water heater with the ideal ratio of Btu input to storage is a challenge for all these reasons. A balance must be achieved between providing the necessary hot water — even during peak periods — and maintaining operational efficiency.

The goal is to select a water heater that delivers safe, reliable hot water with precise temperature control, lower cost of ownership and effective Legionella mitigation. Ensuring long-life, reliability and maintaining a compact footprint are other factors that come into play, as always.

A new approach to sizing and designing water heater systems provides an efficient means to check all these boxes. The theory is designing systems that combine the advantages of both instantaneous and storage water heaters to install the right-sized solution meets the demand efficiently and safely and produces the best possible return on investment (ROI). 


Lower operational costs

By prioritizing Btu input to generate hot water and relying on the stored water only as a supplemental hot water source, dynamic water heaters (shown in Figure 1) reduce tank size compared to a traditional storage water heating approach, and has lower standby heat losses. Unlike instantaneous water tube heaters, which require multiple units to energize (redundant Btu input) to meet over-drive demands, a dynamic water heating system can utilize its moderate amount of stored Btu to accommodate intermittent bursts of 25 gpm demand, for example, without a requirement to energize additional units.

These systems are particularly effective during peak demand, such as early morning showers in hotels and dormitories, and dinner hours at a restaurant. A dynamic system saves on infrastructure cost by avoiding the requirement of instantly available high Btu input to satisfy peak loads.


Superior temperature control

A dynamic water heater controls outlet water temperature within a narrow range due to the buffering effect of available stored water combined with a modulation rate of 4-to-1 and higher to adjust the burner’s Btu input in relation to the hot water demand. No minimum flow rate is required to activate the heater, ensuring hot water is always available.


Improved operation in small footprint

Dynamic water heating systems that are properly sized must address the common issues that plumbers, facility owners and managers specify, as well. These issues include:

  • Longer service life: More advanced dynamic water heaters are constructed with materials like duplex stainless steel alloy, which is highly resistant to corrosion at high temperature and requires no tank lining or anodes. Water heaters utilizing this type of tank have a longer life, while requiring less maintenance and repair.
  • Compact footprint: With real estate at a premium, limiting the space required for the water heaters creates an ancillary financial benefit. Creating more revenue-generating square footage is a goal of every facility owner. Dynamic water heaters are typically small enough to fit through a standard 36-inch doorway and have smaller storage tanks ranging from 25 to 200 gallons.
  • Legionella mitigation: The risk of Legionella in domestic hot water is mitigated by utilizing a 140° F setpoint to aid in neutralizing harmful bacteria. In addition, the stored water in a dynamic water heater is consumed four or more times an hour. A high turnover rate such as this continually introduces fresh water and minimizes the aging of water in the system and the associated decay of free chlorine. The design also reduces the surface decay effect of free chlorine by virtue of a much lower surface-area-to-water volume ratio compared to water tube instantaneous heaters.


Online tools

Proper sizing of the water heater is imperative and can be aided by digital technology. Plumbing contractors and engineers can use web-based applications to quickly, easily and accurately calculate the applicable size of a potential water heater project. Better web-based apps guide plumbing professionals through inputting project specs, including facility type, fixture count and system inputs to provide a recommendation for their optimal system (shown in Figure 2).



Water heating systems for today’s facilities are complex and must meet ever-changing demands. The best approach to installing an optimally sized water heater can be accomplished by utilizing a hybrid system that combines instantaneous and storage water heaters. Such dynamic water heating systems meet demand, while also providing space and operational efficiencies.