DHWS Practices: Pump Size Calculations & Other Design Considerations

To ensure a supply of hot water is provided at every fixture in a timely manner, buildings typically utilize a domestic hot water recirculation system in their plumbing design. DHWR systems are considered a best practice in commercial, multi-family, and institutional buildings, especially if designed with thermostatic balancing technology and accurate pump size calculations and other considerations, as listed in this article.

Such recirculation systems should guarantee occupant comfort, safety, and code compliance, as well as water and energy conservation.

DHWS - DHWS Practices: Pump Size Calculations & Other Design Considerations

When designed properly, a DHWS will provide adequate amounts of water at the prescribed temperature to all fixtures and equipment during all times of use while ensuring a cost-effective, efficient, and lasting installation.

How to Improve Your DHWS Design with Thermostatic Balancing Valves

Understand the Importance of Balancing Domestic Hot Water Recirculation Systems 

A DHWS needs to be properly balanced to ensure hot water is readily available on-demand, on all floors, and at every fixture.

Water is “lazy” and will flow to the path of least resistance. To ensure there is no short-circuiting and all branches have adequate hot water, a restriction point is needed to create only the flow required to offset heat loss in each branch or riser.

Typically relying on multiple branches off the hot water supply line, domestic hot water systems need a device that can respond dynamically to changing needs.

Traditional balancing valves require manual adjustment to create the flow rate needed in each branch.

Challenges associated with manual balancing:

  • Iterative and time-consuming
  • Difficult to properly set to the correct parameters
  • Static and unable to adjust to the changing conditions of a dynamic system.

DHWS are balanced when unoccupied, so “callbacks” are common to rebalance the system when the building becomes occupied.

Implement Thermostatic Balancing Valves

CircuitSolver® self-actuating, thermostatic balancing valves automatically and continuously adjust the flow of DHWR systems to maintain a specified temperature at the end of each branch.

Dynamically designed, these valves adjust in response to heat loss and demand fluctuations without the need for manual balancing, which ultimately reduces labor time and costs and eliminates hot water complaints.

Calculate Flow Rate to Offset Heat loss 

When designing a DHWS, you should calculate the flow required to offset heat loss at the system level. Balancing flow will eliminate the risk of short-circuiting.

Minimizing flow velocity should also be considered to prevent any erosion in copper piping. Erosion can occur when high water velocities and hot water mix in the copper piping.

Ensure Accurate Pump Size Calculations

Inaccurately sizing the recirculation pump in your DHWS can result in many problems that will waste time and money in the long run.

Sizing Too Small can result in not enough flow to maintain temperature throughout the system.

Sizing Too Big tends to be unnecessarily expensive due to the need for larger pumps and wasted energy. Oversized recirculation pumps can wear out copper piping and cause pinhole leaks as well.

To ensure an efficient and economic thermostatically balanced DHWS system, follow ASPE guidelines to determine the recirculation flow rate needed to offset heat loss.

For more information on recirculation pump sizing and demonstration examples of heat loss calculations, check out our Recirculation Pump Sizing Guide.