The design of domestic water and hydronic piping systems are notorious for being entirely code-driven. As engineers, we must provide code-compliant systems while designing to meet the standard of care. The three driving factors to a domestic hot water design are occupant safety, occupant comfort, and energy and water conservation, which is increasingly more important. 

The big question is, can we design a domestic hot water system that is not only safe and comfortable but also saves water and energy? The following defines these three items through the eyes of a mechanical and plumbing engineer:

Occupant safety

The first consideration for most engineers when designing a domestic hot water system is likely the safety of the building’s occupants. Hot water is an integral part of promoting the hygiene and well-being of a building occupant. However, it also can be a source of risk. Legionella, a naturally occurring bacteria, thrives in stagnate, lukewarm water. Through inhalation of contaminated water droplets, building occupants can contract Legionnaire’s Disease, a severe form of pneumonia. 

Increasing the water temperature can mitigate Legionella risk. However, high-temperature water also can create a safety hazard. Most adults will suffer third-degree burns if:

• Exposed to 150 F water for 2 seconds;
• Exposed to 140 F water for 6 seconds;
• Exposed to 130 F water for 30 seconds; or
• Exposed to 120 F water for 5 minutes.

Occupant comfort

In addition to trying to avoid third-degree burns, as occupants, we are extremely aware of how warm water needs to be to feel comfortable. 

For example, we demand different water temperatures for when we wash dishes or take a shower.

Energy and water conservation

A large waste of water and energy can be observed when waiting for the shower to heat up before getting in. Therefore, minimizing wait time for hot water to reach the fixture reduces the amount of water wasted. Even though there is no code requirement, wait times longer than 30 seconds are considered unacceptable and a significant waste of water, pump energy and time. 

To achieve maximum occupant health and comfort, the standard of care would require storing hot water at 140 F to prevent Legionella growth and provide a thermostatic mixing valve to achieve the desired temperature and prevent scalding. However, this doesn’t guarantee timely delivery of warm water. As commonly experienced in residential applications, the water must run for an extensive amount of time before reaching the ideal temperature at the point of use.

So, what’s the solution? Can we prevent wasted water and energy use while also maintaining occupant safety and comfort? Designing a domestic hot water recirculation system is a major contribution to that goal. 

When it comes to domestic hot water recirculation systems, also referred to as return systems, one size does not fit all. Every building is different and requires a unique approach. Using standard details is dangerous because there is nothing standard about domestic hot water systems. Here are some things to consider when designing a system.

Domestic Hot Water Circulating Pumps

When it comes to saving water and energy, domestic hot water circulating pumps are an integral component. Special attention must be given to the sizing of recirculation and distribution pumps to provide adequate flow to maintain the temperature of water in piping while minimizing energy consumption. 

Sometimes recirculation pumping may not be required because the demand at each fixture during peak periods of use during the day is enough to maintain the temperature. For this reason, a demand control configuration, such as using an aquastat, can be used to turn on pumping only when required. 

Speed control devices such as electronically commutated motors or variable-frequency drives can be used to balance the system flow and minimize the energy used to recirculate water.

Recirculation Loop Configuration

The overall domestic water recirculation system is often called a “hot water loop.” All piping in the loop should be insulated to reduce heat loss and prevent temperature loss between two degrees and four degrees. The exact savings depend upon insulation, supply temperature and pipe length. 

The hot water loop can include the following three configurations. First, Dead Legs, which are sections of potable water piping systems that have been altered, abandoned or capped such that water cannot flow through them. Dead legs should be eliminated from all domestic water systems as they are a source of heat loss and promote the growth of Legionella bacteria.

Second, Branches, which are potable water piping sections serve a fixture, or bank of fixtures, without a return to the main distribution system. Branches are inevitable but their lengths should be limited as these sections do not have the added benefit of the loops.

Third, Subloops, which, similar to branches, depart from the main distribution system to serve fixtures. Subloops maintain hot water recirculation by reconnecting to the main hot water loop with a balancing valve.

Balancing Valves

Don’t forget the balancing valves! Since the intent of the hot water recirculation system is to maintain the temperature throughout the distribution system, each branch needs to be balanced to achieve adequate flow for each subloop. Without balancing, water in a recirculation system would follow the path of least resistance. Subloops far from recirculation pump would not be recirculated. 

When it comes to balancing valves, the more, the better. Do not shortchange the system; ensure each subloop has a balancing valve because without them it is difficult to experience the added benefits of the recirculation system. 

Balancing valves can either be pressure-dependent or self-actuating. Pressure-dependent valves — as the pressure changes, the flow will change as well — would be considered the traditional method of balancing in domestic applications. Self-actuating valves are pressure-independent. 

One of the up-and-coming technologies in balancing devices includes thermostatic balancing valves that automatically and continuously maintain a set water temperature at the end of each domestic hot water supply. As the water cools at the end of a hot water loop, the calibrated valve adjusts to allow a greater flow through the valve until the water reaches the desired temperature. 

It solves the problem of delayed hot water delivery. The idea is that the valve only passes water needing to be reheated, whereas manual balancing valves allow water to pass through even if the temperature meets the desired temperature.

Whether the system is composed of manual balancing valves or thermostatically self-adjusting balancing valves, all valves must be tested and commissioned to ensure that domestic hot water return flow rates are providing specified water temperatures circulated through each circuit.

Hot water recirculation loops contribute to occupant safety and comfort, as well as energy and water conservation. Through the process of recirculating unused hot water back to the heating source, constant water supply temperatures are provided to each fixture at minimized wait times. A properly balanced system with sufficiently designed distribution provides significant water savings through timely water delivery, resulting in energy savings that lead to lower operating costs.