Domestic hot water systems have an important role in buildings, especially in places with cold winters. A water heating system that is well designed will provide water at a suitable temperature, while optimizing energy consumption. The design must also prevent health risks, such as the growth of Legionella bacteria.

Many hot water systems have a layout where water is only heated and stored at a central location. When occupants open a shower or faucet, they must wait for water to reach the point of use. This represents not only a waste of water, but also a waste of pumping power.

Recirculation systems require a pump to keep the hot water in movement, and this requires electricity. Plumbing engineers recommend using the most efficient pump available. However, the operating cost is less than having to pump water from a central heating system, and wasting water and energy while it arrives. Like in many MEP installations, a variable speed motor can greatly improve the efficiency of a recirculation pump.


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When hot water circulation is used, controlling the flow rate is important to prevent erosion. The Copper Development Association provides the following recommendations:

  • Eight feet per second for cold water.
  • Five feet per second for hot water.
  • Two to three feet per second, for hot water above 140°F.

How Can Hot Water Recirculation Improve Comfort?

Recirculation systems keep hot water in constant movement through the piping. When someone opens a plumbing fixture, hot water is already close to the point of use. This is an effective water conservation measure, since plumbing fixtures are no longer left open waiting for hot water to reach the point of use. There are also electricity savings, since pumping systems must deliver less water to the upper floors.

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Recirculation also allows better control over the water temperature, making sure it isn’t delivered too hot either. Many old plumbing systems have clogged pipes and oversized water heaters, and water often reaches faucets and showers at an excessively high temperature. The water temperature in a recirculation system can be controlled more accurately, since any unused water is sent back to the heater.

Waiting for hot water can also be bothersome for building occupants, since it wastes their time. However, a recirculation system is always ready to provide hot water on demand.

Preventing the Growth of Legionella Bacteria

Legionella thrives in stagnant warm water, and exposure to it can cause Legionnaires’ disease, which is a more severe form of pneumonia. When plumbing systems are modified, they are often left with closed piping segments that have no exit. These dead legs should be avoided at all costs, since they provide a habitat for Legionella can reproduce.

Legionella bacteria cannot reproduce easily at temperatures above 120°F, and they die at 140°F. However, these temperatures are too high for showering and other applications where the user is in contact with water. An effective solution is mixing the hot water with some cold water before delivering it to the point of use. This reduces its temperature to a safe level.

Improving the Efficiency of a Hot Water Recirculation System

A recirculation system has a lower operating cost if the water heater has a high efficiency and the plumbing fixtures conserve water. There are many types of heating systems, but the lowest operating cost can normally be achieved with a tankless gas heater or a heat pump. Gas heaters with a storage tank tend to have an intermediate efficiency, while electric resistance heaters are the most expensive to use.

To reduce the water consumption of plumbing fixtures, the best recommendation is looking for the WaterSense label from the US EPA. These efficient fixtures offer at least 20% water savings compared to standard fixtures, and their performance has been validated by independent laboratories.

When combined with an efficient water heater and WaterSense fixtures, a recirculation system can offer comfort and healthy conditions, while optimizing energy consumption.

 

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