Knowing The Low-flow Culture: New approaches in water- and energy-efficient plumbing

Photo ©BigStockPhoto/Nikita Sobolkov
Photo ©BigStockPhoto/Nikita Sobolkov

By Geoff McDonell, P.Eng., LEED AP
The last decade of increased awareness and availability of low-water consumption plumbing fixtures and touchless actuators has led to wide industry acceptance. Across the country, building codes have begun to drive even lower water consumption standards. However, as this new low-flow culture takes root, it becomes clear we, as a nation, are not quite there yet. (For more, see Canada Mortgage and Housing Corporation (CMHC) Research Highlight Technical Series 11-101, Monitoring Performance of Retrofitting from Tank to Tankless. See also Brookhaven National Laboratory’s “Actual Savings and Performance of Natural Gas Tankless Water Heaters,” a report prepared for the Minnesota Office of Energy Security in 2010).

Many of us have experienced a long wait for hot (even warm) water to come out of the tap after actuating an infrared sensor or turning the handle. How long is acceptable to wait for the shower to come up to the right temperature before getting in? Surprisingly, there have been many cases where someone has installed instantaneous tankless water heaters in a renovation or new project, only to see no real improvement in energy use or the time it takes hot water to reach the tap.

While the manufacturers of fixtures and trim provide a wide variety of efficient toilets, urinals, faucets, and shower heads, there is still more work needed to accommodate better plumbing piping design to work with all these new low-flow products.

Waste of energy
In spite of the wide availability of low-flow faucets and showerheads, there remains quite a significant amount of water and energy being wasted—in some cases, almost as much as when full-flow faucets and shower heads were used. Much of this comes down to the fact users are leaving the tap running longer to reach their desired water temperature.

The challenges to the engineering and construction community that need to be overcome to provide truly energy and water-efficient domestic hot-water systems include:

  • traditional plumbing piping design approaches and trade resistance to better methods of domestic hot-water piping;
  • plumbing codes requiring larger distribution pipe sizes;
  • many plumbing codes failing to demand pipe insulation in residential houses and suites;
  • codes driving up the need for more energy-efficient service water-heating systems;
  • capital cost resistance for added recirculation pumps to small, residential, domestic hot-water systems (along with the additional piping); and
  • plumbing code ‘minimums’ being used as the maximum standard in many cases.

The main impacts of domestic hot-water distribution design are:

  • water use and wastage, especially in retrofit installations;
  • energy (e.g. electric or natural gas) used to heat the water; and
  • electricity consumed for any recirculation pumps.

The order of the cost impacts to the occupants and building facilities operator are ranked as follows:

  1. Energy used to heat the water—the heat losses are substantial, and hot water has up to 20 times the embodied energy as cold water. Depending on the building, this can be worth thousands of dollars annually.
  2. Electricity to run the recirculation pump can cost a few hundred to a few thousand dollars a year, depending on the building’s size.
  3. Water waste can also be a few hundred to a few thousand dollars annually, depending on the size of the building and local water rates.

Potable water heating, which also can include domestic water booster pump operation in high-rise buildings, generally represents 20 to 30 per cent of energy for a typical residential high-rise building, depending on the building’s location and climate zone. According to a 2005 study sponsored by the Canadian Building Energy End-use Data and Analysis Centre (CBEEDAC), “domestic water heating is estimated to be the second largest energy end-use for Canadian households, accounting for approximately 22 per cent of total household energy consumption.”

Waiting for the warmth
The extended wait time for hot water to get to faucets and shower heads is not limited to residential piping systems; it also frequently occurs with most commercial and institutional buildings that are normally equipped with a full domestic hot-water recirculation system. It does not matter whether the system is a high-efficient condensing gas-fired instantaneous heater, electric hot-water tank, or high-efficient electric or gas-fired tankless instantaneous model, the same question remains for users: How come we are not getting hot water at the tap any quicker?

This situation is especially common in buildings where the older high-consumption fixtures have been swapped out for new water-efficient ones, and the domestic hot-water heater has been upgraded to a new higher-efficient model. This is a huge issue for retrofit installations in both existing residential and commercial buildings, since the existing plumbing system was designed for the original full-flow fixtures. The only way to improve the piping infrastructure would be to tear out walls and ceilings.

This author’s own learning experience was a result of observing a local university going through a campus-wide program of retrofitting building plumbing fixtures with new low-flow equipment and infrared touchless low-flow lavatory faucets in the late 1990s and early 2000s. It took a few years of statistical evidence to pile up, along with complaints to the facilities management office, before design engineers and consultants involved with the work began to realize hot water was not getting to the fixtures quick enough, and a great deal of water was still being wasted while people kept activating the faucets until they got warm water.

How long one has to wait for hot water depends on three factors:

  • distance from the water heater or recirculation take-off branch;
  • diameter of the service piping; and
  • flow rate to the fixture.

The effect of distance is fairly obvious—the further hot water has to flow, the longer it takes to get there. Currently, most provincial plumbing codes require efficient fixtures, but there are no specific requirements regarding ‘wait-time’ for getting hot water to a fixture. The American Society of Plumbing Engineers (ASPE) recommends:

  • acceptable performance: one to 10 seconds;
  • marginal performance: 11 to 30 seconds; and
  • unacceptable performance: more than 30 seconds.
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