December 1, 2012
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:
The main impacts of domestic hot-water distribution design are:
The order of the cost impacts to the occupants and building facilities operator are ranked as follows:
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:
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:
Figure 1 summarizes ASPE data from the association’s Domestic Water-heating Design Manual for a summary based on flow testing with different pipe materials.
Traditionally, even new, large houses with floor areas greater than 390 m2 (4000 sf) are not equipped with any kind of domestic hot-water recirculation system; further, domestic water pipes are not required to be insulated in many jurisdictions. Typically, the domestic hot-water tank/heater is located in a basement or ground-floor utility room, along with the heating/cooling unit (e.g. gas-fired furnace or heat-pump air unit).
Well-planned installations will try to have the hot-water heater as centralized as possible to keep the domestic hot-water pipe runs to the furthest fixtures minimized. Based on Figure 1, even a 12-mm (½-in.) copper tube longer than 7.6 m (25 ft) from the heater to a lavatory on the upper floor exceeds the acceptable wait time, and wastes more than 4.5 L (1 gal) of water.
Figure 2 provides an example of what could be considered a modern, best-practice, hot-water distribution system, using a small recirculation system and minimized branch pipe lengths to the fixtures.
Another constraint for gas-fired domestic hot-water heaters—both instantaneous and tank-type—is the gas-fired appliances must be located where they can be accessed, and have flue routes up through the building or to the outside wall. The instantaneous gas-fired domestic water heaters that can be used as point-of-use hot-water dispensers still need code-compliant clearances and access to the exterior for the flues and combustion air.
Electric point-of-use instantaneous hot-water heaters are less constrained than the gas-fired units, but incur large capital and energy cost impacts due to the amperage draw and Canadian Electrical (CE) Code minimum breaker and wiring requirements. Additionally, the cost of an equivalent kWh of electrical energy in most of Canada is more than the expense of its natural gas equivalent—sometimes almost double.
Whether tankless instantaneous hot-water heaters actually save energy depends on several factors. In a retrofit residential installation with older piping systems and longer un-insulated pipe runs to the fixtures, studies have shown there are very little savings compared to a gas-fired domestic hot-water storage tank.
In a new installation with proper best-practice design and insulated plumbing piping, instantaneous hot-water heaters can show energy savings from reduced standby losses. However, given the relatively low cost of natural gas, and the capital cost of the tankless heater compared to a conventional high-efficiency tank-type water heater, the payback can still exceed 15 years or more.
Since the provincial plumbing codes do not incorporate any wait-time requirements, and since most small residential projects do not involve a professional engineer or plumbing designer, the plumbing contractor is usually the designer/installer of the domestic piping system in the house. Frequently, the lengths of piping from the hot-water heater will be longer than 7.6 m (25 ft) to many of the plumbing fixtures in the house; they are also unlikely to be insulated. The result, therefore, may be the latest in low-consumption fixtures and trim, but still as much water waste as a 40-year-old house that still has its standard plumbing.
Multi-unit residential buildings
Residential apartment buildings and condo high-rises usually have two different styles of domestic hot-water systems:
The good thing is these projects will likely have a consulting engineer and a commercial plumbing design approach, so the potential for poor domestic hot-water piping design is usually minimized.
The compartmentalized system usually consists of a 151- or 227-L (40- or 60-gal) electric heater/tank unit located in a closet of the apartment, with domestic hot-water run-outs (copper or cross-linked polyethylene [PEX] branch pipes from a main to a terminal device or fixture) to the fixtures through the ceiling or walls. Some current designs of PEX piping within the suite include equal pressure manifolds and run-outs that, if the plumbing designer and design engineer are on top of the latest recommendations, will have relatively short run-out branch pipes to the in-suite fixtures.
Central domestic hot-water systems in residential apartment buildings usually have multiple gas-fired heater/tank units located in a rooftop penthouse, and are equipped with a recirculation system. The domestic water piping is generally arranged with hot- and cold-water risers running vertically behind the plumbing fixture groups. Thus, there can be relatively short branch-pipe connections to the individual sinks, showers, and lavatories.
However, the balancing of the recirculation connections to the domestic hot-water risers and the arrangement of how these connections are made can be critical for the difference between providing a short wait time and happy residents, and having a bank of suites wasting large amounts of water to get the right temperature at sinks. The due diligence of the installer, balancing agent, and design engineer of record need to come together to ensure the recirculation system is equipped with proper hydronic balancing valves, and that they have been set up to make certain of proper flows at all domestic hot-water risers.
The next step is to resolve the energy efficiency for operating the system. Running even a fractional horsepower recirculation pump can contribute to a small, but significant, fraction of the building energy consumption, along with the standby heat losses from all that hot water circulating around through pipes insulated to ‘code-minimum’ thickness. The State of California is starting to drive most of the leading-edge North American energy codes and standards, and there is movement toward using a demand-control recirculation pump operation.
The traditional method of domestic hot-water circulation pump control is to use a time-of-day schedule; this means having the recirculation pump running during the normally acceptable occupied hours, which could be up to 16 or 18 hours a day. A demand-controlled pump, on the other hand, would be started and stopped based on a number of options:
From retail shopping centres to high-rise office buildings, commercial buildings also have similar issues. However, they are less affected than residential buildings due to the much lower plumbing fixture count. Nevertheless, the same issues arise for retrofitted low-flow fixtures in an older plumbing system, as well as new installations where the domestic hot-water heat maintenance systems are not designed to best-practice standards.
Commercial office buildings can be ideal project types to specify a demand-controlled domestic hot-water recirculation system using occupancy sensors in the washrooms due to the intermittent use throughout the day. The key design issue involves ensuring the domestic hot-water recirculation connection to each lavatory is made within 2.5 m (10 ft) of the faucet.
Retail projects where the washrooms could have high traffic are best served by a time-of-day domestic hot-water recirculation system, or an electric heat-traced temperature maintenance system in new construction applications. The self-regulating heat tracing is reasonably energy efficient provided the pipe insulation is properly installed, ideally with insulation thicker than the code minimum. The most important thing is to make certain the run-outs to the fixtures requiring hot water are designed properly in the first place for new construction.
A viable option for small local washrooms are point-of-use water heaters, consisting of a local electric heater with a small amount of storage capacity. This can achieve relatively low-energy requirements and quick hot-water flow, but at the expense of lost storage space under the sink and counter (Figure 3).
Properly piped demand-controlled domestic hot-water recirculation pump systems, along with improved and complete pipe insulation application, can provide the following improvements:
Of course, the only way this becomes mainstream design—for everything from a single-family dwelling to high-rise apartments—is for the building codes to incorporate the minimum requirements for energy and domestic hot-water systems performance parameters. (To be blunt, voluntarily designing a low-loss/energy domestic hot-water system would not significantly impact the country’s energy use and water waste since the general ‘budget-cost’ approach is to design a building to simply meet the minimum code requirements. As energy and water costs get higher, there will be some economic drive to try to reduce waste and provide domestic hot-water energy conservation, but the code-minimum standards must be raised for there to be real change).
Geoff McDonell, P.Eng., LEED AP, is an associate partner at Cobalt Engineering LLP in Vancouver. He has more than 30 years of experience in mechanical engineering, and is a registered mechanical engineer in British Columbia and Alberta. McDonell specializes in low-energy mechanical systems, including radiant cooling applications, Passivhaus style designs, and assisting architects with building envelope performance evaluations. A Leadership in Energy and Environmental Design-accredited professional (LEED AP) since 2001, he was a speaker at the first Greenbuild conference. He can be contacted via e-mail at firstname.lastname@example.org.
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