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January 22, 2019
This set of articles will walk you through how to optimize the equipment and systems that homeowners use day-to-day, like hot water, lighting, and appliances.
The first part of Occupant loads deals with domestic hot water, also known as DHW. This article discusses how to use energy efficiency measures to reduce DHW loads at the design phase of a new build or renovation project.
Here’s the evaluation form, so you can rate yourself on your existing understanding of reducing domestic hot water loads. The rating is from 0 (I know nothing about this topic) to 4 (I’m an expert in this topic and can handle complex tasks on the daily). You don’t have to share this with anyone else, so rate yourself honestly!
If you find DHW Energy Efficiency Measures a challenge, read on. There are links to resources in this article.
If you’ve got this part of the competency down, tune in next week for the third part of High Performance Housing, Occupant Loads.
Domestic Hot Water Load Reduction Strategies
In residential installations, water heated for uses other than space heating is called domestic hot water, often abbreviated to DHW. Typical domestic uses of hot water include cooking, laundry, and bathing.
In the kitchen, hot water is needed at the kitchen sink and at the dishwasher, if there is one installed. Sometimes there is more than one sink in the kitchen.
Laundry can require a washing machine and a utility sink with a hot water feed.
Bathrooms require hot water feeds to shower and tubs, as well as to lavatory sinks.
In North American homes, water heating can represent up to 25% percent of the total energy use, more energy than all other household appliances combined. The actual percentage depends on the house type, number of inhabitants, and the lifestyle of those who live there.
As a builder or renovator, it is out of your scope to control occupant use of hot water, but there are several ways to improve the way that hot water is delivered, reducing the hot water energy load through efficient distribution system design.
First let’s review how DHW systems lose heat. All types of DHW systems experience three types of losses that affect energy efficiency:
Firing losses occur when a primary fuel source is converted to heat.
Standby losses occur by keeping water at the delivery temperature in a tank, or by keeping your water heater ready to provide heat at all times
Distribution losses occur after the hot water leaves the heater
Once equipment choices have been made, firing and standby losses have been determined. The next step is to reduce distribution losses. The distribution system includes the plumbing layout and the end uses, that is: the fixtures and the appliances.
The bigger the distribution system, the bigger the losses. When the water heater is all the way on one side of the house and an end use is at the other, more hot water gets stranded in the pipes than when the two are close together.
What do occupants want from their DHW system?
Safety: not too hot, not to cold, no harmful bacteria or particulates
Convenience: adjustable temperature and flow, never run out, hot water now, quiet
As builders and renovators, our goal is to provide occupants with what they want as efficiently as possible. Not doing so puts a financial penalty on occupants that includes not only the cost of the wasted energy in the hot water, but also water and sewer costs.
Between 1970 and today, the median house size has increased by nearly 50 percent, more bathrooms are included in floor plans, and the number of hot water fixtures has increased from six to twelve. In addition, the distance to the furthest fixture increased from 30 to 80 feet - with this doubling of distance, the time to get hot water to fixtures has also doubled. With changes to fixture flow rates - low flow fixtures, flow rates have been reduced by a factor of three, which also reduced the velocity of the water delivery by a factor of 3. That means that, on top of the additional pipe length and the higher number of hot water fixtures, codes and conservation measures have inadvertently compounded the challenge of reducing hot water loads.
So how much energy is used and how much water runs down the drain while waiting for hot water to arrive?
5 gallons a day is 1825 gallons of water wasted per year, that’s 30 to 50 dollars for energy, water and sewer costs.
20 gallons a day is 7300 gallons of water wasted per year, that’s 130 to 200 dollars for energy, water and sewer costs.
One researcher in the US has estimated that there is between $1 and $2 billion dollars per year of energy and water savings to be had in the 20 million existing homes in the States, and $50 to $100 million per year in problem new homes.
Opportunities to reduce DHW use
There are two primary ways to reduce DHW loads without limiting the volume of hot water available to the occupants:
the plumbing system design
the choice of fixtures
Heat loss in the DHW distribution system increases with:
Diameter of pipe
Distance to end use
You can reduce DHW loads - and water use in general - on the supply side by reducing the diameter of the pipes used and reducing the distance to end uses.
How long it takes for hot water to reach the end use depends on three factors: the length of the pipe runs from the water heater, the diameter of the piping, and the flow rate. The effect of pipe length is pretty obvious: the further hot water has to flow, the longer it will take to get there.
It’s a bit counterintuitive on how smaller pipes can improve energy use, because smaller diameter means restricted flow because a small diameter piper holds less water, right? The reality is: the smaller the diameter, the faster hot water will reach the tap.
Let’s look at the math:
50 feet of ½ ″-diameter pipe holds 3 Litres (0.8 US gallons)
50 feet of ¾ ″ pipe holds 5.3 Litres (1.4 gallons)
50 feet of 1″ pipe holds 8.7 (2.3 gallons)
The cooler-to-cold water sitting in the pipe has to flow out before hot water from the water heater reaches the tap. With 50 feet of pipe, it can take a long time for hot water to reach the tap. In the meantime, all the water goes down the drain.The smaller the volume of that slug of cooled water, the faster hot water reaches the end use.
Some codes have increased the minimum size of supply pipes for residential use. The flow-rate of a faucet or showerhead governs how fast the slug of cooler water in the pipes will emptied. A water-conserving bathroom faucet that delivers 0.5 gallons per minute (gpm), and the system consists of 3/4″ piping, a 50-foot run takes about three minutes to get hot water to the sink. In comparison, the same scenario with ½ ” pipe puts hot water at the tap in about a minute and a half.
Smaller diameter piping will also increase the velocity of the water and can help stabilize the water pressure. The faster-moving water loses less heat on the way from the heater to the end use, and arrives with a bit more pressure behind it (bonus points for showers!).
User benefits of smaller-diameter pipes and shorter pipe lengths:
√ Water pressure remains stable
√ Faster delivery of hot water
√ Simpler install = lower cost
√ Easier to remodel/replace
System Optimization: Improve the layout
What is the key to an efficient piping layout? Keep the volume of hot water between the water heater and the end uses as small as possible. That means short runs and smaller diameter pipes. The challenge to creating an efficient piping layout that can reduce DHW loads is this: most buildings have only one source of hot water and the end uses are spread throughout the floor plan.
Some plumbing system design strategies for DHW load optimization:
Keep fixtures as close together as possible
The most efficient design is a “plumbing core” where all bathrooms, kitchens, and other water demands radiate from one chase. This puts limits on the shape of the home and the floor plan. Short runs, with smaller diameter piping will give the best savings of materials, labor, and energy.
Match the plumbing layout to the floor plan
In a small, compact house with stacked bathrooms and centralized kitchen/laundry areas, a home-run system with one main distribution manifold may be a good idea. In a larger two- or three-storey house, or in a long and narrow one-story house, it may be more challenging to group all rooms with plumbing around one chase. Faced with this floor plan challenge, layout the plumbing in a sub-manifold or a zoned trunk and branch system, with a focus on clusters of fixtures.
Where are the pipes?
Hot water pipes may run through cooler parts of the house — a crawl space or joist bay in the basement ceiling, for example. Wrapping the lines with foam insulation reduces energy losses, and keeps the water in the pipes warmer, reducing the amount of time occupants have to wait for hot water at the end use. Pipe insulation also reduces condensation on exposed cold water lines during the summer.
KEY TAKEAWAY: Diameter of the line, distance and speed the water travels affect heat loss
Whatever the approach to the layout, the system needs to be sized properly for anticipated loads to ensure that enough hot water can get to the fixtures being served. Just like sizing the space heating and cooling loads for a house, the loads in each room served determine the amount of hot water required. An optimized layout will be sized to PDL (peak-demand load) for each fixture.
This article from Green Building Advisor goes into more detail about PDL.
Trunk and Branch:
This is the most commonly found type of layout, because it is the easiest to install and reduces the amount of material required. In a trunk-and-branch system, the main supply lines, the “trunks,” carry water to the general area where it will be used. Smaller-diameter tubing, the “branch” lines, get water to showerheads, faucets and other points of use. Often, the systems are not very well laid out, with branches from branches but no well-defined trunk. The primary problem with a trunk and branch system is that the furthest fixtures may not get adequate supply, especially if other fixtures along the same branch are being used. This can also make the wait time for hot water at the furthest fixtures significant. One additional drawback, in terms of repair/replacement issues, is that each fixture must have a close-by shutoff valve.
Typical trunk and branch layouts have a large-diameter (typically ¾ in.) trunk lines to distribute water throughout a house. Smaller branch lines (½ in. and ⅜ in.) tee off to feed individual fixtures. Trunk-and-branch systems also require a large number of fittings, which are costlier, slower to install, and more likely to leak than a single run of pipe.
This layout has all the branches running to a single plumbing ‘core’ or location (typically the service or utility room). Lines to the furthest fixtures can be shorter than a trunk-and-branch system, but the overall system could use more material than a standard trunk and branch system, depending on how the floor plan of the house. The floor plan can also lead to the problem of the furthest fixture having a wait for hot water.
Home run/manifold systems use a large-diameter (¾ in.) main water line to feed the manifold. Smaller lines run from the manifold to each fixture (typically ⅜” or ½” pipe). Any fixture in the house can be shut off at the manifold. The design flexibility of a home run system has a cost, however. Because a dedicated line is going to each fixture, a lot of piping is used, and a lot of holes need to be drilled.
Home run systems make the most sense when fixtures are close to each other. For example, when the manifold supplies kitchen, bath and laundry needs in different rooms but only a few feet away from each other. With only one main hot water line and a couple of short branches, heat losses are kept to a minimum.
Advantages of a home run system:
Stable water pressure when several fixtures are used at the same time
Simplified installation can reduce capital cost of the system
Work well with open building systems, allowing less invasive, easier remodeling
In this layout, rather than one main manifold, two or more plumbing ‘cores’ are created, possibly each bathroom, laundry, and kitchen gets its own submanifold. The simplest system won’t save any water over a trunk-and-branch system, but would have energy savings if the manifolds are kept near the fixtures and the trunk is large enough to ensure adequate supply of water to the fixture.
There are many ways to design submanifold systems which could require far less pipe and drilling than a home-run system.
This Fine Homebuilding article shows these three types of layout using PEX flexible tubing.
Minimum sizing for hot and cold water distribution systems are dictated by your local Plumbing or Building Code. In some codes the minimum diameter is ⅜” and in others, it is ½”. Work with your plumber to ensure that you are code compliant when changing out any piping. Some codes also have tables showing required sizing for manifolds and tubing/piping.
Copper has been the typical water supply pipe for several decades, it is found in millions of homes and is still commonly used in new construction. Rigid copper pipe is used for long runs, and can be used in exterior settings. Flexible copper is used for short runs, typically leading up to the water heater, or for supply in tight spots under vanities, for example. Flexible copper does not do well when exposed to extreme temperatures, and is best kept to interior installations.
Copper is readily available, but the price can fluctuate due to the cost of bulk copper to pipe manufacturers.
PEX is the short name for cross-linked polyethylene, a semi-rigid plastic tubing. It comes in red for hot, blue for cold, and white for any temperature (these colors only aid in installing the piping and to identify lines for repair/replacement, there are no temperature-related differences).
Because PEX is flexible, fewer elbows are required compared to copper, making installation of new or retrofit systems easier. PEX tubing characteristics - flexibility and quick connections - can make lead to cost savings in labour.
Home-run systems are typically plumbed with PEX tubing, although copper tubing is also be used. In the most common configuration, a single length of PEX is run from the manifold to the fixture so there are few joints (opportunities for leaks) compared to a rigid-pipe system.
Copper and PEX: Comparison of Characteristics
Recycled: A good portion of plumbing pipe and fittings are created from recycled copper. Copper is recyclable after the end of it’s useful life in a house.
Repurposed/Reusable: PEX is made from first generation materials. PEX can't be melted down, a process that would be required to produce new tubing. Though PEX can't be recycled to be re-used for tubing, it can still be re-processed to be used for other purposes.
More Rigid: PEX's flexibility is great when you want to go around corners but can be a drawback when stubbing out to a toilet or sink. Special PEX fittings are available for this purpose.
Bendable: Half-inch diameter PEX can make 5-inch radius turns without the application of heat. PEX needs to be supported every 32” horizontally. Vertically it should be supported every 4 to 6 feet.
More Heat Resistant: To connect to high-heat services, like the water heater, copper or special stainless steel braided connectors may be the better choice
More resistant to freeze-breakage PEX typically has a higher PSI rating than copper, and is much more resistant to freeze-breakage than copper or rigid plastic pipe.
Will Not Give Off Toxic Fumes: PEX will melt and emanate toxic fumes in the event of a fire. Copper has a far higher melt point and does not give off toxic fumes. However, sweating and soldering joints and fittings can be a health concern for installers.
More corrosion resistant: PEX pipes are very resistant to chemicals. PEX is not affected by soil or water conditions that cause corrosion. They're much more chemically resistant to damage from external sources. Installation requires no solvents or torches on the job site eliminating fumes and fire hazards.
Cheaper Fittings: Copper fittings, can be between three and ten times less expensive than proprietary fittings for PEX.
Easy to install: PEX can be cut with a razor blade-equipped rotational cutter or a scissors-type of cutter. PEX does not need specialized knowledge on sweating joints, it can be installed with proprietary fitting systems or with a special crimping tool that uses either copper or steel rings that tighten the PEX onto brass fittings.
Value of Copper: Copper is expensive to purchase. However, in retrofits, remodels, and demolitions, salvaged copper pipe can be sold because the material has continued value.
Low Cost: PEX is a substantially lower cost material than copper pipe.
Add Heat Recovery
Another way to save energy in the actual plumbing design is on the drainage side of the system. If you separate the cold only drainage (toilets) from the hot water drainage, and have the showers on one primary drain, you can take the best advantage of a drainwater heat recovery (DWHR) appliance.
Drainwater heat recovery, works best with simultaneous flows, especially showers, where the drain has hot water swirling down the sides of the pipe and hot water is still being demanded by the shower.
The DWHR appliance is a piece of copper drainpipe, 36 to 72 inches in length that can be installed in new construction or retrofits where the appropriate length of vertical primary drain pipe is accessible. The drainpipe has a system of smaller copper tubes that are wrapped around it from bottom to top. The cold water feed is attached to the DWHR unit at the bottom of the DWHR tubes, and strips heat out of the wastewater by conduction through the copper. No waste water touches the incoming cold feed.
For best performance, the DWHR unit should be plumbed to preheat the cold line. This means it is providing pre-warmed water to the cold supply of the showerhead. Less hot water is required to bring the shower up to the desired temperature. Most DHWR units will provide 38 to 42 percent sensible heat recovery for showers.
The second option is to plumb the DWHR unit to preheat the DHW tank. This does is an indirect way of taking advantage of the heat recovery, and the effectiveness of the heat recovery is reduced.
Water Heater Timers
A water heater timer is a small electronic device, that can be installed onto a gas or electric hot water storage tank of any age. It allows the homeowner to set specific on/off periods for hot water production. Storage tank systems require a constant supply of energy to keep the water at deliverable temperature 24 hours a day. A standard hot water tank runs for roughly 3 hours a day. New units may run for a little over an hour a day. Typical households do not have hot water use patterns that require the heater to be able to charge at any time during a 24 hour period. This means most water heaters are 'off' for 20 to 23 hours of any given day.
While the effectiveness of a timer is going to be dependent on the lifestyle and behaviour of the occupants, it can be an option to consider when carrying out an energy retrofit of a house where an older storage tank is not going to be replaced. For occupant use, the savings are best when the household is on a time-of-use rate and set the timer so the water heater will not come on during peak times (typically from 7 or 8 am to 8 or 9 pm), meaning they avoid paying top dollar for electricity.
Storage tank systems are rated on efficiency by the standby loss rate. Heat transfer from the tank to the surrounding air results in a slow but constant leak of energy. New models that have very low standby losses may not benefit from a water heater timer.
How much money could be saved if a water heater was on a timer? First you need to know what the peak and off-peak rates are, and then you need to determine how much energy the water heater will use per hour.
If a 50 gallon electric tank with 95% efficiency requires 4622 kWh/yr, that works out to be about 12.7 kWh/day. If the cost of electricity at peak use is $0.16, it would cost $2.03 a day to run the hot water tank. If the tank was charged during off peak hours, at $0.08, it would cost $1.01 a day. Over the course of a year, that's a savings of $370. Considering a timer installation could be less than $100, this scenario makes economic sense.
Hot Water Circulation Systems
When occupants complain of long wait times for hot water, a circulation pump is often installed as a remedy. These systems use a pump to circulate hot water from the water heater to the tap, reducing the wait time significantly.
There are three categories of hot water circulation systems:
Time and Temperature
All of these hot water circulation systems save water, but they don’t all save energy.
This type of system only works with a water heater tank located in the basement below the hot water fixtures (showerheads, faucets, and spigots). A loop of pipe is installed between the tank and the fixtures with the return line coming from the farthest fixture served.
This return line feeds cooler water into the tank, which sets up a thermosyphon with the hotter water in the top of the tank. Water circulates through the loop continuously, without a pump. While saving water costs, this type of system can potentially lose more energy in the loop than was required to initially heat the water.
Continuous circulation pump systems
This type of system keeps hot water flowing through the plumbing all the time so that hot water is always available with almost no delay. The problem with this approach is that you waste energy, as all of the hot-water piping acts like a radiator, 24/7. Pipe insulation helps, but there is still a significant addition to the hot water load in a house, especially where piping moves through unconditioned or cooler spaces.
Time and temperature systems
Like a continuous circulation system but with controls (timer and thermostat) to anticipate demand. An override allows the pump pump to be activated manually. From an energy load perspective, a time and temperature system cuts waiting time and saves water, but are a net energy increase due to higher standby heat losses: the pipes become extensions of the storage tank.
This type of system pumps to the point of use only when it is needed, minimizing standby losses. Here’s how it works: when there is a demand for hot water in a remote bathroom, for example, the occupant pushes a button to activate a small pump (typically installed in the vanity under the sink. Hot water is pumped from the water heater, and the cooled-off water that’s been sitting in the pipes is pushed back to the water heater, either through a separate piping return that has been installed (most common in new construction), or via the cold-water line (more common in retrofit applications). As soon as hot water reaches the tap, a temperature-controlled switch turns off the pump. On-demand system have a wireless remote or occupancy sensors as controls as well as a manual override. Occupancy sensors willwill lead to some waste since the circulator pump will turn on whether or not hot water will be needed.
At least one company produces and sells a showerhead with a specialized valve that reduces water waste: the shower is turned on, and when hot water reaches the valve, flow is reduced to a trickle. The occupant turns a knob or pull acord to resume full flow when they step into the shower.