Design a compact plumbing distribution system.
- Use a core plumbing strategy with all hot water fixtures positioned near and directly piped to a centrally located water heater to shorten plumbing runs and minimize water use in the home.
- Or, use a central manifold plumbing design with PEX piping runs going from a manifold near a centrally located water heater directly to each hot water use.
- Core and central manifold plumbing strategies must be considered early in the design process for the home.
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.
To reduce water use and energy losses associated with hot water, homes should be designed with plumbing layouts and hot water distribution systems that reduce the amount of time it takes for hot water to get from the water heater to each plumbing fixture. Traditional hot water distribution systems consist of a trunk and branch distribution with a relatively long and large diameter main line feeding smaller branches that flowed directly to plumbing fixtures or split to serve multiple fixtures. Even in relatively small homes of 1200 square feet, the volume to the furthest fixture can exceed 1.5 gallons and the time-to-tap can be more than 90 seconds.
One efficient alternative is a “core” plumbing layout. Core systems utilize a central plumbing core, where plumbing areas (i.e., kitchens, bathrooms, laundry rooms) are placed in close proximity to the water heater. Hot water is piped to each plumbing fixture and appliance in as direct a path as possible. The key to central core designs is to determine the maximum allowable water that can run down the drain before hot water arrives and then work backwards to layout the floor plan and route the piping to achieve this goal. There may be more than one plumbing core, each with its own water heater serving nearby plumbing fixtures and appliances.
There are two basic configurations used to pipe a central core hot water distribution system: Long Trunk-Short Twigs (Figure 1) or Short Trunk-Long Twigs (Figure 2).
In both of these configurations the piping “radiates” out from the water heater and both configurations are found in traditional hot water distribution systems. However, in recent years Short Trunk-Long Twigs has come to be referred to as “home-run manifold” or “parallel piping,” terms that have been popularized by the manufacturers of valved manifolds.
As an alternative to valved manifolds, a non-valved tee could be thought of as a one- port manifold. Using a non-valved manifold with several tees also works well. The manifold may be constructed of either plastic or metal. The key for either configuration is to limit the volume between the water heater and the hot water outlets.
While any piping material can be used for central core systems, the use of flexible piping allows the piping to be installed more quickly than rigid, non-flexible piping because fewer fittings are necessary during installation. Because the flexible piping is supplied as spools of continuous piping, plumbers can lay out relatively long piping runs without needing to install coupling fittings at regular intervals. Furthermore, by virtue of the piping’s flexibility, it can make changes in direction using continuous sweeping turns, eliminating the need for elbow fittings, which are time-consuming to install and contribute to the loss of pressure and heat as water moves through the system.
The EPA WaterSense requirements are that no more than 0.5 gallons of water can be on any one path from the water heater to a plumbing fixture or appliance. This means that the sum of the volume in the trunk, branches and twigs, plus all of the fittings and the fixture supply pipes under sinks, the riser from the shower valve to the showerhead and the volume in the faucets and shower heads cannot exceed this amount. Table 1 in the Compliance section provides information that can be used to calculate the volume in the piping, including the fittings, from the water heater to the angle stop or valve serving the fixture or appliance. The article titled Part 3: Meeting the One-Cup Challenge, as found on the Alliance for Water Efficiency website, provides more details on this method.
How to Design a Core Plumbing Distribution System:
- Designate a proposed location for all hot water plumbing fixtures and appliances. Best practice design specifies that hot water fixtures be as close to the water heater as possible to minimize heat loss in pipes.
- Design plumbing trunks and branches to store as little water as possible by running the smallest diameter pipe allowed by code between the hot water source and each fixture.
- To meet EPA WaterSense criteria, calculate the stored volume between each water source and water use point and ensure that not more than 0.5 gallons of water is stored between the water source and the use point. If the volume between each water source and water use point is more than 0.5 gallons, redesign the floor plan and plumbing layout until it is less than 0.5 gallons. See the Compliance tab for information regarding pipe volume per length for various pipe diameters.
- Record all plumbing design features on a plumbing diagram. Install the plumbing according to the codes outlined in the Compliance tab (including correct pipe diameter and insulation) For more information on pipe insulation, see the guide Pipes in Exterior Walls.
Another efficient plumbing layout method is Demand Plumbing.
Ensure all hot water pipes are adequately insulated throughout the home. Insulation can help save energy on water heating costs, especially in households with multiple hot water draws in a row (example: many morning showers). The 2015 IECC states that all piping ¾ inch nominal and larger shall be insulated. In addition there are certain cases where insulation is also required on smaller diameter piping. For more information see the guide Pipes in Exterior Walls.
Wherever possible, locate both hot and cold water pipes on interior walls as an extra precaution against freezing conditions. When water pipes must be located on the exterior wall of the home, ensure they are located toward the inside of the wall, yet still surrounded by at least 1 inch of insulation. Be aware of interior freezing conditions during long vacation periods.
For builders seeking certification through the U.S. Environmental Protection Agency's WaterSense program, an inspector will test all hot water delivery systems to ensure compliance with the WaterSense new home specification’s requirements using the testing protocol described in the Compliance tab.
No climate-specific information applies.
The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Exhibit 1, Item 4) Hot water delivery systems (distributed and central) shall meet efficient design requirements.
Or, Water heaters and fixtures shall meet efficiency criteria. (See Endnote16 below for a description of this alternative.)
The EPA WaterSense Single-Family New Home Specification (Version 1.2, effective July 24, 2014) requires builders to minimize water wasted while waiting for hot water. Specifically:
To minimize water wasted while waiting for hot water, the hot water distribution system shall store no more than 0.5 gallons (1.9 liters) of water in any piping/manifold between the hot water source and any hot water fixture. In the case of occupant-controlled or occupancy sensor-based recirculation systems, the 0.5 gallon (1.9 liter) storage limit shall be measured from the point where the branch feeding the fixture branches off the recirculation loop, to the fixture itself. To verify that the system stores no more than 0.5 gallons (1.9 liters), verifiers shall calculate the stored volume using the piping or tubing inside diameter and the length of the piping/tubing.
System options include manifold-fed systems; structured plumbing systems; core plumbing layouts, and on-demand recirculation systems. The following requirements apply to recirculation systems:
- Recirculation systems must be based on an occupant-controlled switch or an occupancy sensor, installed in each bathroom which is located beyond a 0.5 gallon stored-volume range from the water heater.
- Recirculation systems which operate based on “adaptive” scheduling, meaning that they “learn” the hot water demand profile in the home and adapt their operation to anticipate this profile, are permitted at this time, and do not require the use of occupant-controlled switches or occupancy sensors.
- Recirculation systems that are activated based solely on a timer and/or temperature sensor are not eligible.
Table 1 shows the volume of water for various pipe widths and lengths and maximum pipe length allowed between the water source and the use point for various pipe sizes to meet the 0.5-gallon maximum required by EPA WaterSense, Version 1.2.
The EPA document Inspection and Verification Guidance for WaterSense Labeled New Homes, and the ZERH program, requires a test to ensure that hot water waste is minimized in these certified homes.
To account for the additional water that must be removed from the system before hot water can be delivered, no more than 0.6 gallons (2.3 liters) of water shall be collected from the hot water fixture before hot water is delivered. Recirculation systems must be based on an occupant-controlled switch or an occupancy sensor. Recirculation systems that are activated based solely on a timer and/or temperature sensor do not meet this requirement. Recirculation systems which operate based on “adaptive” scheduling, meaning that they “learn” the hot water demand profile in the home and adapt their operation to meet this profile, are permitted at this time. To verify that the system meets the 0.6 gallon (2.3 liter) limit, verifiers shall first initiate operation of occupant-controlled or occupancy sensor-based recirculation systems, if present, and let such systems run for at least 40 seconds. Next, a bucket or flow measuring bag (pre-marked for 0.6 gallons) shall be placed under the hot water fixture. The hot water shall be turned on completely, a digital thermometer placed in the stream of water just where it meets the water being collected, and the starting temperature recorded. Once the water reaches the pre-marked line (approximately 24 seconds for a lavatory faucet), the water shall be turned off and the ending temperature reading at the same location recorded. The temperature must increase by 10 °F. Only the fixture with the greatest stored volume between the fixture and the hot water source (or recirculation loop) needs to be tested.
DOE Zero Energy Ready Home Endnote 16, Alternative to Testing: Water heaters and fixtures in single family homes and in multifamily dwellings with their own independent water heater meet the following efficiency criteria: a. Gas water heaters, if present, shall have an Energy Factor ≥ 0.90 or a Uniform Energy Factor ≥ 0.87 b. Electric water heaters, if present, shall have an Energy Factor ≥ 2.2 or a Uniform Energy Factor ≥ 2.2 c. All showerheads and bathroom sink faucets shall be WaterSense labeled. d. The hot water distribution system shall store no more than 1.2 gallons between the hot water source and the furthest fixture. This shall be verified by either 1) a calculation using the piping or tubing interior diameter and the system length based on plans, or 2) by a field verification test, using the protocol described in Endnote 15, which demonstrates a minimum temperature rise of 10 °F by the time 1.4 gallons of water is delivered to the furthest hot water fixture. These provisions do not apply to multifamily buildings with central hot water delivery systems. These project types must instead satisfy the Efficient Hot Water Distribution provision instead (see the DOE Zero Energy Ready Home National Program Requirements for information on multifamily systems and additional advisories).
Section R403.3 (R403.4 in 2015 and 2018 IECC) Mechanical system piping capable of carrying fluids above 105 °F (41 °C) or below 55 °F (13 °C) shall be insulated to a minimum of R-3.
Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)
Follow the requirements for plumbing distribution found throughout the IRC, especially including Chapter 29, Water Supply and Distribution.
Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)
Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.
Follow the requirements for plumbing diameter found in Chapter 6, Water Supply and Distribution, of the 2012 Uniform Plumbing Code.
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The following authors and organizations contributed to the content in this Guide.
Traditional hot water distribution systems consist of a trunk and branch layout. This entails a relatively long, large-diameter main line feeding smaller branches that flow to plumbing fixtures or split to serve multiple fixtures. Even in relatively small homes of 1,200 square feet, the volume to the furthest fixture can exceed 1.5 gallons and the time to the tap can be more than 90 seconds. One efficient alternative is a compact hot water plumbing layout where the home is designed so that all rooms using water (e.g., kitchen, bathrooms, and laundry room) are placed in close proximity to the water heater. This results in hot water piping to each plumbing fixture and appliance in as direct a path as possible.