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Hot Water Distribution

The hot water distribution system in a house introduces heat losses to the domestic hot water system as a whole, but the magnitude of those losses can vary significantly depending on the geometry and insulation added to the pipes. The total losses introduced by the distribution system can vary from 2% to 25% [1]. Adding pipe insulation is an easy, inexpensive measure, but is not practical in retrofits. In fact, any efforts to improve the hot water distribution system in a house are truly only applicable to new construction.

There are two main geometry categories that are commonly used in homes; trunk and branch, and manifold or home-run systems. Trunk and branch is the most typical plumbing arrangement. A larger diameter trunk for both cold and hot water runs through the house and smaller branch pipes come off the trunk to supply water to individual end uses. This distribution system is the simplest in terms of installation and pipe length required, but the large trunk diameter means that it takes more water to get hot water from the water heater to the fixture.

A diagram example of trunk and branch water distribution system in a house. A large trunk line goes from the water heater throughout the house and the smaller branch lines bring water from the main trunk to the specific end-uses.

Diagram of a trunk and branch distribution system in a house.
The thicker trunk line is a large diameter pipe and the thinner
branch lines are a smaller diameter pipe
Credit: www.epa.gov

Home-run distribution systems run a single supply line for each fixture, which reduces the number of connections (additional heat losses) and allows the pipe diameter to be much reduced, which reduces wait time to get hot water. Considerably more plumbing is required for this set up.

A plumbing manifold for hot and cold water for a home-run distribution system is shown. Each line goes directly to each end-use, rather than using a larger trunk line to distribute water throughout the house.

Plumbing manifold for hot and cold water home-run distribution using PEX piping

Credit: www.energy.ca.gov

 

There are also hybrid distribution systems that combine the trunk and branch arrangement with the manifold idea: a smaller manifold supplies each room but a trunk still supplies the house water. Recirculation loops are another option that is primarily used to reduce wasted water since the recirculation loop keeps hot water in the lines, but energy used by the pump usually offsets any gains achieved by avoiding wasting water.

Recommended quantities to measure in a field test of a hot water distribution system:

  • Water temperature using a thermocouple at the outlet of the water heater
  • Water temperature at a few fixtures in the house, including the fixtures the farthest away from the water heater.
  • Stop watch to measure how long it took for hot water to reach fixture. This would be done during a short term test, rather than long term monitoring.
  • Flow meters at the same fixtures with the outlet temperature being measured. These could be used to measure how much water is wasted waiting for hot water during long term monitoring.

Reference:

1. Krigger, J. and Dorsi, C. Residential Energy: Cost Savings and Comfort for Existing Buildings. 5th Edition. Helena, MT: Saturn Resource Management Inc., 2009; pp. 220.