5.5 B5.3. Recirculation System Plumbing Designs

A recirculation system can have one or multiple recirculation loops. Each recirculation loop consists of many pipe sections, which are connected in sequence to form a loop. Each pipe section could have different pipe diameter, length, and location. The compliance software shall use six pipe sections, with three supply pipe sections and three return pipe sections, to represent a recirculation loop.

When multiple recirculation loops exist, all recirculation loops are assumed to be identical. The compliance software shall provide default and standard recirculation system designs based on building geometry according to the procedures described in the following sections. The default design reflects typical recirculation loop design practices. The standards design is based on one or two loops and is used to set recirculation loop heat loss budget.

The first step of establishing recirculation system designs is to determine the number of recirculation loops, Nloopk, in water heating system k. The standard design has one recirculation loop, Nloopk =1, when Nunit <= 8, or two recirculation loops, Nloopk =2 for buildings with Nunit > 8. The proposed design is allowed to specify more than one loop only if the design is verified by a HERS rater.

Otherwise, the proposed design can only be specified to have one recirculation loop.

The standard and default recirculation loop designs are based on characteristics of the proposed building. There could be many possibilities of building shapes and dwelling unit configurations, which would determine recirculation loop pipe routings. Without requiring users to provide detailed

dwelling unit configuration information, the compliance software shall assume the proposed buildings to have same dwelling units on each floor and each floor to have a corridor with dwelling units on both sides. Recirculation loops start from the mechanical room (located on the top floor), go vertically down to the middle floor, loop horizontally in the corridor ceiling to reach the dwelling units on both ends of the building, then go vertically up back to the mechanical room. At each dwelling unit on the middle floor, vertical branch pipes, connected to the recirculation loop supply pipe, are used to provide hot water connection to dwelling units on other floors above and below.

Both the standard and default recirculation loop designs are assumed to have equal length of supply sections and return sections. The first section is from the mechanical room to the middle floor. The second section serves first half branches connected to the loop and the third section serves the rest of the branches. The first and second sections have the same pipe diameter. Pipe size for the third section is reduced since less dwelling units are served. Return sections match with the corresponding supply pipes in pipe length and location. All return sections have the same diameter. For both the standard and default designs, mechanical room is optimally located so that only vertical piping is needed between the mechanical room and the recirculation pipes located on the middle floor. Pipe sizes are determined based on the number of dwelling units served by the loop, following the 2009 Uniform Plumbing Code (UPC) pipe sizing guidelines. The detailed recirculation loop configurations are calculated as following.

Pipe Length in the mechanical room (ft):                                                  Lmech=8

Height of each floor (ft):                                             Hfloor=user input floor-to-floor height (ft)

Length of each dwelling unit (ft):

Lunit =                  (see Equation 7)

Length of recirculation pipe sections (ft):

Len

= Len  = L           + H

×  Nfloor

1              6         mech           floor         2

Equation 27

Len   = Len  = Len   = Len = L                                                               ×

Nunitk

Method WH-LOOPLEN

2              3              4              5

unit

4 × Nloopk × Nfloor

Pipe diameters for recirculation loop supply sections depend on the number of dwelling units being served and return section diameters depend only on building type, as follows --

Dia1, Dia2, and Dia3: derived from Table B-4 based on Nunit1, Nunit2, and Nunit3

where

Dia4 = Dia5 = Dia6 = 0.75 in for low-rise multi-family building and hotel/motel less than four stories

Dia4 = Dia5 = Dia6 = 1.0 in for high-rise multi-family and hotel/motel more than three stories

Equation 28 Method WH-LOOPSZNunit1 = Number of dwelling unit served by the loop section 1 =

Nunitk Nloopk

Nunit2 =              Nunit1

Nunit3 =

Nunit1

2

Note that Nunit values are not necessarily integers.

Branch pipe parameters include number of branches, branch length, and branch diameter. The number of branches in water heating system k is calculated as (note: not necessarily an integer)

The branch pipe diameter shall be determined as follows:

The branch length includes the vertical rise based on the number of floors in the building plus four feet of pipe to connect the branch to the recirculation loop.

Proposed designs shall use the same branch configurations as those in the standard design. Therefore, compliance software does not need to collect branch design information.

Table B-4. Pipe Size Schedule

B6. High Rise Residential Buildings, Hotels and Motels

Simulations for high rise residential buildings, hotels and motels shall follow all the rules for central or individual water heating with the following exceptions.

For central systems which do not use recirculation but use electric trace heaters the program shall assume equivalency between the recirculation system and the electric trace heaters.

For individual water heater systems which use electric trace heating instead of gas, the program shall assume equivalency.

B7.  Energy Use of Individual Water Heaters

Once the hourly adjusted recovery load is determined for each water heater, the energy use for each water heater is calculated as described below and summed.