I'm reposting my question below as it has been a few days with no reply.
Thanks!
~Bill
William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
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bbishop at pathfinder-ea.com
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Bill,
As far as I understand G3.1.3.5 stems from the following prescriptive
requirement of 90.1:
6.5.4.1 Hydronic Variable Flow Systems. HVAC
pumping systems that include control valves designed to
modulate or step open and close as a function of load shall be
designed for variable fluid flow and shall be capable of reducing
pump flow rates to 50% or less of the design flow rate.
There is also this example in 90.1 User Manual:
Example 6-LLL-Variable Flow Hydronic System
Q
A hot water system has two-way valves at most coils, but occasional
three-way valves are
provided at the end of branches to ensure flow through them. Does this
design comply
with the Standard?
A
Yes, provided the total flow through three-way valves does not exceed 50% of
design
flow. While these end-of-line valves are allowed, they are not usually
required except
perhaps in very large campus systems. Water piping is generally designed for
water
velocities that are high enough so that the time it takes for chilled or hot
water to leave
the plant and reach the control valve will be seconds or minutes, a small
enough time
that the system will not be "starved" and no discomfort will result. To
minimize energy
use in variable flow systems, limit the use of three-way valves to one or
two to prevent
pump dead heading.
I typically follow the approach that you described on LEED models, but
arguably the minimum flow can be increased to 0.5 based on the above. I am
curious to hear what others think!
Maria
I'm with Maria!
6.5.4.1 is also referenced from the Chapter 11 system description footnotes,
which I've found helpful for other "gray" areas regarding baseline hydronic
requirements. The same text provides guidance for head/efficiency ratings
which correspond with 22 and 19 gpm.
I have also interpreted a minimum flow rate of 0.5 from that passage in the
past, though I confess I haven't been terribly consistent in applying it to
my baseline models over the years.
~Nick
PS: Bill, why so impatient??? You know we don't operate on a timely
schedule around here ;).