Appendix G "riding the pump curve"

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I'm reposting my question below as it has been a few days with no reply.

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7


As far as I understand G3.1.3.5 stems from the following prescriptive
requirement of 90.1: 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


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?


Yes, provided the total flow through three-way valves does not exceed 50% of

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

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 Karpman's picture
Joined: 2011-09-30
Reputation: 0

I'm with Maria! 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.


PS: Bill, why so impatient??? You know we don't operate on a timely
schedule around here ;).

Nick Caton2's picture
Joined: 2014-11-17
Reputation: 0