Reusing An Existing Closed Loop Water System
These installations are quite commonplace, but they usually lack a proper checklist.
How often does an existing closed loop hot water heating or chilled water cooling system get reused as part of a building renovation project, and/or used to expand the area served by the system coverage to provide additional heating or air conditioning? It happens probably every day somewhere, and yet I can’t say I have ever seen a quality control engineering checklist to standardize the process, to maximize the reuse of the pipe distribution and to reuse or replace existing pump(s). Hindsight is always 20/20, so why isn’t there a checklist to avoid past errors and omissions, as well as to improve on system design from site-specific lessons learned?
To start, the design engineer should document the existing basis of design and review it with the existing facility personnel to get a sign-off on the original criteria and design assumptions. As the new design of the closed-loop system is being finalized, the engineer should meet with the facility personnel to get sign-off that the new basis of design is adequately addressing past deficiencies, proposed energy conservation measures, and O&M design intent.
There are a number of checklist tasks to consider as the engineer gives the existing water system a renewed lease on O&M life.
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Use as-built drawings to initiate the basis of design and have the design engineer and an O&M technician walk the site confirming the existing conditions.
- Pipe distribution “things to look for”
- At the terminal units: are the coils piped correctly (e.g., supply connection is on downstream side of multi-row coils)?
- Are there adequate shut-off valves as well as drain valves?
- Is there adequate air-control?
- Does the system function based on 3-way valves, 2-way valves, combination of 3-way and 2-way control valves, or no control
valves at terminal units?
- Survey closely for unmarked bypass valves that may have been installed during the initial construction and used at that time to flush/clean segments of supply to return piping. (Note: on two energy retrofit projects it was determined that one project had 12 undocumented by-pass valves left open and the second project had 15 undocumented by-pass valves left open.)
- Original system design
- What were the original design parameters (e.g., antiquated fixed supply-to-return temperature difference; 20ºF for heating
systems and 10ºF for cooling systems)?
- Was the existing pump head conservatively selected resulting in the discharge balancing valve set 15% or more closed? (Note:
This is analogous to having one foot on the gas and the other foot on the brake.)
- Is there a balancing valve throttled back at an existing VSD pump discharge, introducing unnecessary system friction head?
Let the VSD do all the reducing of pump head.
- Are there chronic operating parameters that need to be addressed (e.g., coils tripping out on freeze protection, excessive pump
noise and/or vibration)?
- Complete a hydraulic model of the closed-loop system.
- Begin with a hydraulic model of the existing conditions and compare it to the existing design conditions to challenge the original design assumptions (e.g., conservative pump head can be an obvious culprit)
- Complete a water balance to record pertinent flows and head pressures to compare with the original design hydraulic model.
- Reassess existing conditions and consider improvements (e.g., is an existing primary-secondary-tertiary pumping strategy applicable for the new design?).
- Based on the adjusted existing condition hydraulic model (computer simulation with actual water readings) add in the new engineered system to come up with the final closed loop system design for optimum performance.
- Based on the new system hydraulic design, can the existing pump be reused (e.g., review pump curve)?
- If existing pump can be reused, does it need a more energy-efficient motor?
The draft of this designer checklist can easily be increased to cover a broad range of issues, concerns, requirements, and compliance. Heating and cooling each have unique needs, too. Once completed, the checklist can always be continuously improved based on lessons learned and other people’s input, so that reusing a closed-loop system will be a cost-effective solution. For more on developing checklist tasks for water systems, read next month’s column on reusing an open water system. ES
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