A senior buildings services engineer recently asked me why his variable flow hydronic systems never delivered the energy savings he modelled and expected. My rambling answer follows.
Variable flow hydronic systems are ubiquitous in North America due to their perceived energy efficiency. However in my experience they rarely deliver on energy efficacy due to equipment and control valve over sizing plus a consistent disregard of how to Commission and optimize them. Variable flow hydronic systems are absolutely not, a start-up and walk away Commissioning activity.
A variable flow hydronic system is a complex group of interacting, interrelated elements forming a complex system. Commissioning them requires a holistic, whole system approach, so lets outline a logic process get this done. BTW; all logic processes start and finish with a logic diagram IMHO.
First things first, YES, you do have to proportionally balance a variable flow hydronic system. Why? Because a balanced system matters at times of maximum simultaneous demand.
Second things second, the DP sensor controlling the variable pump set must be installed about 2/3’s of the distance from the pump to the end of the pipework distribution network and NOT in the mechanical room.
The Commissioning process in summary is:
Installation and pre-commissioning tasks – whole system
- Install the whole system including the entire pipework distribution network, all pumps, power, VFD’s, controls plus other system interfaces.
- Quality control checks, pipework pressure testing, controls programming plus uploading of controls “code” and system start up.
- Complete and verify system dynamic flushing and chemical treatment.
- System, filled, vented & pressurized.
Dynamic commissioning tasks – whole system
- Start up and preliminary commissioning of the pump VFD’s.
- Controls point to point testing.
- Calculate and apply system diversity (if applicable).
- Complete proportional balancing with diversity applied at maximum simultaneous demand flow rate (System VMax).
- Test and determine the optimum (i.e. index circuit satisfied at lowest possible pump speed) DP set point with diversity applied.
- Measure and record the pressure drop across the system strainers (clean) and chiller at V Max.
- Test the system on minimum demand (V Min) and record the pressure at the system DP sensor plus the VFD speed.
- Test, determine, set and record pump VFD V Max speed, Vmin Speed and trip speed.
- Test and validate all controls sequences of operation i.e. morning start-up, Vmax, Vmin, night set back, temp reset on minimum demand, fire interlocks etc.
- Test and ensure, were applicable, that the primary flow is greater than the secondary flow at V Max and V Min and two mid points.
- Set parameters and alarms for “set points out of range” on the BAS.
- Complete all Functional Performance Tests (FPT’s) and demonstrate to the authority with jurisdiction.
So how many and who are the players required to commission a variable flow hydronic system?
- The main contractor / GC – to coordinate the specialist sub-contractors, provide access etc.
- The Commissioning engineer / technician / authority – please don’t call him or her an “agent”!
- The mechanical contractor – typically the installation is their responsibility.
- The TA&B specialist – to measure, test and determine empirically, the optimum system set points.
- The controls specialist – probably the most important player.
- Specialists from vendors such as the VFD, chiller, controls, cooling tower.
It should be noted that the empirical setting up of the VFD and DP sensor (TA&B, CxA and Controls people working together at the same time) are critical if there is any hope of achieving the system energy targets for the building. The set & operational control points that are required to be determined, set, locked in and recorded (with clean strainers) are as follows:
- Optimum set point @ DP sensor with diversity applied @ V Max
- VFD Hz @ V Max – Noted for record with clean strainers
- Pressure @ DP sensor with system @ V Min
- VFD Hz @ V Min – Noted for record with clean strainers
- Chiller temperature set point
- System water temperature set point @ maximum simultaneous demand
- System water temperature set point @ minimum simultaneous demand (set back, on low load if applicable)
- Time schedule
Call me old fashioned, but I believe building systems should be set-up for optimum operation to meet the design intent. This is the basic working assumption by building owners when they employe design and construction teams.
Remember all building energy models assume system optimization with no defects. It is on buildings services design engineers to specify the level of systems (HTG & CHW) Commissioning outlined above.
How many systems do you know of that have been set to work to this level of testing and optimization?