Hydronic Balancing — What Actually Happens on Commissioning Day
The design schematic says every terminal unit gets its design flow. Commissioning day says the variable speed pump is hunting, two AHU coil valves are jammed open, the dynamic balancing valve at level 4 was installed backwards, and the BMS is reading 22°C when the room is 17°C. Here's the actual sequence, the actual measurements, and where the design assumptions break down on a real system.
David Okafor
CIBSE Commissioning Specialist + LCC + NVQ Level 4 · 25 yrs · Independent Commissioning Manager (ex-Hoare Lea M&E + ex-MJN Colston)
The design has its first principles — the physics. So does delivery. These are the three a good practitioner carries onto every job; get them in your head and the rest becomes easy. The read from David Okafor on this topic:
the basics that make everything else easy
Balanced means witnessed. A flow rate you didn't watch get measured didn't happen. The whole commission rests on real readings at real terminals — not the sheet the installer handed you.
the order of operations — what comes before what
Flush, fill, vent, set the index (worst) circuit, then work back to the pump and prove it under load. Don't balance a dirty system, and don't balance at one load and call it done — people set static flows and leave, and on a variable-flow system that's only right at one operating point.
what to look ahead for — anticipate, don't react
On variable flow, assume the balance will drift the moment loads change — so check whether PICVs were specified and installed the right way round before you start chasing flows. Calibrate the BMS sensors before handover, not after the occupants complain they're cold.
On site — the practice points
- 1
Witness, don't trust the sheet
If you didn't watch the flow being measured, it didn't happen. 'Balanced' on the commissioning sheet is worth nothing without a witnessed reading at each terminal unit.
- 2
Pump curve vs system curve
Variable speed pumps hunt because the system curve isn't where the design said it would be. Pressure-independent control valves fix this — but only if they were specified at design stage AND installed the right way round.
- 3
BMS calibration before sign-off
The BMS thinks it knows the room temperature. It doesn't. Calibrate every sensor on the system against a witnessed reading before you hand the system over. Otherwise occupants are cold and the BMS thinks everything is fine.
Site walkthrough video
Hydronic Balancing — What Actually Happens on Commissioning Day
Mock — live video activates when content arrives
The single thing new starters struggle with most: turning a drawing into work on site. This is how David Okafor reads the hydronic schematic and the terminal-unit schedule — design flows and valve types:
On the drawing
design flow 0.18 l/s per FCU
What it means on site
that's the target at each terminal — but only reachable if you set the index circuit first. Balance the easy near circuits first and you'll never get flow to the far ones.
On the drawing
2-port control valves shown
What it means on site
this is a variable-flow system — a fixed manual balance is only correct at one load. If the designer hasn't specified PICVs, expect drift and flag it.
On the drawing
BMS sensor points
What it means on site
the BMS reads what its sensors tell it, and they're often 3–5°C out of calibration. Calibrate against a witnessed reading before sign-off, or the building runs cold while the BMS says it's fine.
This is the delivery half.
Same topic, two credential chains. The chartered engineer sets the intent; here's how it's made real on site.
Delivery knowledge is verified by a Master-Trade or chartered validator and passes the AI pre-publication check. Codes are the floor; judgement is the value.