OPTIMUS DISTRIBUTOR
- On a variable orifice balancing valve, the same pressure reading means a different flow at each handwheel setting, so the setting must be recorded with every reading.
- Albion Valves UK, in a 2013 article, puts fixed orifice accuracy at usually plus or minus 5% of design flow, against plus or minus 10% or more for variable orifice systems. Pettinaroli's PICV manual gives plus or minus 5% against plus or minus 20%.
- Armstrong, which makes fixed orifice commissioning valves, says a variable orifice valve reaches plus or minus 5% accuracy only when it is fully open, and a valve throttled to balance its branch is no longer fully open.
- Both types need straight pipe around them for a true reading: Albion asks for five pipe diameters upstream and two downstream.
- Pettinaroli suggests a Venturi flow measuring device on every terminal unit and riser, even in a PICV system, so each design flow can be checked and recorded.
On this page10 sections
01What is the difference between a fixed and a variable orifice balancing valve?
The difference is where the flow is measured. A fixed orifice balancing valve reads flow across an opening that cannot be adjusted, while a variable orifice balancing valve reads it across the adjustable seat that also sets the flow, so its reading depends on the handwheel setting. Les Littlewood of Albion Valves UK, writing in Modern Building Services in 2013, defines a fixed orifice as an aperture that cannot be adjusted and a variable orifice as one that can.
Fixed and variable orifice balancing valves are usually double regulating valves. In Albion's definition, a double regulating valve can be fully closed at its set position and re-opened to that same position. A fixed orifice double regulating valve is called an FODRV, and a variable orifice one a VODRV.
Albion puts a balancing valve's job this way: "Balancing valves create additional resistance in each branch to direct water in the required proportion to each terminal unit." How that fits with pressure independent control valves (PICVs) is covered in our guide to hydronic balancing and PICV for commercial buildings.
02How does a balancing valve turn a pressure reading into a flow?
A balancing valve turns a pressure reading into a flow through its flow coefficient, Kvs. The engineer measures the pressure difference, called the signal, across the valve's two test points and converts it with the maker's chart or formula into litres per second. Albion's data sheet for its ART 250 variable orifice double regulating valve, published by Hydraquip, gives the formula as flow in litres per second equals Kvs times the square root of the signal in kPa, divided by 36. The same sheet defines the flow coefficient in cubic metres per hour at 1 bar differential.
On a variable orifice valve, the Kvs changes with the handwheel. Albion's ART 250 sheet lists a different Kvs at each handwheel position, so a pressure reading means nothing until the setting is known. On the DN50 size, the listed Kvs runs from 10.2 at position 2 to 48.2 at position 8. Worked through Albion's own formula, the same 4 kPa signal gives almost five times the flow at position 8 as at position 2:
| Handwheel position | Kvs listed by Albion | Flow from a 4 kPa signal |
|---|---|---|
| 2 | 10.2 | 10.2 x 2 / 36 = 0.57 l/s |
| 8 | 48.2 | 48.2 x 2 / 36 = 2.68 l/s |
A fixed orifice reads the same way, but the opening being measured does not move when the valve is adjusted. On Pettinaroli's Terminator, the design flow is measured with the valve's Venturi and a digital pressure gauge set to the valve type, size and Venturi. The Venturi itself can be swapped for another size so the pressure signal suits the flow.
The ART 250 sheet also gives a sizing rule: with the valve fully open, the design flow should give a signal of at least 1 kPa, the maximum signal is normally under 5 kPa but can reach 10 kPa, and the flow velocity should not exceed 3 m/s at design flow.
03Fixed vs variable orifice balancing valve: which measures flow more accurately?
A fixed orifice balancing valve measures flow more accurately. Albion said in 2013 that fixed orifice is generally accepted to give a tolerance usually of plus or minus 5% of design flow, against plus or minus 10% or more in variable orifice systems. Pettinaroli's PICV technical manual puts the gap wider:
"flow measurement through a variable orifice is always less accurate than a calibrated fixed orifice devices (±20% against ±5%)."
Fratelli Pettinaroli, Pressure Independent Control Valves Technical Manual
The setting explains the gap. Armstrong, describing its own fixed orifice commissioning valves in a 2007 Modern Building Services product review, says variable orifice valves can achieve plus or minus 5% accuracy only when they are fully open. A balancing valve that has been throttled to balance its branch is no longer fully open. All three figures are the valve makers' own published statements, not independent test results.
| Point | Fixed orifice | Variable orifice |
|---|---|---|
| Where flow is measured | Across a fixed aperture or Venturi (Albion, Pettinaroli) | Across the adjustable seat that also regulates (Albion) |
| Typical tolerance, Albion 2013 | Usually plus or minus 5% of design flow | Plus or minus 10% or more |
| Tolerance, Pettinaroli manual | Plus or minus 5% | Plus or minus 20% |
| When the accuracy holds | Over the entire adjustment range (Armstrong's fixed orifice valves, 2007) | Only when fully open (Armstrong, 2007) |
| Where it is usually specified | Nearly always in the UK (Albion) | More common in continental Europe and North America (Albion) |
04When does a variable orifice balancing valve still make sense?
A variable orifice balancing valve still makes sense where the project specification calls for one. Albion says system specification dictates which type is used, and notes that variable orifice valves are more common in continental Europe and North America. These sources describe UK, European and North American practice; on a Malaysian project, the project specification decides.
Variable orifice valves also come in large flanged sizes. Pettinaroli's UK catalogue lists its PESB1 variable orifice static balancing valve from DN15 to DN50 and, flanged, from DN65 to DN300. The PESB1 has a graduated scale for whole turns and tenths of a turn. An adjustable travel stop returns it to its setting.
On any variable orifice valve, record the handwheel position next to every pressure reading, because the flow chart depends on it.
Albion also notes a separate choice in how measurement is fitted. Some systems install a metering station apart from the regulating valve. A combined valve and metering station has a single body with no joint between valve and metering station, which Albion says reduces potential leak points and is more compact and cheaper than a two-piece commissioning set.
05How much straight pipe does a balancing valve need?
A balancing valve needs straight pipe on both sides, because fittings and bends close to it create turbulence that throws off the reading. In Albion's example, that is five pipe diameters upstream and two downstream, and the upstream length matters more. Albion's ART 250 data sheet gives the same minimum for its variable orifice valve: five diameters of straight pipe without intrusion upstream and two downstream.
Albion adds four site points worth putting in the method statement:
- In ceiling voids, handwheels, levers, test points and drains should point down so they can be reached for commissioning.
- Chilled-water valves and pipework should be insulated with effective vapour seals so condensation does not damage the ceiling.
- All air should be removed from the system first, because air can seriously affect commissioning results.
- Commissioning should only be carried out when the system is cold.
Clean water matters too: how to keep dirt out of the valves before balancing starts is in our guide to flushing a chilled-water system without sending dirt through the control valves.
06Does a system with PICVs still need flow measurement?
A system with PICVs still benefits from flow measurement. Pettinaroli's manual suggests fitting a Venturi flow measurement device on every terminal unit and riser, so each design flow can be checked and recorded at commissioning, because a calibrated fixed orifice gives an accurate, repeatable reading. The manual also says all terminal units should have a calibrated Venturi fixed orifice for accurate and repeatable measurement of the flow rate.
What a PICV removes is the balancing exercise. Pettinaroli's manual says the traditional proportional balancing of flow rates through branches is eliminated, and the task becomes setting the required flow at each PICV. It also warns that static balancing and PICVs need careful thought when used together: a terminal with static balancing and no dynamic protection only has full design flow during commissioning, and its flow changes at part load.
In a PICV system, the fixed orifice is a measuring point, not a balancing device. How a PICV holds its flow is explained in PICV explained, and the choice between mechanical and electronic types in mechanical PICV vs electronic PICV.
07Which Pettinaroli valves measure flow?
Pettinaroli's balancing range covers both measuring methods: the Terminator, which measures across an interchangeable Venturi, the PESB1 variable orifice valve, and a separate Venturi flow measurement valve. Optimus Distributor supplies Pettinaroli's HVAC and data centre range; tell us the model and size and we confirm what we can supply.
- Terminator: a patented static balancing valve with a quarter-turn ball, full shut-off, an interchangeable Venturi for flow measurement and a memory stop. Pettinaroli's UK catalogue lists it from 1/2" to 2", for minus 10 to plus 120 degrees C and up to 25 bar, with water or water and glycol up to 50%.
- PESB1: the variable orifice static balancing valve above, DN15 to DN50, and DN65 to DN300 with EN 1092-2 PN16 flanges.
- Flow measurement valve: a Venturi flow measurement device with a full bore isolation valve, listed from 1/2" upwards with a choice of Venturi sizes.
- SB2V and SB2VA: ductile iron static balancing valves with a test port, 2 1/2" to 16", on Optimus's own Pettinaroli industrial valve flyer.
Pettinaroli's data centre kits for small and medium CRAH units list Venturi devices among their parts, and its kits for big CRAH units an isolation ball valve with an integrated Venturi; more in valve kits for data centre CRAH units. The full range is on our Pettinaroli HVAC and data centre page, and the brand on our Pettinaroli page.
08What should a balancing valve enquiry state?
A balancing valve enquiry should state the design flow and pipe size at each valve and the measuring method the specification asks for, because those decide the valve size and, on a fixed orifice valve, which Venturi is fitted. With these six lines, the valves can be matched before anything is ordered:
- Each valve's location (terminal, branch or riser) and how many.
- The design flow at each valve, and the pipe size and connection type.
- Fixed orifice, variable orifice or either, as the specification says.
- Whether the system uses PICVs, and where flow has to be measured.
- The fluid (water or a water and glycol mix) and the operating temperature.
- Whether the valves sit in a ceiling void or a plant room, and any insulation needed.
Send these six lines on WhatsApp, or browse more guides on our insights page.
09What do engineers ask us about balancing valves?
Is a fixed orifice valve always the better choice?
A fixed orifice valve is the more accurate measuring point, but not always the specified one. Albion says the system specification dictates the type, and variable orifice valves are more common in continental Europe and North America. Follow the specification, and on a variable orifice valve record the handwheel setting with every reading.
Why does my variable orifice reading look wrong?
Check the handwheel position first. A variable orifice valve's Kvs changes with its setting, so a reading converted with the wrong position gives the wrong flow. Then check the straight pipe around the valve and any air left in the system.
Can I measure flow at a PICV instead of fitting a Venturi?
Pettinaroli's manual suggests a Venturi flow measurement device on every terminal unit and riser, because a calibrated fixed orifice gives an accurate and repeatable flow reading. The PICV's own test points are used to check the differential pressure across the valve.
Can a Terminator be set for a different flow later?
Yes. Pettinaroli says the Terminator's components can be adapted to regulate different flow rates with the same valve body. Its Venturi can be changed for another size, so the pressure signal taken from the orifice suits the new flow and the reading stays as accurate as possible.
10Sources
Read 28 October 2026. Product features and accuracy figures are the manufacturers' own published statements: confirm every detail against the current datasheet for the model you are buying.
- Les Littlewood, Albion Valves UK, Striking the perfect balance, Modern Building Services, 3 June 2013 (definitions, accuracy, where each type is specified, straight pipe, site practice).
- Albion Valves, ART 250 variable orifice double regulating valve, technical data sheet 10349, via Hydraquip (flow coefficient, Kvs by position, sizing, installation).
- Simplifying the art of commissioning, Modern Building Services, 8 December 2007 (Armstrong on fixed and variable orifice accuracy).
- Fratelli Pettinaroli, Pressure Independent Control Valves Technical Manual (flow measurement in PICV systems, static and dynamic balancing).
- Fratelli Pettinaroli, UK catalogue 2025, printed pages 32 to 34 and 40 (Terminator, PESB1, flow measurement valve).
- Fratelli Pettinaroli, HVAC Water distribution 2024 catalogue, printed pages 46 and 52 to 53, and the Optimus Pettinaroli industrial valves flyer (the manufacturer's own documents, held by Optimus Distributor).