If a coil keeps heating or cooling while its control valve is commanded closed, the control valve leakage class is rarely the cause. ANSI/FCI 70-2 lets a Class IV valve pass 0.01% of its rated capacity under test conditions, about a quarter of a litre per minute on a typical 3-inch valve. Flow big enough to change a room temperature usually means the plug is not seating, so check close-off, stroke, control signal and seat condition first.
What a control valve leakage class allows
ANSI/FCI 70-2 (current edition 2021) sets six seat leakage classes and the shop tests behind them. As the Fluid Controls Institute puts it in its guidance on the standard, control valves are not isolation valves, and they will leak. The class defines how much, on a new valve, under test conditions. Class IV is typical of metal-seated single-seat valves and Class VI of resilient seats, but the standard does not tie a class to a design.
| Class | Maximum seat leakage | Test medium and pressure |
|---|---|---|
| I | No test (by agreement) | None |
| II | 0.5% of rated capacity | Air or water, 45 to 60 psi (3 to 4 bar) or the maximum operating differential if lower |
| III | 0.1% of rated capacity | As Class II |
| IV | 0.01% of rated capacity | As Class II |
| V | 0.0005 ml/min of water per inch of seat diameter per psi of differential (air option: 4.7 ml/min per inch) | Water at the maximum service differential, or air at 50 psi (3.45 bar) |
| VI | Set by seat size, e.g. 0.90 ml/min of air for a 3-inch seat | Air or nitrogen, 50 psi (3.45 bar) or the maximum rated differential if lower |
The international counterpart is IEC 60534-4 (2021 edition; BS EN IEC 60534-4:2022 in the UK), which also sets Class IV at 0.01% of rated capacity. European HVAC data sheets often quote leakage as a percentage of kvs instead. Expressed as a flow coefficient, Class IV is 0.01% of Cv or kvs, so a sheet quoting “max. 0.05% of kvs” allows five times the Class IV figure.
Worked example: Class IV in litres and watts
A 3-inch globe valve with a rated Cv of 100 (kvs about 86.5, since Cv = 1.156 × kvs) serves a chilled water coil. Using the basic equation for water, Q = Cv × √ΔP:
- Rated capacity at a 50 psi (3.45 bar) water test: 100 × √50 ≈ 707 US gpm (161 m³/h).
- Class IV allowance: 0.01% of 707 gpm ≈ 0.071 gpm, about 16 L/h.
- Heat carried at a 10°F (5.6 K) coil rise: 500 × 0.071 × 10 ≈ 350 Btu/h, about 0.1 kW.
- Design flow at a 5 psi (0.34 bar) valve drop: 100 × √5 ≈ 224 gpm, about 1.1 million Btu/h (330 kW).
The full allowance is about 0.03% of design flow. The standard says its test cannot predict leakage at other conditions, so this is a scale check. Still, a closed valve that delivers enough capacity to shift supply air or room temperature is passing far more than Class IV permits.
Close-off: the usual reason the plug lifts
Close-off rating is the highest differential a specific valve and actuator combination can close against and hold tight. It depends on actuator force and valve construction, not on the body’s pressure rating. Checking it against the design valve drop is not enough. With a constant-speed pump the worst case is low load, when most valves are shut and the pump climbs its curve toward shutoff head.
A September 2005 ASHRAE Journal article on variable-flow systems notes that even with variable-speed pumps, valve differentials can approach shutoff head if the pump runs on its backup across-the-line starter or the differential pressure control fails. It recommends selecting close-off against pump shutoff head, or 1.5 times pump design head when shutoff head is unknown.
Example: a constant-speed pump has an 80 ft design head and a 100 ft shutoff head (43 psi, about 300 kPa). A valve and actuator combination rated for 25 psi (172 kPa) close-off can be pushed off its seat at low load. Specify at least 43 psi (300 kPa); with shutoff head unknown, 1.5 × 80 ft = 120 ft, about 52 psi (360 kPa).
Close-off is not a throttling limit
The same article separates close-off from the dynamic (modulating) rating: its example valve closes against 100 psid but tolerates only 25 psid while throttling. Above that, the plug and seat can erode or wire-draw until the valve passes, and high differential near the seat also drives control valve cavitation. Data sheets often list them separately, e.g. Δps (close-off) and Δpmax (maximum differential while controlling).
Field checks before you replace the valve
| Symptom | Likely cause | Check and fix |
|---|---|---|
| Coil active, actuator shows closed | Differential above close-off, or a damaged seat | Measure valve differential at low load and compare with the published close-off; upgrade the combination or cut system differential |
| Indicator short of closed at a 0% command | Signal range mismatch, uncalibrated stroke, override left on | A 2-10 V output into an actuator set for 0-10 V parks it 20% open; match the ranges, recalibrate, release the override |
| Leakage grows over months, noisy near closed | Oversized valve throttling close to the seat | Check opening and differential at low load; reselect a smaller Cv |
| Passing after flushing or start-up | Debris on the seat; small-port valves are most exposed | Clean strainers, inspect the seat, replace scored trim |
| Chatter near closed on a new installation | Body fitted against its flow arrow | Refit to the arrow; the leakage class is tested from the normal or specified inlet |
Specification and submittal checklist
- Name the standard (ANSI/FCI 70-2 or IEC 60534-4) and the class the valve is actually tested to, not just “tight shut-off”.
- Set close-off from pump shutoff head (or 1.5 × design head), not from the design valve drop.
- Check the modulating limit (dynamic rating or Δpmax) separately.
- Rate the body for static head plus pump shutoff head plus fill pressure.
- Require close-off data for the exact valve and actuator combination in the submittal.
- Specify fail-safe position and signal range, and add isolation valves; they are tested differently (see our API 598 vs EN 12266 comparison).
Need a valve schedule checked?
ValveAtlas supplies HVAC valves and actuated valve packages for projects in Canada, the US, the UK and EMEA. Send us the pump curve and valve schedule and we will help match leakage class and close-off to each valve. Reach us through the contact page, at info@valve-atlas.com.
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