Sizing HVAC control valves: Kvs, valve authority and a worked example

Control valves are where the BMS logic finally meets the water. Get the valve right and the loop is easy to tune. Get it wrong, usually by making it too big “to be on the safe side”, and it’ll hunt however long you spend on the PID settings.

2-port and 3-port valves

A 2-port valve simply throttles. As it closes, less water goes through the coil and through that part of the system. A system built on 2-port valves is a variable flow system, so something has to cope with the flow dropping off, normally a variable speed pump controlled on differential pressure.

A 3-port valve keeps the flow through the circuit roughly constant and changes how it’s split. A mixing valve has two inlets and one outlet, and blends hot flow with cooler return water to set the temperature leaving it. You’ll see these on weather-compensated heating circuits. A diverting valve has one inlet and two outlets, and sends the water either through the coil or round it through a bypass.

Lots of 3-port valves can be piped either way, but the port markings matter. The common port is usually marked AB. Swap the ports and the valve works backwards.

Kv and Kvs

Kv is the flow, in m³/h, that goes through a valve with exactly 1 bar across it. Kvs is that figure with the valve fully open, and it’s the number you’ll find on the datasheet.

The relationship you’ll use is:

Kv = Q ÷ √ΔP, with Q in m³/h and ΔP in bar

or, if you prefer kPa:

Kv = 10 × Q ÷ √ΔP, with Q in m³/h and ΔP in kPa

Valve authority

Authority is about how much say the valve really has over the flow. For a 2-port valve:

authority = ΔP across the fully open valve ÷ (ΔP across the fully open valve + ΔP across the rest of the controlled circuit)

The rest of the circuit is everything whose flow the valve is controlling: the coil, its branch pipework and fittings.

The usual design target is 0.5 or better. With poor authority, say 0.2, the valve hardly changes the flow until it’s nearly closed. Practically all the control happens in the last few per cent of stroke, and the loop hunts. Oversizing is by far the most common way to end up there.

Worked example: a heating coil valve

Take an AHU heating coil that needs 50 kW across a 20 K temperature difference, for example 70 °C flow and 50 °C return. The coil and its branch pipework drop 20 kPa at design flow.

Work out the flow. Flow = power ÷ (specific heat × ΔT) = 50 ÷ (4.19 × 20) ≈ 0.60 kg/s. Call a kilogram of water a litre, and that’s 0.60 L/s, or about 2.15 m³/h.

Pick a target pressure drop. For an authority of 0.5, the valve wants to drop roughly the same as the circuit, so 20 kPa.

Calculate the Kv. Kv = 10 × 2.15 ÷ √20 ≈ 21.5 ÷ 4.47 ≈ 4.8.

Choose a real valve. Valves come in standard Kvs steps, commonly 1.0, 1.6, 2.5, 4.0, 6.3, 10, 16 and up. So compare the two either side:

Kvs	Pressure drop at 2.15 m³/h	Authority
4.0	(2.15 ÷ 4.0)² bar ≈ 29 kPa	29 ÷ (29 + 20) ≈ 0.59
6.3	(2.15 ÷ 6.3)² bar ≈ 12 kPa	12 ÷ (12 + 20) ≈ 0.37

The Kvs 4.0 gives you decent authority, as long as the pump can find the extra 9 kPa over the original target. The 6.3 is easier on the pump but noticeably harder to control. That trade-off is exactly why valve sizing and pump selection need to be done together, not by two people who never speak to each other.

Valve characteristics

The characteristic describes how the flow changes as the valve opens. With an equal percentage characteristic, each step of opening adds a similar percentage to the flow. That suits heating and cooling coils, because a coil’s output rises steeply at low flow and then flattens off. The two curves roughly cancel out, so heat output ends up close to proportional to valve position. A linear characteristic, where flow rises in step with opening, is often used on the bypass port of 3-port valves.

PICVs

A pressure independent control valve combines a control valve with a built-in differential pressure regulator. Its flow at any given position stays put however the pressure elsewhere in the system changes, which gives you full authority in effect.

You don’t size a PICV on Kvs. You set its maximum design flow. That’s why they’re so popular on fan coil and chilled beam jobs: they make balancing much simpler. They still need a minimum differential pressure across them to work, though, so the pump and its DP control still have to be right.

Don’t forget the actuator

The valve is only half of it. Check that the actuator:

• can close against the system. It has to shut the valve against the maximum differential pressure the pump can produce. If it can’t, the valve lets water through when it’s meant to be shut.
• is quick enough for the loop. A slow actuator can’t keep up with a fast loop.
• matches the controller’s signals. Is it 0–10 V or 2–10 V, and does the strategy use position feedback?
• fails the right way. Where there’s any frost risk, heating coil valves are usually specified to fail open on power loss. Check what the spec asks for and what the actuator actually does.
• gets full travel. Does 100% command really mean fully open?

Signs you’ve got the wrong valve

The loop hunts at low load but behaves itself at full load. The valve does all its work in the first 10 or 20% of travel. The coil’s warm with the valve shut, which means it’s passing or the actuator’s too weak. Or the temperature does the opposite of what the output suggests, which usually means the ports are swapped or the action is backwards.

For more of these, have a look at hydraulic problems that get blamed on the BMS. And once the valve’s right, tuning the loop gets a whole lot easier.