The plain-English difference
Kv is the flow through a regulating valve at a stated opening, when the pressure drop across that valve is 1 bar. Change the opening (a lockshield turn, a TRV pin position, a control-valve stroke) and Kv changes with it.
Kvs is the special case: the valve is fully open, and the pressure drop is still 1 bar. It is the manufacturer’s capacity figure — the most the valve body can pass in that test. Every Kvs is a Kv; not every Kv is a Kvs.
| Kv | Kvs | |
|---|---|---|
| Valve position | A stated opening (partly open or a preset) | Fully open |
| Test pressure drop | 1 bar | 1 bar |
| What it answers | How much flow at this setting? | What is the valve’s maximum capacity? |
Datasheets sometimes print a small Kv table against turns, percentage open, or a preset number. The last row of that table is Kvs.
The formula and units
For water, with consistent trade units:
Kv = Q / √ΔP
The same form is used for Kvs when the valve is fully open:
Kvs = Q / √ΔP
- Kv or Kvs — flow capacity, in m³/h
- Q — volume flow, in m³/h
- ΔP — pressure drop across the valve, in bar
Rearranged when you already have two of the three:
Q = Kv × √ΔP
ΔP = (Q / Kv)²
Keep the units as written. If the drop is in kPa, divide by 100 to get bar (100 kPa = 1 bar). If the flow is in litres per minute, multiply by 0.06 to get m³/h (1 m³/h = 16.7 L/min). US catalogues sometimes quote Cv in gpm at 1 psi; a common conversion is Kv ≈ 0.865 × Cv.
Try the numbers
Enter any two values. The third is calculated from the formula above.
A worked radiator example
Take a 1.2 kW radiator on a traditional 20 K drop (about 70/50 or 75/55). The site heat formula is the same one used on the kW to flow rate calculator:
kW = (flow in m³/h × ΔT) / 0.86
So the radiator needs:
Q = 1.2 × 0.86 / 20 = 0.0516 m³/h (about 0.86 L/min)
If the pressure available across the valve at that flow is 0.1 bar (10 kPa):
Kv = 0.0516 / √0.1 = 0.16
That is the opening the valve must provide. A compact 15 mm TRV body is often listed around Kvs 0.5–0.8 when fully open — plenty of capacity for this radiator. The lockshield or preset is there so the valve does not run wide open and steal flow from the rest of the circuit.
Drop the same radiator to a 10 K ΔT (a heat-pump-style 50/40 circuit) and the flow doubles:
Q = 1.2 × 0.86 / 10 = 0.103 m³/h → Kv = 0.33 at the same 0.1 bar
Lower mean water temperature means more water through the same emitter. The valve still has to be able to pass that higher Kv, which is why a body that was fine on a hot boiler circuit can look tight on a low-ΔT design. Check the manufacturer’s chart rather than assuming the old setting still works.
Turn the formula the other way. A valve with Kvs 0.60, fully open, at 0.1 bar passes:
Q = 0.60 × √0.1 = 0.19 m³/h (about 3.2 L/min)
At 20 K that is roughly 4.4 kW — more than a typical domestic panel needs. Capacity is rarely the problem on a single radiator. Getting the right flow on every radiator is.
Why installers care
Kv and Kvs are design numbers, not a DIY setting on the boiler. They show up when you size or commission a circuit:
- Capacity — will the fully-open valve (Kvs) pass the design flow at the pressure the pump can spare? If not, the radiator never reaches output, however open the head is.
- Balancing — lockshields and presettable TRV bodies are set to a lower Kv so each radiator takes its share. The worked example above is that calculation: design Q, available ΔP, required Kv.
- Authority — a control valve only changes flow usefully if a fair slice of the circuit’s pressure drop sits across that valve. A common target is about 0.3–0.5 (valve ΔP divided by the drop around that circuit). An oversized body (Kvs far above the needed Kv) drops very little pressure until it is almost shut, so the TRV or actuator hunts. An undersized body cannot hit design flow even wide open.
Pump head, pipe size and radiator output all sit in the same sum. Use the manufacturer’s Kv chart for the actual valve, and the volume flow rate and pipe size tools for the rest of the circuit. This page is a reference, not a commissioning procedure.
Sources and related pages
Definitions and the formula above follow Grundfos — Kv and Kvs values.
The old hub page pointed at a Drayton TRV4 user and installer guide. The current public copy is the Drayton TRV4 Installation & User Guide (PDF), also listed on the TRV4 Classic product page. Use the manufacturer’s flow-capacity sheet for Kv figures, not the fitting guide.
See also kW to flow rate · Volume flow rate Q · Radiator correction factor · Heat meters · TRV pin stuck.
Guidance only. Valve selection, balancing and heating work need a competent person; gas work in the UK needs a Gas Safe registered engineer. Always use the current manufacturer data for the valve in front of you.