Tools · Valve Authority

Valve Authority Calculator Hydronics

A control valve only controls flow if it claims a healthy share of the circuit's pressure drop — that share is its authority.
Authority β = ΔPvalve, open ÷ (ΔPvalve, open + ΔPrest) compares the drop across the wide-open valve at design flow to the total drop across the part of the branch that varies as the valve strokes. High authority keeps control across the whole stroke; low authority front-loads it into the first crack of travel, and the loop hunts. Size the valve's Cv first, then check authority here.

Inputs are seeded with an example — edit them to your numbers.

Input

Use any consistent pressure unit — authority is a ratio, so the units cancel. The "rest of the branch" is everything whose drop changes as the valve strokes: the coil, the balancing valve, the fittings on that controlled circuit.

Output

Valve authority β —
Enter both pressure drops to see the verdict.

Installed characteristic

An equal-percentage valve is chosen so that, fighting the circuit's rising resistance, its installed behaviour lands near the linear diagonal. That only works when authority is high. Watch the green curve pull away from the diagonal as β drops — below ~0.25 it bows up hard and the valve does all its work in the first quarter of travel.

Installed characteristic at the computed authority Flow versus valve travel. The dashed diagonal is ideal linear control; the blue curve is the valve's inherent equal-percentage characteristic; the green curve is how the valve actually behaves at the calculated authority β. 0 100 0 100 Valve travel % Flow %

What the number means

βControl quality
≥ 0.5Good — installed characteristic stays close to linear.
0.25 – 0.5Marginal — distorted, front-loaded stroke.
< 0.25Poor — little real control, prone to hunting.

Low authority means the valve does almost all of its throttling in the first sliver of stroke — the loop hunts and the rest of the travel does nothing. Oversizing on Cv is the quiet killer: a valve far bigger than the duty barely cracks open, so it takes almost none of the circuit's drop and authority collapses. Size for the duty, then confirm the valve claims enough of the branch to actually control.

Valve authority FAQ

What is valve authority?

Valve authority (β) is the share of a controlled circuit's pressure drop that the wide-open control valve takes — β = ΔP valve, open ÷ (ΔP valve, open + ΔP rest). It measures how much real control the valve has over flow across its stroke.

What is a good valve authority?

Aim for β ≥ 0.5. Between 0.25 and 0.5 control is marginal and the installed characteristic is distorted; below 0.25 the valve barely controls flow and the loop tends to hunt.

How do you calculate valve authority?

Divide the pressure drop across the wide-open valve at design flow by the total drop across the parts of the branch that change as the valve strokes — the valve plus the coil, balancing valve, and fittings on that circuit — β = ΔP valve ÷ (ΔP valve + ΔP rest). It is a ratio, so any consistent pressure unit works.

Why does low valve authority cause hunting?

With low authority most of the flow change happens in the first sliver of valve stroke, so a small movement swings flow a lot. The valve effectively acts on/off, the loop overshoots, and it oscillates around setpoint.

How does oversizing a valve affect authority?

An oversized valve — Cv far larger than the duty — barely cracks open at design flow, so it takes very little of the circuit's pressure drop and authority collapses. Size the Cv to the actual duty first, then confirm authority.
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