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BTU Logic

Tools / Valve Sizing

Valve Sizing

Sizes from the coil out. Enter the design flow, the coil and piping drop, and a trim allowance; the valve drop is estimated as a share of that branch resistance, which fixes the authority you are designing for. From there you get the Cv required, and entering the catalogue size you can actually buy re-grades the authority on the drop that valve really takes. A close-off check is there for the differential the shut valve has to hold.

Cv sizing helpers are approximate and depend on your assumptions (fluid, ΔP, flow, valve characteristics). Not a substitute for manufacturer selection software, hydronic design by a qualified professional, or stamped drawings. Verify against manufacturer data before specifying or installing valves.
1 · Coil or process

Standard trim will not drop much less than 0.5 psi at design flow. It is counted into the branch resistance below, and it floors the estimated valve drop so a very low-drop coil cannot ask for a Cv no real valve has. Set it to 0 to leave it out entirely.

Glycol figures are a starting point only — specific gravity moves with concentration and temperature. Use your fluid supplier’s data for anything you are specifying.

Flow from load (water)

40.00 gpm

2 · Valve pressure drop

Valve authority β is the share of the branch’s total drop that the fully open valve takes: β = valve ΔP ÷ (valve ΔP + everything else). It is what decides how much of the stroke actually controls flow — at low authority the installed characteristic distorts and the valve does its work in the first crack of travel. A ratio of 1 gives the valve the same drop as the rest of the branch, which is β = 0.50; 0.5 gives β = 0.33. Below about β = 0.25 the valve controls badly. More drop across the valve buys authority and costs pump head.

4 · Valve you selected (optional)

Catalogue sizes come in steps, so the valve you can buy is rarely the Cv the calculation asked for. Enter it and the authority is re-worked on the drop that valve really takes at design flow.

Close-off (optional)

2 · Estimated valve ΔP

Branch resistance (coil + trim)5.50 psi
Estimated valve ΔP5.50 psi
Total branch drop11.00 psi
Authority β this implies0.500

Good — the valve holds enough of the branch to keep its installed characteristic.

3 · Required Cv

Cv17.06
Kv14.75

What it takes to pass design flow at the estimated ΔP. Look this up in a manufacturer’s table and bring the size you can buy back to step 4.

Cv is the flow of 60 °F water in gpm at 1 psi of drop; Kv is the metric equivalent (Cv ≈ 1.156 Kv). Flow from load uses the standard 500 × ΔT water constant. Size from a manufacturer’s published table — this gives you the Cv to look up, not a valve selection.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms