A. Two-Position Valves
Normally selected "line size" to keep pressure drop at a minimum. If desirable to reduce valve below line size, then 10% of "available pressure" normally used to select valve.

B. Proportional Two-Way valves
Usually selected to take a pressure drop equal to at least 50% of the "available pressure" (that is, the pump pressure differential available between supply and return mains with design flow at the valve location). As "available pressure" is often difficult to calculate, normal procedure will be to select the valve using a pressure drop at least equal to the drop in the coil or other load being controlled (except where small booster pumps are used) but never less than 4 psi (34 kPa).
When design temperature drop is less than 60°F (33^oC) for conventional heating systems, higher pressure drops across the valve are needed for good control results. See following table.

Conventional Heating Systems -------------------------------------------------
  Design Temp Recommended Multiplier on Drop °F (°C) Pressure Drop
  Load Drop (% of Available Pressure)* --------------------------------------------------
  60 (33) or more 50% 1 x load drop 40 (22) 66% 2 x load drop 20 (11) 75%
  3 x load drop --------------------------------------------------- * Recommended
  minimum pressure drop - 5 psi (34 kPa) Secondary Circuits with
  Small Booster Pumps 50% of available pressure difference (equal
  to drop through load or 50% of booster pump head) 

C. Proportional Three-Way Valves
Recommended Pressure Drop - Bypass applications: 50% of "available pressure," or equal to pressure drop through the load at full flow.

Three-way valves in the return used to control heat output by throttling water flow to the load (bypass applications) are controlling output in the same manner as throttling two-way valves, and must be selected using the same high pressure drops if good control results are to be obtained.

Recommended Pressure Drop - Constant flow applications: 20% of "available pressure" or equal to 1/4 of the pressure drop through the load at full flow.

Three-way valves used with individual pumps to control heat output by varying water temperature to the load (constant flow applications) are controlling output by mixing two water sources at different temperatures, and do not require high pressure drops for good control results.

In most cases the required Cv falls between two valve sizes. If the pressure drop of the smaller is acceptable for the application, select the smaller valve.

 Where: Cv = Coefficient of flow
    gpm = U.S. gallons per minute (60°F/15.6°C) P = Differential
    pressure in psi (pressure drop)

Other forms of this formula are:

and

These formulas can be used to calculate one of the three values if the other two are known.

Flow coefficients ( C_v's) for valve bodies are generally found on the manufacturer's valve body data sheets.

A. Two-position Zone Valves and Direct Radiator Valves
Use a minimum of 10% of inlet pressure (psig).

B. Proportional Control Valves
Low pressure (15 psig or less) P of 80% of gauge inlet pressure.

When C_v required is between two valve sizes and closer to the smaller valve size, re-size for C_v using 42% of the absolute inlet pressure as pressure drop. Use the valve that is larger than the calculated C_v.
For steam pressures greater than 15 psig: 42% of the absolute inlet pressure.

When C_v required is between two valve sizes, select the larger size.

NOTE: Do not size steam valves on higher system pressures using a pressure drop greater than 42% of the absolute inlet pressure.

Where: Cv = Coefficient of flow Q = lbs per hour of steam P
  = Differential pressure in psi (pressure drop) P2 = Outlet pressure in
  psia (absolute) psig + 14.7 = psia (absolute) K = 1 + (0.0007 x °F
  super-heat)

NOTE: K normally is 1 (K = 1 for saturated steam)
Other forms of the formula are:

NOTE: K normally is 1 (K = 1 for saturated steam)

These formulas can be used to calculate one of the values if the others are known.