December 19, 2025
In HVAC control valve selection, one of the most misunderstood notations engineer's encounters is: “7/8” O.D” - Many professionals mistakenly assume this directly defines the control valve size or Pipe Size. In reality, it does not.
Let’s break it down clearly.
What Does 7/8” O.D Actually Mean?O.D = Outside DiameterWhen you see 7/8” O.D, it typically refers to: Copper tube outside diameterCommonly used in: 1. Heating / Cooling coils2. Small AHU or FCU connections3. DX or hydronic coil piping
Important:This is NOT the valve size and It is only the tube size of the coil connection.Copper Tube Size vs Pipe Size (Critical Difference)Copper tubing and threaded pipe sizes do not match 1:1.Copper Tube O.D Approx. Nominal Pipe Size 7/8” O.D 1/2” Nominal Pipe Size
A 7/8” O.D copper tube is typically adapted to a 1/2” NPT (threaded) control valveThis is achieved using:1. Sweat-to-thread adapters2. Reducers / transition fittingsSo… How Do We Actually Size the Control Valve?Never size a valve based only on O.D. Valve sizing must be based on:1. Required Flow (GPM) - 2. Pressure Drop Across Valve - Typically 10–30% of total system DP Ensures good valve authorityPractical Example 1. Coil connection: 7/8” O.D copperCalculated Cv: 1.2Manufacturer valve availability:1/2” valve Cv range: -> Suitable3/4” valve: -> OversizedCommon Mistakes to Avoid 1. Selecting valve equal to copper O.D2. Upsizing valve “to be safe”3. Ignoring Cv and focusing only on pipe size4. Assuming coil connection size = valve sizeThese mistakes often lead to:Poor temperature control or Valve hunting or Noise issues or Low valve authorityKey Takeaway 7/8” O.D defines the coil tube size — NOT the control valve size1. Control valves are sized by Cv and flow2. Pipe size is a secondary check3. Adapters handle the physical connection
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