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Understanding 7/8” O.D in Coil Cutsheet and Its Relation to Control Valve Sizing. A Common HVAC Controls Confusion – Simplified

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 Diameter
When you see 7/8” O.D, it typically refers to: Copper tube outside diameter
Commonly used in:
1. Heating / Cooling coils
2. Small AHU or FCU connections
3. 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 valve

This is achieved using:
1. Sweat-to-thread adapters
2. Reducers / transition fittings

So… 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 authority

Practical Example
1. Coil connection: 7/8” O.D copper
Calculated Cv: 1.2
Manufacturer valve availability:
1/2” valve Cv range: -> Suitable
3/4” valve: -> Oversized

Common Mistakes to Avoid
1. Selecting valve equal to copper O.D
2. Upsizing valve “to be safe”
3. Ignoring Cv and focusing only on pipe size
4. Assuming coil connection size = valve size

These mistakes often lead to:
Poor temperature control or Valve hunting or Noise issues or Low valve authority

Key Takeaway
7/8” O.D defines the coil tube size — NOT the control valve size
1. Control valves are sized by Cv and flow
2. Pipe size is a secondary check
3. Adapters handle the physical connection

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