Pump Unions Explained
Pump Unions Explained
Not sure which pump union fits your hot tub pump? This guide explains the most common union sizes, collar dimensions, socket sizes and thread types used across the hot tub and spa industry.
Need Replacement Pump Unions?
We stock a wide range of hot tub pump unions, collars, tailpieces and plumbing fittings suitable for many of the most popular spa and hot tub brands.
SHOP PUMP UNIONSWhat Size Pump Union Will Fit My Pump?
The quickest way to identify a pump union is to measure the outside diameter of the pump wet end connection where the union collar screws onto the pump.
For standard buttress thread unions:
- 63mm outside diameter = 1.5" union
- 79mm outside diameter = 2.0" union
- 93mm outside diameter = 2.5" union
In most hot tubs, the 2" pump union is by far the most common size.
Understanding The Different Parts Of A Pump Union
A complete pump union consists of two main components:
- Union Collar - The threaded nut that secures the fitting to the pump.
- Tailpiece Socket - The section that connects to the pipework.
When replacing a damaged union, it is important to identify both the collar size and the socket size to ensure compatibility.
Pump Union Size Chart
| Union Size | Collar ID | Thread Type | Socket Size |
|---|---|---|---|
| 2.0" | 79mm | Buttress | 60mm |
| 1.5" | 63mm | Buttress | 48mm |
| 1.0" | 57mm | Buttress | 33mm |
Outside Diameter Reference Guide
The advertised union size does not directly relate to a physical measurement on the union itself.
Typical outside diameters are:
- 1" union = approximately 57mm
- 1.5" union = approximately 71mm
- 2" union = approximately 81mm
- 2.5" union = approximately 105mm
Measuring the outside diameter is often the easiest way to identify an unknown pump union.
Collar ID Dimensions
- 1.5" Union = 63mm Collar ID
- 2.0" Union = 79mm Collar ID
- 2.5" Union = 92-93mm Collar ID
The collar measurement should be taken across the threaded opening where the union screws onto the pump wet end.
Socket (Tailpiece) Dimensions
- 1.0" = 33mm Socket ID
- 1.5" = 48mm Socket ID
- 2.0" = 60mm Socket ID
- 50mm Pipe = 50mm Socket ID
- 63mm Pipe = 63mm Socket ID
- 32mm Pipe = 32mm Socket ID
Always measure the pipe size and socket size carefully before ordering replacement unions.
Understanding Pipe Sizes
The tailpiece socket size can vary depending on the pipework used by the manufacturer.
For example, a union with a 48mm socket ID is typically designed to suit 1.5" pipe. Although referred to as 1.5" pipe within the spa industry, the actual dimensions differ from the nominal size.
Because of these variations, it is always best to measure the existing union rather than relying solely on the pump model.
Need Replacement Pump Unions?
We stock a wide range of hot tub pump unions, collars, tailpieces and plumbing fittings suitable for many of the most popular spa and hot tub brands.
SHOP PUMP UNIONS