What Exactly is 1.5 Plumbing

10 February 2017  |  Superior Wellness

What Exactly is 1.5" Plumbing as it doesn't measure that?

Hot Tub Plumbing ImageThis is a question we get all the time! The answer is easy everything that is 1.5" plumbing will fit together, so to make this easier to understand if you want to fit 1.5" to 2" hot tub plumbing you will need a reducer.

So you're asking where does the 1.5" come from? Well the size relates to the internal diameter of a straight piece of piping, and it's obviously an Americanism as we use Metric and all the original hot tubs came from America.

It is, however, possible to get hot tub's that have been plumbed in metric pipework, but these are rare, they tend to be made in Europe rather than America, China and Canada who all thankfully make parts that interconnect.

Hot Tub Plumbing is boring Right? No Wrong!

Once the penny drops on the sizing repairing hot tubs can be fun, connecting couplings, bends and manifolds with glue can be very therapeutic, be careful thou the glue is highly odorous, the reason being is the solvent used to make them set. Its also good to note at this point that glue used on hot tub plumbing is different to domestic plumbing glue it is designed to take the chemicals and temperature in a 24 hour 365 environment, where, as, domestic is not.

Why not buy specific plumbing and not use from a plumbers yard.

The answer to this is one of experience, and includes plumbers yard glue and parts, sadly over the years I have had to re-repair many hot tubs due to bad material choice, the glue will last 6 months and the pipework/fittings that are thinner will swell with the pressure like little balloons and pop, this is bad considering the amount of water that can be drained to the inside of the tub quickly - however if you are quick enough to catch it you can always take the pumps and part apart and dry them out as it will simply trip the electrics and not actually brake anything.

Ok Here is the boring bit

1.5" pipe measures 41.5mm internally and 48.5mm. Externally which means the internal diameter of the fittings are 48.5mm and the union thread size is 6.1cm making the outside of the union 7.4cm.

Bored yet?

2" plumbing measure 2" internally (52mm) and 60mm outside, the size of the union thread is 7.6cm and the external is 9cm.

Connection types

Hot tubs can be made of all sorts of fantastic parts, but from experience to create an extremely well engineered hot tub the main ingredients seem to ignore complex expensive parts and keep it simple. Anything freak unique will always break!

The types tend to be:

straight connections or sockets

90% or Elbows


Sweep which is a longer version of either a 45% or 90%


Reducers or extenders

Adapters for changing from Imperial to metric



End Caps

End Inserts

Unions to connect to heaters or pumps

Socket unions where you can connect 2 pieces of plumbing with a thread.

Gate valves or stop valves or slice valves

One way or check valves

Numerous air and water valves and diverters.



Spiggot - meaning designed to fit another connection

Socket - meaning the pipe will fit on the inside

Glue used is called 'Solvent weld' or 'Cement'


The most common sizes are American hot tubs tend to be made in 2" plumbing and Chinese in 1.5" plumbing.


Hot Tub DIY

We sell a great range of Hot Tub Plumbing Parts, click on the link below to see the parts and spares.

Hot Tub Plumbing