How to make a hot tub part 2 - Design Plumbing & Filtration

12 March 2020  |  Superior Wellness

How to make a hot tub part 2 - Design, Plumbing and Filtration

How to design a hot tubThis article continues from part 1 of how to make a hot tub, and includes essential design tips to consider in the early planning stages before costly mistakes are made. Building a hot tub is always a fun project to do, the following tips and tricks are from an engineer with many years of experience building and working on hot tubs / spas in the UK.


Its crucial to take everything into consideration before spending and committing to a project like building a hot tub, so after you have considered your materials to make the body of your hot tub its time to think layout of seats, if you plan on loungers, foot well suctions for pumps and possibly where the filter will be mounted.

Hot tub filtration

There are 3 main types of filters on hot tubs, inline canisters for paper filters, skimmers (also with paper filters) and sand filters. Both the inline canister filter and sand filter will require 2 suctions in the footwell, where as a skimmer is classed as 1 suction and will require a second suction in the foot well with a 'pressure by pass valve' fitted (this helps 2 speed pumps draw water through the filter rather than the second suction when on low speed).

Sand filters are not always the best solution but would make sense on a commercial spa, where as a family spa would probably use a paper filter based filtration system. If the spa / hot tub is situated outside we would recommend a skimmer filter (to catch the leaves) but inside you shouldn't need a skimmer so a canister filter or sand filter would be a better idea.

Canister filters need isolation valves either side to enable you to remove the paper filter for cleaning and they should never be fitted above the water level because you will get issues with air locks.


Start thinking about what you want to include in you project.

  1. All pumps need 2 suctions unless they are drawing from a filter (see above note on filters).
  2. Pumps should always be fitted below water level.
  3. Air bubbles from air jets go up so are always based on the seats or loungers, where as water jets are placed on the seat backs, also on loungers and in the foot wells.
  4. Always take into consideration water displacement for when people get in!
  5. If planning a lounger the person wants to be under the water and the water jets don't want to blast the water out the tub when the jets come on.
  6. Be subtle with lighting, remember the water will be clear most of the time so you only need a little light subtly done not Blackpool illuminations. After the first few months gimmicks like flashing multicoloured LED's will probably no longer get used.
  7. Music - if your planning on putting music in your spa do your home work on the correct system because they can get expensive quickly. A better method is to have a system on the walls near the spa and will probably sound better than built in speakers.
  8. Waterfalls and water features - Things that shoot water into the air or trickle away in the corner can sound and look nice but are hard to get right, think about the pump sending the water and how you will control this as well as when the lids on will it empty your spa of water?

Air Jets

Air jets are really easy and fun, however difficult to fit in a concrete hot tub and easy to fit under a bench in a wooden, fibreglass or tiled spa. The reason is the thickness of thread, any more than a 10mm thickness and the thread won't be long enough to attach most, so 12cm building blocks will be impossible to seal.

As a rule of thumb a 700 watt air blower can power a max of 10 jets and a 900 around 14 jets, if you have to fewer air jets the back pressure on the pump can be so great that you will burn the air blower motor out prematurely and to many may see some jets not working at all. All air blower will require a none return valve to stop the water going back into the motor and you can fit air blowers above the water line if required.

A common misconception is that air from air jets will cool the water down in a spa, this is only true in cases of bad engineering, because if the air blower is situated near water jets it will draw the hot air from them.

Water Jets

Water jets come in many sizes, on concrete (or spas built with block) you are restricted to gunite style jet bodies where as on most other custom built hot tubs the wall thickness is so thin you can use any jets. Wooden hot tubs are the only exception to this because they will need a jet body thread length of around 30mm to be able to fit through the wood so something like a 'Waterway Polyjet' with a long wall fitting is best for them.

Jets in general

The rest (bulk) of the jets are designed to work with a maximum wall thickness of around 15mm, these consist of anything from small jets called 'cluster jets' and go all the way up to single large jets called volcano jets that are designed to be very powerful. When thinking about location of jets, think about the different muscle groups in the human body and think about the distances from each other, for example the shoulders or lumbers of a persons back muscles or jets on the thighs and back of calf's, from experience jets going on to the spine are not that effective and i think its a bad idea to have them around the sensitive areas of the body like genitals and under arms. Feet and arms are a good idea as well if you have the space, also try and mix up the different seats or lounger for a different experiences but don't add to many because that would only make the person feel like they are levitating on water.


Its possible to do several types of plumbing depending on material and jets being used.


  • you can only have 1 pump on one group of plumbing, never 2 pumps because the motors will burn out.
  • Most hot tubs are based on imperial plumbing, this means they are measured internally (I/D) Internal Diameter.
  • Water diverter's can be used to move water from a pump from one area to another or both whilst sitting in the hot tub.

Loop topology

This is one complete circuit of plumbing going all the way around and connecting back on itself most commonly seen in commercial spas using commercial polyjets and similar through jets like gunite. These tend to have larger pipes like 2" I/D connecting them together. all the jets will have to be the same height unless you start using fittings to move them up and down accordingly. Also good to point out this would be a bad idea for loungers, however you could do this separately with air.

Building a hot tubThis image shows a loop of gunite jets being installed.

Star topology

Most commonly seen on mass produced hot tubs where the water goes to a manifold in a large pipe then splits of on 3/4" pipe to go to the back of the jets. This means the jets can be anywhere, any size and all you would have to think about is how many work with the pumps you are fitting.

Hot Tub PlumbingThis image shows jets linked to manifolds in a star topology.

Tree topology

This isn't really used on hot tubs, however it is possible and would be used in a similar capacity to air ducting, larger pipes supplying larger jets then reducing to smaller pipes on smaller jets. (this can make for a very complicated custom hot tub).

Conclusion on jets

You should consider a good combination of jets including air specific jets where possible.

Plumbing jets

All water jets need air to work, this creates the viciousness in the water for the massage.

There are 2 methods of getting air to come out of water jets.

All water jets have a basic venturi built inside, the jet body (built into the tub) is built with 2 pipes connected, 1 larger for water and 1 smaller for air which is drawn into the water by the venturi.

The first method consists of an air valve connected to an air manifold directly connected to the back of the jets for the air venturi to work.

The second method has similar jets working with venturi's inside, usually larger pipes and is nicknamed the turbo method, this has no air valves but instead the water is pushed in using an hot tub specific air blower, this method is rarer and usually seen in commercial spas built inside.

Warning:  you cannot pull water for 2 plumbing circuits from 1 air manifold they would need to be separate, because if 1 jet is switched of you will only pull water from the second pumps plumbing.


How many jets can a pump run and an introduction to electrical control systems will be included in the next article: 'How to Build a Hot Tub: Part 3 Electrics

This article was written by Jon Pennington at Hot Tub Help Limited, if you need help on components please get in touch directly at [email protected]