A pool and a spa together: what can be shared, and when it goes wrong

The question always arrives at the same moment in a project, when there is already a budget on the table: if we are going to build a plant room anyway, with its pump, its filter and its heater, why not hang the spa off it too and save a whole set of machinery?

It is a reasonable question, the saving is real on paper, and the answer is no. Until recently that answer was an engineer’s opinion you had to defend against a supplier. Since May 2026 it is written down.

What the rule says, in one line

A spa is a similar structure for this rule: the resolution defines that category by expressly naming spas, whirlpool baths or tubs and hydrotherapy pools. So if you build a pool and a spa today, each one carries its own recirculation system. This is not a design recommendation: it is the text of the technical annexe.

Why: three incompatibilities, not a fussiness

The rule does not explain its reasoning, so I will, because understanding it is what lets you argue with anyone who proposes the opposite. There are three, and each would be enough on its own.

1. Turnover times differ by a factor of ten

This is the hard argument, and it is purely numerical. The same resolution sets how long the system should take to pass a volume equal to all the water in the shell through the filter:

Half an hour against four or six. A plant sized to turn over fifty cubic metres in five hours cannot turn over fifteen hundred litres in thirty minutes and go on doing its own job: they are two different flow rates, two different filtration velocities and two different duty regimes. You can alternate with valves — do one and then the other — but then while the spa recirculates the pool does not, and the rule asks the pool for between eight and twenty-four hours of circulation a day.

2. The temperatures are incompatible and water does not divide

A spa is used hot: the resolution caps the water at 40 °C. A pool is used at a temperature for prolonged bathing, which in trade practice is around twenty-seven or twenty-eight degrees — that last figure is comfort judgement, not a code figure: the resolution only sets the ceiling.

Twelve degrees of difference across volumes that differ by thirty or forty times. If the two shells share water, there are not two temperatures: there is one, and it will be whatever the mixture gives. Heating the spa ends up heating the pool, which is like paying to heat fifty cubic metres in order to get into one and a half.

3. The spa’s chemistry drags the pool’s along with it

The bather load per litre of a spa is two orders of magnitude greater than a pool’s, and at forty degrees the disinfectant is consumed far faster. A spa needs aggressive dosing and frequent water changes; a pool needs stability.

With common water you have to choose which of the two regimes applies, and both outcomes are bad: either you dose for the spa and over-chlorinate fifty cubic metres, with the cost and the smell that implies, or you dose for the pool and the spa falls short precisely where the microbiological risk is greatest.

The spillover spa, which is the elegant trap

The arrangement most often proposed and the one that looks best in a render: the spa raised a few centimetres above the pool, spilling into it as a cascade. One body of water, two levels, and that sound of falling water everybody likes.

It is a legitimate solution when designed in full knowledge of what it does, and a trap when sold as "the spa that comes free with it". Because spillover means exactly what it looks like: the spa’s water is the pool’s water, with all three consequences above at once.

The right way to do it exists and consists of being able to isolate the spa: three-way diverter valves that close the spillover and put the spa on a closed circuit of its own while it heats and is used, and return it to spillover when the visual effect is wanted. That works, but look at what it implies: its own circuit, its own pump, a heater with its own setpoint and a set of valves somebody has to operate correctly. Which is to say, almost everything that was meant to be saved.

What is never shared, however much you want to

There is a part of a spa that admits of no technical argument because it has no equivalent in a pool:

• —The jet pump. Jets call for a lot of flow at little head for short spells, which is the opposite of what a filtration pump does. It is a different pump, with its own circuit, and it does not filter: it moves water from the shell back to the shell.
• —The air blower. What makes the water bubble is injected air, not water. It is a separate piece of plant, noisy, with its own electrical installation.
• —The heater setpoint. Even if the heater were physically the same appliance, it cannot hold two temperatures at once. Either it has two heat exchangers and two controls, or it does one thing at a time.

It is worth knowing because it dismantles the commonest sales argument. When you are told "the machinery is shared", the machinery being shared is the filtration set, which is only part of a spa’s equipment. The jets — that is, the thing you buy a spa for — are never shared.

What is shared, and where the real saving is

This is the distinction that settles the subject: you share the room, not the circuit. And almost all the saving sought by whoever asks the original question lies precisely in the room.

Seen that way, the saving from sharing the room, the supply, the store and the controls is substantial and perfectly lawful. What is not saved is one small pump and one small filter, which is the lesser part of the budget and the part that causes most trouble if left out.

Safety is never shared

This point stands apart because it is the only one where sharing produces not an inconvenience but a hazard.

Each shell needs its own two submerged drains, hydraulically balanced, at the deepest part, at least 0.90 m apart, with their anti-entrapment cover, their vacuum release system and their emergency stop button. The rule requires them per tank, not per installation.

And there is a hydraulic consequence worth understanding, because it is counter-intuitive. If two shells are connected to a single pump and that pump is sized for the total flow, the flow passing through each grating depends on where the valves happen to be set. Half-close the pool branch and the whole of the pump’s flow goes through the spa drains, which are smaller. The velocity at the opening rises, and it rises in precisely the shell where people sit on top of the drain.

The three arrangements, compared

When it goes wrong: three recurring cases

1. 1The spa that takes half a day to heat up. This is the classic symptom of a shared heater with common water: the appliance is trying to raise the temperature of the entire pool in order to raise the spa’s. It gets discovered on the first electricity bill.
2. 2The pool that is lukewarm in the morning and smells of chlorine. The same problem from the other side: dosing is being set for the spa’s demand and the large shell receives all of it.
3. 3The water that goes cold as soon as the pump stops. When the spa is heated by spillover and the spillover depends on the filtration pump, switching filtration off at night — which is what the rule and common sense both advise — leaves the spa with neither turnover nor heat. People solve it by leaving the pump on twenty-four hours a day, which is exactly the consumption they were trying to avoid.

What to ask for before signing

1. 1The hydraulic schematic of both circuits, on a single drawing, with the valves labelled. If the spa has no pump of its own drawn on it, it has no independent circuit.
2. 2The turnover calculation for each shell, with its flow rate. That is two calculations, not one.
3. 3The temperature setpoint of each, and how both are held at the same time.
4. 4The drain detail for each shell, with the 0.90 m separation dimensioned on the drawing.
5. 5The sizing of the equipment room for both sets of plant, with maintenance clearances. It is covered in the plant room.
6. 6The breakdown of what is shared and what is not, in writing. It is the table in this article applied to your project.

Where this comes from

Whoever leads this line of work in the studio brings fifteen years of designing hydraulic systems and bodies of water, and between 2019 and 2022 built pools, spas and waterfalls with the systems behind them — pipe networks, filtration plants, pumps and plant rooms — in the Bahamas, Bermuda, Turks & Caicos and Miami. A pool and a spa sharing a terrace is, in that sort of project, the normal situation and not the exception.

The conclusion from having seen them work and fail is the same one the Colombian rule now states, and it comes down to one line: share the room, not the water. Everything else is an argument about how much to save today in order to pay for it every month.