Pool and Hot Tub Plumbing: Why They Should Stay Separate (and What You Can Share)

Last updated: September 2026

I get asked an awful lot about combining pool and hot tub plumbing. “Can I just add some jets to my pool pump?” “Can the hot tub share the pool’s filter and heater?” Usually the question comes after a pool installer has said “no problem”. In this post I’ll explain why the two systems do different jobs, the problems you’ll hit if you try to merge them, and the parts you genuinely can share.

Pools and hot tubs do different jobs

In general, pool plumbing parts are designed to move a large volume of water slowly, to filter and heat it. Hot tub plumbing parts are designed to move a small volume of water quickly, through jets. Almost every problem with combining them comes back to that one difference.

Part	Swimming pool	DIY hot tub
Pump	Self-priming filtration pump, often variable speed, built for volume at low pressure	Centrifugal jet pump (often 3HP or more) for high flow through the jets, below the water line
Filter	Usually sand	Usually cartridge
Controls	Timer or pool automation	Spa pack with topside buttons for jets, blower and lights
Water pipe	Sized to the pool’s filtration flow	2in water lines, sized to the jets’ flow
Air	None	1.5in air lines from a blower or venturi
Outlets	Wall returns	Jets (plus returns on a separate loop)

The pump

I’ve got a video on my YouTube channel, above, that explains the difference between swimming pool pumps and hot tub pumps. In essence, pool pumps are designed to move large amounts of water slowly through a sand filter and then a heater, and back to the pool. Hot tub pumps can have dual roles, but they’re essentially designed to push a smaller body of water quickly through the jets.

The flow from a swimming pool pump generally isn’t enough to run, say, a 16-jet hot tub. Each gunite jet needs around 10 GPM, so 16 jets need about 160 GPM, delivered after the friction of the pipes and fittings. You might have a 0.33HP or 0.5HP pool pump. That’s a lot smaller than the 3HP dedicated jet pump you’d find on a hot tub, and it’s that size for a reason. Even bigger pool pumps don’t deliver the same jet performance per horsepower. I’ve covered why in Swimming pool pumps: the shocking truth.

What about variable-speed pumps?

Variable-speed pumps are brilliant for pool filtration. ENERGY STAR explains that halving the pump speed cuts its energy use to about an eighth, which is why they save so much on running costs. But the flow drops with the speed, and slow flow is exactly what jets don’t want. Run fast enough to drive the jets and most of the saving disappears. So a variable-speed pump makes sense on the circulation and heating side, not as the jet pump. There’s more in Can I use a variable speed pump in a hot tub?

And how do you switch the jets on?

With a spa pack, you press a button on the topside and the jets come on. A pool pump on a timer has no such control. You could add pool automation (more on that in Can I use pool parts on a hot tub?), but it’s a lot of cost and complication for something a spa pack does out of the box.

So when your pool guy says you can “just add the jets onto the pool pump”, he or she is wrong. It won’t work well, if at all.

The filter

Hot tub filters are generally cartridge filters. Pool filters are generally sand filters. That’s a big difference in itself.

Sand filters are restrictive at jet-pump flows. As a rough guide, many domestic sand filters top out at around 80 GPM. Check the rating on yours, but if the jets need 160 GPM and the filter only passes half that, you’ll never power them all properly. Spa cartridge filters are chosen to pass the jet pump’s flow.

Sand filters also don’t filter as finely. Trouble Free Pool puts sand at 20–40 microns and cartridges at 10–15 microns. On the other hand, sand copes well with big volumes of water and heavy debris.

That said, you can use a sand filter on a hot tub, on a separate heating and filtration loop with its own pump. What you can’t do is put a sand filter on the jet plumbing. I’ve covered this in Can I use a sand filter on my DIY hot tub? and Cartridge or sand filter?

Can I heat my hot tub with my pool heater?

This is a question I get regularly. The answer is generally yes, but you need either a separate system or at least some diverter valves to switch the suction and return between the pool and the hot tub.

In spa mode, you draw water from the hot tub, through the pump and filter, divert it through the heater and return it to the tub. In pool mode, the valves switch back. Pool automation systems handle this with motorised valves. Pentair’s IntelliCenter, for example, uses IntelliValve actuators to direct the flow between pool and spa.

What I don’t suggest is returning the heated water through the jets. Give the hot tub its own set of wall returns for the heated water, and keep the jets on their own jet pump. If the jet pump is running, its pressure can overpower the heating loop’s flow at the jets, and the heater will stop with a flow error. With separate returns, the two never fight.

It sounds more complicated than it actually is, but it needs designing properly. Size the heater for the pool (a small tub heats quickly on a big pool heater). See Determining the perfect heater size for your pool or hot tub.

What about a spillover into my pool?

This is another question I get asked a lot. A spillover (or overflow) spa sits above the pool and pours into it. There are a few things to think about in the plumbing:

• How are you going to heat the tub? If you’re pumping pool water into the tub to make it overflow, you’re constantly replacing hot tub water with cooler pool water. It’ll work, but the heating efficiency drops dramatically if you’re trying to keep the two at different temperatures.
• Same temperature? If you’re happy to run the pool and tub at the same temperature, a spillover works fine.
• Heating the pool from the tub. The other way it works is to let hot water from the tub overflow and warm the pool. The downside is that there’s no real control over the pool’s temperature. The thermostat is on the hot tub, not the pool.
• Shared water means shared chemistry. A spillover spa and its pool are one body of water. That’s simpler to look after, but you can’t run the hot tub at its own chemical levels. Separate systems can each have their own. See Hot tub chemical levels.

A common approach is to run spa mode (the spa’s suction and returns only, heater on) when you want a hot soak, and spillover mode when you want the look. There’s more on the options in Adding a spill-over hot tub to an existing swimming pool and on flow in How to correctly size a swimming pool overflow or spillover.

Pipe sizes

Depending on your pool’s design, the chances are its pipe sizes won’t match the hot tub’s.

Pipe sizes. On a typical DIY gunite-jet tub the water lines are 2in and the air lines are 1.5in, which matches the connections on gunite jet bodies. The 2.5in pipe is the sleeve for the jet bodies through the wall. The sizes are smaller than this on plastic shell tubs, but that’s not what we’re talking about here.

On a pool, the suction lines might be 2in while the returns are often smaller, commonly 1.5in. They won’t line up with the hot tub side without adaptors. It’s all doable, but it’s one more thing to plan. And if either side uses metric pool parts, remember that 63mm is not 2in. See Hot tub pipe: the ultimate guide.

Whatever you share, keep the water speed sensible. Pool and spa standards cap it at 8 ft/s in returns and 6 ft/s on the suction side of filtration piping. A big jet pump pulling through a pool-sized suction line breaks that quickly.

Skimmers, returns and drains

Skimmers and pool returns can be used on both hot tubs and pools. They’re the same parts. But a hot tub jet is not the same as a pool return.

Hot tub jets mix water from the pump with air, either from a blower or drawn in by venturi, and the jet nozzles are designed to focus that into a massage. That’s what gives the jet its punch and the tub its bubbles. A pool return doesn’t do much more than look neat and deliver filtered, heated water back into the pool.

Main drains are shared kit too, but suction safety matters even more on a hot tub. You’re sitting right on top of the drains, with a powerful pump behind them. Fit two drains at least 1m (3ft) apart with certified anti-entrapment covers rated for your flow. See Installing a main drain.

What you can share, and what you shouldn’t

Share it?	Part
Yes	Heater (with diverter valves), skimmer type, wall returns, main drains (per tub), sanitiser system on the circulation loop
With care	Pool automation instead of a spa pack, and a filtration pump and sand filter on a separate hot tub heating loop
No	Jets on the pool pump, a sand filter on the jet loop, pool returns in place of jets, or heated water returned through the jets

Common mistakes

• Adding jets to the pool pump and expecting hot tub performance.
• Putting the jet flow through a sand filter.
• Returning heated water through the jets, then chasing heater flow errors. See Hot tub FLO error.
• Designing a spillover without deciding how the tub will be heated.
• Mixing pipe sizes and metric and imperial parts without planning the adaptors.
• Undersized suction on the jet pump.

Planning a pool and hot tub combo?

With the right planning, you can combine a lot, sharing a heater for example. What you can’t do is just “add some hot tub jets” onto your pool pump. If you’re adding a tub to an existing pool, start with Can I add a hot tub to my existing swimming pool? and see how one customer did it in How to add a hot tub to an existing pool. For design ideas, see Hot tub pool combo design considerations.

Combos are where I see the most plumbing mistakes, and they’re expensive to fix once the concrete’s in. My hot tub and pool combo plumbing consultancy works through your pumps, valves, heater and pipe sizes with you. If you’re starting from scratch, the combo design consultancy covers the whole design. Or try the free Tub & Pool Designer Pro software and I can turn it into architectural CAD and a 3D plumbing design.

FAQs

Can I add jets to my swimming pool pump?

Not if you want proper hot tub jets. Gunite jets need around 10 GPM each, and a pool filtration pump isn’t built to deliver that at jet pressure. Give the jets their own jet pump.

Can a pool and hot tub share a heater?

Yes, with diverter valves (manual or motorised) that switch the suction and returns between the pool and the tub. Return the heated water through wall returns, not the jets.

Can a hot tub and pool share a filter?

They can share a filter on the circulation and heating side. Don’t put the hot tub’s jet flow through a pool sand filter, because it’s too restrictive.

Will a spillover spa stay hot?

Only if you stop the pool water flowing into it while you heat it, or run both at the same temperature. Most combos use a spa-only mode for heating.

Are hot tub jets the same as pool returns?

No. Jets mix air and water and focus it into a massage. Returns simply put filtered water back into the pool.

Planning a combo? Tell me about it in the comments below. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi