Self-Priming Hot Tub Pump Options: What to Do When the Pumps Sit Above the Water Line

Last updated: September 2026

If you can’t build the control room for your DIY hot tub below ground, you’re going to need a self-priming hot tub pump, or a way round the problem. Standard hot tub pumps have to sit below the water line. Put them above it and they won’t prime.

The catch is that big, powerful self-priming pumps that can drive a full set of hot tub jets are hard to find. In this post I’ll explain why, and walk through the options you do have, from splitting the jets across smaller pumps to a priming circuit on a standard jet pump.

What is a self-priming pump?

A normal hot tub jet pump is a centrifugal pump. It’s brilliant at throwing a lot of water through the jets at pressure, but its impeller is designed to move water, not air. If the pump is full of air, the impeller just spins and nothing moves. That’s why hot tub pumps are fitted below the water line, where gravity keeps them full. This is called a flooded suction.

A self-priming pump is a centrifugal pump with a built-in chamber that holds water. On start-up it churns that water with the air in the suction line, separates the air out and pushes it out of the discharge, until it’s pulling solid water. Once it’s running, it pumps normally. Most swimming pool pumps are self-priming, with a strainer basket and a clear lid on the front.

Two things to know:

• It still needs water to start. Fill the strainer housing before the first start, and after any time it’s been drained.
• It has limits. Every self-priming pump has a maximum suction lift in its specs. The higher it sits above the water and the longer the suction pipe, the longer it takes to prime, and at some point it won’t. Any air leak on the suction side will stop it priming at all.

There’s more on why control room position matters in Hot tub control room position and Hot tub control room design considerations.

Why are large single-speed self-priming pumps hard to find?

We used to be able to get hold of large single-speed self-primers. These days they’re few and far between, especially in the USA. The main reason is regulation. Under the Department of Energy rules that took effect in July 2021, new or replacement inground pool filter pumps over 1.0 total horsepower must, in practice, be variable speed to meet the efficiency standard. Commercial pumps over 5.0 THP are excluded, but those are big, expensive units built for large pools.

So most manufacturers’ self-priming ranges are now variable speed pool pumps, and those are designed for filtration: moving volume at low pressure. For example, Pentair’s SuperFlo VS tops out at 2.2 THP and around 130 GPM on its published curve, and that’s with almost nothing to push against. Rules differ outside the USA, but the same trend towards variable speed pool pumps is there. I’ve explained why variable speed pumps aren’t right for jets in Can I use a variable speed pump in a hot tub?

Why this is a problem for DIY hot tub builders

For us, flow is everything. Each gunite jet needs around 10 GPM (that’s what Waterway rates its jet internals at), so a 16-jet hot tub needs about 160 GPM at the jets, after the friction of the pipes and fittings. If all we can get are smaller self-priming pool pumps, there just isn’t enough flow for one pump to run all 16 jets.

Pool pumps also give less jet performance per horsepower than hot tub pumps. I learned that the hard way on the build in my priming circuit post, where a 2HP plus a 3HP self-priming pump couldn’t match one 5HP jet pump. There’s more in Swimming pool pumps: the shocking truth.

What are the options?

Option 1: split the jets across smaller self-priming pumps

Instead of trying to run a 16-jet system on one pump, split it into two 8-jet circuits, each with its own self-priming pump. Each pump then only needs to deliver around 80 GPM, which is much easier to find. Run them on separate pump outputs from the spa pack, and each circuit gets its own button.

Things to check:

• The spa pack needs two pump outputs, each rated for the pump you’re fitting, and a supply big enough for both. See Hot tub wiring explained.
• Voltage and speeds. A spa pack switches a pump on and off, or between two fixed speeds. A single-speed self-primer is simplest. A variable speed one will just run at whatever speed its own controller is set to.
• Size by the pump curve, not the horsepower. See How to correctly size a hot tub pump.

Option 2: a priming circuit on a centrifugal jet pump

The second option is to keep a proper centrifugal jet pump and use a small self-priming pump to prime it. I’ve written this up in full in Using a priming circuit on a centrifugal hot tub pump, but here’s how it works, using the colour-coded pumps and ball valves in this diagram:

1. Prime (pumps in series). Close the pink and turquoise ball valves and open the yellow one. The self-priming circulation pump now draws water from the tub and forces it through the centrifugal pump and back to the tub. I left it running for a couple of minutes to push all the air out, then shut the system off.
2. Switch (pumps in parallel). Close the yellow ball valve, then open the turquoise and pink ones. The pumps are now in parallel, so the flow can increase.
3. Run the jets. The 5HP centrifugal jet pump is already full of water, so it can run. Bingo!

Why does it stay primed? Once the loop is completely full of water and sealed, with both ends under the water in the tub, the water can’t drain back because air can’t get in to replace it. It’s like putting your finger over the end of a straw. The catch is that it has to be airtight. A weeping union, a pump lid O-ring or a poor suction joint will let air in, and you’re unprimed again.

Option 3: check valves (if you can still reach the pipes)

If you’re at the design stage, or the pipes are still accessible, the simplest fix is a check valve (non-return valve) on the suction side. Fill the pump and pipes once with a hose, and the check valve holds the water in when the pump stops. It adds a little friction, and check valves can stick, but it’s cheap. It’s also what I wish we’d been able to do on the build in my priming post.

Option 4: design it out

The best option by far is to avoid the problem. Put the control room, or at least the pumps, below the water line, and make sure the pipes between the tub and the pumps stay below it too. A pump pit with drainage is often easier than you think. It’s something I’d sort out on paper before a single pipe goes in.

The options compared

Option	Best for	Downsides
Pumps below the water line	Any build, at the design stage	A deeper control room or pump pit needs drainage
Split jets, two self-priming pumps	Above-ground control rooms	Two pumps, two pack outputs, less punch per HP
Priming circuit	When a big jet pump is essential and the pipes are buried	More pumps, valves and joints. Must be airtight
Check valves	When the pipes are still accessible	Extra friction, can stick, prime once by hand

For the circulation side, filtration and heating, it’s much easier. Small self-priming circulation pumps are widely available and work well above the water line. See How to add a circulation pump to a hot tub and How many pumps do you need?

Troubleshooting a self-priming setup

• Won’t prime: fill the strainer housing, check the lid O-ring is clean and seated, and make sure the water level is above the skimmer mouth and drains. Check the pump isn’t beyond its rated suction lift.
• Primes, then loses it overnight: there’s an air leak on the suction side. Check the unions, the lid and any joint above the water line.
• Weak or surging jets: air still in the system, or a pump too small for the number of jets. See How to deal with a hot tub air lock.
• Pump running with no flow: switch it off. Running dry can damage the seal quickly.
• Noisy, gravelly sound: the pump is starved of water. See Noisy hot tub pump.

Want it drawn up for you?

Pump position, pipe depth and pump sizing are exactly the things a proper plumbing design gets right before the concrete goes in. The Tub & Pool Designer Pro plumbing design service lays out the pumps, pipe sizes and routes for your build. If you’re already mid-build and stuck with pumps above the water line, the hot tub plumbing consultancy will help you work through it.

For standard sizes, my DIY hot tub plans include the plumbing layout and control room position. Pumps are available from parts4tubs.co.uk in the UK and parts4tubs.com in the USA.

FAQs

Do I need a self-priming pump for my hot tub?

Only if the pump, or the pipes to it, sit above the water line. Below the water line, a standard centrifugal hot tub pump is the better choice.

Can a self-priming pool pump run hot tub jets?

Yes, but expect less jet performance per horsepower. Most are sized for filtration, so you may need to split the jets across two pumps.

How many jets can one self-priming pump run?

Allow about 10 GPM per jet at the jets, then check the pump’s curve at your system’s head. For example, 8 jets need around 80 GPM.

Why can’t I find a big single-speed self-priming pump?

In the USA, efficiency rules since 2021 mean most new inground pool pumps over 1 total HP are variable speed. Large single-speed self-primers are mostly commercial.

How do I keep a pump primed above the water line?

Use a check valve on the suction side, or a sealed priming circuit, and make sure every suction joint is airtight.

Stuck with a control room above the water line? Tell me about it in the comments below. I read them all.

Cheers,
Andi

You can also find me on my socials;

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi

 

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