Using a Priming Circuit on a Centrifugal Hot Tub Pump Above the Water Line

Last updated: September 2026

I’ve said many times on this blog and on my YouTube channel that the position of your control room is really important. Standard hot tub pumps need to sit below the water line. This post is the story of a build where, despite a design that said exactly that, the pumps effectively ended up above it, and how I used a priming circuit to get a centrifugal jet pump working anyway.

It’s a fix, not a first choice. But if you’re stuck with a pump that won’t prime, it might just get you out of trouble.

Why hot tub pumps won’t prime above the water line

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 liquid, which is far denser than air. If the pump is full of air, the impeller just spins in it and nothing moves.

That’s why the pumps in a shop-bought tub sit in the base. If the pump is below the water line, gravity fills it with water every time, and it never has to lift water from the tub. This is called a “flooded suction”. I’ve covered it in detail, including a quick test you can do before you commit, in Hot tub control room position and Hot tub control room design considerations.

Background: what went wrong on this build

This was a project I was involved in from the outset, and I’d custom designed it with a below-ground control room. Everything was designed around the control room being below the water line of the hot tub, and the builder understood the principles.

However, when the pipes were laid to and from the control room, they weren’t laid far enough below the water line themselves. There was a concrete base in the way, so the builder couldn’t get them any lower, and he thought they’d be OK.

They weren’t.

The initial result

As you’d expect with a centrifugal pump, we couldn’t get the system to prime. No matter how much water we poured into the pipes with a hosepipe, it wouldn’t prime, and there was no action at the jets.

Hindsight

Hindsight is a wonderful thing. If we’d known the pipes weren’t deep enough, we could have fitted non-return (check) valves on the pipes to and from the tub. We could then have filled them with a hosepipe once, and they’d have stayed full, and the pump primed, even when it stopped. Without them, when the pump switched off, the water simply ran back out of the pipes into the tub.

But we didn’t have that luxury. The pipes were set in concrete, the final pool slabs had been laid, and the last thing the builder wanted to do was rip them up. If you’re at the design stage, fit the check valves. It’s by far the simplest answer.

The problem: distance and power

There was a significant distance between the control room and the hot tub. We’d allowed for this in the design by moving up to 3in pipe to cut friction and keep the flow up, and all the calculations had been done. But they were based on a 5HP centrifugal jet pump with some serious power.

Our first attempt was to switch to two self-priming pumps: a 2HP circulation pump (because of the distance) running alongside a 3HP self-priming jet pump, which was one of the largest and most powerful models on the market.

The result was not enough pressure. As I’ve said many times, swimming pool pumps don’t match the flow of hot tub pumps, so a 2HP plus a 3HP pool pump was never going to perform like a 5HP jet pump. (There’s more on why in Swimming pool pumps: the shocking truth.) The builder knew it would be trial and error because of what had happened with the pipes.

With the water flow that weak, the jets and the air blower ended up working against each other. Water was forced into the air lines rather than out of the jets, and there was little to no action in the tub. The run was simply too long for a 3HP self-priming pump to drive all of the jets.

Bigger self-priming pumps do exist, but they’re commercial units for huge swimming pools and cost an absolute fortune. That wasn’t something the client was going to entertain.

The solution: a priming circuit

The idea came from a YouTube video I found after many hours of researching the problem. In it, a chap used a hosepipe to prime a centrifugal pump so he could use it above ground. He had non-return valves in his setup so it stayed primed when it was disconnected and turned off.

I couldn’t add non-return valves to the buried pipework on this project, so I needed another way. I took the same principle but, instead of a hosepipe, used a self-priming circulation pump to prime the centrifugal jet pump.

A self-priming pump can do what the jet pump can’t. It keeps a reservoir of water in its own casing, and it uses that to separate out the air and push it out of the discharge until it’s pulling solid water. So the plan was to use it to draw water up from the tub, push it through the centrifugal jet pump and back to the tub, until the whole loop was full of water. Then, with a few ball valves, I’d switch the flow round so the jet pump draws its own water, not in series with the smaller pump, which couldn’t supply enough flow.

When the jet pump switched on, it was already full of water. It was primed, even though it was above the water line. Job done. It worked.

How the priming circuit works, step by step

I’ll refer to the colour-coded pumps, pipes and ball valves in my diagram below.

• Purple pump: the 5HP centrifugal jet pump.
• Red pump: the 2HP self-priming circulation pump.
• Yellow pipe: from the lower drains in the tub.
• Turquoise pipe: the jet returns to the tub.

1. Prime (pumps in series)

Close the pink and turquoise ball valves and open the yellow ball valve. This puts the self-priming pump and the centrifugal pump in series. The self-priming pump draws water from the tub and forces it through the centrifugal pump and back to the tub through the pipework. I left it running for a couple of minutes to push all the air out, then shut the system off.

2. Switch the valves (pumps in parallel)

Close the yellow ball valve, then open the turquoise and pink ball valves. This puts the two pumps in parallel, where the flow can increase. Pumps in parallel add flow, which is what jets need. Pumps in series add pressure. There’s more on that in How many pumps do you need?

3. Run the jet pump

Because the jet pump and its pipes are already full of water, the 5HP centrifugal pump can now run. Bingo.

Why it stays primed

It’s tempting to think the self-priming pump “pressurises” the system, and that’s what holds the water in place. That’s not really what’s going on.

Once the loop is completely full of water and sealed, with both ends under the water in the tub, the water can’t fall back out because air can’t get in to replace it. It’s the same as putting your finger over the top of a straw full of water. The weight of the air pressing on the tub’s surface holds the column up, the same way a siphon works. It can do that for up to about 10m (33ft) in theory, far more than any control room needs.

The catch is the word sealed. If air can get in anywhere (a weeping union, a pump lid O-ring, a poor suction joint, or a big enough bubble collecting at a high point), the water drains back and you’re unprimed again. That’s why a priming circuit only works on plumbing with perfect joints. It’s also why a check valve on the suction side is the standard way to stop a stopped pump siphoning empty.

Your options if the pump sits above the water line

Option	When it fits	Downsides
Move the pumps (and pipes) below the water line	At the design stage. Always the first choice.	A deeper control room needs drainage.
Check valves and a way to fill the lines	When the pipes are still accessible	Adds some friction. Check valves can stick, and you need to prime once by hand.
Self-priming pumps	Smaller tubs, or circulation duties	Less flow per HP, pricier (“swimming pool tax”), and big ones are commercial. See self-priming hot tub pump options.
A priming circuit (this post)	When the pipes are buried and a big jet pump is essential	More pumps, valves and joints. It only works if the plumbing is airtight, and the valves must be set in the right order.

Troubleshooting a priming circuit

• Won’t prime at all: check the water level is well above the skimmer mouth and drains, and that the self-priming pump’s own basket housing is full of water before you start it. Self-priming pumps usually need their housing filled on first start.
• Primes, then loses it overnight: there’s an air leak on the suction side. Check the pump lid O-ring, the unions and any joint above the water line.
• Jets weak or surging: there’s air still in the system. Run the priming step for longer, and see How to deal with a hot tub air lock.
• Water in the blower: the air lines need a check valve and a Hartford loop. See Water in a hot tub air blower.
• Pump running but no flow: switch it off. A centrifugal pump running dry can damage its seal quickly. Check the valve positions before trying again.

Conclusion

It isn’t ideal, and I’d always design for one approach from the start: a below-ground control room with standard hot tub pumps, or an above-ground one with self-priming pumps. But this priming circuit does work. It can make a centrifugal pump prime above the water line when you’ve run out of other options.

The real lesson is in the design. The pump being below the water line isn’t enough on its own. The pipes between the tub and the control room must stay below it too. If you’re adding a separate circulation loop anyway, see How to add a circulation pump to a hot tub. To check your pump will cope with a long run, see How to correctly size a hot tub pump.

Want it drawn up for you?

This whole problem came down to a few inches of pipe depth. That’s exactly the sort of thing a proper plumbing design catches before the concrete goes in. The Tub & Pool Designer Pro plumbing design service lays out pumps, pipe sizes and routes for your build. If you’re already mid-build and stuck, the hot tub plumbing consultancy is there to help you work through it.

For standard sizes, my DIY hot tub plans include the plumbing layout and control room position. For pumps and spares, head to parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

Can a hot tub pump be above the water line?

A standard centrifugal hot tub pump shouldn’t be, because it can’t lift water to prime itself. Above the water line you need a self-priming pump, check valves or a priming circuit.

What is a flooded suction?

It’s when the pump sits below the water level, so gravity keeps it full of water. That’s how standard hot tub pumps are designed to run.

Why does my hot tub pump lose its prime?

Usually because air is getting in on the suction side (a lid O-ring, a union or a joint) or because the water level has dropped. If the pump is above the water line, water drains back when it stops.

Can I use a check valve to keep my hot tub pump primed?

Yes. On the suction side, a check valve holds the water in the line when the pump stops. You’ll need to fill it once, and it adds a little friction.

Can I replace a hot tub jet pump with a pool pump?

You can, but expect less jet power for the same horsepower. Pool pumps are built for volume at low pressure. See why pool and hot tub plumbing should remain separate.

Stuck with a pump that won’t prime? Drop your setup 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