Using a Priming Circuit Above Ground on a Centrifugal Hot Tub Pump

priming pump circuit

I’ve mentioned many times on this blog and on my YouTube Channel that the position of the control room is really important. I actually have a full article here that explains the difference between above ground and below ground control rooms here.

In this post, I am going to explain how I used a priming circuit to “fix” a control room issue on an above ground build.


A recent project that I was involved with from the outset had the plan of a below ground control room. Everything was designed around the control room being below the water line of the Hot Tub.

The builder understood the principals involved and I had custom designed the project so everything was clear from the outset.

However, what transpired afterwards was that when the pipes were laid to and from the control room, they were not far enough themselves under the waterline. The builder could not get them any lower as there was a concrete base and he thought that they would be ok.

This was not the case.

The Initial Result

As you would expect if you are using a centrifugal pump (regular hot tub jet pump) they are not able to suck air so we could not get the system to prime.

No matter how much water we tried to fill the pipes with using a hose pipe, we just could not get the system primed up. There was no action on the jets.


Hindsight is a wonderful thing. What we could have done if we had known that the pipes were not deep enough would have been to put a non-return or one way valve on the pipes going to and from the tub.

In that way, we could have filled them initially with water using a hose pipe and then they would have stayed primed once the pump had run for a couple of minutes even when it stopped.

Basically, when you shut off a centrifugal pump it is not pressurised, so the water flows out of the pipes – this was what was happening on this build.

With a non-return valve it would not have done this and would have stayed primed.

However, we did not have that luxury as the pipes were set in concrete and the final pool ground sabs had been laid and the last thing the builder wanted to do was rip them up.

The Problem

The problem we had on this build was that there was a significant distance to the hot tub from the control room.

We had allowed for this in the design moving the pipes to 3” to reduce water friction and loss of flow. Calculations had all been done. However, this was centred around a 5HP jet pump with some serious power.

Initially, we switched to a two self-priming pump setup one circulation and one jet. We had a 2HP circulation pump (because of the distance) and that was used in parallel with a 3HP self-priming jet pump. This is one of the largest and most powerful models on the market.

The result – not enough pressure. As I have said many times, swimming pool pumps do not match the flow rates of hot tub pumps so combining a 2HP and a 3HP swimming pool pump was not going to give us the same result as a 5Hp jet pump.

Just a note here is that the builder knew the solution was going to be trial and error because of what he had done with the pipes.

When you don’t have enough pressure on your jets, the jet pump actually works against the air blower and everything grinds to a halt. Water is forces into the air lines rather than the jets and there is little to no action in the tub.

In this instance, the distance was just too far and the 3HP jet pump was simply not powerful enough to power all of the jets.

This was an issue as larger self-priming pumps are available for huge swimming pools but they are commercial grade and cost an absolute fortune. This was not something the client was going to entertain.

The Solution – a Self-Priming Circuit

This principal of this solution came out of a video I watched on YouTube after many hours of researching the issue. In that video, a guy used a hosepipe to prime up a centrifugal pump so he could use it above ground.

He has some non-return valves in there so that it stayed primed when it was disconnected and turned off.

Knowing that I could not add any non-return valves to this project, I needed another way of doing this.

What I did was take that principal and instead of using a hosepipe, I was going to use a self-priming circulation pump to prime up the centrifugal jet pump.

When a centrifugal pump primes, it uses gravity to keep the water in the pipes and in the pump. When a self-priming pump primes, it creates a pressurised system to hold the water in place.

This was exactly what I needed. I needed to use the self-priming pump to draw the water up from the hot tub through the pipes, push it through the centrifugal jet pump and then use the self-priming pump to pressurise and hold all of that water in place.

Then, through a series of ball valves, I would then put the flow back together so that the centrifugal jet pump was drawing its own water in (not running in series with the self-priming pump as there would not be enough flow)

When the centrifugal jet pump turned on, it already had water, was already primed even through it was above ground.

Perfect – job done. The solution worked.

priming pump circuit


In this explanation, I am going to refer to the color coded pumps and gate valves in the diagram below.

Priming Circuit Hot Tub Pump

  • Purple pump – this is the 5hp centrifugal jet pump
  • Red Pump – this is the 2HP Self Priming Circulation Pump
  • Yellow Pipe is from the lower drains in the tub
  • Turquoise pipe is the jet returns to the tub

On initial setup, to prime the system, the pink and turquoise ball valves would be closed and the yellow ball valve open.

This then placed the self-priming pump and the centrifugal pump in series. The self-priming circulation pump is forcing water through the centrifugal pump and back to the tub through the pipework.

I left it running for a couple of minutes, then shut off the system.

As it is pressurised, the water does not flow back into the tub and the pump stays primed.

The yellow ball valve was closed, turquoise ball valve open and pink ball valve opened too. This now puts the pumps in parallel where the flow rate can now be increased. As the system is pressurised, the system remains primed.

Now, the 5HP centrifugal jet pump can operate. Bingo!


Whilst it is not ideal and I would definitely opt for one or the other in terms of an above ground or a below ground setup – this self-priming circuit method can be used to make a centrifugal pump prime above ground.

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 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.

Today, I've helped over 1000 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

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