How to Install a Hydrostatic Relief Valve in a Pool or Hot Tub

Last updated: September 2026

Installing a hydrostatic relief valve sounds complicated. It’s got a complicated name, after all. But on a DIY hot tub or plunge pool it’s really just three plastic parts screwed together and set into the floor before you pour the slab. The hard part is remembering to do it at the right time, because once the concrete’s in, it’s too late.

This guide walks through the parts, where they go and how to install them, using the Certikin kit I supply in the UK and the USA. If you’re not sure why you need one in the first place, read Why do I need a hydrostatic relief valve? first.

What the valve does, in one paragraph

A pool or hot tub hydrostatic relief valve is a much simpler thing than the valves of the same name used in marine and offshore work. It’s a normally closed one-way valve in the floor. With water in the tub, the water above helps keep it shut. If the tub is empty and groundwater builds up under the slab, the valve opens and lets that water into the tub, so the pressure can’t crack or lift the floor. It doesn’t let pool water out into the ground. There’s a full explanation, with a diagram, in the “why” post, and more on video on my YouTube channel.

The parts

Hydrostatic relief valves come in different shapes and forms depending on the manufacturer. I’ll explain the installation for the Certikin one, because it’s the model I supply in Europe and the USA through parts4tubs.co.uk and parts4tubs.com. Other makes work in the same way, but always follow the instructions that come with yours.

1. The main drain (the housing)

The first part is the main drain body. It’s a key part of the system, because it’s the housing the valve sits in. It also means you can get to the valve later to clean or replace it. Just remember you’ll need to empty the tub or pool first. Certikin sells it as a concrete main drain kit with the hydrostatic relief valve, collection tube, plug and fittings, which is the easiest way to get everything in one box. It also comes with an anti-vortex grille.

2. The valve

Next is the valve itself. It’s normally closed, and the water pressure above helps to keep it closed. When the tub or pool is empty and there’s no pressure on top, if hydrostatic pressure builds up underneath, the valve opens to let water in and release the pressure. That’s what prevents the damage. On the Certikin system it’s a separate screw-in part, the HD34, which fits the standard HD33 sump, so it can be replaced on its own.

3. The collection tube

The final part is the water collector, or collection tube. It’s a perforated, spike-shaped tube that screws into the base of the drain and sits in the gravel under the floor. That’s where it collects the groundwater from. It must not be concreted in place, as that would defeat its purpose.

Where does it go?

At the lowest point of the floor. On most hot tubs and plunge pools that’s the main drain position, usually central in the deepest area. The valve goes in one of your main drains, and you only need one valve for a typical DIY tub.

If the drain is also a suction outlet for your pumps, the usual anti-entrapment rules still apply: a suitable drain cover, and on a pool, two drains a set distance apart. I’ve covered that in Installing a main drain. If you’re not using it as a drain at all, you can simply cap off the 2in pipe connection and it becomes a housing for the valve.

How to install a hydrostatic relief valve, step by step

This fits into the base build-up described in my in-ground build guide and concrete slab guide: compacted sub-base, sand blinding and a DPM, insulation, then a reinforced slab of about 150mm (6in). Here’s the drawing I use to show how the parts sit in the slab:

Step 1: Make a gravel pocket for the collection tube

Once your sub-base is down and compacted, dig out a pocket where the drain will go, a bit deeper and wider than the collection tube. Fill it with clean, free-draining gravel or stone, not sand or soil. This is the water collection area. The gravel lets any groundwater under the floor find its way to the tube. If you’re on clay, a bigger pocket is better.

Step 2: Assemble the valve and collection tube

These are screw-in connections, and you don’t glue them, just in case you need to change them in the future. Wrap the threads of both the valve and the collection tube with PTFE tape and screw them into the drain body. To be honest, you could glue in the collection tube, because once it’s under the slab you won’t get to it without breaking out the drain, which you don’t want to do. But the valve must not be glued, because it’s the part you might need to replace.

Step 3: Get through the DPM and insulation

The collection tube has to reach the gravel, so it goes through the DPM and any insulation board. Cut a neat hole just big enough for the tube, push it through, and tape the DPM tightly around it so the concrete doesn’t run down into the gravel.

Step 4: Set the height, level and position

This is the bit that matters for the finish. The drain rim needs to end up flush with your finished floor, so set it proud of the concrete by the thickness of your finishes: render, membrane, adhesive and tiles. My drawing says about 25mm (1in) on average for render and mosaic, but work it out from your own build-up. Check it’s level and square with the shell, then fix it firmly. Support it on blocks, or wire it to the rebar so it can’t float up or shift when the concrete goes in.

Step 5: Connect and test the pipework

If you’re using it as a main drain, solvent-weld the 2in outlet pipe back towards the control room (see how to make a PVC solvent weld joint). Where a pipe runs through the concrete, a puddle flange helps stop water tracking along the outside of the pipe, as shown on the drawing. Pressure test the drain line before you pour. If you’re not using the drain, cap off the outlet. For the wider plumbing layout, see DIY hot tub plumbing.

Step 6: Protect it and pour

Leave the grille on or tape over the top of the drain so no concrete or slurry gets inside. Grit on the valve seat is the most common reason these leak. Pour the slab as normal, working the concrete around the drain body so there are no voids, and trowel it neatly to the flange. Don’t let concrete get down around the collection tube.

Step 7: Waterproof and tile up to it

Render the floor to the drain flange, then take your pool-rated membrane into the drain as the membrane manufacturer details it. Penetrations like drains are where a membrane system is most likely to fail, so this is worth doing carefully. There’s more in my waterproofing, grout and tile adhesive guide.

Step 8: Check the valve before you fill

Before the first fill, take the grille off and check the valve. It should be clean, free of grit and properly seated. Then refit the grille and do your flood test, as described in the build guide. It’s far easier to sort out a problem now than once the tub is tiled and full. When you do fill it, see How to fill a hot tub the correct way.

Replacing the valve later

Because the valve screws in, replacing it is simple. Drain the tub (in dry weather, not after heavy rain, when the groundwater is high), take off the grille, unscrew the old valve and screw in a new one with fresh PTFE tape. Clean the seat first. If water starts coming up through the drain while it’s out, that’s groundwater, and it’s a sign you picked a wet day. Let it settle, or refill the tub and wait for a drier spell. There’s more on this, and on spotting a leaking valve, in Why do I need a hydrostatic relief valve?

Common mistakes

• Forgetting it until the concrete’s ordered. Put it on the “before you pour” list with the drain, sleeves and rebar. My DIY hot tub shopping list includes it.
• Concrete around the collection tube, or no free-draining gravel under the drain.
• Gluing the valve into the drain.
• Setting the drain flush with the slab, so it ends up sunk below the finished floor once the render and tiles go on.
• A drain that moves during the pour because it wasn’t fixed down.
• Not pressure testing the drain line before it disappears under concrete.
• Grit on the valve seat at first fill.

For more build pitfalls, see Avoid these 7 DIY hot tub mistakes.

Want it drawn up for you?

My DIY hot tub plans show the drain and hydrostatic relief valve position, the full plumbing layout and a parts list, with the video course to walk you through it. Building something bigger? There are plunge pool plans and swim spa plans too. For an unusual shape or a hot tub and pool combo, I can draw up architectural CAD and a 3D plumbing design for you.

FAQs

When do you install a hydrostatic relief valve?

Before you pour the floor slab, at the same time as the main drain. It has to connect to a gravel bed under the slab, so it can’t sensibly be added afterwards.

Where should the hydrostatic relief valve go?

At the lowest point of the floor, normally inside the main drain in the deepest area of the tub or pool.

Should I glue the hydrostatic relief valve in?

No. Use PTFE tape on the threads so you can unscrew and replace it later. The collection tube underneath can be glued if you want, because you won’t be able to reach it anyway.

Can I use the main drain as a drain as well?

Yes. The valve sits in the bottom of the sump and the drain works as normal. If you don’t need a drain, cap off the outlet and it’s just a housing for the valve.

Does an above-ground hot tub need one?

No. A tub built fully above ground on a slab has no groundwater around it. Anything sunk into the ground, even partly, should have one.

Got a question about fitting yours? Drop 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