Why Do I Need a Hydrostatic Relief Valve in My Pool or Hot Tub?

Last updated: September 2026

A hydrostatic relief valve is one of the cheapest parts of an in-ground pool, plunge pool or hot tub build, and one of the easiest to forget. It sits hidden in the floor, does nothing at all for years, and then one wet winter it’s the thing that stops your empty shell cracking or lifting out of the ground.

This post explains what the valve does, why groundwater is such a problem for a concrete shell, when you really need one and how to look after it. If you’re ready to fit one, I’ve covered that step by step in How to install a hydrostatic relief valve.

What is a hydrostatic relief valve?

A hydrostatic relief valve (often shortened to HRV, and sometimes called a hydrostatic release valve) is a small, normally closed one-way valve set into the floor of an in-ground pool or hot tub. On most DIY builds it screws into the bottom of the main drain sump. Underneath, a perforated collection tube goes down into a bed of gravel or hardcore below the slab.

The valve is held shut by a spring and by the weight of the water in the tub. It only opens when the water pressure underneath the floor is higher than the pressure inside. When that happens, groundwater flows up through the gravel and collection tube, through the valve and into the shell. The pressure equalises, and the floor doesn’t take the load. As one pool builder puts it, it gives the groundwater “a relief point, so instead of pushing on the pool it enters the pool, equalizing pressures”.

Two things it is not:

• It doesn’t drain pool water into the ground. The valve only opens inwards. In normal use it stays shut.
• It has no float switch or power. It’s a simple mechanical check valve. There’s nothing electrical to fail.

Some pools, mostly in the USA, use screw-in hydrostatic plugs in the floor instead, which you open by hand when you drain. In The Swim describes both types, and says the automatic spring-loaded valves are the kind usually found in the main drain pot. On a DIY hot tub or plunge pool, the automatic valve in the main drain is the one I’d use.

Why groundwater is such a problem

Concrete is heavy, but water pushes hard. For every metre of groundwater above the underside of your slab, the water pushes up with roughly a tonne on every square metre of floor (about 62lb per square foot for every foot of depth). A 150mm (6in) reinforced slab weighs about 360kg per square metre. So only around 360mm (14in) of groundwater above the underside of the floor is enough to match the weight of the slab itself.

When the tub is full, that’s not a problem. The water inside is pushing down harder than the groundwater is pushing up. When the tub is empty, there’s nothing holding the floor down except its own weight and whatever the walls add. That’s when you get:

• A cracked or heaved floor, often around the drain or along a construction joint.
• Cracked render and membrane, which then leaks once you refill.
• A shell that lifts (“pops”). This is rarer on a heavy concrete tub than on a fibreglass pool, but it happens, and there’s no easy fix.

There’s a second reason it matters on a DIY build. The pool-rated membranes most of us use (Ardex S 7 Plus in the UK and Europe, Laticrete Hydro Ban in the USA) go on the inside of the shell. They’re designed to hold water in, not to stop groundwater pushing through from behind. Laticrete says Hydro Ban isn’t recommended where there’s negative hydrostatic pressure. I’ve gone into that in my waterproofing, grout and tile adhesive guide. Give groundwater an easy way in through the valve and it won’t try to force its way through the membrane.

Do you need one?

On an in-ground concrete, block or ICF tub, plunge pool or pool, I’d fit one every time. It costs very little, and once the slab is poured you can’t add one without breaking out the floor. Whether your local code or your engineer calls for one varies, so don’t rely on that to decide. Here’s how I’d weigh it up:

Your situation	Hydrostatic relief valve?
In-ground tub or pool, high water table, or you hit water while digging	Essential. Also talk to an engineer about drainage around the shell.
In-ground on clay, on a slope, or at the bottom of a garden that holds water	Essential. Clay holds water around the shell after heavy rain even if the water table is normally low.
In-ground on free-draining sand or gravel	Strongly recommended. Cheap insurance for the day you drain it after a wet week.
Sunk into a basement floor	Yes, along with proper basement waterproofing. See Build a hot tub in your basement.
Partly sunk (half in the ground)	Yes. Anything below ground level can see groundwater pressure.
Fully above ground on a slab, a stock tank or a wooden tub	No. There’s no groundwater around the shell.

If you hit water while you’re digging, stop and think before you go any further. I said the same in my in-ground build guide. A high water table doesn’t rule out an in-ground tub, but it’s worth getting an engineer to look at it. They may add perimeter drainage or a sump, not just the valve.

Where does it go?

At the lowest point of the floor, which on most builds means inside the main drain. That’s the neatest arrangement, because the drain sump acts as the housing and you can reach the valve later by taking off the drain cover. The gravel or hardcore bed under the slab acts as the collection area, and the perforated collection tube connects it to the valve.

On a DIY hot tub, the valve goes in at the same time as the main drain, sub-base, DPM and rebar, before you pour. It’s on the “before you pour” checklist in my concrete slab guide and the in-ground build guide for that reason. For drain positions and anti-entrapment covers, see Installing a main drain.

Draining an in-ground tub safely

This is where the valve earns its keep, but it’s also where people get caught out. Some rules I’d follow:

1. Drain in dry weather, not after days of rain or during a thaw, when the groundwater is at its highest.
2. Don’t leave it empty for longer than you need to. Clean it, fix it and refill it. That’s one reason many in-ground owners keep the tub full and running over winter. See Winterizing a hot tub.
3. If water starts coming up through the drain as you empty it, that’s the valve working. It means the groundwater is higher than the floor. You can’t pump the tub dry against it. Either refill and wait for a drier spell, or get the groundwater down first (a pool contractor can do this with a sump or well point).
4. If you have manual plugs rather than an automatic valve, open them when you drain. Pool builders are blunt about it: if hydrostatic relief valves or caps are present, they must be opened when draining.
5. If there’s no valve at all and you’re not sure about the groundwater, don’t drain it. Ask a pool professional first.

For the full process, see How to drain and clean your hot tub.

Looking after it

There isn’t much to do, which is the point. When the tub is drained:

• Take the drain cover off and look at the valve. It should be seated, clean and free of grit.
• Check the seal (usually an O-ring) isn’t split, flattened or twisted. A bit of grit on the seat is enough to stop it sealing.
• If it’s damaged, unscrew it and fit a new one. It’s a cheap part. On a Certikin drain, the valve is a separate screw-in part, sold on its own as the HD34 hydrostatic relief valve, which fits the standard HD33 sump.
• Before you refill, make sure it’s seated properly. It’s much easier to fix now than once the tub is full.

Don’t try to “test” it by pushing on it or standing a bucket of water on it. Pressure from above closes a hydrostatic valve. It’s designed to open only with pressure from below.

Is my hydrostatic valve leaking?

It can happen. If grit gets on the seat, the O-ring perishes or the valve sticks slightly open, water can drain out of the tub into the gravel underneath. Aquaman Leak Detection lists unexplained water loss and cloudy water after storms among the signs, and notes the valve is often overlooked because owners don’t know where it is.

That said, out of all the tubs and plunge pools I’ve helped with, very few leak from the structure. Nine times out of ten, a leak is in the plumbing. Before you blame the valve, work through my hot tub leak guide. If the level keeps dropping with the pumps off and the plumbing checks out, the main drain and its valve are the next place to look.

Common mistakes

• Leaving it out and planning to “add one later”. You can’t, not without breaking out the floor.
• Concreting over the collection tube, or letting slurry fill the gravel around it. The valve then has nothing to draw from.
• No free-draining gravel under the slab, so there’s no collection area for the tube to sit in.
• Gluing the valve in, so it can’t be replaced.
• Grit on the valve seat when you first fill, causing a slow leak from day one.
• Draining after heavy rain and then fighting the tub as it refills with groundwater.

There are more build pitfalls in Avoid these 7 DIY hot tub mistakes.

Planning your own build?

My DIY hot tub plans include the main drain, hydrostatic relief valve and the rest of the plumbing, all drawn up with dimensions and a full parts list, plus the video course. There are also plunge pool plans if you’re going bigger. See How to build a DIY plunge pool for that side of things.

FAQs

What does a hydrostatic relief valve do?

It lets groundwater into an empty or part-empty pool or hot tub when the groundwater pressure under the floor is higher than the pressure inside. That stops the pressure building up and cracking or lifting the shell.

Does a hot tub need a hydrostatic relief valve?

An in-ground or partly sunk concrete, block or ICF tub should have one. A tub built fully above ground, a stock tank or a shop-bought hot tub doesn’t need one.

Where is the hydrostatic valve in my pool?

Usually inside the main drain at the deepest point. Take the drain cover off with the pool drained, and you’ll see the valve screwed into the bottom of the sump. Some pools use separate plugs set into the floor instead.

Can a hydrostatic relief valve leak?

Yes. If the seal perishes or grit gets on the seat, water can drain out into the ground. Check it and clean the seat whenever the tub is empty, and replace the valve if it’s damaged.

Can I add a hydrostatic relief valve to an existing pool?

Not easily. It has to connect to a gravel bed under the slab, so retrofitting means cutting through the floor. On a pool that already has a main drain with a sump, a professional may be able to fit one, but on a new build, just put it in before the pour.

Got a question about groundwater or your own build? 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