A hydrostatic valve is a pressure-relief valve set into the floor of a concrete pool, usually in or near the main drain. When a pool is empty and groundwater is high, the valve lets water in from under the shell so the pressure does not push the pool up out of the ground. It is the reason an empty pool does not float, and it is why you should not drain a pool yourself without checking first.
Safety first: do not empty a concrete pool after heavy rain, or when the ground is wet, unless you know the valve is working and the groundwater is low. A drained pool can lift, crack or shift, and that damage is expensive.
What the valve does
Water in the ground around a pool pushes against it from below and the sides. When the pool is full, the weight of the water inside balances that pressure. Take the water out and the balance changes. If the groundwater is high, it can push the shell upward, a bit like an empty boat in a river.
The hydrostatic valve gives that groundwater a controlled way in. Water flows through it into the pool, which equalises the pressure. Because it sits at the lowest point, it only needs to open when there is a real pressure difference.
Where it sits
In most pools that have one, the valve is in the floor at the deepest point, in or next to the main drain. Look for a small plug or cap, often with a cover or a key-type fitting. Not every pool has one. Older pools in particular may not, and some have valves that are blocked, painted over or buried.
Why empty pools can lift
A few conditions make it more likely:
- High groundwater after heavy or prolonged rain, or a high water table on low-lying blocks
- Clay and poorly draining soil that holds water around the pool, as in parts of western Sydney
- Sloping blocks where water collects uphill and runs down towards the pool
- A blocked or failed valve, or a pool without one
When it goes wrong, the shell can crack, shift or lift. Even if it settles back, the damage may show up as cracks and leaks later. It is a problem you want to avoid rather than fix.
Signs the valve may be a problem
You cannot always tell from above, but some signs raise questions about the valve and the ground around it:
- Cracks in the floor near the main drain, or around the base of the walls
- A pool that has shown movement or cracking after past periods of heavy rain
- Water seeping into the pool when it is empty, which can be the valve working or a sign of high groundwater
- A valve cap that is missing, seized or covered over by an earlier surface
None of these is proof of a fault. They are good reasons to have it looked at before any work that involves emptying the pool.
Why we check before we drain
This is the first step of any resurface. Before a pool is emptied we look at the ground conditions, recent weather and the valve. If the valve is not working, it is dealt with. If groundwater is high, we wait or plan around it. It is one of the less glamorous parts of a job, and it is a good example of how a proper resurface starts well before any surface goes on. Our pool resurfacing page covers the whole process.
What you should do as the owner
- Do not drain a concrete pool yourself, especially after rain or in wet weather
- If you are unsure whether your pool has a valve, ask us to check before any work
- If the pool has been empty and the ground is wet, do not leave it empty. Call someone straight away
- Tell us about any past issues, such as movement or cracking after rain
If you are already seeing cracks or water loss, our guides on pool crack repair and pool leak repair are good places to start. For the planning side, how long pool resurfacing takes explains what affects the timing, and weather and ground conditions are part of it.
Book an inspection early, and we will check the valve and ground conditions before any draining is planned. That gives you the widest choice of dates, whether you want to swim by November, by Christmas or later in summer.
