Most pool owners have not heard of a hydrostatic valve and wouldn’t know why they need one.
A hydrostatic valve prevents the build-up of ground water under the swimming pool exerting upward pressure on the shell. The hydrostatic valve opens inwards allowing ground water to flow into the bottom of the swimming pool to relief the pressure.
Without a hydrostatic valve to relieve the pressure, the pool can be moved or uplifted out of the ground damaging the pool structure and plumbing. The movement usually only happens when the pool is empty.
Pools located in a low lying area, near a beach or at the bottom of a valley near a creek or river will have a higher chance of movement by ground water pressure.
Table of Contents
What is a Hydrostatic Valve?
A hydrostatic valve is a one-way pressure relief valve that is installed at the deepest point of an in-ground swimming pool. The valve is designed to relieve ground water pressure under a swimming pool. It’s located in the main drain under the drain lid which sits at the bottom of the pool.
Hydrostatic valves come in two sizes: 40mm & 50mm
Hydrostatic valves cost about $20
Why Swimming Pools Need a Hydrostatic Valve
Ground water pressure from the rising water table can exert upward pressure on in-ground swimming pools. The water table rises and ground water pressure increases after periods of heavy rain.
A hydrostatic valve prevents the build-up of ground water under the swimming pool exerting upward pressure on the shell. The hydrostatic valve opens inwards allowing ground water to flow into the bottom of the swimming pool to relief the pressure. When this happens you will notice dirty water at the bottom of your pool which is evidence the valve is working.
There is a hidden danger when an inground pool without a hydrostatic valve is emptied of water. When the weight of the water is removed the pressure underneath exerts an upwards pressure on the bottom of the pool.
Without a hydrostatic valve to relieve the pressure, the pool can be moved or uplifted out of the ground damaging the pool structure and plumbing. The movement usually only happens when the pool is empty.
Concrete pools always have a hydrostatic valve built into the lowest point of the pool at the deep end which releases pressure from under the pool.
Not all fibreglass shell pools have a hydrostatic valve fitted to relieve the upward pressure from the ground water.
Fibreglass & Vinyl Lined Pools
Vinyl lined & fibreglass swimming pools are light weight structures and have a greater chance of lifting or moving due to increased ground water pressure. It’s important to empty a fibreglass pool slowly to allow for equalization of ground water pressure. It is common for the floor of fibreglass pools to buckle upwards when empty of water. If the pool in not fitted with a hydrostatic valve leave some water in the pool if possible.
Chance of movement by ground water pressure
Swimming pools located on high ground or at higher altitude away from rivers or streams will have a reduced chance of movement by ground water pressure.
Pools located in a low lying area, near a beach or at the bottom of a valley near a creek or river will have a higher chance of movement by ground water pressure.
The lower in altitude and closer to a water source the higher the chance of ground water reaching your pool because you are closer to the water table.
Draining Water from an In Ground Pool
When you drain your pool down to the last few inches of water you cannot remove it with a pump or suction hose. You can open the hydrostatic valve by lifting it up and wedging it open to drain the remaining water to the ground. The valve is on a spring that opens upwards into the pool. It’s designed to release pressure underneath into the pool but we can use it to drain water out of the pool by lifting the valve and holding it open until the excess water drains away.
Please read our article on How to Drain Water from a Swimming Pool
Swimming Pool Leak
All swimming pools lose water due to evaporation but if your pool water is dropping more than 1cm per day you may have a leak.
One of the most common sources of leaks is the hydrostatic valve. It should be one of the first things to check if you suspect you have a leak in your pool.
The valve is a simple mechanism but it can cause leaks when old and deteriorated. The most common issue is the rubber seals deteriorating causing pool water pressure to leak around the valve’s screw fitting.
Another issue occurs if the valve has ever been pushed open by ground water pressure. The dirty ground water contains small particles of grit or stone that can lodge in the valve’s opening. The valve will not be able to fully close and form a water tight seal causing a leak.
The only way to detect a leak in the hydrostatic valve is with some water soluble dye or food coloring. You need to swim to the bottom of the pool with a diving mask. Put a few drops of dye around and above the main drain. If the dye is sucked away into the drain there is a leak and it’s time to replace the valve.
Please read our article on How to Find and Fix a Swimming Pool Leak
How often to change the Hydrostatic Valve?
The hydrostatic valve is constantly immersed in pool water and the rubber seals may deteriorate over time due to the action of pool chemicals.
I recommend replacing the hydrostatic valve every time you change your swimming pool water.
All in-ground swimming pools should be drained, cleaned and refilled every 5 or 6 years to resolve the issue of total dissolved solids (TDS) accumulating in the pool water.
All substances added to the pool water increases the total dissolved solids TDS in your pool. This includes all the normal pool chemicals such as chlorine, shock, pH reducer, algaecide, etc. and sweat, cosmetics, hairspray, sunscreen, etc. from swimmers. Over time the TDS level will increase and eventually the water will be over saturated with dissolved solids.
There is no product that you can add to the pool water to lower the level of total dissolved solids (TDS) in the water. When the TDS is too high you will need to drain or partially drain your pool and refill with fresh water.
Why do I need to lower the TDS?
A high TDS level in your pool water will reduce the effectiveness of chlorine and other chemicals. Your pool water will be more difficult to keep chemically balanced and sanitized resulting in:
- Scaling & stains on the pool surface
- cloudy or haziness
- hard water
- calcium deposits
- green water as the chlorine is ineffective
- salty tasting water
- running the pump longer using more electricity
Your pool water may be perfectly chemically balanced but you still get the above problems because the TDS is too high. The addition of more chlorine or other chemicals will not resolve the problem.
How to Install a Hydrostatic Relief Valve
There is no need to drain the water from the pool to replace the valve. Swim down to the drain wearing a face mask. Remove the drain cover and unwind the hydrostatic valve from its position with a valve tool.
Be careful because the pool water will flow rapidly out of the pool and into the drain. Don’t allow any small objects near the drain as they will be sucked away.
Fit a new valve into the drain and screw tight with a valve tool. Replace the drain cover.
How to Replace the Drain Cover on my Pool?
Always switch off the power to the pump before replacing the drain cover.
If you have water in the pool swim down to the drain wearing a face mask.
Some drain covers contain a weight and just sit over the drain.
Other covers twist on and off. Screws fastened other drain covers.
Simply remove the old cover and replace with a new one.
Conclusion
If you are renovating your swimming pool don’t let pool contractors overcharge you for replacing a hydrostatic valve. Pool contractors can charge hundreds of dollars to remove and replace a hydrostatic valve. That is completely unacceptable. The valves only cost around $20 and take a minute to replace.
If you have any questions or comments fill out the comments section below. And we will get back to you as soon as possible.