Water is the driver of nature

Once the inspector had completed inspecting our glorious temporary fence, it was time to finally get some water into the pool. A milestone to be sure.

Since the fibreglass shell is not strong enough to support the hydro-static pressure of the water, the outside must be back-filled at the same rate that water is added.

Although the backfill for the finished pool will fill the gap between the shell and the retaining walls, the base of the pool (at the deep end) is sufficiently below top of the concrete footing that we can backfill the first 600mm without the retaining walls in place. Beyond that, we will need to complete and core fill the walls first. Back-filling once the walls are completed will be a much more difficult task as the backfill material (i.e. crushed rock) must be shoveled over the wall.

Backfill at deep-end

Initial backfill between the shell and footing at the deep-end of the pool.
Firing up the filtration system

Pools with only a skimmer must wait until the pool is full to run the filtration system. Because we have two wall suction points near the bottom of the deep-end wall, we can run the filtration system with little water present.

Despite a successful pressure test, I was keen to run the system prior to irreversibly back-filling the gap between the pool shell and retaining walls. So as soon as the water was above the suction points, we started up the filtration system.

With only around 600mm of water in the pool, the trip from the water surface via the wall suction fittings to the pump involves a vertical rise of nearly 3 metres. This presents a challenge for the pump, which needs to pull the water up* those 3 metres. Centrifugal pumps are designed to push water through them, not air.

* In reality, it needs to pump the air, thus lowering the air pressure between the water in the pipe and the pump, causing the water level to rise

Our pump is self-priming. This means that it can pump some air out of the system and thus bring water up from below the level of the pump. It does this by mixing water (provided by filling the pump with water first) with the air and pumping that mixture. Self-priming is not a panacea and there is a limit to the amount/pressure of air it can deal with. After filling the pump with water and running it for a few minutes, it became clear that the 3 vertical metres and around 10 metres of 50mm air filled piped was beyond it.

I lent my assistance by removing the pump basket cover, pouring as much water as possible down the intake pipe, quickly replacing the cover and switching the pump on. Water poured down the pipe will rapidly make it's water down the pipe and out the suction fittings so it's imperative to get the cover back on and pump running without delay.

After 20-30 seconds, water began surging into the basket from the intake pipe and within a minute or so, the basket was full of water and several fountains of water were shooting out of the pool walls:

Filtration system running

Filtration system running for the first time.
Bubbles spells troubles

Initial relief that everything was running smoothly was replaced with concern when I noticed small bubbles entering the pump basket from the intake pipe.

Bubbles in the pump basket

Bubbles entering the pump basket from the intake pipe.

Bubbles like this typically indicate a leak on the suction side of the system (i.e. in the pipe between the pump and the suction fittings). Having already performed a successful pressure test on these pipes, I was pretty confident that there were no leaks.

I soon narrowed in on the likely source of the bubbles. We have a 3-way diverter valve immediately prior to the pump inlet, allowing balancing of the water drawn from the skimmer vs the wall suction fittings. At this stage, the wall suction inlet is drawing water from the pool, while the skimmer inlet is completely open to the atmosphere.

The flap in these diverter valves is designed to provide a water tight seal, not air tight. Compounding this is the greater than usual pressure differential across the flap caused by the wall suction inlet drawing water from several meters below the valve level. The only way to verify this hypothesis would be to cap the skimmer pipe and see if the problem resolves, but with the successful pressure test as evidence, I was comfortable that the flap was the source of the bubbles and that it would resolve itself once the skimmer was connected and pool was full of water.