How to Close an Inground Pool

When should you close an inground pool?

Once the water stays below about 65°F, and colder is better. Algae grows slowly in cold water, so a pool closed while it is still warm gives algae the run of the winter under the cover. Some experts hold out for 60°F, where algae barely grows at all.

The temperature rule is the same as an above ground pool's. What differs is the deadline. An above ground pool's freeze risk is its exposed hoses and pump, which come off and go indoors. An inground pool's freeze risk is buried PVC that cannot be moved, so the lines have to be cleared before the first hard freeze.

In a climate that freezes only occasionally there is a third option: leave the pool running. Moving water is far less likely to freeze, so an automation system with freeze protection can carry a short cold snap without any of this.

What you need

Step 1: Balance the water

Test and adjust a few days ahead, while the pump can still circulate what you add. Aim for a pH of 7.4 to 7.6 and total alkalinity of 80 to 120 ppm.

Calcium hardness is the one number that genuinely depends on what your pool is made of, and most guides publish a single range for all three surfaces without saying so:

  • Gunite, concrete or plaster: it needs calcium. Water low in calcium dissolves it out of the plaster. 250 to 650 ppm is the working range, with 350 to 550 the usual target.
  • Vinyl liner: a liner is PVC and contains no calcium, so the liner itself needs none. Keep 200 ppm or so anyway if you have a gas heater or tile grout at the waterline, both of which do care.
  • Fiberglass: keep it below about 120 ppm, and do not add calcium.

Add a stain and scale product next if your fill water carries metals. A closed pool sits still for months, which is exactly the condition that lets dissolved metal settle onto the surface and stain it.

Step 2: Shock, wait, then the algaecide

Shock in the evening and run the pump for a full turnover. Then wait. The chlorine has to come back down, to somewhere under 4 to 5 ppm, before the algaecide goes in, because high chlorine breaks polyquat down and you lose the thing you paid for. That wait can be two to five days after a chlorine shock, so do not plan to shock and cover on the same afternoon.

Then the algaecide, polyquat 60 for preference, at a double dose if the cover is mesh, because mesh lets light through and light grows algae. An enzyme or winter pill goes in last, and is optional.

Dissolve granular shock in a bucket first and pour it around the perimeter. Granules that sit on a vinyl liner bleach it.

Do not park a chlorine floater under the cover on a vinyl liner pool. With the pump off there is nothing to move it, so it sits in one spot, and trichlor is about as acidic as anything you own. It will bleach and eat the liner underneath it. If your closing kit includes a floater, check whether it is the non-chlorine kind. A white plaster pool tolerates one better than vinyl does, but it still has no circulation to move it.

Step 3: Clean the pool and the filter

Skim, brush and vacuum on closing day, so nothing organic sits and rots under the cover all winter. Backwash a sand or DE filter. Take a cartridge out, hose it clean and let it dry completely before it goes away.

Step 4: Lower the water

How far depends on the cover, and the two answers are further apart than most guides admit. For a solid winter cover, a few inches below the skimmer opening, commonly 4 to 6. For a safety cover, the cover maker's own manual wins, and the number that matters is a floor rather than a target: several brands void the warranty below about 18 inches from the top of the pool, because the water is what holds the cover up under a snow load.

Never drain an inground pool. The water holds the shell down against the groundwater around it, and this is the opposite of the above ground reason. How far to lower it, in detail.

Step 5: Clear the lines

This is the step with no above ground equivalent, and the one that most rewards doing properly. Buried PVC that holds water and then freezes splits, and it splits where you cannot see it.

Use a high volume, low pressure line blower. It takes only about 3.5 psi to clear a pipe eight feet below the water line, and a blower makes roughly 6, so this is a job about air volume rather than air pressure. An air compressor is the wrong tool in the other direction and can over-pressurize a fitting.

Blow each skimmer, then each return, then the main drain, plugging each line as its air breaks the surface. The main drain is held with a trapped column of air rather than a plug: you blow until bubbles rise over the drain, then close the valve while the blower is still running, and the air pressure keeps the water out all winter. The full blowout, line by line.

Step 6: Drain the equipment

  • Pump. Take out both drain plugs and let it empty. Store the plugs in the pump basket so they are there in spring.
  • Filter. Open the drain and leave it open all winter. Set a multiport valve between positions so the tank cannot hold water. Open the air relief.
  • Heater. Open its drain plugs. A little water left in a heat exchanger is what cracks it.
  • Salt cell. Take the cell out and store it indoors. Leave the control box where it is; it is not the part that freezes. A salt chlorinator has already stopped making chlorine by now anyway, since output falls away below about 60°F water and stops below 50°F.
  • Chlorinator. Empty it and open its drain. Never put antifreeze in a chlorinator.

Step 7: Put the cover on

A solid cover lies on the water and is held down by water tubes laid end to end round the edge. It keeps everything out, including light, and it needs a cover pump to take the rain and snowmelt off the top. It is not a safety cover and does not hold a person.

A mesh safety cover anchors into the deck, drains through, and needs no tubes and no pump. It lets light through, which is why the algaecide dose doubles under one.

Never use bricks or concrete blocks to hold a cover down. They tear the cover, and if somebody goes through, the blocks go in with them.

The mistakes that cost the most

  • Draining the pool. Covered above, and it is the expensive one.
  • An air compressor instead of a blower. Wrong pressure profile, and it can split a fitting.
  • Shocking and covering the same day. The algaecide goes in after the chlorine falls, not before.
  • A trichlor floater left under the cover. It bleaches a liner in one spot.
  • Standing water on a solid cover. It is a drowning risk and it drags the cover into the pool. Keep the cover pump on it.
  • Assuming a solid cover is a safety cover. It is not, and neither is a cover with water tubes on it.