What chemicals do you need to close a pool for winter?

The five categories are: baking soda (if total alkalinity is below 60 ppm), a pH adjuster — muriatic acid to lower or soda ash to raise — if pH is outside 7.4–7.8, calcium chloride if calcium hardness is below the minimum for your surface type, pool shock to bring FC to the shock level, and a 60% polyquat winter algaecide added 24–48 hours after shocking. Exact amounts depend on your pool volume and current readings — use the pool closing calculator for your specific doses.

Do I need to blow out pool lines when winterizing?

In-ground pools in climates where ground temperatures drop below 32°F need their lines blown out (or filled with pool antifreeze) to prevent freeze damage. A single freeze-thaw cycle can crack PVC pipes, fittings, and equipment housings — repairs typically run hundreds to thousands of dollars. Above-ground pools and pools in warm climates (hardiness zones 9–10) generally do not need lines blown — draining the equipment and plugging the returns is sufficient.

What order do you add pool closing chemicals?

Add closing chemicals in this order: (1) baking soda to raise alkalinity if needed — wait 30 minutes; (2) muriatic acid or soda ash to adjust pH — wait 30 minutes and retest; (3) calcium chloride to raise hardness if needed — wait 30 minutes; (4) pool shock — broadcast around perimeter with pump on, run pump 24+ hours; (5) winter algaecide — 24–48 hours after shocking, once FC drops below 5 ppm. Do not add algaecide when FC is high; chlorine degrades it rapidly.

Can I close my pool if the chemistry is off?

No. Closing with unbalanced water is the most common cause of spring problems. Acidic water (low pH, low alkalinity) attacks plaster, etches grout, and corrodes metal equipment all winter even with no pump running and no swimmers. Basic water with high calcium causes calcium carbonate scale on tile lines, inside plumbing, and on heater elements. Always balance before closing — the chemistry you put in at closing is what your pool will sit in for 5–6 months.

How do you close a saltwater pool for winter?

A saltwater pool closes the same way as a chlorine pool for chemistry — shock, balance, algaecide — with two additional steps. First, remove and store the salt cell: the thin titanium plates are fragile in freezing temperatures and the cell housing needs to be drained. Store the cell indoors. Second, shock with liquid chlorine rather than cal-hypo; cal-hypo is incompatible with salt chlorinator housings. Leave the salt in the water — it stays through winter and does not degrade.

How to close a pool for winter: step-by-step checklist

Q: When should you close a pool for winter?

Close when water temperature consistently stays below 60°F (15°C). At that point algae growth effectively stops, and keeping the pool open — with active chemistry maintenance — costs more than it protects. In most of the northern US that's late September through October. Do not wait for a hard frost: closing in cold but above-freezing conditions is far easier than dealing with iced-over equipment.

10 min read · August 16, 2026 · By Jeffrey Thompson

Closing a pool for winter is mostly a chemistry job — balanced water going into winter is what protects your pool surface, plumbing, and equipment through months of no maintenance. The physical steps (lowering water level, blowing lines, draining equipment) are secondary to getting the chemistry right first. Here's how to do both.

Pool closing checklist — 8 steps

Clean the pool — vacuum, brush walls and steps, skim leaves
Test and balance chemistry — pH 7.4–7.8, TA 60–90 ppm, CH at minimum for your surface
Shock — raise FC to CYA × 40%, minimum 10 ppm
Add winter algaecide — 24–48 hours after shocking, once FC drops below 5 ppm
Lower the water level — below skimmer opening (mesh cover) or 4–6 inches below (solid cover)
Blow out lines and plug returns — or fill lines with pool antifreeze if you can't blow
Drain and winterize equipment — pump, filter, heater, salt cell
Install the winter cover — water bags or anchors around the perimeter

When to close your pool

Close when water temperature stays consistently below 60°F (15°C). At that temperature, algae growth effectively stops — and a well-closed pool with balanced chemistry will stay clear through winter without any active treatment.

Do not wait for a frost. Closing in late September or October when temperatures are dropping but above freezing is far easier than closing in November when equipment is icing up. In most of the northern US and Canada, the window is mid-September through October depending on your climate zone.

What happens if you close too late?

A light frost can crack pump housings, split PVC fittings, and fracture filter tank heads overnight. These are not cheap repairs. Closing two weeks earlier than you think you need to costs almost nothing compared to the damage a single hard freeze can do to open equipment and full lines.

Step 1: Clean the pool before anything else

Closing a dirty pool amplifies every chemistry problem. Leaves and debris consume chlorine as they decompose; algae spores settle on walls and grow through winter even under a cover. Before touching a single chemical:

Vacuum the floor — remove all visible debris and fine sediment
Brush walls, steps, and any algae-prone areas (shaded corners, behind ladders)
Skim leaves and floating debris
Remove accessories: ladders, rails, diving boards, solar blankets, and toys
Clean the skimmer basket and pump basket
Backwash or clean the filter (sand: backwash; cartridge: hose off; DE: backwash and recharge)

A clean pool at closing is a clean pool in spring. Every leaf you leave in the water is a brown stain waiting to form.

Step 2: Test and balance chemistry

This is the most important part of closing. The chemistry you put in at closing is what your pool will sit in for 5–6 months. Unbalanced water doesn't stay unbalanced — it reacts. Acidic water attacks plaster and corrodes metal. Basic water deposits calcium on tile, in plumbing, and inside heaters. Test all five parameters:

Parameter	Closing Target	Why It Matters
pH	7.4 – 7.8	Below 7.2 corrodes; above 8.0 scales
Total Alkalinity	60 – 90 ppm	Buffer against pH crash; <60 allows pH to drop sharply
Calcium Hardness	Plaster: 200–400 ppm Vinyl/fiberglass: 150–300 ppm	Soft water pulls calcium from plaster surfaces
CYA	30 – 50 ppm	CYA doesn't degrade over winter; close with too much, open with too much
Free Chlorine	See shock step	Raise to shock level before closing — not a maintenance target

The pool closing calculator takes your current readings and pool volume and generates an exact chemical shopping list in the right order — shock dose, pH adjuster, baking soda, and calcium amounts calculated for you.

Step 3: Shock the pool

The closing shock serves two purposes: it destroys any remaining algae and bacteria before the pump shuts down, and it leaves a residual FC high enough to protect the water through the first weeks of winter before it fully dissipates. The target is the same as a regular shock treatment:

Closing shock target = CYA × 40% (minimum 10 ppm FC)

Subtract your current FC from the shock target to find how much you need to add. Liquid chlorine (10–12.5% sodium hypochlorite) is the best choice — it dissolves instantly, adds no CYA, and doesn't affect calcium hardness. Cal-hypo (65%) works too; it does add some calcium, which the closing calculator accounts for.

Add shock last, after all other chemical adjustments. Shock affects pH readings, and high pH significantly reduces chlorine effectiveness — balance first, then shock.

Run the pump while adding and for at least 24 hours after
Add at dusk to reduce UV degradation
Broadcast granular shock around the pool perimeter — never pour directly into the skimmer

Step 4: Add winter algaecide

A 60% polyquat algaecide suppresses algae growth through winter when there is no active chlorine residual. It is not a substitute for shocking — you need both. Timing is critical: add algaecide 24–48 hours after shocking, once FC drops below 5 ppm. High chlorine degrades algaecide rapidly, so adding it when FC is still at shock level wastes the product.

Polyquat vs. copper-based algaecide

Use 60% polyquat for winter — it's non-foaming and leaves no staining residue. Copper-based algaecides (25% or "PolyQuat 60 with copper") can stain surfaces over a long winter, especially if pH drifts and copper precipitates out of solution. Stick to polyquat.

Step 5: Lower the water level

How far you lower water depends on your cover type:

Mesh safety cover: Lower water to 12–18 inches below the skimmer opening. Mesh covers allow rainwater to pass through, so the water level will rise through winter — starting lower prevents overflow and cover strain.
Solid vinyl cover: Lower water to 4–6 inches below the skimmer opening. Solid covers keep rain out, and a small pump or drain is used to remove water that pools on top.
Above-ground pool (solid cover): Lower 4–6 inches below the return fitting.

Do not drain below the main drain on in-ground pools — this can create hydrostatic pressure that pops the pool out of the ground in high water table areas. Use a submersible pump or the multiport valve set to waste.

Step 6: Blow out and plug the lines

This is the step that prevents freeze damage. Any water left standing in underground plumbing can freeze, expand, and crack PVC pipe — including pipes buried two or three feet underground. A plumber's snake-style drain rod can't fix a burst pipe under a deck.

Blowing out lines (in-ground pools in freezing climates)

Connect a shop vac or air compressor (2–5 CFM minimum) to the skimmer or dedicated blow-out port
Close the main drain (if applicable) and blow through each skimmer and return line until only air comes out — typically 3–5 minutes per line
Insert a rubber plug or expandable plug into each return fitting while air is still flowing, to keep water from being sucked back in
Remove the skimmer basket and install a Gizzmo (threaded foam plug) in the skimmer to absorb ice expansion
Plug the skimmer opening

Pool antifreeze (alternative to blowing lines)

If you can't blow lines, pour pool-grade antifreeze (propylene glycol — not automotive ethylene glycol antifreeze, which is toxic to animals) into the skimmer and return lines. Use a winterizing kit that includes plugs and antifreeze sized to your plumbing.

Step 7: Drain and winterize equipment

Draining the pump and filter is as important as blowing the lines. These items sit above ground and are fully exposed to freezing temperatures.

Pump

Turn off the pump and GFCI breaker
Remove all drain plugs (typically 1–2 on the pump housing, 1 on the pump trap)
Store drain plugs indoors — you will lose them if you leave them near the equipment
Leave pump housing cover open or slightly ajar to allow complete drainage and airflow

Filter

Sand filter: Set multiport valve to "winterize" (or between positions) so it cannot accidentally cycle. Remove the drain plug at the bottom of the tank.
Cartridge filter: Remove and clean the cartridge; store it indoors. Remove the drain plug.
DE filter: Backwash, then open the filter tank and remove DE grids. Hose off and store indoors. Remove all drain plugs.

Heater

Gas/propane heater: turn off the gas valve. Open the drain plug on the heater header (often at the bottom rear). Leave the plug out all winter to allow full drainage.
Heat pump: most have self-draining headers — consult your manual. Turn off the breaker.

Salt chlorinator (SWG)

Remove the salt cell from the plumbing housing
Clean with a mild acid wash if calcium deposits are visible (1 part muriatic acid : 10 parts water)
Store the cell indoors — the titanium plates in the cell are fragile in freeze-thaw cycles
Drain the cell housing and plug the union connections
Leave the salt in the pool — it does not degrade and will be there when you open in spring

Automation systems and timers

Turn off the pump timer or set it to "off" — you don't want the pump cycling on a cold January morning with no water to prime
Turn off the chemical feeders or automatic chlorinators and remove any remaining tablets or granular product

Step 8: Install the winter cover

A properly installed cover is your last line of defense against leaves, sunlight (which accelerates algae), and debris accumulation over winter. A cover that sags into the water or fills with debris defeats most of the benefit.

Mesh safety cover: Anchor points attach to anchors drilled into the deck. Pull taut — a cover that's loose will sag under leaf accumulation and allow debris to contact the water surface.
Solid vinyl cover: Use water bags or cover weights (not bricks — they tear covers) around the perimeter. Add an air pillow under the center of the cover to prevent ice sheet formation and absorb ice expansion.
Above-ground pools: Most use a solid winter cover with an air pillow underneath. Secure with a cable and winch system.

For solid covers: check the cover every 2–4 weeks through winter and pump off any water that accumulates on top. More than a few inches of water on a solid cover strains the cover and the water bags.

Record your closing readings in the app

PoolChem Tracker stores your closing chemistry as a timestamped baseline — so when you open next spring, you can see exactly where the water was and what it will need.

Closing a saltwater pool

A saltwater pool closes the same way as a chlorine pool for chemistry — balance, shock, algaecide — with a few additional steps specific to the salt chlorinator:

Shock with liquid chlorine, not cal-hypo. Cal-hypo can react with salt chlorinator housings.
Remove and store the salt cell indoors. The titanium plates are fragile in freeze-thaw. Clean the cell before storing if calcium scale is present.
Drain the salt cell housing and plug the union connections so nothing enters the cell line over winter.
Leave the salt in the water. Salt does not degrade or evaporate. The salt level at opening will be close to what you closed with, adjusted only for any rainfall dilution.

One exception: if you're in a climate where the pool barely freezes or never freezes (zones 9–10), you may not need to blow lines or fully drain equipment — check with local pool professionals for your region's standard practice.

Common pool closing mistakes

Closing with unbalanced chemistry. Low pH eats plaster all winter. High calcium plus high pH deposits scale inside your heater and on tile lines. Fix the chemistry first — it's the entire point of the closing process.
Adding algaecide right after shock. High FC (the shock residual) degrades polyquat rapidly. Wait until FC drops below 5 ppm before adding algaecide.
Leaving debris on the pool floor. Leaves and organic material that decompose all winter leave brown stains that require acid washing or draining to remove. Clean before closing.
Not removing all equipment drain plugs. One forgotten drain plug in the pump housing can crack the housing in a hard freeze. Go methodically — pump, filter, heater, anything with a drain plug.
Closing too late. Waiting for the last warm swim of the year costs almost nothing in terms of timing — but closing after the first freeze can mean cracked equipment and expensive repairs before you've even started.
Skipping the cover pump on a solid cover. A solid cover with 6 inches of water sitting on it through winter strains the material and the weights. Check and pump it periodically.
High CYA with no plan. CYA does not degrade over winter. If you close at 80 ppm, you will open at 80 ppm — which means your minimum FC target next season will be 6 ppm before you add a single chemical. If CYA is above 70 ppm, plan for a partial drain at spring opening.

Frequently asked questions

When should you close a pool for winter?

When water temperature consistently stays below 60°F. At that point algae growth stops, and the cost of maintaining open-pool chemistry outweighs any benefit from occasional swims. In most of the northern US, that's mid-September through October depending on your climate zone. Close before the first hard frost — not after.

Do I need to blow out pool lines?

In-ground pools in climates where ground temperatures drop below 32°F: yes, absolutely. Underground pipes hold water in every low point and fitting, and a single freeze-thaw cycle can crack PVC lines, return fittings, and equipment unions. Above-ground pools and pools in warm climates (Florida, southern California, Arizona) generally do not need lines blown — draining the equipment and plugging returns is sufficient.

Can I close my pool myself?

The chemistry steps and above-ground closing are fully DIY. Blowing lines on an in-ground pool requires a shop vac or small air compressor and some familiarity with your plumbing layout — it's DIY-able on most pools with a straightforward plumbing design. Complex or unusual plumbing (multiple skimmers, in-floor cleaning systems, waterfalls with separate lines) is worth having a pool professional handle at least once so you understand the right sequence.

What should CYA be when closing?

Ideally 30–50 ppm. CYA does not degrade over winter — whatever you close with, you open with. If CYA is above 70 ppm, plan to drain 20–30% of your water at spring opening before adding CYA-producing tablets. Closing with high CYA and then adding trichlor tablets for the first weeks of spring is a fast path to needing a partial drain mid-season.

How long does it take to close a pool?

For a DIYer working alone: most of a day. Allow 2–3 hours for cleaning, testing, and chemical additions (with wait times between additions), then another 2–4 hours the following day for water level adjustment, line blowing, equipment draining, and cover installation. Rushing any stage — especially the chemical wait times — tends to produce a worse result in spring.

Use the pool closing calculator to get exact chemical doses based on your pool volume and current readings. The calculator outputs a numbered shopping list in the correct addition order.