How to Winterize Your Fiberglass Pool in Ontario (Step by Step)

Winterizing your pool is not optional in Ontario. With temperatures regularly dropping to -15 to -25 degrees Celsius through January and February, any water left in your plumbing lines, pump, or filter will freeze, expand, and crack whatever is holding it.

The good news is that winterizing a fiberglass pool is straightforward. It takes a few hours, and if done correctly, your pool will be ready to open cleanly the following spring.

This article walks through every step of the process so you know exactly what is involved, whether you plan to do it yourself or hire someone.

When should you close your pool in Ontario?

The best time to close a pool in Ontario is when the water temperature drops consistently below 15 degrees Celsius (about 60 degrees Fahrenheit). For most of Southwestern Ontario, that means sometime in October, usually after Thanksgiving weekend.

There are two common mistakes with timing:

• Closing too early (September or early October). If the weather stays warm after you close, the covered pool becomes a warm, dark environment that encourages algae growth. You may open the pool in spring to find green water and heavy staining that takes days to clean.
• Closing too late (November). By this point, overnight temperatures can dip below freezing. If you have not blown out the lines and the temperature drops suddenly, you risk cracked pipes before you even get started.

Watch the forecast, not the calendar. Some years, October stays warm well past Thanksgiving. Other years, cold weather arrives early. The water temperature is a better guide than the date.

One practical tip: most pool service companies get very busy in the last two weeks of October. If you plan to hire someone, book your closing appointment in September to secure your preferred date.

Step-by-step winterization process

Here is the complete process for winterizing a fiberglass pool in Ontario. Each step matters, and skipping any of them can create problems you will not discover until spring.

Step 1: Clean the pool thoroughly

Before you start the closing process, get the pool as clean as possible. Anything left in the pool will sit under the cover for five months.

• Skim the surface to remove all leaves and debris.
• Brush the walls and floor, paying attention to corners, steps, and the waterline.
• Vacuum the bottom. A manual vacuum or robotic cleaner both work.
• Empty the skimmer basket and pump strainer basket.
• Clean or backwash the filter. If you have a cartridge filter, remove the cartridge, hose it off, and let it dry before storing it indoors.

A clean pool at closing means a much cleaner pool at opening. Every leaf that sits on the bottom for five months creates an organic stain that can be difficult to remove in the spring.

Step 2: Balance the water chemistry

The water stays in the pool all winter, and it needs to be properly balanced to protect the gel coat surface through the off-season.

Target these levels before closing:

Parameter	Target for Closing
pH	7.2 – 7.6
Total alkalinity	80 – 120 ppm
Calcium hardness	200 – 400 ppm
Free chlorine	1 – 3 ppm (before adding winterizing chemicals)

Low pH water is acidic and will slowly etch the gel coat surface over the winter months. High pH water causes calcium scaling. Both are preventable with a quick test and adjustment before closing.

Run the pump for at least 2 hours after adding any balancing chemicals so they mix thoroughly through the water.

Step 3: Lower the water level

Lower the water level to approximately 10 to 15 centimetres (4 to 6 inches) below the bottom of the skimmer opening. You can use the pump's waste setting, a submersible pump, or a siphon hose.

The water level needs to be below the skimmer so that ice cannot form inside the skimmer body and crack it. It also needs to be below the return jets so those lines can be fully drained.

Do not drain the pool completely. A fiberglass pool shell is designed to have water in it. The weight of the water holds the shell down against groundwater pressure from below. An empty fiberglass pool in an area with a high water table can actually float out of the ground. This happens more often than people expect, and it is extremely expensive to fix.

Step 4: Blow out the plumbing lines

This is the most important step in the entire winterization process. Any water left in the plumbing lines will freeze, expand, and crack the pipes. A single cracked pipe underground can cost $500 to $2,000 to repair in the spring.

Here is how it works:

1. Disconnect the pump, filter, and heater from the plumbing.
2. Use a shop vac set to blow mode or an air compressor to force air through each plumbing line individually.
3. Blow air through the skimmer line until you see bubbles coming up from the pipe at the equipment pad. This confirms the line is clear.
4. Blow air through each return line. You will see air bubbles coming out of the return jets in the pool wall.
5. While the air is still blowing, plug the return jets with threaded winterizing plugs. This seals the lines and prevents water from seeping back in.
6. Plug the skimmer line from the equipment side, or install a skimmer guard (Gizzmo) that absorbs the pressure of expanding ice in the skimmer.
7. If you have a main drain, blow that line out as well. Some installers plumb the main drain line so it can be blown from the equipment pad.

This step requires confidence and the right tools. If you are not sure whether all the water is out of the lines, this is the one step worth hiring a professional for.

Step 5: Add winterizing chemicals

Winterizing chemicals prevent algae growth and protect the water quality while the pool sits covered for five months.

A typical winterizing chemical kit includes:

• Winterizing algaecide. A concentrated, non-foaming algaecide that stays active for months. Pour it directly into the pool water.
• Oxidizer or shock treatment. A non-chlorine shock or calcium hypochlorite shock to kill any existing bacteria or algae before closing. Add this at least a few hours before covering.
• Stain and scale preventer. This helps prevent mineral staining and calcium buildup on the gel coat over the winter.
• Enzyme treatment (optional). Breaks down organic material like pollen and body oils that settle to the bottom over the winter.

A winterizing chemical kit for a residential pool typically costs $50 to $100 CAD at a pool supply store. Read the dosing instructions on each product, as the amounts vary by pool volume.

Step 6: Disconnect and store equipment

Any equipment that holds water needs to be drained and either disconnected or protected.

• Pump: Remove the drain plugs from the pump housing and the pump basket lid. Store the drain plugs in the pump basket so you do not lose them. Some homeowners disconnect the pump entirely and store it indoors.
• Filter: Open the drain plug on the filter tank. For cartridge filters, remove the cartridge, clean it, and store it indoors. For sand filters, set the valve to the winterize position and remove the drain plug.
• Heater: Drain the heater by opening its drain plugs. Disconnect any water supply lines. If the heater is outdoors, cover it with a breathable cover to protect it from snow and ice.
• Salt cell: Remove the salt cell from the plumbing and store it indoors. Inspect it for scale buildup and clean it with a mild acid solution if needed before storing.
• Chlorinator: Remove any remaining chlorine tablets. Rinse and dry the chlorinator.
• Automation controller: If your system has an outdoor controller, most can stay in place. Turn off the breaker to the pool equipment panel.

Label your drain plugs and small parts with tape and a marker. Five months from now, you will be grateful you did.

Step 7: Install the winter cover

The winter cover keeps debris, leaves, and snow out of the pool and blocks sunlight that would encourage algae growth.

There are two main types of winter covers:

Cover Type	Cost (CAD)	Pros	Cons
Standard tarp-style cover	$150 – $400	Affordable, widely available	Collects water and debris on top, needs water bags or anchors
Safety cover (mesh or solid)	$2,000 – $5,000	Anchored securely, supports weight, lasts 10 – 15 years	Higher upfront cost, requires anchor installation
Automatic cover (if already installed)	N/A (already part of the pool)	Easy to close, acts as safety cover year-round	Must be properly maintained to avoid mechanism damage in winter

A safety cover is the better long-term choice. It is anchored into the patio with brass anchors and can support the weight of snow, ice, and even a person walking across it. It also keeps the pool cleaner through the winter because it does not sag into the water like a tarp cover.

If you use a tarp-style cover, you will need water bags or sandbags around the edges to hold it down. You will also need a cover pump to remove standing water from the top of the cover after rain or snowmelt. Standing water on a tarp cover can cause the cover to collapse into the pool.

Should you DIY or hire a professional?

Both are reasonable options, and the choice depends on your comfort level and your time.

	DIY	Hire a Professional
Cost	$50 – $100 (chemicals only)	$300 – $500 (chemicals may be extra)
Time	3 – 5 hours	1 – 2 hours (they have done it hundreds of times)
Equipment needed	Shop vac or air compressor, winterizing plugs, water test kit	Included
Risk	Higher if lines are not fully cleared	Lower (experienced crew, done it many times)

If this is your first pool, consider hiring a professional for the first year's closing. Watch what they do, ask questions, and learn the process. Many homeowners switch to DIY after seeing it done once by a professional.

The one step where professional experience matters most is blowing out the plumbing lines. If there is any water left in a line and it freezes, you will not know until spring when you find a cracked pipe underground. Getting the lines fully clear is not difficult, but it requires the right tools and knowing what to listen and look for.

What happens if you skip winterization?

If you leave a pool un-winterized through an Ontario winter, here is what you can expect:

• Cracked plumbing lines. Water in the underground pipes freezes and expands. PVC pipe cracks under this pressure. Repairing underground pool plumbing typically costs $500 to $2,000 CAD per line, depending on how deep the pipe is and where the crack is located.
• Damaged pump and filter. Water inside the pump housing and filter tank freezes and can crack the housings. A new pump costs $1,500 to $2,500 CAD. A new filter can cost $800 to $1,500 CAD.
• Cracked skimmer. If the water level is not lowered below the skimmer, ice forms inside the skimmer body and cracks it. Replacing a skimmer in a fiberglass pool is a difficult repair that can cost $500 to $1,500 CAD.
• Heater damage. Water trapped in the heat exchanger freezes and cracks the copper or titanium tubes. A new heat exchanger costs $1,500 to $3,000 CAD, and some heaters cannot be repaired economically once this happens.
• Green, stained pool in spring. Without winterizing chemicals and a cover, the pool becomes a breeding ground for algae. Opening a severely neglected pool can take a week of chemical treatment and multiple filter cleanings.

The total cost of freeze damage from a skipped winterization can easily reach $3,000 to $8,000 CAD. That is far more than the $300 to $500 it costs to have it done properly.

What are the most common winterizing mistakes?

Even homeowners who winterize their pools regularly can make mistakes that cause problems in the spring. Here are the ones we see most often:

Not getting all the water out of the lines

This is the most common and most expensive mistake. If you blow out the lines but miss one, or if you do not blow long enough for the air to push all the water out, that remaining water can freeze and crack the pipe. Use a shop vac or compressor with enough power, and watch for air bubbles at each return jet and skimmer to confirm the line is clear.

Closing too early

Closing in September or early October while the water is still warm gives algae a head start. The pool sits covered in warm darkness for weeks before the temperature drops enough to slow algae growth. By spring, you may find a pool full of green water and stains.

Not balancing the water before closing

The water sits in the pool for five months. If the pH is low (acidic) all winter, the gel coat takes the damage. Five months of acidic water can cause more etching than a full swim season of properly balanced water.

Draining the pool completely

Some homeowners think draining the pool is the safest option. For a fiberglass pool, it is the opposite. The water provides weight that holds the shell in place. An empty pool in an area with groundwater can literally lift out of the ground. This is called "pool pop" and it is one of the most expensive problems to fix.

Forgetting to remove the salt cell

Salt cells left in the plumbing over winter can trap water inside. If that water freezes, the cell cracks. Salt cells cost $800 to $1,200 to replace. Removing it takes two minutes.

Not using a cover pump

If you use a tarp-style cover, rain and snowmelt collect on top. The weight of the standing water can pull the cover into the pool, dumping all that dirty water in. A small cover pump ($50 to $100 CAD) on top of the cover prevents this.

Frequently Asked Questions