How to Winterize Your Pool in Maryland and Virginia

In Maryland and Virginia, most pools are open from late April through early October — roughly 20–24 weeks. What happens in the other 28 weeks matters just as much to your pool's condition and your wallet. A properly winterized pool is protected from freeze damage, arrives in spring with manageable chemistry, and saves you hundreds of dollars in chemical costs and repairs. An improperly closed pool often opens in the spring with green water, damaged equipment, and cracked plumbing. Here's how to do it right.

When to Close Your Pool in Maryland and Virginia

Most pool professionals in the DMV area recommend closing when consistent overnight temperatures drop below 60°F — typically late September to mid-October depending on the year. Closing too early (when water is still warm) invites algae growth under the cover all winter. Closing too late risks a surprise freeze before you've winterized the lines.

Watch the 10-day forecast in late September. When you see a pattern of overnight lows consistently below 55–60°F, it's time. Most homeowners in Northern Virginia and Maryland close between September 25 and October 15.

Step 1: Balance Water Chemistry (1 Week Before Closing)

Starting with balanced water prevents surface staining and scaling during the months the pool is covered. Test and adjust to these targets:

• pH: 7.2–7.6
• Total Alkalinity: 80–120 ppm
• Calcium Hardness: 200–400 ppm
• Free Chlorine: 2–4 ppm (slightly higher than normal to account for the months ahead)

Do not add winterizing chemicals at this stage — those go in at the final step.

Step 2: Deep Clean the Pool

Vacuum the floor to waste (bypassing the filter to remove as much debris as possible). Brush all walls, steps, and corners. Skim the surface thoroughly. Clean the waterline tile to remove any mineral deposits that would harden over winter. Empty and clean all skimmer baskets and the pump strainer basket.

A clean pool going into winter means less work in spring and less chance of staining from decomposing organic material sitting against the surface for months.

Step 3: Backwash and Clean the Filter

For sand filters: backwash thoroughly, then set the valve to "winterize" or remove the drain plug to allow water to fully drain from the filter tank. For cartridge filters: remove and clean the cartridge, then store it indoors in a clean, dry location. For DE filters: backwash, disassemble, clean the grids, and store grids indoors if possible.

Do not store water in the filter tank over winter — trapped water can expand during a hard freeze and crack the tank.

Step 4: Lower the Water Level

For pools with mesh safety covers that allow rain and melting snow to pass through, lower the water level 12–18 inches below the skimmer. This prevents the skimmer from filling with water that freezes and cracks the skimmer housing — one of the most common (and avoidable) freeze damage repairs in Virginia and Maryland.

For solid covers, a smaller water drop (4–6 inches below the skimmer) is typical, since water accumulation on top of a solid cover needs somewhere to go.

Step 5: Blow Out and Plug the Plumbing Lines

This is the most technically demanding step and the most important for freeze protection. Water left in underground plumbing lines can freeze and expand, cracking PVC pipes and fittings. To blow out lines:

• Use a dedicated wet/dry shop vac or an air compressor to blow air through each line until no water exits at the pool end.
• Start with the skimmer lines, then the main drain, then each return line.
• Insert a winter plug (Gizzmo or rubber stopper) into each fitting as you blow it out to prevent air from escaping and water from re-entering.
• Plug the skimmer with a black Gizzmo expansion absorber — these are designed to absorb ice expansion rather than crack the skimmer body.

If you're not confident about this step, a professional closing is well worth the cost. Blown plumbing repairs are significantly more expensive than the cost of a professional closing visit.

Step 6: Winterize the Equipment

Remove drain plugs from the pump, filter, heater, and any other equipment that holds water. Store plugs somewhere you'll find them in spring (many homeowners tape them inside the filter tank lid). If you have a heater, follow manufacturer instructions for winterizing the heat exchanger — particularly important for gas heaters.

Salt chlorinators should be disconnected and stored indoors if the cell will be exposed to freezing temperatures. Most salt cells are rated for temperatures down to about 32°F but not repeated hard freezes.

Step 7: Add Winterizing Chemicals and Cover

With the pool at the correct water level and lines blown out, add a winter algaecide and possibly a phosphate remover (especially if you've battled algae during the season). Some homeowners add a stain and scale inhibitor as well. Follow product dosage instructions based on your pool volume.

Install your winter cover — mesh safety cover, solid cover, or automatic cover — and secure it according to the manufacturer's instructions. For above-ground pools, install the air pillow in the center of the pool before covering to absorb ice expansion.

One Winter Check-In Is Worth It

Mid-winter, check the cover for damage, pooling water, or ice accumulation. Check the water level — if it's dropped significantly, the cover may not be sealing properly, or there may be a slow leak. Adding a pound of non-chlorine shock in January or February on a warm day (above 40°F) can help keep the water from developing algae under the cover.

Beltway Pools offers professional pool closing and winter opening services throughout Virginia, Maryland, and DC. If you'd prefer a technician to handle it, contact us in early September to get on our fall schedule before it fills.

Common Winterization Mistakes to Avoid

The most expensive spring repairs in Virginia and Maryland pools almost always trace back to one of these closing mistakes:

• Closing too early with warm water: Water above 60°F under a cover is an ideal algae incubator. A pool closed in August that looks fine at closing can open in late April with a massive algae problem requiring expensive chemical correction.
• Skipping line blow-out or doing it partially: Any water remaining in above-ground plumbing lines at the time of a hard freeze will expand and can crack fittings, manifolds, or the skimmer body. This is the single most common preventable winter repair in this region.
• Not removing all water from equipment: Pump strainer housings, filter tanks, heater heat exchangers, and salt cell housings can all hold residual water. Every piece of equipment with a drain plug needs to be drained before a hard freeze.
• Using the wrong cover type: A tarp or standard solar cover is not a winter cover. Winter mesh safety covers are engineered to allow water to pass through while blocking debris; solid covers require a pump to manage water accumulation. Using the wrong cover type leads to cover failure, debris contamination, or unsafe water accumulation on top of the cover.
• Adding too little (or no) algaecide at closing: Winter algaecide provides months of protection under the cover. Many homeowners skip this step to save money, then spend $200–$600 on algae correction products at spring opening — far more than the $30–$60 the closing algaecide would have cost.

Frequently Asked Questions

When should I close my pool in Virginia and Maryland?

When consistent overnight temperatures drop below 60°F — typically late September to mid-October in Northern Virginia and Maryland. Watch the 10-day forecast and close before a sustained cold pattern sets in, but not so early that you’re closing warm water.

Do I need to drain my pool completely for winter?

No — a complete drain is not recommended and can actually cause damage (empty inground pools can “pop” out of the ground due to hydrostatic pressure). Lower the water level to 12–18 inches below the skimmer for mesh covers, or 4–6 inches below the skimmer for solid covers.

What chemicals should I add when closing a pool?

At minimum: a winter algaecide dosed for your pool volume, and a stain and scale inhibitor if your fill water is hard. Many pool professionals also add a phosphate remover and a non-chlorine shock at closing to reduce the organic load the pool will carry over winter.

Should I hire a professional to close my pool, or do it myself?

The line-blowing step is where most DIY closings go wrong. If you have the right equipment (an air compressor with sufficient CFM, or a dedicated pool vac/blower) and understand the sequence, DIY closing is feasible. If you’re unsure about any step — particularly the blow-out — a professional closing costs $150–$350 and is well worth avoiding a $500–$2,000 freeze damage repair in spring.