Maintaining a clean, safe, and inviting swimming pool requires more than just regular skimming and pH checks. One of the most critical maintenance tasks is shocking the pool—a process designed to eliminate contaminants and restore water clarity. However, pool owners often face confusion about the proper filtration settings during this process: should you recirculate or filter when shocking a pool?
This article explores the differences between the recirculate and filter settings, explains the chemical process behind shocking, and provides practical guidance on which setting is best suited for your specific situation. Whether you’re a seasoned pool owner or new to pool maintenance, this detailed guide will help you make an informed decision for optimal water quality.
Understanding Pool Filtration Systems: The Basics
Before diving into the specifics of shocking, it’s essential to understand how your pool’s filtration system functions. Filtration systems are the heart of pool maintenance, continuously working to remove debris, bacteria, and other contaminants from the water. Most residential pools use one of three main filtration types:
- Cartridge filters
- Sand filters
- DE (diatomaceous earth) filters
Each system operates with different efficiencies and maintenance requirements, but they share a common purpose: to keep your pool water clean and clear. Filtration systems are typically controlled by a multiport valve that allows you to select from different operational modes like filter, backwash, rinse, recirculate, waste, and closed.
What Is the Filter Setting?
The filter setting is the usual mode used during daily pool operation. It sends water from the pool through the pump, then through the filter medium, which captures debris and contaminants before returning the cleaned water to the pool. This setting is ideal for ongoing filtration because it helps maintain chemical balance and clarity.
What Is the Recirculate Setting?
The recirculate setting bypasses the filter and returns the water directly back into the pool without passing through the filtration system. In this mode, the pump continues to run and circulate water, but no filtration or debris removal occurs. This setting is typically used when the pool water needs to be mixed quickly or during chemical treatments that could clog or damage the filter.
What Does It Mean to Shock a Pool?
Shocking a pool involves adding a large dose of chlorine or non-chlorine oxidizers to the water to destroy contaminants such as body oils, sweat, lotions, and ammonia compounds that form chloramines—byproducts that can irritate the skin and eyes and cause unpleasant odors. Pool shock also helps eliminate algae and bacteria, restoring the sanitizing power of your regular chlorine levels.
Types of Pool Shock
Not all pool shocks are created equal. The type you use influences the filtration choice:
- Calcium Hypochlorite – High chlorine levels, effective against algae and bacteria. Leaves calcium residue. Requires filtration after use.
- Sodium Dichlor – Stable in sunlight, often used in outdoor pools. Dissolves quickly, less likely to cloud the water.
- Potassium Peroxymonosulfate (Non-Chlorine Shock) – Used for oxidation without adding chlorine. Safe to use anytime, but doesn’t kill algae or bacteria effectively.
- Lithium Hypochlorite – Fast-dissolving, useful in cold water, but more expensive and slightly less effective than calcium hypochlorite.
When Should You Shock a Pool?
Shocking is not a daily activity but a necessary weekly or bi-weekly maintenance task—or more often during heavy usage, after a rainstorm, or during algal blooms. Signs you need to shock include:
- Cloudy water
- Strong chlorine smell (a sign of chloramines)
- Eye or skin irritation
- Visible algae growth
- After heavy swimmer use (e.g., pool parties)
Should You Recirculate or Filter When Shocking a Pool? Breaking Down the Options
Now to the core question: should you use the recirculate or filter setting when shocking your pool?
The answer is not universal, but depends on several key factors including the type of filter you have, the type of shock you’re using, and your water chemistry. Let’s assess each option carefully.
Shocking While Recirculating
Using the recirculate setting during shocking bypasses the filter, allowing the shock to mix more rapidly throughout the pool. This option is often recommended when:
- Using calcium hypochlorite shock – This type of shock can cloud the water and cause scaling, which could strain or clog your filter. Turning the valve to recirculate during initial mixing and reaction helps prevent filter damage.
- Dealing with an algae bloom – Recirculating the water quickly distributes the shock throughout the pool, helping to ensure all algae are exposed to high doses of chlorine.
- After heavy contamination – If the pool has been exposed to excessive organic matter (e.g., after a party or heavy rain), recirculating first ensures even shock distribution before allowing the filter to do its job.
However, it’s generally only advised to recirculate at the beginning of the shocking process—approximately 1 to 2 hours—before switching back to filter mode. Without filtration following the shock, contaminants won’t be removed, and long-term water clarity could suffer.
Shocking While Filtering
Using the filter setting throughout the shocking process means the water is both being treated with chlorine and filtered as it cycles through. This option can be appropriate in certain situations:
- When using a non-chlorine shock (e.g., potassium peroxymonosulfate)
- If you’re using sodium dichlor, which dissolves quickly and doesn’t cloud the water
- If your filter is in excellent condition and has recently been cleaned
- When water clarity is already decent and no scaling risks are apparent
The advantage here is that debris, dead algae, and oxidized compounds are removed from the water simultaneously, leading to faster normalization of water quality and less manual labor. However, this approach is not without risks.
Risks of Filtering During Shocking
Filtration during shocking—especially with calcium-based shocks—can lead to:
- Clogged filter media – Calcium hypochlorite shock can cloud the water and leave behind calcium deposits that reduce filter efficiency.
- Increased pressure in sand or cartridge filters – This may cause damage or necessitate premature backwashing.
- Poor contact time for the shock to work – Running the filter too soon might pull the chlorine into the filter before it has fully oxidized contaminants.
Best Practices for Shocking Your Pool: Step-by-Step
The following best practices ensure you shock your pool effectively, whether you choose to recirculate or filter.
Step 1: Test Water Chemistry
Before shocking, always test your pool water for:
- pH (Ideal: 7.4–7.6)
- Alkalinity (Ideal: 80–120 ppm)
- Calcium Hardness (Ideal: 200–400 ppm)
- Chlorine Levels (You generally want a free chlorine level no higher than 1–3 ppm before shocking)
Adjust these levels as needed to maximize the shock’s effectiveness.
Step 2: Choose the Correct Shock Type
Decide whether to use chlorine-based or non-chlorine shock based on your situation:
| Shock Type | Best For | Post-Shock Filtration |
|---|---|---|
| Calcium Hypochlorite | Heavy algae problems, strong sanitization | Recirculate first, filter later |
| Sodium Dichlor | Swimming after a few hours, clear water maintenance | Filter continuously |
| Potassium Peroxymonosulfate | Daily maintenance, non-chlorine users | Filter continuously |
Step 3: Pre-Dissolve and Dilute the Shock
Especially when using calcium hypochlorite or lithium hypochlorite, pre-dissolve the shock in a large bucket of water before adding it to the pool. This step ensures even distribution and minimizes the risk of bleaching or cloudiness on the pool surface.
Step 4: Circulate the Water
At the start, most professionals recommend running the pump in recirculate mode for 1–2 hours. This allows the shock to mix thoroughly throughout the pool without subjecting it to filtration too soon, which can lower chlorine levels in the water and interfere with oxidation.
Step 5: Switch to Filter Mode
After initial mixing, switch the valve to filter. This ensures that:
- Dead algae and debris are removed from the water
- Oxidized particles are filtered out, promoting clarity
- Chemical balance improves faster as filtered water supports even distribution
Keep the filter running for at least 8–12 hours post-shock, possibly longer if water clarity is slow to return.
Step 6: Brush the Pool Surfaces
Brushing the walls, steps, and floor during and after shocking helps dislodge stubborn algae and contaminants, allowing the chlorine to work more effectively and improving overall filtration efficiency.
Step 7: Monitor and Retest
After 24 hours, retest the pool water. Wait for free chlorine levels to drop to 3 ppm or below before allowing swimmers back in. If the water is still cloudy or has a strong chlorine odor, additional filtration or partial backwashing may be needed.
Real-World Scenarios: Choosing Between Recirculate and Filter
Let’s apply what we’ve learned to various real-world pool care situations to see which mode is best.
Scenario 1: Algae Outbreak (Green Pool)
You wake up to find your pool has turned green overnight—signs of an algae outbreak. In this case, the priority is rapid chlorine distribution to kill the algae.
- Recirculate: Run in recirculate mode for 1–2 hours after shocking.
- Switch to filter: After initial mixing, resume filtration to remove dead algae.
- Brush daily: Prevent reinfection by loosening debris and exposing all areas to chlorine.
Scenario 2: Weekly Maintenance Shock
You’re performing your routine weekly shock with a non-chlorine oxidizer or sodium dichlor.
- Filter continuously: Water clarity and filter load risk is minimal.
- No need to recirculate: You can shock directly and let the pump run normally.
- Maintain normal filtration schedule: No backwashing or downtime required.
Scenario 3: After a Heavy Rainstorm
Your pool has been inundated with organic material from storm run-off, or debris has accumulated from a clogged skimmer basket.
- Recirculate for 1 hour: To ensure even shock distribution.
- Continue filtering: After recirculation to remove contaminants.
- Total filtration time: At least 12 hours post-shock to clear debris-laden water.
Tips for Maintaining Optimal Post-Shock Conditions
After you’ve shocked your pool, the work doesn’t quite end there. Here are some additional best practices to get the best results:
Clean or Backwash the Filter
If you used calcium hypochlorite or noticed increased pressure in your filter system:
- Backwash sand filters
- Clean or replace cartridge filters if needed
- Ensure DE filters are properly recharged
Neglecting this step can lead to reduced efficiency, poor water quality, and even damage to the system.
Keep the Water Moving
Running the pump continuously for 24 hours after shocking helps:
- Ensure even chemical distribution
- Maximize filtration benefits
- Return the water to safe swimming conditions more quickly
Don’t Overdo It
Too much shock can imbalance pH, irritate skin, and reduce overall pool life. Follow manufacturer dosing instructions carefully, and only increase dosage if signs of severe contamination or algae persist.
Know When Not to Run the Filter
In special cases, such as when the filter is broken or during winterization, avoid running the filter until the situation is corrected. Always consider safety and system longevity alongside pool cleanliness.
Conclusion: Striking the Right Balance Between Recirculate and Filter
Shocking a pool is not just about the chemicals you use—it’s also about how you circulate and filter these chemicals once they’re in the water. The choice between recirculating and filtering depends on your pool type, the filter system you have, and the kind of shock you’re applying.
Generally speaking, the best practice is to begin the process in recirculate mode for 1–2 hours to allow the shock to distribute evenly without interfering with the filter. Then, switch to filter mode to remove the oxidized debris and bring the pool back to crystal-clear condition.
Combining smart circulation decisions with routine water testing and maintenance will keep your pool clean, safe, and ready for every swim. So, the next time you ask, “When shocking a pool, should I recirculate or filter?” remember: choosing based on your unique situation will lead to the best outcome.
Additional Resources
- How to Test Pool Water: A Step-by-Step Guide
- Understanding Chloramines and How to Remove Them
- Pool Maintenance Schedule Checklist
- How to Backwash a Pool Filter Step-by-Step
By staying informed and proactive, you’ll keep your pool in top condition year-round.
What does it mean to shock a pool?
Shocking a pool involves adding a high dose of chlorine or a non-chlorine chemical to the water to eliminate contaminants such as bacteria, algae, and combined chlorine (also known as chloramines). This process helps to restore the effectiveness of the sanitizing agents in the pool and ensures the water remains safe and clean for swimmers. Pool shocking is an essential part of regular maintenance, especially after heavy use or exposure to environmental debris like rain or wind.
The frequency of shocking can vary based on usage, water chemistry, and environmental factors. Typically, most pools benefit from being shocked once a week during peak swimming seasons or after events that add a lot of contaminants to the water, such as parties or storms. Keeping up with this practice helps prevent algae growth and maintains clear, healthy water that’s comfortable for all swimmers.
Why is circulation important when shocking a pool?
Proper circulation ensures that the shock treatment is evenly distributed throughout the pool water, which is crucial for effectively eliminating algae and bacteria. Without good circulation, pockets of untreated water can form, allowing contaminants to survive and potentially cause problems later. Running the pool pump helps to circulate the water and the added chemicals, increasing the efficiency of the shock treatment.
In addition, adequate circulation helps to distribute the sanitizer to all areas of the pool, including hard-to-reach spots like corners and the deep end. This ensures a consistent chemical balance and promotes faster sanitization. Ideally, the pump should run for several hours after shocking to allow for complete coverage and optimal results, regardless of whether you choose to filter during this process or not.
What is the role of the filter during pool shocking?
While the filter does not directly contribute to the chemical process of shocking, it plays an important role in the overall cleanliness of the pool. After shocking, the dead algae and other debris in the water can be captured by the filter as water passes through it. This helps to clear the water more quickly and reduces turbidity, making the pool look clean and inviting again.
However, it’s important to monitor the filter pressure during this time. Shocking can stir up a lot of debris, which may clog the filter and reduce its efficiency. If the pressure rises significantly, it may be necessary to backwash or clean the filter to maintain proper water flow and filtration effectiveness after the shocking process is complete.
Should I recirculate or filter when shocking my pool?
When shocking your pool, the decision between recirculate and filter settings depends on your specific system and goals. In general, the recirculate setting bypasses the filter and keeps the pump and plumbing system active, allowing chemicals to mix without forcing water through a potentially clogged filter. Some pool owners prefer this option immediately after shock application to prevent immediate strain on the filter.
However, most experts recommend using the normal filter setting during shocking to allow the system to circulate the chemicals while simultaneously capturing debris and contaminants. This maximizes the efficiency of both the shock and filtration processes. As long as your filter is clean and functioning properly, the filter setting is often the better choice for overall effectiveness and faster results.
What happens if I shock without proper circulation?
Shocking a pool without adequate circulation can result in an uneven distribution of chemicals, leaving some areas of the pool inadequately treated while other areas have too much sanitizer. This imbalance may allow algae and bacteria to persist in certain parts of the pool, leading to unsightly growth and potential health hazards for swimmers. Ineffective circulation also delays the time it takes for the water to clear and return to a balanced state.
Moreover, poor circulation can create “dead zones” where water doesn’t move sufficiently, which can become breeding grounds for algae and bacteria over time. To avoid this, it’s essential to ensure the pump is running long enough to circulate water throughout the entire pool system. For best results, pool owners should aim to circulate water for at least 8 to 12 hours after shocking, or even longer in cases of severe contamination.
How long should the filter run after shocking a pool?
After shocking your pool, it’s important to run the filter system for at least 8 to 12 hours to ensure thorough circulation and filtration of the pool water. This allows the chemicals to reach all parts of the pool and gives the filter enough time to capture dead algae and other debris that the shock has loosened from surfaces. The exact time may vary depending on pool size, the type of shock used, and the initial condition of the water.
For pools with severe algae blooms or cloudy water, running the filter for 24 hours or more is often advised. Continuous operation during this time helps speed up the recovery process. However, it’s crucial to monitor the filter pressure and clean or backwash the filter if needed to prevent clogging and ensure optimal performance throughout the post-shocking period.
What are the differences between recirculate and filter settings on a pool valve?
The recirculate setting on a multiport valve bypasses the filter media, sending water back into the pool without passing through the sand or cartridge that traps debris. This setting is useful for adding heat to the pool or mixing chemicals without risking clogging the filter. However, it does not effectively clean or clarify the water since it skips the natural filtration process.
In contrast, the filter setting sends water through the filter media before returning it to the pool, which helps remove debris, dead algae, and other contaminants. This setting is the standard mode for everyday pool operation and is generally recommended during and after shocking unless there’s a specific reason to bypass the filter, such as a damaged or clogged system. Choosing the right setting ensures both chemical and physical cleanliness for optimal pool maintenance.