Trichlor Restart Dose for a 15,000-Gallon Vinyl Liner Pool After Two Weeks Away

You pull into the driveway after two weeks of vacation, walk around back, and your pool is staring at you in a shade of green you don't love. It happens to almost every pool owner who's been away without a house-sitter willing to test and dose chemicals twice a week.

The short answer: for a 15,000-gallon vinyl liner pool that has gone cloudy or lightly green after two weeks, add 2 to 3 pounds of granular trichlor shock broadcast evenly across the water with the pump running. If the water is visibly dark green, skip trichlor for the heavy shock and use cal-hypo or liquid chlorine to avoid spiking your cyanuric acid - then return to trichlor once the pool is clear. This article walks through how to assess what you're dealing with, how to dose correctly, what vinyl-specific rules to follow, and how to get back to swimming as fast as possible.

Step 1: Test Before You Add Anything

Before you reach for any chemical, test the water. You need four numbers: free chlorine, pH, cyanuric acid (CYA), and a rough visual read on algae. After two weeks without attention, expect free chlorine to be near zero and pH to have drifted upward toward 7.8 or higher. A high pH matters a lot here because chlorine efficiency drops sharply above 7.6. At pH 7.8, roughly 30 percent of your chlorine is in the active hypochlorous acid form; at 7.2, that number jumps to about 66 percent. So if your pH is high, lower it with muriatic acid before shocking, not after.

Your CYA reading will tell you which product to reach for. Trichlor contains about 57 percent available chlorine and about 9 percent CYA by weight, meaning every pound of trichlor adds roughly 0.5 ppm of CYA to 15,000 gallons. If your CYA is already sitting at 60 ppm or above after a summer of tablet use, using a large trichlor shock dose will push it higher and make the chlorine you add less effective. In that case, cal-hypo or liquid chlorine is the smarter heavy-shock product. If CYA is in the 30 to 50 ppm sweet spot, trichlor is perfectly fine for the restart.

Step 2: Assess the Severity

Not every neglected pool is in the same condition, and the dose depends heavily on what you're looking at:

• Cloudy but still blue: Free chlorine dropped but algae hasn't bloomed yet. A single 2-pound dose of trichlor shock broadcast evenly across 15,000 gallons will spike chlorine to roughly 12 to 15 ppm and handle this quickly.
• Light green tint: Early algae growth. You're looking at a 3-pound trichlor dose, pH adjusted to 7.2 first, pump running 24 hours, and a brush-down of the walls and floor before adding chemicals.
• Dark green or opaque: A full algae bloom. Trichlor alone is not the right tool here. Use 3 to 4 pounds of cal-hypo (65 to 73 percent) to hit a breakpoint chlorine level of 20 to 30 ppm, then follow up with trichlor tablets once the pool clears. Trying to reach those numbers with trichlor in a 15,000-gallon pool would require so much product that your CYA could climb 4 to 5 ppm in a single treatment.

Trichlor Dose by Condition: 15,000-Gallon Pool

Why These Numbers Matter for Vinyl Liners

Vinyl liner pools need a little extra care when you're adding concentrated chlorine products. The main risk isn't from chlorine itself in solution - it's from undissolved granules sitting on the liner floor. Trichlor granules are acidic and can bleach or weaken vinyl in a localized spot if they sit there for even a few hours. The fix is simple: pre-dissolve your trichlor granular shock in a 5-gallon bucket of pool water before you pour it in, then walk the perimeter and pour it evenly with the pump running so it disperses immediately. Never add granular trichlor to the skimmer. Trichlor and residual cal-hypo in a skimmer basket is a combination that can ignite.

Another vinyl-specific consideration: keep your pH between 7.2 and 7.4 when chlorine levels are elevated. High chlorine plus low pH is hard on liner colors and seams over time. A good habit is to check both numbers together. For broader guidance on keeping a liner in good shape, the Vinyl Pool Liner Maintenance Tips post on this site covers water chemistry balance in the context of liner longevity.

Step 3: The Restart Process, Start to Finish

1. Test free chlorine, pH, and CYA. Write the numbers down.
2. If pH is above 7.4, add muriatic acid and retest in 2 hours before shocking.
3. Brush the walls, steps, and floor to knock algae loose from surfaces.
4. Pre-dissolve the trichlor granular shock in a bucket of pool water.
5. Pour the solution evenly around the pool perimeter with the pump running.
6. Run the pump continuously for at least 24 hours.
7. Backwash or clean your filter after 12 hours - the filter will load up fast pulling dead algae and debris.
8. Retest free chlorine after 24 hours. If it's still below 3 ppm, the demand isn't met and you need a second dose.
9. Once free chlorine holds at 3 ppm or above for two consecutive tests, resume your normal tablet routine.

A Worked Example: The Typical Two-Week Return

Say your 15,000-gallon pool has a light green tint, your test reads free chlorine at 0 ppm, pH at 7.9, and CYA at 45 ppm. First, add about 13 ounces of muriatic acid (31.45 percent) to bring pH down to around 7.2 and wait two hours. Then pre-dissolve 3 pounds of granular trichlor shock in a bucket and pour it around the pool. That dose delivers roughly 17 ppm of available chlorine into 15,000 gallons of water, which is well into breakpoint territory for light algae. Run the pump overnight, backwash in the morning, and test again. In most cases the pool is clear blue within 36 to 48 hours. If free chlorine has dropped below 3 ppm after 24 hours and you can still see cloudiness, add another pound of trichlor and run the pump another 12 hours.

One thing worth noting: after a two-week gap in tablet dosing, your feeder or floater is probably empty. When you reload it, think about how many weeks are left in the swim season. If you're past midsummer and CYA is already at 45 ppm, each additional trichlor tablet pushes CYA up by about 0.5 ppm per 10,000 gallons per tablet. A full season of weekly tablet use in a 15,000-gallon pool can add 20 to 30 ppm of CYA on top of whatever you started with - worth tracking so you don't hit 80 ppm by Labor Day.

A Note on Line Buildup When Reopening

After two weeks of little-to-no circulation, biofilm and scale can start forming inside return lines and jet fittings, particularly if the pump ran only sporadically or not at all. If you notice reduced flow from returns or a musty smell when the pump first kicks on, it's worth running a line cleaner through the system. We make our Spa Jet Line Cleaner for exactly this kind of situation - it flushes biofilm out of plumbing lines before you shock, so you're not fighting dirty water from inside the pipes at the same time you're fighting algae in the pool.

Liner wrinkles can also appear after extended periods of low circulation or uneven water pressure from jets. If you notice new folds along the floor or walls when the pool was fine before you left, that's worth addressing before the liner sets further - the post on how to prevent pool liner wrinkles explains the usual causes and how to smooth them out.

Frequently Asked Questions

How much trichlor shock do I add to a 15,000-gallon pool after two weeks?

Add 2 to 3 pounds of granular trichlor shock to a 15,000-gallon pool that has gone cloudy or lightly green after two weeks. Pre-dissolve it in a bucket before adding, and run the pump for at least 24 hours straight. If the water is dark green, use cal-hypo or liquid chlorine for the initial heavy shock instead - the volume of trichlor needed to hit 20 to 30 ppm would push CYA too high.

Can I use trichlor tablets to restart a pool instead of shock?

Trichlor tablets dissolve slowly and deliver roughly 3 to 5 ppm of chlorine over several days in a floater or skimmer, so they can't produce the rapid spike you need after a two-week absence. For a restart, you need free chlorine to hit 10 to 20 ppm immediately to kill algae and oxidize organics. Shock first with granular product, then resume tablets once the water is clear and holding chlorine.

What should CYA be in a 15,000-gallon vinyl liner pool using trichlor?

Keep CYA between 30 and 50 ppm for an outdoor vinyl liner pool relying on trichlor for maintenance chlorine. Below 30 ppm, UV degrades your chlorine too fast. Above 60 ppm, your effective chlorine drops enough that you need to maintain higher free chlorine levels to stay protected. Each pound of trichlor adds about 0.5 ppm of CYA to a 15,000-gallon pool, so it accumulates gradually over the season.

Is trichlor safe for vinyl pool liners at high doses?

Trichlor is safe for vinyl liners as long as it's fully dissolved before it contacts the surface. Undissolved granules sitting on the liner floor are acidic enough to bleach or weaken the vinyl in that spot. Always pre-dissolve granular trichlor in a bucket of pool water and pour it evenly around the perimeter with the pump running - never pour it directly into the skimmer or dump it in one spot.

How long after adding trichlor shock can I swim in a 15,000-gallon pool?

Wait until free chlorine tests at 1 to 3 ppm before swimming - typically 8 to 24 hours after a standard shock dose, depending on sunlight exposure, water temperature, and your CYA level. Higher CYA will hold chlorine longer and slow the drop-back to safe swimming levels. Always test with a reliable kit before anyone gets in rather than going by time alone.

Getting your pool back after two weeks away mostly comes down to testing first, adjusting pH, then hitting it with the right trichlor dose for what you actually see in the water - 2 pounds for cloudy, 3 for lightly green, and cal-hypo if it's truly gone. Keep the pump running and brush everything, and you'll be swimming again within a day or two.

If you're not sure which product fits your situation, we're happy to help. AquaDoc carries pool chlorine and shock options suited for vinyl liner pools at every severity level, so you can find the right fit and get back in the water without guessing.