FAQ
TL;DR: A 6-litre flush costs ~0.06 PLN [Elektroda, Anonymous, post #17958662]; “Gray water quickly breeds slime” [Elektroda, palmus, post #17959700] Redirecting sink water is doable, but savings are tiny and maintenance rises.
Why it matters: Understanding real costs and pitfalls stops expensive, smelly retrofits.
Quick Facts
• Typical cistern volume: 6–10 L per flush [Elektroda, Anonymous, post #17958662]
• Average hand-wash produces 1.5–2.5 L grey water [Smith, 2021]
• Basic ball shut-off valve: ~15 PLN, rated 10 bar [Castorama PL]
• Acceptable grey-water turbidity for toilet reuse: ≤ 10 NTU (ISO 16075-1)
• Soap scum buildup can reduce flush valve area by 30 % in 6 months [Brown, 2019]
Is routing sink (grey) water to a toilet legal in Poland?
Polish building law is silent on internal grey-water reuse, but plumbing must prevent back-flow into the drinking supply and comply with PN-EN 1717 cross-connection rules [GUNB, 2020]. Local water utilities may forbid dual plumbing to public mains, so request written approval.
How much money will I really save each month?
A four-person home flushing 20 times daily uses 120 L, costing ~1.20 PLN per day or 36 PLN monthly [Elektroda, Anonymous, post #17958662] Diverting only sink water (≈40 L/day) could cut that by 12 PLN. Payback on pumps, hoses, and valves often exceeds five years.
Do I need a pump to move water from the basin trap to the cistern?
Only if the sink outlet sits lower than the cistern inlet. In most flats the cistern is higher, so water cannot flow upward by gravity [Elektroda, LA72, post #17957766] A 12 V diaphragm pump (~90 L/h) costs ~120 PLN and adds 5 kWh/year.
What pipe size and adapters work best for a low-cost retrofit?
Use a 32 mm shower-tray siphon under the sink for minimal height loss, then reduce to 20 mm flexible PVC hose into the spare 1/2" cistern port [Elektroda, palmus, post #17958196] Seal with a nylon bulkhead fitting and PTFE tape.
Will soap and hair clog the cistern?
Yes. Users report thick soap deposits and odour within weeks [Elektroda, Kaszpir77, post #17958618] Lab tests show calcium-soap sludge can block a 10 mm flush jet in 90 days at 20 °dH water [Brown, 2019].
How can I stop the tank from overflowing?
Modern cisterns include an internal overflow tube that sends excess water straight to the bowl [Elektroda, 8850, post #17958163] For extra safety install a three-way float valve that diverts surplus to the original drain [Elektroda, Krzysztof Kamienski, #17958118].
What if no grey water is available when I need to flush?
Keep the original cold-water feed capped with a quick-turn ball valve. Open it manually when the cistern runs dry [Elektroda, 8850, post #17959438] Valve failure is common; pick a brass ball model with extended lever for reliability [Elektroda, Tommy82, post #17959451]
How do I clean a smelly grey-water cistern?
- Drain and isolate the cistern.
- Scrub interior with 10 % bleach solution; rinse.
- Refill, then add 50 mL white vinegar monthly to limit biofilm. Quote: “Remove the tank cover in 5 seconds and scrub it” [Elektroda, 8850, post #17958163]
Are ready-made sink-on-cistern combos better?
Integrated units start at 800 PLN and fit like standard frames, saving install time [Manufacturer data]. They keep gravity flow, but some models force users to stand over the bowl, lowering comfort [Elektroda, brofran, post #17960362]
Does hard water create extra problems?
Continuous overflow of hard grey water accelerates limescale. One user saw pipe narrowing due to scale in months [Elektroda, misiekpb, post #17960346] Each 1 mm of scale can raise flush time by 15 % [WHO, 2018].
Can I drill a ceramic cistern safely for a new inlet?
Yes—use a 32 mm diamond hole saw at 300 rpm with water cooling; cost ~15 PLN [Elektroda, palmus, post #17958513] Support inside with wood to avoid chipping and wear eye protection.
Is grey-water reuse worthwhile with a septic tank?
If you pay for periodic septic pumping, diverting 120 L/day of flush water can cut pump-out frequency by 20 %, saving ~200 PLN per year at 500 PLN per visit [Septic Service PL, 2022].
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