How do I choose between whole-house and under-sink water filtration?

Short answer

Choose under-sink filtration if your goal is great-tasting, safe drinking and cooking water at one faucet with minimal cost and plumbing work. Choose a whole-house system if you want to treat all water in the home to address issues like sediment, chlorine odor, staining, scale, or well-water microbes. Many homes benefit from a combo: whole-house sediment or softening plus an under-sink RO or carbon system for drinking.

What each option does

• Under-sink (point-of-use): Treats water at a single location, usually the kitchen. Options include carbon block (taste, chlorine, some lead), ultrafiltration (bacteria cysts), and reverse osmosis RO (dissolved solids, many contaminants). Lower cost, easiest maintenance, minimal pressure impact.
• Whole-house (point-of-entry): Filters all water entering your home. Common types: sediment filters, carbon tanks for chlorine/odor, water softeners for hardness, iron/manganese filters, and UV for microbiological control. Improves bathing, laundry, and plumbing lifespan, but higher cost and more plumbing.

Typical specs
- Whole-house flow: 7–15 gpm; main line: 3/4 in or 1 in; pressure drop: 5–15 psi
- Under-sink flow: 0.5–1.5 gpm (RO dispenses from a small faucet via storage tank)
- RO waste ratio: ~3:1 to 1:1; tank capacity commonly 2–4 gallons
- Filter life: sediment 3–6 mo; carbon 6–12 mo; RO membrane 2–3 yrs; UV lamp yearly

How to decide (step-by-step)

1. Test your water. Use a lab kit or request a report from your supplier. For wells, get a full panel: bacteria, iron, manganese, hardness, pH, nitrates. For city water, look for chlorine, chloramines, lead, and hardness.
2. Set goals.
   • Taste and drinking only: under-sink carbon or RO.
   • Chlorine odor in showers and whole house: whole-house carbon.
   • Scale on fixtures or appliances: softener or conditioner whole-house.
   • Sediment staining or well grit: whole-house sediment filter.
   • Microbial risks on a well: UV after pre-filtration; RO for drinking.
3. Match technology.
   • Lead: certified under-sink carbon block or RO.
   • Nitrates, high TDS: RO (under-sink) or specialized whole-house media.
   • Hardness: softener (ion exchange) whole-house.
4. Size for flow and pressure. Whole-house systems must match your peak flow; oversized housings reduce pressure drop. Under-sink systems are fine at the kitchen but may feed the fridge if spec allows.
5. Plan maintenance. Filters need changing on schedule. If you are not likely to maintain whole-house equipment, choose simpler under-sink units.
6. Budget. Under-sink: 60–600 plus 40–200 per year. Whole-house: 200–1,200 for cartridge/tank systems; softeners 600–1,500; UV 250–600; pro install can add 200–800.

Tools and materials

• Under-sink:
  • Adjustable wrench, basin wrench, drill and step bit if adding a faucet hole
  • Tee stop valve or dual-outlet stop, 1/4 in tubing and push-to-connect fittings
  • Plumber’s tape, mounting screws, leak detector tabs
• Whole-house:
  • Pipe cutter (copper, PEX, or PVC compatible), deburring tool
  • PEX crimp or expansion tool, or solder kit for copper, or PVC primer/cement (per pipe type)
  • Two ball valves for a bypass loop, unions, pressure gauges before/after, mounting bracket, pipe hangers, bucket

Quick install snapshots

• Under-sink carbon or RO
  1. Shut water at the stop valve, open faucet to relieve pressure.
  2. Mount the filter bracket on cabinet sidewall; keep cartridges vertical and accessible.
  3. Add a tee to the cold-water stop; run 1/4 in tubing to the filter inlet.
  4. Install the dedicated faucet on the sink or use an adapter to feed fridge if allowed.
  5. For RO, connect drain with an air-gap faucet to meet code and prevent backflow.
  6. Flush per manufacturer directions and check for leaks.
• Whole-house cartridge housing
  1. Shut main, drain lines, and cut into the main after the meter and before branches.
  2. Build a bypass loop with three ball valves (inlet, outlet, bypass) for service.
  3. Mount housings on a solid backer at eye level with clear bowls facing down.
  4. Add pressure gauges before and after to monitor pressure drop.
  5. Restore water, purge air, and inspect for leaks.

Safety and code considerations

• Shut off and relieve pressure before cutting. Wear eye protection and gloves.
• Support heavy tanks; do not let housings hang from piping alone.
• Use an air-gap drain for RO and UV per local code. Prevent cross-connections.
• Disinfect lines and RO tank at initial install and filter changes.
• If you have high static pressure over 80 psi, add a pressure-reducing valve; thermal expansion tank may be required on closed systems.

Common mistakes to avoid

• Undersizing whole-house filters, causing low shower pressure.
• Skipping pre-filtration before UV or RO, leading to fouling.
• Ignoring filter change intervals; clogged carbon can channel and underperform.
• Connecting RO to a fridge with incompatible flow requirements.
• Installing on the hot side by mistake; most filters are cold-water only.

Practical examples

• City water with chlorine taste and dry skin: whole-house carbon plus under-sink carbon at the kitchen if you want extra polishing.
• Well with iron staining and sediment: whole-house sediment and iron filter; UV for microbes; optional under-sink RO for drinking.
• Older home with possible lead: certified lead-reduction under-sink carbon or RO for drinking; replace lead service line where possible.

When to call a pro

• Galvanized or mixed piping you are not prepared to cut and adapt.
• Need for soldering near combustible materials, or limited access spaces.
• Complex well treatment trains (iron, pH correction, softener, UV) needing proper sequencing.
• Code-driven requirements such as air-gap drains, backflow prevention, or permits.

Tips for best results

• Add shutoff valves and unions before and after each whole-house unit for easy service.
• Label filter change dates and keep spare cartridges on hand.
• Install leak sensors under sinks and near whole-house housings.
• Track performance: TDS meter for RO, and pre/post pressure gauges for whole-house.

Bottom line: pick the solution that targets your contaminants and usage. For most city homes, under-sink filtration covers drinking needs affordably; add whole-house only if you want to improve showers, laundry, plumbing longevity, or have specific well-water issues.