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Install 3-in-1 hot-water tap & tank

  • Writer: Robert Costart
    Robert Costart
  • May 29
  • 4 min read

Updated: Jun 4



📝 Overview


• Installing an under-sink boiler tank and 3-in-1 monobloc tap that dispenses filtered near-boiling, standard hot and cold water.

• Skill level: 4 / 5 (plumbing + fixed-electrical connection).

• Typical duration: 3–4 h hands-on.




⚠️ Safety First


  1. Isolate hot and cold supplies and unplug/turn off the socket before starting to prevent scalds, flooding and electric shock.

  2. Near-boiling water (≈ 98 °C) can cause severe burns—wear heat-resistant gloves and keep children clear.

  3. The tank is an electrical appliance; it must be earthed via an RCD-protected 13 A socket or fused spur. 

  4. Steam vents through the tap; never fit additional valves on the boiling outlet or tank pressure will rise dangerously. 

  5. Follow the Water Supply (Water Fittings) Regulations 1999—fit service valves and check valves where required. 




🧰 Tools & Materials

Item

Purpose

Adjustable & monobloc box spanners

Undo old tap, tighten stud nut

35 mm (or maker-specified) hole-saw & drill

Cut/resize sink hole if needed

Pipe cutter, deburr tool, olives

Prepare 15 mm copper tails/valves

Full-bore lever isolation valves + double-check valve

Mandatory service & backflow protection

Flexible braided tap tails & ¼” boiling hoses

Connect tap to supplies & tank

PTFE tape & sanitary silicone

Seal threaded joints and tap base

Scale-control filter kit

Protect boiler element in hard-water areas

RCD-protected 13 A socket / fused spur

Safe power for the tank

Bucket, towels, torch, kneeling pad

Catch drips, see clearly, work comfortably


📋 Step-by-Step


1. Plan the layout


  • Action: Measure cupboard space: leave at least 150 mm clear above and around the tank for ventilation; confirm a nearby RCD socket.

    • Why: Prevents overheating and allows cartridge changes.

    • Pro-tip: If space is tight, mount the tank sideways using the manufacturer’s brackets (check manual first).



2. Isolate water & power


  • Action: Close service valves (or main stopcock) and turn off the socket/fuse. Open the existing tap to depressurise the lines.

    • Why: Makes the work area safe and dry.



3. Remove the old tap (if present)


  • Action: Undo flexi tails, loosen the retaining nut with a monobloc spanner, lift out the tap.

    • Why: Clears the sink deck for the new 3-in-1 unit.



4. Prepare the sink hole


  • Action: Clean existing 35 mm hole or drill a new one through stainless/composite at slow speed; deburr.

    • Why: Ensures the tap gasket seats flat.

    • Pro-tip: Clamp plywood beneath thin sinks to stop breakout.



5. Assemble the new tap


  • Action: Screw braided tails and boiling/vent hoses into the tap body; slide on the rubber base gasket.

    • Why: Bench assembly is easier than in a cupboard.

    • Pro-tip: Lubricate O-rings with a smear of silicone grease for a smoother swivel.



6. Mount and align


  • Action: Feed hoses through the sink; have a helper keep the spout centred while you tighten the stud nut, rubber then metal horseshoe.

    • Why: Locks the tap rigidly and compresses the seal.



7. Fit isolation and check valves


  • Action: Cut the 15 mm cold feed and install a full-bore lever valve, plus a double-check valve if not integral to the tap. Repeat for the hot feed.

    • Why: Meets Regulation 4 & 5 requirements and aids future servicing. 



8. Install scale-control filter


  • Action: Mount filter head vertically, flush the cartridge until the water runs clear.

    • Why: Removes debris and protects the tank element.



9. Position and connect the tank


  • Action: Stand the tank upright on its rubber feet; connect the cold inlet from the filter, the boiling outlet to the dedicated hose, and the vent hose to the tap’s return.

    • Why: Maintains the open-vent, pressure-free system. 

    • Pro-tip: Colour-code hoses with cable ties to avoid mix-ups.



10. Prime the system


  • Action: Open the isolation valve; lift the tap’s boiling lever until water flows steadily.

    • Why: Fills the tank and expels air before switching on.



11. Make the electrical connection


  • Action: Plug into the RCD socket or connect to a fused spur (13 A, earthed) and switch on.

    • Why: Powers the heater safely. 



12. Set temperature & test


  • Action: Select the maker’s default (usually 98 °C) and wait for the “ready” light. Run boiling, mixed hot and cold functions for one minute each while checking every joint.

    • Why: Confirms heating, flow and leak-tightness.

    • Pro-tip: Leave kitchen roll beneath fittings overnight—any hidden seepage shows as a stain.




🔍 Completion Checks


  • Tap base sealed, sits rigid; spout swivels smoothly.

  • Boiling water dispenses in a steady, non-spitting stream; LED shows ready.

  • All hose and valve joints remain perfectly dry after 10 min at operating pressure.

  • Tank exterior stays just warm, not hot; ventilation slots unobstructed.




👷‍♂️ When to call a pro instead


If you lack an RCD-protected spur, your mains pressure exceeds the tap’s limit (often 5 bar) and needs a pressure-reducing valve, or cupboard space demands pipe rerouting through stone worktops, a qualified plumber/electrician can ensure full compliance and keep your guarantee intact.

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