Intro

On most jobs, swapping a water heater looks simple. Disconnect, lift out, drop in. But small misses can cost you hours, callbacks, or worse, safety risks. This guide walks UK contractors through swapping a water heater the right way. You’ll get clear steps, code hints, and practical checks. We cover like-for-like swaps, basic upgrades, and when to bring in certified help. If you’re swapping a water heater tomorrow, use this as your checklist. If you’re training a new hand, this is a solid start.

Quick Answer

Swapping a water heater usually takes 3–6 hours for like‑for‑like. Isolate, drain, remove, set the new unit, reconnect, vent/commission, and test. Gas work needs Gas Safe. Unvented cylinders need G3 competence. Follow manufacturer instructions, record readings, and issue paperwork. That’s how you avoid leaks, CO risks, and callbacks.

Table of Contents

• Key Takeaways
• Planning the Swap
• Tools and Parts You’ll Actually Use
• Step-by-Step: Swapping a Water Heater
• Venting, Gas, and Electrical Safety
• Commissioning, Testing, and Handover
• Pricing, Proposals, and Paperwork
• Common Mistakes to Avoid
• FAQ
• Conclusion

Key Takeaways

• Most like‑for‑like swaps take 3–6 hours; cylinders can be 1 day.
• Gas Safe for gas heaters. G3 for unvented cylinders. Part P for electrics.
• Record 5 key numbers: pressures, temperatures, flow rate, leak test, CO/CO₂.
• Always fit discharge pipework to G3 and manufacturer specs.
• Quote clearly and get sign‑off before you start to avoid change orders.

Planning the Swap

When you’re swapping a water heater, the plan saves the day. Check space, ventilation, services, and access before you price. Take photos. Note model numbers and connections. Measure pipe sizes: many homes have 15 mm hot/cold, 22 mm gas. Check incoming mains pressure (aim for 1.0–1.5 bar at the tap) and flow (10–12 litres/min is common in small homes).

• Confirm type: vented, unvented cylinder, combi, or storage multipoint.
• Confirm fuel: gas, electric immersion, or LPG.
• Check permits and competence: Gas Safe, G3, Part P where needed.
• Plan disposal: old unit can weigh 40–80 kg dry; a 210 L cylinder is heavy.

If you’re also improving your professional proposals, see our guide on “professional proposals” and “pricing strategies” to tighten your numbers.

Tools and Parts You’ll Actually Use

Most jobs need the basics and a few lifesavers. When you’re swapping a water heater, have this ready:

• Adjustable spanners, pipe cutters, olives, PTFE, jointing compound.
• Drain hose, buckets, dust sheets, lifting straps.
• 15 mm/22 mm fittings, isolation valves, flexi hoses (rated), brackets.
• Approved discharge pipework, tundish (for unvented), clips.
• Scale reducer or filter if hardness is high (common over 200 ppm).
• Flue parts (gas), CO alarm, analyser, leak detector spray.
• Multimeter, RCD tester (electric), thermometer, pressure gauge.

Tip: Carry 300–500 mm lengths of 15 mm and 22 mm copper and a few elbows. Those save you 30–45 minutes on most swaps.

Step-by-Step: Swapping a Water Heater

This is a like‑for‑like outline. Follow the manufacturer instructions for the exact model. For gas or unvented, only a competent person should do the work.

1. Explain scope to the client and get sign‑off. Agree location, venting, and timings.
2. Isolate supplies. Water off at the stopcock. Gas off at the meter. Power off at the breaker.
3. Drain down. Open hot taps to vent. Attach a hose to the drain. Expect 50–150 litres depending on tank size.
4. Protect the area. Sheets down. Clear 600–900 mm of working space if you can.
5. Disconnect services. Label wires. Cap gas line safely. Catch residual water.
6. Remove old unit. Use straps. Team lift if over 35 kg. Mind sharp edges.
7. Prep the base and fixings. Level up. Many wall units need 2–4 solid fixings. Maintain clearances per MI (often 50–150 mm sides; follow the book).
8. Set the new heater. Dry‑fit first. Check flue route or vent path now.
9. Pipework connections. Keep runs neat. Use new isolation valves. Don’t reduce a 22 mm gas feed without design approval.
10. Discharge pipework (unvented). Fit tundish in a visible spot. Size and route per G3 and MI. No traps. Terminate safely.
11. Electrical/gas reconnection. Test earth continuity. Gas tightness test before lighting. Record results.
12. Fill and vent. Open cold in. Bleed air at hot taps. Check for leaks 10–15 minutes.
13. Commission. Set temperature (usually 60 °C). Check flow (aim 10–12 litres/min small homes). Balance if needed.
14. Final checks. Flue integrity test (gas). CO alarm within 1–3 metres as required. Safety devices operate.

When you’re swapping a water heater in tight cupboards, pre‑build the valves on the bench. That can save 20–30 minutes on site.

Venting, Gas, and Electrical Safety

Swapping a water heater safely matters more than speed.

• Gas: Only Gas Safe engineers should work on gas. Do a tightness test and flue integrity test. Record analyser readings. Follow MI for flue lengths and elbows.
• Unvented (G3): Temperature/pressure relief must discharge via a tundish to a safe point. Pipe size and fall matter. Do not reduce sizes below MI.
• Electrical: Follow Part P. Confirm isolation. Test RCD. Many heaters need a dedicated 16–32 A circuit. Check the plate.
• Combustion air and clearances: Keep minimum clearances. A common rule is 300 mm under opening windows for terminals, but always follow MI and the site layout.

If any of this looks marginal, stop and redesign. Swapping a water heater isn’t worth a safety shortcut.

Commissioning, Testing, and Handover

Commissioning is where many callbacks start. Take 10 extra minutes.

• Fill and purge air. Listen for kettling. Re‑bleed as needed.
• Set temperature: 60 °C protects against legionella and scalding risk when used with a mixing valve.
• Check pressures: Cold mains 1.0–1.5 bar typical; PRV often 3.0 bar on unvented. Match expansion vessel pre‑charge to MI.
• Flow rate: Log litres per minute at a hot tap. Note 10–12 L/min for small homes, 14–16 L/min for larger combis.
• Safety devices: Lift TPRV test lever. Confirm tundish discharge and stop. Test CO alarm.
• Paperwork: Benchmark log, Gas Safe certificate, electrical test note, and user instructions.

Walk the client through the basics in 3–5 minutes. Show isolation points. Set expectations: first heat can take 30–60 minutes.

Pricing, Proposals, and Paperwork

Clear paperwork reduces back‑and‑forth by half. When you’re swapping a water heater, spell out what’s included: make/model, valves, discharge pipework, flue parts, disposal, and any electrical or Gas Safe paperwork. State exclusions and day rates for extras.

• Site survey: 20–30 minutes with photos and notes.
• Fixed price for like‑for‑like. Variations only for hidden issues (e.g., undersized gas line).
• Payment terms: deposit on booking; balance on completion.

Capture details fast using tools like Donizo. You can speak notes, add photos, and generate a clean proposal in minutes. Clients sign with e‑signature, and you convert to an invoice in one click. If you’re improving admin, see our content on “invoice templates” and “project timelines”.

Common Mistakes to Avoid

• Rushing the discharge pipework. G3 is strict for a reason. Follow MI.
• Reusing old, crusty flexis. Fit new rated hoses and washers.
• Skipping isolation valves. Add them. You’ll thank yourself next time.
• Ignoring scale. In hard areas, add a scale reducer. Saves elements.
• Guessing gas sizes. Don’t. Confirm pipe sizing or redesign.
• Not logging readings. Record at least 5 numbers: pressures, flows, temperatures, gas tightness, analyser.

When you’re swapping a water heater under time pressure, slow down on steps 9–12. Those steps prevent 90% of leaks and callbacks.

FAQ

How long does swapping a water heater take?

Most like‑for‑like swaps take 3–6 hours. A simple wall unit can be 3–4 hours. An unvented cylinder with discharge work often takes a full day. Add time if you’re moving location, rerouting flues, or upgrading electrics.

Do I need special certification to do the swap?

Yes for certain types. Gas replacements need Gas Safe. Unvented cylinders need G3 competence. Electrical wiring may require Part P compliance. If you’re not certified, bring in the right person. Don’t risk it.

Can I reuse the old flue or pipework?

Only if it meets the new manufacturer’s instructions and is in good condition. Many times flue parts must be replaced. Replace worn flexis, valves, and washers. Keep pipe sizes the same or better.

Should I fit a mixing valve on the hot outlet?

It’s good practice. Set the cylinder to 60 °C to control bacteria, then use a thermostatic mixing valve to deliver safer temperatures at taps. This reduces scald risk, especially for families.

What do I do with the old heater?

Follow local waste rules. Many recyclers take steel cylinders and copper coils. Drain fully, remove hazardous parts, and provide a waste transfer note when required. Build disposal into your price.

Conclusion

Swapping a water heater is simple work done carefully. Plan the job, follow MI and UK rules, record readings, and hand over cleanly. Next steps: 1) Build a standard checklist for surveys and commissioning, 2) Create a clear proposal template for like‑for‑like swaps, 3) Track your readings on every job. Platforms such as Donizo help you capture details, send proposals with e‑signatures, and invoice fast. By tightening your process, you cut callbacks, protect margins, and finish on time more often. Stay safe, stay tidy, and keep it professional.