Practical, code-aware guide to swapping a water heater. Steps, tools, timeframes, safety checks, and pricing tips for UK contractors. Includes FAQs and checklists.
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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.
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.
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).
If you’re also improving your professional proposals, see our guide on “professional proposals” and “pricing strategies” to tighten your numbers.
Most jobs need the basics and a few lifesavers. When you’re swapping a water heater, have this ready:
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.
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.
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.
Swapping a water heater safely matters more than speed.
If any of this looks marginal, stop and redesign. Swapping a water heater isn’t worth a safety shortcut.
Commissioning is where many callbacks start. Take 10 extra minutes.
Walk the client through the basics in 3–5 minutes. Show isolation points. Set expectations: first heat can take 30–60 minutes.
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.
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”.
When you’re swapping a water heater under time pressure, slow down on steps 9–12. Those steps prevent 90% of leaks and callbacks.
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.
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.
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.
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.
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.
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.
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