You've signed the quote, picked the boiler, the date's in the diary — now what actually happens when our van pulls up outside your house?
This is the hour-by-hour breakdown for a standard same-position combi swap in Lancashire — the most common job we do, and the one most customers book. Bigger jobs (system conversions, unvented cylinders, boiler relocations) run longer, but the structure is the same.
Before install day — what we do
- 48 hours before: we order your boiler, MagnaClean filter, flue extensions, chemicals and any consumables direct to our depot. Nothing is delivered to your house — keeps your driveway clear.
- 24 hours before: a quick text to confirm arrival window (usually 8:00–8:30am) and the engineer's name.
- The night before: we recommend you turn the heating off about an hour before bed so the system is cool when we arrive. Saves us 20 minutes of waiting in the morning.
8:00–8:30am — Arrival & isolation
The engineer (usually Shamshul or Alhamza) arrives in a sign-written Gas Tech van. First job: laying down dust sheets from the front door through to the boiler location. We don't walk muck through your hallway and we don't leave any behind.
Then we:
- Isolate gas at the meter and electric at the consumer unit (label-tagged so other circuits stay on)
- Isolate water at the stopcock
- Drain the heating circuit into a waste hose run to the outside drain — no buckets in the kitchen
- Drain the boiler itself
That's usually 30–45 minutes. By 9:15am the old boiler is off the wall.
9:30–11:30am — Pipework & flue
For a same-position swap, most existing pipework is reusable but we always:
- Renew gas, flow, return, cold-mains and DHW tails in 22mm copper (the new Worcester/Baxi/Vaillant boilers all need 22mm to perform)
- Fit isolation valves on every tail so the next engineer (us, hopefully) doesn't have to drain the system again to do a service
- Re-route or replace the flue if the manufacturer's clearance rules have changed
- Fit a MagnaClean Professional 2 magnetic filter on the return — required for Worcester Bosch 12-year warranty (and a good idea anyway)
This is the noisiest hour of the job — pipe cutting, fittings being made up, the occasional drill. By 11:30am the new boiler is on the wall and tight to the bracket.
11:30am–1:00pm — Hang, flue, connect, gas-tighten
The boiler goes onto its bracket. Flue is fitted and sealed. Condensate pipe is run (44mm internal, externally lagged where it leaves the property — see our frozen condensate guide for why this matters).
Then we pressure-test the gas, water and heating circuits separately. Any leak at this stage gets fixed before we even think about lighting it.
1:00–1:30pm — Lunch (we sort our own)
We don't expect tea, biscuits or sandwiches — we bring our own and usually eat in the van. A glass of cold water is always welcome though.
1:30–3:30pm — Flush & commission
This is where corner-cutting installers save 2 hours and you pay for it 6 months later. Every Gas Tech install includes:
- A chemical hot flush on the existing radiators (or a full powerflush if specified in the quote)
- Filling and venting every radiator individually
- Adding Sentinel X100 system inhibitor — keeps the heat exchanger free of sludge and is required for warranty
- Adding Sentinel X400 cleaner if a chemical flush was specified
- Commissioning the boiler per manufacturer instructions — gas rate, flue analysis, CO/CO₂ ratio, flow temp setpoint
- Balancing radiators by flow-temperature differential (10–12°C across each rad)
The flue gas analyser readings get logged on the Benchmark commissioning checklist that lives inside the boiler casing.
3:30–4:30pm — Handover & paperwork
The boiler is hot, the radiators are hot, the shower is mains-pressure. Now we sit down at the kitchen table and:
- Walk you through the controls (boiler display, thermostat, smart-thermostat app if fitted)
- Show you how to repressurise the system from the filling loop
- Hand over the Benchmark commissioning certificate, the Gas Safety Certificate (CP12), and the Building Compliance Certificate — all stored inside the boiler casing
- Register the manufacturer warranty for you online before we leave (up to 12 years on Worcester Bosch Greenstar 8000)
- Email digital copies of everything to you the same evening
By around 4:30–5:00pm we're packing the van, dust sheets are folded and lifted, your old boiler is on its way to a metal recycler, and your driveway is exactly as we found it.
What if it's a bigger job?
- System → combi conversion: typically 1.5–2 days. Day 1 is removing the cylinder, cold tank, F&E tank and re-piping. Day 2 is fitting and commissioning the combi.
- System boiler + new unvented cylinder (Megaflo / Telford): usually 2–3 days. G3-qualified install includes T&P valve, expansion vessel, tundish, and an annual servicing label.
- Boiler relocation: add half a day to a day for the new pipe runs and flue route.
Common questions on the day
“Can I work from home?” Yes — water's off for about 2 hours, electric is only off briefly for the boiler circuit. Most customers carry on working in another room.
“Will the house get cold?” In summer, no. In winter, an electric heater or two helps — we'll bring one if you let us know in advance.
“Do I need to be in?” Yes for the arrival and the handover. The middle 4 hours you can pop out — we'll text if anything comes up.
“When does payment happen?” Final invoice goes via email the same evening. Payment by bank transfer or card on completion — never beforehand for the full amount.
Ready to book a fixed-price install?
If you're in Burnley, Blackburn, Preston, Nelson, Colne, Accrington or anywhere in Lancashire and you want a properly run install day instead of a corner-cut one, call 01282 914 044 or book a free survey. Fixed-price quote within 24 hours, install usually within the week.