Rehan owns a portfolio of five 2-bed mid-terraces around the BB9 postcode of Nelson. Every year in July he rolls all five CP12 gas safety certificates into one week, and every year we fit or repair one or two of the older boilers at the same time. This case study covers the July 2026 batch — specifically the property on Every Street where the old Baxi Duo Tec combi finally gave up.
The property
Late-Victorian stone-fronted 2-bed mid-terrace in BB9. Original boiler was a 2011 Baxi Duo Tec combi in the kitchen. Six radiators, one small bathroom, on-mains pressure around 2.5 bar. Water hardness in BB9 is hard — around 270 ppm — which is the local killer for heat exchangers.
The CP12 fail
On the annual landlord gas safety check, the Baxi failed at the flue gas analysis stage: CO/CO₂ ratio out of spec, likely a partial heat-exchanger blockage from scale. Combustion was on the edge of dangerous. We issued the CP12 as an “At Risk” with recommendation to replace, capped the gas at the meter, and gave Rehan two options: £480 repair (heat-exchanger flush + new fan) with no warranty on parts, or a full replacement.
The fixed-price quote
Rehan went with replacement. We specified a Baxi 400 Combi 30 kW — a landlord-favourite in Lancashire for its 7-year warranty, generous cylinder-of-parts kit and manufacturer's next-day breakdown response. Total fixed price: £1,780, including the boiler, MagnaClean Professional 2, new Baxi wired programmer, chemical flush of the whole system, filling loop, and the fresh CP12 issued on completion.
The install
- Day 1 (morning): Isolated services. Removed old Baxi Duo Tec. Descaled the plate heat exchanger side of the return pipe with a magnetically-flushed cartridge.
- Day 1 (afternoon): Fitted the new Baxi 400. Wired the new programmer. Fitted MagnaClean on the return.
- Day 2 (morning): Full chemical hot flush of the system using Fernox F3. Overnight soak on the return, drained, refilled, dosed with F1. Commissioned to Baxi spec.
- Day 2 (afternoon): Fresh CP12 issued to Rehan with a clean bill of health, tenant walk-through, warranty registered.
Total spend, all in
£1,780 for the install + £75 for the CP12 = £1,855. A 7-year Baxi manufacturer's warranty and 12 months of workmanship warranty from us. See our landlord CP12 guide for what a portfolio landlord should be paying across Lancashire.
Why hard water in BB9 matters
Nelson, Colne, Brierfield and the top end of Burnley (Pendle water catchment) sit in one of the harder-water belts in the North West — 260-290 ppm typical. Scale kills heat exchangers on cheap combis in 8-10 years. A MagnaClean plus a cartridge scale inhibitor on the cold main is essential to hit the full manufacturer warranty. Every install we do in BB8 / BB9 / BB10 / BB12 has both fitted as standard — read our hard water guide for the science.
Landlord in Nelson, Colne or Brierfield? Book a batch CP12 + fixed-quote install via the instant boiler quote, or see our Nelson boiler installer page.
