Radiators are toasty, but you turn the shower on and it's freezing? It's one of the most frustrating combi-boiler faults — and one we see multiple times a week across Lancashire. The good news: it's almost always one of 5 specific faults, and all of them are repairable.
Here's exactly what's likely wrong, in order of how often we see it, plus rough costs for 2025 Lancashire repairs.
1. Stuck diverter valve (60% of cases)
Inside every combi boiler is a 3-port motorised valve called the diverter valve. When you open a hot tap, it physically redirects the boiler's output from the radiators to the hot water side. If its motor sticks, or the rubber diaphragm seizes, hot water stops flowing — but heating works fine.
Telltale signs:
- Heating works perfectly, but no hot water at any tap
- Sometimes you can get a few seconds of hot water then it goes cold
- Boiler fires up when you open a tap, but the heat doesn't reach the water
Fix cost: £140–£220. Most diverter valves can be reconditioned (cartridge swap) — sometimes a full valve replacement is needed on older models.
2. Faulty DHW thermistor / NTC sensor (20% of cases)
The DHW (Domestic Hot Water) thermistor measures hot-water flow temperature. If it fails or sends bad readings, the boiler may refuse to heat the water even though the valve is OK.
Fault codes that point here:
- Worcester:
C7-038,E9 - Baxi:
E133,E168 - Vaillant:
F.75,F.13 - Ideal:
F2
Fix cost: £95–£150 (the sensor itself is cheap, it's the diagnosis time).
3. Low DHW flow rate (10% of cases)
Combi boilers need a minimum incoming water flow to fire up the DHW side. If your incoming mains has weakened (common in older Lancashire terraces in BB10/BB11/BB12), or if the inlet filter has clogged with limescale, the boiler simply can't produce hot water — even though heating works because it's a closed loop.
Tests we run:
- Mains flow rate at the kitchen tap (we want at least 12 L/min)
- Inlet filter inspection (we clean these on every service)
- DHW plate heat exchanger blockage test
Fix cost: Filter clean: £85. Plate heat exchanger replacement: £180–£280.
4. Limescale-blocked plate heat exchanger (8% of cases)
Common in hard-water areas (Ribble Valley, Nelson, Colne especially — water hardness ~250ppm). The plate heat exchanger inside the boiler gradually clogs with calcium, restricting hot-water flow.
Signs:
- Hot water comes through but at a much lower flow rate than a year ago
- The water is luke-warm rather than properly hot
- Old boiler (5+ years) in a hard-water area without a scale reducer
Fix cost: Plate descale: £140. Full plate replacement: £220–£280. Adding an inline scale reducer afterward: £85–£120.
5. Air-locked DHW circuit (rare, 2%)
After a system drain-down or repair, air can become trapped in the DHW circuit. Symptoms vanish after a few minutes of running the hot tap full-blast.
Fix: Free-ish — usually just a 10-minute job to bleed the system properly.
What to do right now
- Open the kitchen hot tap on full. Listen — does the boiler fire up?
- Look at the boiler display. Any fault code? Note it down.
- Check the pressure. Below 1 bar? Re-pressurise first (sometimes that's the only problem).
- Try every hot tap. All taps cold = boiler issue. One tap cold = local plumbing/tap fault.
- Call us. Most no-hot-water faults need a Gas Safe engineer's diagnostic — DIY repairs on gas appliances are illegal in the UK without Gas Safe registration.
How much does it cost to fix in Lancashire?
Most no-hot-water faults cost between £85 (diagnostic) and £280 (full plate heat exchanger) in 2025 prices. Our fixed-price callout includes a full diagnostic and you only pay for parts/labour you agree to before we proceed.
Same-day callouts across Lancashire
If you've got no hot water and we can be there same-day from our Burnley base, we cover: Burnley, Padiham, Nelson, Colne, Brierfield, Clitheroe, Accrington, Blackburn, Preston, Bury, Rochdale, Rawtenstall, Rossendale, Bacup, Todmorden.
Call 01282 914 044 or use our 24/7 emergency line 07475 978 821.