Common causes
- A faulty diverter valve — on a combi boiler, this switches flow between heating and hot water; if it's stuck, one or the other stops working.
- A blocked plate heat exchanger — sludge or scale can restrict flow so badly the boiler can't heat water properly, sometimes triggering it to overheat instead. This was exactly the fault we found and fixed on an Ariston Evo combi in Stourbridge — read the case study.
- A failed immersion heater — on electric and Pulsacoil-type boilers rather than standard gas combis. We diagnosed and replaced a corroded immersion heater on a Gledhill Pulsacoil in Erdington — see that job too.
- A faulty thermistor or PCB fault — less common, and something that needs proper diagnostic equipment to confirm rather than guesswork.
What's safe to check yourself
Open a hot tap and listen: does the boiler actually fire? If it doesn't, check the pressure gauge (below 1 bar can stop it) and try a single reset per your manual. If it fires but the water never gets properly hot, that points at the plate heat exchanger or a sensor rather than anything you can reach — don't remove panels or go any further yourself.
Not just standard gas combis
A lot of general "boiler repair" companies focus almost entirely on mainstream gas combi brands. We also work on electric and Pulsacoil-type systems, which have their own specific failure points — worth mentioning if you've been told elsewhere that your system is too unusual to fix.
Pricing
Diagnosis and first hour: £99. Parts — such as a plate heat exchanger or immersion heater — are quoted before fitting.
What we test when we arrive
Every diagnosis follows the same discipline as our emergency callouts — our advice article on what we check on every callout covers it in full, but in short: the engineer confirms pressure, power and gas supply first, then tests the hot water circuit specifically — whether the boiler fires when a hot tap opens, whether the diverter valve actually moves, what temperatures the thermistors report against reality, and whether flow through the plate heat exchanger is restricted. Where anything gas-side is in question, gas tightness and inlet working pressure are checked, and combustion is verified with a flue gas analyser where relevant.
Likely repairs
The parts that most often end this particular fault are the diverter valve (or just its diaphragm on some models), the plate heat exchanger, a thermistor, or — on electric and Pulsacoil-type systems — the immersion heater. Each is quoted before fitting, and most repairs carry a 12-month parts warranty. The Stourbridge Ariston Evo job and the Wolverhampton Biasi Advance job in our case studies are worked examples of exactly this diagnosis: on the Biasi, the diverter valve came off the van on the first visit and the sludged plate heat exchanger behind it was replaced on the second.
Symptoms not quite matching? Try the Boiler Problem Finder.
Frequently asked questions
Why do I have heating but no hot water?
On a combi boiler that pattern points strongly at the diverter valve — the component that switches flow between the heating and hot water circuits — or at a blocked plate heat exchanger restricting the hot water side. Both need proper diagnosis to confirm, not guesswork.
Why does my hot water go cold after a few seconds?
Water that starts warm and quickly turns lukewarm or cold is a classic sign of a partially blocked plate heat exchanger — scale and sludge restricting flow so the boiler can't sustain the heat transfer. That's exactly the fault we found on an Ariston Evo in Stourbridge.
Do you work on electric and Pulsacoil-type systems?
Yes — including failed immersion heaters, which many general boiler repair firms won't touch. We diagnosed and replaced a corroded immersion on a Gledhill Pulsacoil in Erdington; the case study is on this site.
How much will the repair cost?
Diagnosis is £99 including the first hour. If a part such as a diverter valve, plate heat exchanger or immersion heater is needed, it's quoted before fitting, and most repairs carry a 12-month parts warranty.