Boilers 5 min read2 February 2026
If radiators get hot but the taps run cold, the boiler is firing — the problem is in how it routes heat to the domestic hot water side. Three parts do that job, and they fail in distinct ways.
Diverter valve failure
The diverter valve directs hot water either to the radiators or to the taps. When the cartridge sticks in the heating position, you get exactly this symptom. It is the single most common cause and, on most makes, a same-visit repair.
Blocked plate heat exchanger
In hard-water pockets and on sludged systems, the plate heat exchanger scales up. Hot water then runs lukewarm at first, then not at all. Replacing the exchanger and adding a magnetic filter with proper inhibitor stops it recurring.
Flow sensor or thermistor fault
If the boiler cannot detect that a tap has opened, it simply never fires for hot water. A flow sensor or a failed thermistor is usually an inexpensive part but needs the right diagnostic to confirm rather than swapping components in hope.
Need help with this?
We handle this work every week. See Heating & Boiler Services in Glasgow or browse all our services.
Frequently asked questions
Can I fix a diverter valve myself?
No — it is inside a sealed gas appliance and must be worked on by a Gas Safe registered engineer.
How long does the repair take?
Most diverter and sensor repairs are completed in one visit of one to two hours.
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Power Flushing: When You Actually Need One