Radiators are warming up normally but every hot tap runs cold? The boiler is firing, the pump is moving water, and yet the shower will not run hot. That very specific pattern points to one of two faults inside the boiler, and we diagnose both routinely.
In a combi boiler, hot water priority is handled by a diverter valve. When you open a tap, the boiler should sense flow, fire the burner, and rotate the diverter to send primary water through the DHW plate heat exchanger. If the diverter motor sticks in the central-heating position, you get heating but no hot water. In an older system boiler or open-vented setup with a separate cylinder, the same symptom can be caused by a failed DHW microswitch on a Honeywell V4073A mid-position valve, a seized motor on a Drayton or Sundial 3-way valve, or a wiring centre fault that never sends the call-for-hot-water signal to the actuator. Either way, the boiler is healthy and the fix lies in the controls or the valve assembly. For combi-specific causes such as a scaled plate heat exchanger or a failed flow switch, see our guide to no hot water from a combi boiler.
Our engineers stock OEM diverter valve assemblies for Worcester Greenstar, Vaillant ecoTEC, Ideal Logic, Baxi DuoTec, and Viessmann Vitodens, along with Honeywell V4073A, Drayton ZA6, and Sundial mid-position and 3-way valve motors and powerheads. We arrive with the right diagnostic kit, multimeter, manometer, and live PCB-data interface where supported, and quote in writing before any parts are fitted.
Where this service applies
- Combi boiler delivering full radiator heat but no hot water at any tap
- System boiler with a hot-water cylinder that has gone cold while the radiators stay warm
- Honeywell V4073A mid-position valve clicking but not switching (failed DHW microswitch)
- Drayton or Sundial 3-way valve powerhead with no audible motor when a tap is opened
- Programmer set to hot water but no demand reaching the wiring centre
- Cylinder thermostat reading correctly but never triggering the valve actuator
- Hot water that worked yesterday and is dead today with no error code displayed
- Combi with a stuck-shut diverter, often paired with overheating radiators (always-on DHW route)
How we deliver
No surprises, no upselling. The exact path every job follows.
- 1
System type identification
Combi, system boiler with cylinder, or open-vented gravity? Each has a different DHW path, and the fix differs accordingly. - 2
Programmer and controls check
Confirm hot water is calling at the programmer, the cylinder stat is wired correctly, and the wiring centre is passing the signal to the valve. - 3
Live valve operation test
For mid-position and 3-way valves, we manually trigger the actuator and verify both microswitches with a multimeter under load. - 4
Combi diverter assessment
On a combi, we listen for the diverter motor stroking on DHW demand and feel for the temperature change on the secondary heat exchanger flow pipe. - 5
Written quote
Customer is given a fixed-price written quote covering parts and labour before any chargeable work begins. - 6
Repair or replacement
Powerhead, microswitch pack, or full valve body replaced as required. Combi diverter assemblies fitted with new gaskets and O-rings. - 7
Refill, inhibitor, and recommission
System refilled, vented, and dosed with fresh Sentinel X100. Pressure set to the boiler's cold-fill specification. - 8
Hot water and heating verification
Cylinder reheat or DHW tap-side temperature confirmed. FGA recorded on the boiler. Written hand-over emailed to the customer.