Heating System Repair: Diagnosing and Resolving Complex Central Heating Faults in Residential Property

Case Study

Hampstead, London NW3

When a tenant reports hot water but no heating, the fault could lie anywhere in a complex system of valves, wiring, and controls. This case study follows All Services 4U's methodical diagnosis and repair of a central heating system in a Hampstead flat, where incorrect wiring and a faulty zone valve had left the property without heating during winter.

The Reported Issue

The tenant reported that hot water was functioning normally but the heating system would not operate. Additionally, there were concerns about the magnetic sludge filter and leaking valves. The property featured a Megaflo unvented cylinder—a common configuration in London flats where combination boilers are unsuitable and space for traditional header tanks is limited.

This symptom pattern—hot water working but heating failing—typically points to the heating circuit controls rather than the boiler itself. The boiler is clearly capable of firing and heating water; the question is why that heat isn’t reaching the radiators.

Diagnostic Process

Our heating engineer attended site and conducted systematic fault-finding across the heating system components. The investigation revealed two distinct issues working in combination to prevent heating operation.

Issue 1: Incorrect Junction Box Wiring

The junction box controlling the heating system circuits had been wired incorrectly. In an S-Plan or Y-Plan heating system, the junction box coordinates signals between the programmer, room thermostat, cylinder thermostat, and zone valves. Incorrect wiring here can prevent the heating circuit from receiving the signal to operate, even when all other components are functional.

Issue 2: Faulty Zone Valve

The zone valve controlling the heating circuit had failed. Zone valves are motorised valves that open to allow hot water to flow through either the heating circuit or the hot water circuit (or both) depending on demand. When a zone valve fails, it typically fails closed—meaning no flow to the circuit it controls.

The Repair

The engineer removed the junction box, corrected the wiring to ensure proper signal routing, and replaced the faulty zone valve with a new unit. This dual repair addressed both the control signal issue and the mechanical obstruction.

Component	Issue Found	Action Taken
Junction Box	Incorrect wiring	Rewired to correct configuration
Zone Valve	Failed closed	Replaced with new unit
Heating Circuit	No flow	Restored to operation
Hot Water Circuit	Functioning	Verified continued operation

Following these repairs, the engineer verified that the initial issues were resolved and the system was operating correctly.

Gallery

Additional Finding

During testing, the engineer identified a further issue: the filling loop was leaking when opened. The filling loop is used to repressurise the sealed heating system when pressure drops—a routine maintenance task. A leaking filling loop, while not preventing system operation, requires attention to prevent water damage and maintain system pressure.

The client was informed that additional work would be necessary to replace the filling loop component and drain the system accordingly. The client accepted this recommendation and agreed to schedule a follow-up appointment.

Understanding Zone Valve Systems

Zone valves are critical components in heating systems that serve multiple circuits. They allow independent control of heating and hot water, ensuring that calling for hot water doesn’t unnecessarily heat the radiators, and vice versa.

Common zone valve failure modes include:

Failure Mode	Symptom	Cause
Motor failure	Valve won’t open	Electrical fault or wear
Stuck closed	No flow to circuit	Mechanical seizure
Stuck open	Circuit always on	Spring failure
Microswitch failure	Boiler doesn’t fire	No end-switch signal

The failed zone valve in this case had seized closed, preventing any flow to the heating circuit even when the system called for heat.

Compliance Framework

Heating system work operates within established regulations and standards that ensure safety and proper function.

Regulation/Standard	Application
Gas Safety (Installation and Use) Regulations 1998	Work on gas-connected systems
Building Regulations Part L	Energy efficiency requirements
BS 7593	Code of practice for treatment of water in domestic hot water central heating systems
Manufacturer specifications	Component-specific requirements
Unvented hot water storage systems (G3)	Megaflo cylinder compliance

Proactive Heating Maintenance

This case demonstrates how multiple small issues—wiring errors, component wear—can combine to cause system failure. Regular heating system servicing can identify developing problems before they cause complete loss of heating:

Annual boiler service
Zone valve operation checks
System pressure monitoring
Magnetic filter maintenance
Radiator balancing and bleeding

All Services 4U provides comprehensive heating services from emergency repairs through planned maintenance programmes. Our engineers hold the relevant qualifications for work on both gas and unvented hot water systems.

Service Category: Heating System Repair
Location: Hampstead, London NW3
Sector: Residential / Private Rental
Resolution: Zone valve replaced, wiring corrected, follow-up scheduled
Reference: L4L-802323

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