No Hot Water but Heating Works: Diverter Valve Diagnosis

Summary

"Heating's fine but there's no hot water" is one of the most diagnostically useful complaints a customer can give you, because it immediately rules out the components shared by both circuits. If the boiler fires, the pump runs, the gas is on and the radiators get hot, then the fault must lie in the part of the system dedicated to making hot water specifically. That narrows the search dramatically — and on a combi boiler it points first at the diverter valve.

A combi boiler has no stored hot water. On demand, it diverts the heated primary water away from the radiators and through a plate heat exchanger to instantly heat the mains water going to the tap. The diverter valve is the mechanical switch that makes this happen. Its default resting position is usually the heating side; it should move to the DHW side the instant you open a hot tap. When it sticks, jams with debris, or its actuator/motor fails, it stays on heating — so the heating still works perfectly while hot water disappears. This is by far the most common cause of the symptom on a combi.

But the diverter is not the only candidate, and the right diagnosis depends entirely on the type of boiler. On a combi, also suspect the plate heat exchanger (scaled up in hard-water areas) and the DHW flow sensor/turbine that tells the boiler a tap has opened. On a system or regular (heat-only) boiler with a stored cylinder, the same "heating works, no hot water" pattern usually means a problem with the cylinder's motorised valve (a stuck mid-position or 2-port zone valve), the cylinder thermostat, or the wiring/programmer for the hot-water circuit. This article gives a boiler-type-aware decision tree, the tests that confirm a diverter fault, and the realistic fixes.

Key Facts

• Combi diverter valve — switches primary flow between heating and DHW. Default position is typically heating; stuck-on-heating = no hot water but heating fine.
• How a combi makes hot water — opening a hot tap is sensed by a flow sensor/turbine; the boiler fires, the diverter sends primary water through the plate heat exchanger to heat the incoming mains water.
• Plate (DHW) heat exchanger — scales up in hard-water areas; symptoms are weak/lukewarm hot water, fluctuating temperature, or whistling/kettling on demand.
• DHW flow sensor/turbine — if it fails to detect the tap opening, the boiler never fires for hot water even though everything else works.
• Minimum DHW flow — a combi typically needs around 2.5–3 litres/minute flow to trigger hot water; below that it won't fire for DHW. Low mains flow can mimic a boiler fault.
• System/regular boiler with cylinder — "heating works, no hot water" usually = motorised valve (2-port or mid-position/3-port) stuck on heating, cylinder thermostat failed, or programmer/wiring fault on the HW channel.
• Mid-position (3-port) valve — a single valve serving both CH and HW; if its actuator sticks, it can favour one circuit. A classic cause of HW failure with CH working (and vice versa).
• Cylinder thermostat — if it fails open or is set too low, the boiler never heats the cylinder even though heating runs normally.
• Programmer / time clock — separate HW and CH schedules; HW simply switched off or mis-programmed is a frequent "non-fault".
• Gas Safe registration is legally required for any work on the gas boiler (Gas Safety (Installation and Use) Regulations 1998).
• Diverter cure — sometimes a stuck valve frees with cleaning; more often the diverter cartridge or actuator is replaced. Hard-water/sludge accelerates failure, so cleanse and inhibit per BS 7593.

Quick Reference Table

Detailed Guidance

Step one: identify the boiler type

The diagnosis forks completely on boiler type, so establish it first. A combi has no cylinder and heats water on demand — diverter and plate exchanger territory. A system boiler works with an unvented or vented cylinder and a sealed system. A regular (heat-only) boiler uses a cylinder plus a feed-and-expansion tank in the loft. For system and regular boilers, the "no hot water, heating fine" fault lives around the cylinder and its valves, not a diverter. (See hot water systems for distinguishing the three.)

Decision tree: no hot water but heating works

NO HOT WATER, HEATING WORKS
│
├─ What BOILER TYPE?
│
├─ COMBI (no cylinder)
│   │
│   ├─ Open a hot tap. Does the boiler FIRE / display HW demand?
│   │     │
│   │     ├─ NO (no fire on hot demand)
│   │     │     ├─ Check HW FLOW at tap ≥ ~2.5–3 l/min?
│   │     │     │     ├─ Flow too low → mains/flow restriction (see low pressure)
│   │     │     │     └─ Flow OK → DHW FLOW SENSOR/TURBINE fault → replace
│   │     │
│   │     └─ YES (boiler fires) but water stays COLD
│   │           ├─ Does the DIVERTER ACTUATOR move when tap opens?
│   │           │     ├─ NO movement → DIVERTER VALVE STUCK on heating
│   │           │     │       → Free/clean or replace diverter cartridge/actuator
│   │           │     └─ YES it moves, but DHW out of plate exch is cool
│   │           │           → SCALED/BLOCKED PLATE HEAT EXCHANGER
│   │           │               → Descale or replace; fit scale reducer
│   │           └─ Water LUKEWARM/fluctuating → plate exchanger scaling
│   │
│   └─ Cleanse & inhibit system (BS 7593) — sludge jams diverters
│
└─ SYSTEM / REGULAR (stored cylinder)
    │
    ├─ Is the PROGRAMMER calling for HOT WATER (HW on/boost)?
    │     └─ NO → switch HW on / fix schedule  → FIXED?
    │
    ├─ With HW calling, is the MOTORISED VALVE moving to HW?
    │     ├─ Actuator dead / stuck on CH only
    │     │       → 2-PORT or MID-POSITION VALVE fault
    │     │       → Test actuator/microswitch; manual lever test; replace head/valve
    │     └─ Valve opens to HW but cylinder stays cold → continue
    │
    ├─ CYLINDER THERMOSTAT calling and within spec?
    │     ├─ Stat failed / set too low / no continuity → replace/adjust
    │     └─ Stat OK → check WIRING / call-for-heat to boiler on HW circuit
    │
    └─ Check programmer/wiring centre HW channel and terminals

Confirming a combi diverter fault

The diverter is the headline suspect, but confirm rather than assume — the cartridge is a chargeable part. With a hot tap running:

• Listen and feel for the actuator moving when the tap opens. No movement on demand strongly implicates a stuck valve or dead actuator.
• Compare pipe temperatures at the diverter/heat exchanger. If primary flow is still heading to the heating circuit and not through the DHW plate exchanger when hot water is demanded, the valve is favouring heating.
• Check the boiler doesn't error. Some boilers flag a diverter or flow fault explicitly — cross-reference the model's fault codes.

If the valve is jammed by sludge or scale, cleaning can free it, but a worn cartridge or failed actuator needs replacing. Because sludge and limescale are what jam diverters in the first place, the durable fix includes cleansing and inhibiting the system to BS 7593 and, in hard-water areas, fitting a scale reducer.

System and regular boilers: it's the cylinder side

With a stored cylinder, the equivalent of the diverter is the motorised valve. A 2-port zone valve on the HW circuit that fails to open, or a mid-position (3-port) valve whose actuator sticks favouring heating, gives exactly this symptom. The other two common culprits are a cylinder thermostat that has failed or is set too low (so it never calls the boiler to heat the cylinder) and the programmer/wiring simply having the hot water channel off or faulty. Always check the simple things — is HW actually switched on at the programmer? — before condemning a valve.

Don't overlook the cheap causes

Before quoting a diverter or valve, rule out:

• Programmer set wrong — hot water schedule off, or an "all-day/once/twice" setting the customer didn't expect.
• Low mains flow — a combi that won't fire for hot water may simply not be getting the minimum flow to trigger; check the flow rate at the tap (see low pressure for boiler-side pressure and the mains-pressure article for incoming flow).
• Thermostatic mixing valve / scald-protection valve at the outlet stuck or maladjusted, limiting delivered temperature.

Frequently Asked Questions

How do I know if it's the diverter valve or the heat exchanger?

Both are combi faults but behave differently. A stuck diverter gives no hot water at all (or hot water only when the heating is off) because the primary flow never reaches the DHW exchanger — and the diverter actuator won't move when you open a hot tap. A scaled plate heat exchanger still passes some heat, so you get weak, lukewarm or fluctuating hot water, often with whistling/kettling on demand. No hot water at all points to the diverter; poor-quality hot water points to the exchanger.

Can I fix a stuck diverter valve myself?

No — this is gas boiler work and must be carried out by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998. Beyond the legal requirement, freeing or replacing the cartridge involves draining and re-pressurising the sealed system and working inside the appliance. A registered engineer can also identify whether the underlying cause is system sludge, which needs cleansing to prevent the new part jamming too.

My boiler is a system boiler with a cylinder — is it still the diverter?

Almost certainly not. System and regular boilers don't have a combi-style diverter. The equivalent fault is a motorised valve (a 2-port zone valve or a mid-position 3-port valve) that won't open to the hot-water circuit, a failed cylinder thermostat, or the programmer having hot water switched off. The diagnosis lives around the cylinder, not inside the boiler.

Why does sludge cause hot-water failure?

Magnetite sludge and limescale circulate through the boiler and settle in narrow passages — including the diverter valve seat and the plate heat exchanger's fine waterways. A diverter cartridge that should glide between positions jams on grit; an exchanger's channels block. That's why the lasting cure is not just the part swap but a system cleanse and inhibitor dose to BS 7593, plus a magnetic filter and (in hard-water areas) a scale reducer.

Is no hot water from a combi an emergency?

It's an inconvenience rather than a safety emergency, provided the boiler is otherwise operating safely and not locking out on a fault that suggests combustion problems. If the boiler is showing fault/error codes, smells of gas, or the case is hot, treat it as urgent and call a Gas Safe engineer. Loss of hot water alone, with heating still working, is a routine repair.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — only a Gas Safe registered engineer may work on a gas boiler's gas circuit or internal components.

• BS 7593:2019 — central heating system cleaning, inhibitor dosing and filtration; addresses the sludge/scale that jams diverters and exchangers.

• Building Regulations Approved Document G (Sanitation, hot water safety and water efficiency) — hot water temperature/scald safety, relevant to mixing valves and cylinders.

• Building Regulations Approved Document L — heating system efficiency and controls.

• BS EN 12828 — design of water-based heating systems (system/valve context).

• Boiler manufacturer technical literature — definitive diverter, exchanger and flow-sensor fault diagnosis and part numbers for the specific model.

• Gas Safe Register — legal requirement for gas boiler repairs

• HSE — Gas Safety (Installation and Use) Regulations 1998 — statutory framework

• BSI — BS 7593:2019 — system cleaning and inhibitor code of practice

• GOV.UK — Approved Document G — hot water safety requirements

• no hot water — full no-hot-water diagnostic with separate combi, system and regular boiler paths

• hot water systems — distinguishing combi, system and regular boilers (which determines the diagnosis)

• powerflush — cleansing the sludge that jams diverter valves and blocks heat exchangers

• low pressure — boiler pressure/filling-loop checks when the system won't deliver