No Heating But Hot Water Works: Motorised Valve, Thermostat & Programmer Diagnosis

Summary

This fault pattern — hot water fine, heating dead — is one of the most reliably diagnostic in the central heating world. Because both functions share the same boiler and pump, the fault must lie in whatever controls or directs flow specifically to the heating circuit. The most common system configurations in UK domestic properties are Y-plan (one mid-position valve, also called a 3-port valve) and S-plan (two 2-port motorised valves, one for heating, one for hot water). The diagnostic approach differs depending on which system is installed.

A mid-position valve in a Y-plan system is the single most common cause of this fault. The valve uses a three-position actuator (off, hot water only, heating + hot water) and is operated by wiring from the programmer and room thermostat. When the actuator fails internally — usually the motor, the microswitch, or the capacitor — the valve can get stuck in the HW-only position, giving exactly the symptom described.

Don't overlook the simple controls first. A room thermostat set too low (below room temperature), a programmer that has lost power or had its heating programme deleted, or a wireless thermostat with a flat battery are extremely common and take under a minute to check. Always eliminate these before opening the valve head.

Key Facts

• Y-plan system — one 3-port mid-position valve (Honeywell V4073, Drayton MA1 or similar); controls both heating and HW from a single body
• S-plan system — two separate 2-port motorised valves; one for heating zone, one for HW cylinder; fault is almost always the heating valve
• Mid-position valve actuator — contains a synchronous motor, a microswitch, and a capacitor; any of the three can fail
• Common valve fault — actuator jammed in HW-only position; can often be freed by manual lever on the valve body (test only — not a fix)
• Room thermostat call — if the room thermostat is above the set point (room is already warm enough), the heating will not run; check the setpoint before diagnosing a fault
• Programmer/timer — if the heating is in a timed 'off' period, hot water can still run if the HW programme is active
• Wireless thermostats — flat batteries in the wireless receiver or controller are an extremely common cause; replace AA/AAA cells first
• Wiring fault — if valve has power (motor hums but doesn't move) or has no power at all, check the wiring connections at the programmer, cylinder stat, and junction box
• Boiler Plus (2018) — requires all new boiler installations to have TRVs on all radiators (except where a room thermostat is located), plus a time and temperature control
• TRV override — if all radiator TRVs are closed (set too low), the system may appear not to be heating even if the valve is working correctly; check TRV positions
• Pump operation — the pump runs for both heating and HW in most systems; if the pump were failed, both circuits would be affected — not just heating
• Cylinder thermostat — if the cylinder stat is set too low and the HW is already at temperature, an S-plan system will close the HW valve and demand from heating next; this is normal
• Fan coil units or UFH — additional zone valves may be installed; check all zone valve positions if the system has multiple heating zones

Quick Reference Table

Detailed Guidance

Step 1: Eliminate the Controls First

NO HEATING — HOT WATER WORKS

Step 1: Check the programmer/timer
├── Is heating in a programmed ON period?
│   ├── NO  → Reprogram or advance the time. Test.
│   └── YES → Continue to Step 2
│
Step 2: Check the room thermostat
├── Turn setpoint to 30°C (maximum)
│   ├── Does heating come on within 2 minutes?
│   │   ├── YES → Setpoint was too low; explain to customer
│   │   └── NO  → Continue to Step 3
│   │
Step 3: Wireless thermostat?
├── Replace batteries in handset AND receiver
│   ├── Does heating come on?
│   │   ├── YES → Battery was flat
│   │   └── NO  → Continue to Step 4
│
Step 4: Check TRV positions
├── Are all TRVs set to max (5 or *)?
│   ├── NO  → Open all TRVs; check if heating runs
│   └── YES → Continue to motorised valve diagnosis

Step 2: Identify the System Type

Look at the airing cupboard or plant room. A Y-plan has one valve body with two pipe connections and three wires. An S-plan has two valve bodies, each with two pipe connections and two or three wires. Trace the flow pipes to identify which valve serves heating.

MOTORISED VALVE DIAGNOSIS

Y-plan (mid-position 3-port valve):
├── Locate valve — usually on return to boiler in airing cupboard
├── Find the manual lever/override (usually a coloured tab)
├── Manually push lever to 'heating' position
│   ├── Does heating now run?
│   │   ├── YES → Actuator is faulty; replace actuator head
│   │   │         (motor, microswitch, or full head replacement)
│   │   └── NO  → Fault is upstream (programmer, stat, wiring)
│   │              OR downstream (pump, boiler)
│
S-plan (2-port valves):
├── Identify the HEATING valve (usually labelled CH)
├── Check indicator — is the light showing open?
│   ├── NO LIGHT → Valve not being called; check wiring/controls
│   └── LIGHT ON → Valve open but no flow; check pump
├── Manual override — open valve manually
│   ├── Heating runs → Actuator fault or wiring to actuator
│   └── No change   → Pump or boiler fault

Y-Plan Mid-Position Valve — Detailed Diagnosis

The mid-position valve (3-port) moves between three positions controlled by voltage on specific terminals:

• Orange wire — live feed from programmer heating demand
• White wire — live feed from cylinder thermostat (HW demand)
• Grey wire — mid-position microswitch output (boiler/pump live)

The most common failure mode is the capacitor inside the actuator head failing, preventing the motor from completing its travel to the mid (heating) position. The valve will stay in HW-only position even when a heating demand is present.

Test by removing the actuator head (usually a plastic cap that unclips) — this reveals the manual lever. In heating mode (testing), push the lever manually. If the heating circuit responds, the actuator is confirmed faulty. Order a direct replacement (Honeywell or Drayton make generic replacement heads that fit standard bodies).

Replace the actuator head only unless the valve body itself is corroded or mechanically stuck. Bodies rarely fail.

S-Plan 2-Port Valve — Detailed Diagnosis

In an S-plan system, each valve has an indicator LED and a manual lever. If the heating valve's LED is off when a heating demand is being made, trace back through the wiring to identify where the signal is lost:

1. Check the programmer output terminal for the heating call (should be live 230V in timed ON period)
2. Check the wire between the programmer and the room thermostat
3. Check the wire from the room thermostat output to the heating valve actuator
4. Check the valve actuator terminals directly

If the LED is on (valve open) but no heating is occurring, the pump may have failed, or the boiler is not firing. Since HW is working, the boiler is firing and the pump is running — so double-check that the heating valve is actually physically open (listen for the valve to operate when it should).

Programmer and Wiring Faults

After 10–15 years, programmer terminals corrode. Remove the programmer front and check the wiring connections — the heating channel output terminal is the most common failure point. Some older Horstmann and Randall programmers develop relay failures internally; replacement is the repair (programmers are consumables, not service items).

Smart thermostats (Nest, Hive, Tado) have their own diagnostic apps. If a Hive shows 'Heating On' but heating is not running, the fault is in the wiring between the Hive receiver box and the motorised valve or boiler.

Frequently Asked Questions

How do I know if I have a Y-plan or S-plan system?

Count the motorised valves in the airing cupboard. One valve = Y-plan. Two valves = S-plan. If there are two valves, one will be labelled or positioned on the hot water cylinder flow; the other serves heating. Some systems have zone valves for additional rooms or underfloor heating — these will be additional to the main S-plan pair.

The manual override on the valve works but the actuator doesn't — do I need to replace the whole valve?

Usually only the actuator (head) needs replacing, not the valve body. Actuator heads clip or screw onto the valve body. Make sure you match the brand and series — Honeywell heads fit Honeywell bodies, Drayton heads fit Drayton bodies. Generic/compatible heads are available from merchants.

The room thermostat is calling for heat but the boiler isn't firing — is the problem still the valve?

If the boiler isn't firing at all, check whether the boiler has power and no lockout codes. If the boiler fires for HW but not heating, the problem is still in the heating control chain (valve or wiring). If the boiler won't fire for either, the fault is in the boiler or its immediate controls.

After replacing the actuator, do I need to rebalance the system?

No — replacing the actuator does not affect the valve body or pipework. However, if you drained any water or disturbed the system, check system pressure and bleed radiators as normal.

Regulations & Standards

• Building Regulations Part L (England) — Boiler Plus (2018) — New heating installations must include time and temperature control; TRVs on all radiators except in the room where the main thermostat is sited

• BS EN 14336:2004 — Heating systems in buildings: installation and commissioning of water-based heating systems

• Wiring Regulations BS 7671:2018+A2:2022 — All wiring, including 230V controls and valve actuators, must meet IET requirements

• Honeywell Home UK — Valve Installation Instructions — Y-plan and S-plan wiring diagrams

• Drayton Technical Documentation — Mid-position and 2-port valve service guides

• HHIC Boiler Plus Compliance Guide — Heating controls regulations

• Heating and Hotwater Industry Council — System Design — Y-plan vs S-plan configuration reference

• heating controls — Full heating controls overview including Boiler Plus

• system design — S-plan vs Y-plan system configurations

• no hot water — When hot water is also affected

• radiator cold bottom — When some radiators are cold