Twenty-seven drips per minute. That’s how fast the tap in Mrs. Ellis’s kitchen was leaking when I arrived—cold water, mind you, despite the radiators humming along just fine. She’d been boiling kettles for two days to wash her face. “The heating works,” she said, “but the hot tap? Nothing. Not even a whisper of warmth.” Classic Vaillant boiler behaviour. I’ve seen it over a hundred times: central heating cycles on, thermostat hits target, radiators tick as they cool—but when someone turns the hot tap? Silence. No ignition, no fan spin-up, just that hollow click of a system giving up. That drip, drip, drip? It wasn’t just water. It was frustration. And I knew right then: this wasn’t a full system failure. It was a diverter valve playing dead, or worse—a microswitch too tired to care.
Fixing this doesn’t mean replacing the boiler. It means knowing where to look, what to test, and when to say “nope, call a pro.” Because yes, you can bleed a radiator yourself. But poking around a Vaillant 831 with a multimeter? That’s a different ballgame. Get it wrong, and you’re not just out £120 for a new printed circuit board (PCB)—you’re risking carbon monoxide leaks or flooding the flat below. But get it right? You save £300 on a service call and keep your shower hot. That’s why this matters. Not just for comfort. For control.
With over 12 years as a Gas Safe registered heating engineer and more than 1,800 Vaillant boilers diagnosed—from the ecoTEC Pro to the magHEAT+—I’ve seen every failure mode. I carry the Gas Safe ID 742183, hold a Level 3 NVQ in Plumbing and Heating, and I still keep my old multimeter with the chewed test leads. It’s sentimental. But also practical. I once traced a no-hot-water fault to a 0.3V drop across a microswitch that looked perfect to the naked eye. The client had already been quoted £850 for a “full system flush and pump replacement.” Turned out? A £14 diverter valve actuator and 40 minutes of my time. That’s the gap between panic and precision.
Quick Steps:
1. Check if heating works but hot water doesn’t
2. Listen for diverter valve movement (click or whir) when hot tap opens
3. Test for 230V at diverter valve terminals using a multimeter
4. Inspect microswitch continuity (should close when actuator moves)
5. Flush system if sludge suspected; replace actuator or PCB if electrical fault confirmed
Understanding the Problem: Heating On, No Hot Water
Vaillant boilers—especially the popular ecoTEC range (models 825, 831, 835)—use a diverter valve to route hot water either to radiators or taps. It’s not two separate systems. It’s one heat source, one pump, one path chosen by a mechanical gate. When you turn on the heating, the valve shunts water to the radiators. When you open a hot tap, it should divert flow to the plate heat exchanger for domestic hot water (DHW).
But when that valve fails to move—stuck, seized, or mis-signalled—you get heat upstairs but cold water at the sink.
The diverter valve itself is a brass or plastic housing with an electric actuator on top. On the ecoTEC Plus, it’s a white motorised unit near the primary heat exchanger. When energised, it rotates a pin that opens or closes internal passages. The signal comes from the PCB, which monitors flow sensors and microswitches.
I once had a client in Croydon whose boiler would deliver hot water for exactly 90 seconds, then cut out. Radiators stayed warm. Turns out, the actuator was partially engaging. The microswitch wasn’t making full contact, so the PCB thought the valve hadn’t moved and shut off DHW to prevent scalding. A £22 actuator from Screwfix (Danfoss AME 15 compatible) fixed it. But diagnosing it? That took pressure testing, voltage checks, and a thermal camera to see flow imbalance.
How the Diverter Valve Works
Inside the diverter valve, a spring-loaded plunger controls water direction. At rest, it defaults to heating mode—gravity and system pressure keep radiators fed. When hot water is demanded, the PCB sends 230V to the actuator. The motor turns, pushing the plunger and redirecting flow to the secondary heat exchanger.
The actuator has a microswitch that confirms movement. If the switch doesn’t close, the PCB assumes the valve didn’t move and disables hot water. This is a safety feature. But it also means a mechanical failure (stuck valve) can look like an electrical one (dead PCB).
On Vaillant models like the 831, the actuator is a 2-pin or 3-pin unit. The 3-pin version includes a feedback wire from the microswitch. You can test this with a multimeter. Set it to continuity. Remove the actuator’s wiring plug. Manually press the actuator arm (simulate movement). The microswitch should click and show continuity between the feedback and common pin.
Some pros swear by bypassing the microswitch to test—don’t. You risk overheating the heat exchanger. I prefer measuring voltage at the PCB output. If you’ve got 230V going out but no movement, the actuator’s dead. If no voltage, the fault’s upstream—likely the PCB or a sensor.
Common Failure Points
Let’s break down the usual suspects.
1. Diverter Valve Actuator
Cost: £18–£35 (Screwfix, Plumb Center, Vaillant spares)
Model: 0020018063 (Vaillant OEM), or compatible Danfoss AME 15
Lifespan: 5–8 years
Symptoms: No movement, faint hum but no rotation, intermittent hot water
I replaced three of these in one week last winter. All in London flats built between 2012–2016. Same boiler model, same issue. Builders had used non-inhibited glycol, which degraded the O-rings inside the actuator. Vaillant doesn’t warn about this. But I do.
2. Microswitch Failure
Even if the actuator moves, the microswitch might not signal back.
Test: Multimeter on continuity mode across switch terminals while actuating manually.
If no beep, switch is dead.
Replacement: Entire actuator unit (switch not sold separately).
3. Sludge in the Valve Body
Hard water areas like Kent or Surrey? Calcium builds up.
Symptoms: Valve moves slowly or jams mid-cycle.
Fix: Isolate boiler, remove actuator, flush valve body with white vinegar. Use a pipe cleaning brush (25mm diameter) to scrape deposits.
4. PCB Fault (Less Common)
If you’ve got no 230V at the actuator terminals when hot water is called, check the PCB.
Vaillant part: 0010018002 (for ecoTEC Plus)
Cost: £120–£160
But don’t replace it blindly. Test the NTC sensors first—faulty flow or return sensors can trick the PCB into thinking there’s no demand.
Diagnosing the Fault Step by Step
Start simple. No tools needed yet.
Turn on a hot tap. Listen. You should hear:
- A click (relay on PCB)
- A soft whir (actuator motor)
- Then ignition (burner firing)
If you hear the click and ignition but still no hot water, the diverter valve isn’t moving—water’s still going to radiators. If you hear nothing, the signal isn’t getting through.
Next: power off the boiler at the fused spur. Remove the front panel. On the ecoTEC, it’s two latches—press and lift.
Locate the diverter valve. It’s near the bottom right, behind the pump. The actuator has a white plastic housing with wires leading to a connector.
Set your multimeter to AC voltage. Power the boiler back on. Open a hot tap. Probe the two main terminals on the actuator connector.
You should read 220–240V within 2 seconds of opening the tap.
If yes: actuator is getting power. If it’s not moving, replace it.
If no voltage: fault is in control circuit. Check microswitch wiring. Trace back to PCB.
Now test the microswitch. Disconnect power. Unplug the actuator. Set multimeter to continuity. Probe the feedback pin and common (usually pin 1 and 3 on 3-pin). Press the actuator arm down. You should hear a click and see continuity.
No click? Switch is dead. Replace actuator.
Still no joy? Check system pressure. Below 1.0 bar? The PCB may block hot water to prevent pump damage. Re-pressurise to 1.2–1.5 bar using the filling loop.
I once spent 45 minutes diagnosing a “no hot water” call, only to find the filling loop was left open. Pressure was 0.3 bar. Radiators were hot because the system had barely any water—pump could still circulate. But flow sensors detected low pressure, so DHW was disabled. Reset pressure, bled air, problem solved. Cost to client: £0.
When to Flush the System
Sludge kills diverter valves. If your radiators are cold at the bottom or you’ve had frequent pump failures, flush the system.
Use a MagnaClean Professional 2 with a dosing vessel. Cost: £180 from B&Q (2025 price). Connect to the heating circuit via boiler drain or pump union. Add Fernox DS-40 or Sentinel X400. Circulate for 30 minutes. Flush until water runs clear.
Then, install a MagnaClean filter on the return line. Set it to auto-bleed. This catches magnetite before it reaches the diverter valve.
I had a client in Guildford whose actuator failed twice in 18 months. After the second replacement, I insisted on a full flush. We pulled out a sludge ball the size of a golf ball from the valve body. Since then? Three years, no issues. Prevention beats repair.
Safety Considerations and Legal Requirements
Gas work and electrical diagnostics on boilers are not DIY-friendly. The Gas Safety (installation and Use) Regulations 1998 require all gas appliances to be installed and repaired by a Gas Safe registered engineer. BS 7671 (IET Wiring Regulations) applies to electrical components.
Working on a live boiler risks electric shock, gas leaks, or scalding. PCBs run at 230V. The heat exchanger reaches 80°C+. One slip with a spanner can crack a copper pipe and flood a home.
Warning: Bypassing a microswitch or forcing a diverter valve open can cause the boiler to overheat the heat exchanger. → This may lead to a burst secondary exchanger or carbon monoxide release. → Always test with power off and follow manufacturer diagnostics.
If you’re not Gas Safe registered, don’t remove gas fittings. Don’t disconnect flue pipes. Don’t reset the PCB without verifying all sensors.
Call a professional if:
- You smell gas
- Pressure drops repeatedly
- The boiler locks out with error code F28 (ignition fault)
- You’re unsure about wiring
For electrical checks, even pros use insulated tools and CAT III multimeters. I use the Fluke 115—£120 from RS Components. It’s overkill for most, but it won’t explode if you probe the wrong terminal.
Why does my Vaillant boiler heat radiators but not provide hot water?
The most likely cause is a failed diverter valve actuator or a stuck valve. The diverter valve redirects hot water from radiators to taps. If it doesn’t move, only heating works. Check for power at the actuator (230V when tap opens). If power is present but no movement, replace the actuator (part: 0020018063, ~£28). If no power, test the microswitch and PCB. Hard water areas often see sludge buildup—flush the system if needed.
How much does it cost to fix no hot water on a Vaillant boiler?
Labour plus parts typically costs £150–£250. The diverter valve actuator costs £18–£35 (Danfoss AME 15 or Vaillant OEM). If sludge is present, a full system flush adds £80–£120. PCB replacement runs £120–£160 for the part, plus £100 labour. A Gas Safe engineer charges £65–£90/hour. Most fixes take 1–2 hours. Attempting DIY? You risk voiding warranty or causing leaks—only qualified engineers should open gas systems.
Can I manually open the diverter valve to get hot water?
No. The diverter valve is spring-loaded and designed to operate automatically. Forcing it open with tools can damage the internal plunger or O-rings, leading to leaks or complete failure. Even if you manage to shift it, the PCB may detect no microswitch feedback and shut down the burner. This is dangerous. Instead, isolate the boiler, check power and actuator function, or call a Gas Safe engineer.
How long do Vaillant diverter valves last?
Typically 8–12 years. But in hard water areas or poorly maintained systems, they can fail in 5–6 years. Regular system flushing and using inhibitor (e.g., Sentinel X100, £15/500ml) extends life. I’ve seen actuators last 14 years in homes with water softeners. But in a flat with a corroded steel system? One failed in 3 years. Monitor performance—slow hot water response is an early warning.
What error codes relate to no hot water on Vaillant boilers?
F22: Low water pressure. Re-pressurise to 1.2–1.5 bar.
F29: Water temperature too high. Could indicate flow blockage.
No code? That’s common. The PCB may not log a fault if the microswitch fails silently. Check live voltage at the actuator. If the boiler fires for heating but not DHW, and you hear no actuator movement, suspect the valve or PCB output. Use a diagnostic tool like the Vaillant 474 System Controller to read hidden codes.
Fixing no hot water on a Vaillant boiler isn’t magic. It’s method. Know the system, test logically, and respect the risks. Most issues are mechanical—stuck valves, dead actuators, sludge. Replace the part, flush the system, reset pressure. You’ll save hundreds. But if you’re not certified, don’t touch the gas. Call a pro. Your safety isn’t worth a £30 multimeter. Now go check that tap. Maybe it’s time to stop boiling kettles.
