Common boiler room control faults – pump not running, sensor error, thermostat not responding

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Common heating room control faults – pump not running, sensor error, thermostat not responding

The boiler room controller is the brain of the entire heating system. When it works correctly, you barely notice it – heat arrives on time, the pump runs, the boiler switches on exactly when needed. But when something stops working, chaos ensues: cold radiators, the boiler firing at full blast while the house stays cold, or the opposite – gas being burned, the pump circulating hot water, yet the thermostat insists everything is fine. Over the years I've seen dozens of such jobs, and most faults tend to repeat. This article is a practical overview of what most commonly fails, how to recognize it, and what you can realistically do about it – on your own or with the help of a technician.

Note – a boiler room controller is not just a wall thermostat. It's a system that includes the control unit, the circulation pump, outdoor and clamp-on temperature sensors, possibly weather-compensated control, mixing valves, and other components. The fault can be anywhere in this chain, which is why diagnostics need to be done systematically.

Why boiler room controllers fail at all – a view from practice

Heating systems operate in unfavorable conditions. Moisture, dust, vibrations from the pump, power supply fluctuations, rusty water in the pipes, surges during storms – all of this gradually wears down the electronics and mechanics. Add to that the fact that most boiler rooms are in basements or utility rooms, where the temperature can rise to 40 °C in summer and drop to 5 °C in winter. Electronics suffer under such conditions.

Another factor: many controllers have been in operation for 10, 15, even 20 years without any maintenance. Capacitors in power supplies age, contacts oxidize, capillaries in mechanical thermostats change characteristics. When a fault then occurs, the owner is surprised – the controller "always worked, after all." Yes, it did work. But without preventive maintenance, every system eventually reaches the end of its service life. You can read more about preventive care in the article Maintenance and calibration of thermostats and controllers – how to prevent faults and extend service life.

The chart above reflects my experience from service jobs – it's not precise manufacturer statistics, but a real reflection of what technicians deal with most often. Pump faults and sensor errors account for the majority of service calls.

Pump not running – step-by-step diagnostics and solutions

This is one of the most common faults customers report. The symptoms are clear: the boiler is firing (or should be), but the radiators are cold. Or the pump makes noise, but heat isn't being transferred. There can be several causes.

Pump is "seized" after the summer break

This is by far the most common case I encounter every autumn, when people first start the heating. The circulation pump has been standing still all summer. Oxidation on the shaft and rotor literally "sticks" the pump – it receives power, the motor hums, but doesn't turn. The solution is surprisingly simple: on most pumps (Grundfos, Wilo, DAB), there's a slotted screw on the side of the head for a flathead screwdriver. Unscrew it, insert the screwdriver, and manually turn the shaft. Once released, the pump usually starts on its own.

Preventive recommendation: every summer, even if you're not using central heating, let the pump run for at least 30 minutes. Most modern controllers have a "summer pump protection" function – it activates the pump for a few minutes every few days precisely to prevent it from seizing.

Pump power supply fault from the controller

The boiler room controller switches the pump via an output relay contact. If the relay sticks (permanently closed) or stops switching (the pump never starts), the problem is in the controller itself, not the pump. Verification is simple: measure the voltage at the pump output terminals on the controller. If the controller signals that the pump should be running but there's no 230 V at the output – the relay is damaged. If voltage is present but the pump doesn't work – the problem is directly in the pump or in the wiring.

Failed capacitor in the pump

Older single-phase pumps use a running capacitor. When the capacitor fails, the motor hums but doesn't turn (the same symptom as a seized shaft). Replacing the capacitor costs a few euros and an electrician can do it in half an hour. Diagnosis: when manually turning the shaft, the motor either doesn't start at all or turns very slowly.

Pump is turning but not heating

If the pump runs but heat isn't arriving, it could be air trapped in the system. Bleeding valves on the radiators and on the pump itself should be opened until water starts to flow out. Another reason could be a clogged filter screen before the pump – a clogged filter causes a dramatic drop in flow, the pump turns, but the water barely moves.

Temperature sensor fault – how to detect and fix it

Temperature sensors are relatively simple components, but when they fail, they can cause mysterious behavior in the entire system. The controller may behave as if the temperature were much higher or lower than it actually is – and act accordingly.

Types of sensors and their common faults

Modern controllers most often use NTC thermistors (resistance decreases with temperature) or PT100/PT1000 resistance sensors. An example of a well-designed outdoor sensor is the ADEX B Outdoor Sensor, designed for ADEX controllers, which measures the outdoor temperature for weather-compensated control. Such sensors have a defined resistance characteristic – for example, at 0 °C, NTC sensors typically have around 10 kΩ, at 20 °C around 5–6 kΩ, at 80 °C under 1 kΩ. You can verify these values with a multimeter.

The most common sensor faults:

Broken wire – the controller reports "OC" (open circuit) or a maximum fault. This is caused by mechanical damage to the cable, poor connector insertion, or an oxidized terminal.
Short circuit in the wires – the controller reports "SC" (short circuit) or shows a temperature much higher or lower than actual. This happens when insulation is damaged, especially where a cable runs over sharp edges or was embedded in a wall without a conduit.
Moisture in the sensor – outdoor sensors are exposed to rain and frost. If water gets into the sensor housing and freezes, the ceramic NTC component can be mechanically damaged or short-circuited. Symptom: the sensor measures correctly in summer but starts reporting errors in winter.
Characteristic drift – after years of operation, the sensor's characteristic can shift. The controller may measure, for example, 3–5 °C differently than the actual temperature. You won't notice this without comparing it to a reference thermometer.

Practical sensor diagnostics with a multimeter

Disconnect the sensor from the controller. Measure the sensor's resistance at room temperature (e.g. 20 °C). Compare the measured value with the resistance table for that sensor type in the manufacturer's manual. A deviation of more than 5% from the tabulated value indicates a damaged or aged sensor. If the multimeter shows infinite resistance – the wire is broken. If it shows zero resistance – there's a short circuit.

With ADEX controllers (for example the ADEX Comfort 6 Controller), the controller displays an error code directly on the display – for example "Er1" for an outdoor sensor fault, "Er2" for a clamp-on sensor fault, and so on. These codes are always described in the operating manual and significantly speed up diagnostics.

Outdoor sensor and weather-compensated control

If you use weather-compensated control, the outdoor sensor is crucial. An incorrectly mounted, shaded, or damaged outdoor sensor causes the controller to set the heating water temperature incorrectly – the house is either overheated or underheated, even though everything "works." Read more on this topic in the article Weather-compensated control – how it works and when it pays off. The outdoor sensor must be placed on the north side of the building, protected from direct sunlight and precipitation, but exposed to the outside air. If the sensor is hidden behind a blind or above a window where warm air escapes, it measures incorrectly.

Thermostat not responding – causes and solutions

"Thermostat not responding" is another common problem, but it hides several different situations. Let's tell them apart.

Room thermostat not switching on the boiler

A room thermostat works as a simple switch – when the temperature drops below the set value, it closes a contact that signals the boiler or controller to start heating. If the thermostat isn't responding, verify it like this:

Increase the set temperature significantly above the actual room temperature (at least 5 °C). You should hear the relay click.
If you don't hear a click – the thermostat is damaged, or the battery is dead (for wireless and digital models).
If you do hear a click, but the boiler doesn't start – the problem is in the wiring between the thermostat and the boiler or controller.
Measure the resistance at the thermostat's output terminals: it should be close to 0 Ω when closed, and infinite when open.

For boiler rooms, it's important to understand the difference between a simple room thermostat and a comprehensive controller. This difference is well described in the article Room thermostat vs. boiler room controller – which solution is better for my home. In short: a room thermostat only handles the temperature in one room, while a boiler room controller manages the entire system, including the pump, boiler, weather compensation, and zones.

Digital thermostat with a dead display

If a digital thermostat has no display at all, the first thing to do is replace the batteries. It sounds trivial, but in practice this is the cause of every fifth service call. For thermostats powered from terminals (230 V or low voltage), check whether power is arriving. A blown fuse in the distribution board, or a disconnected neutral – the controller stays silent as the grave, but the problem lies outside it.

Thermostat measures incorrectly – off by several degrees

This is a tricky fault because the thermostat "works," it just measures incorrectly. Causes:

The thermostat is placed on a sunny side or near a heat source (TV, lamp, radiator).
The thermostat's internal NTC sensor has drifted – its characteristic can change after years of operation.
A draft from an improperly installed cable conduit in the wall is cooling the thermostat's sensor (the cable runs through the outer wall and draws in cold air).

Solution for drafts: seal the cable conduit with foam sealant. Solution for poor placement: move the thermostat to a more representative location. Read more about correct placement of clamp-on sensors in the article Clamp-on thermostat for pipes – how to properly position and set the sensor.

Boiler room controller – typical error states and their interpretation

Modern controllers display an error code when a fault occurs. Knowing what a given code means is half of the diagnostics. Let's look at examples from products I encounter most often.

ADEX controllers and their error messages

Controllers of the ADEX series, such as the ADEX midi Controller or the ADEX Comfort R Controller, have a well-developed diagnostic system. Typical error states:

Er1 / E1 – outdoor temperature sensor interrupted. The controller switches to a backup mode with a fixed water temperature.
Er2 / E2 – boiler or tank clamp-on sensor fault. The controller may disable boiler switching as a protective measure.
Er3 / E3 – room sensor or thermostat fault. The controller either ignores the room input or stops.
bL – device lockout after a fault (e.g. after a prolonged sensor interruption). Requires manual reset confirmation.

After every sensor or wiring repair, don't forget to restart the controller – either by cycling the power off/on, or with the Reset button (if available).

Controller not communicating with the boiler

With modern condensing boilers, the controller communicates with the boiler via a digital bus (OpenTherm, eBus, Modbus). If the bus cable is damaged or connected with reversed polarity, the boiler "doesn't see" the controller and operates in standalone mode (or stops). Symptom: the controller shows settings, but the boiler doesn't respond – and doesn't even show a fault. Solution: check the bus wiring, polarity, and possibly reset both devices.

Diagram of a typical boiler room controller wiring with a pump

The diagram above illustrates the basic wiring of a boiler room controller. The controller is a "crossroads" – it receives signals from sensors (outdoor sensor, clamp-on sensor, room thermostat) and, based on them, controls the outputs for the boiler and pump. If any input signal drops out or is faulty, the controller reacts incorrectly – or doesn't react at all.

Faults related to power supply and the electrical circuit

Many faults that look like a controller or pump problem actually have an electrical cause outside the devices themselves.

Loss of phase or neutral

Boiler rooms are usually powered from a dedicated circuit. A blown breaker or a loose neutral terminal will cause the controller to stop working, even though its LEDs are lit (powered from a different circuit). Always check the power supply directly at the controller's terminals – 230 V L-N.

Surges and protection

Summer storms can destroy power supplies and input circuits of controllers. If the controller stops working after a storm, check the fuse on the PCB (if accessible) and the input resistance. Installing a class D (type 3) surge protector in the boiler room distribution board costs about €20–30 and can save a controller worth several times more.

Ground fault in sensor wiring

Sensors are powered by low voltage (typically 5 V DC from the controller's bus). If a sensor wire touches a grounded part (boiler, piping), a ground fault occurs, which the controller interprets as a sensor fault or extreme temperature. Diagnosis: disconnect the sensor from the controller and measure the resistance between each wire and ground – it should be high (kΩ to MΩ). If it's low or zero, the wiring has a ground fault.

Special cases and more complex scenarios from practice

Controller is cycling – the boiler switches on and off too often

The so-called "short cycle" is a state where the boiler starts up, heats quickly, and shuts off right away – then again after a few minutes. Causes can include: too narrow thermostat hysteresis (set too tight), a stuck clamp-on limiter, too little water in the system (low pressure), or too high boiler output for a small house. Solution: increase thermostat hysteresis (e.g. from 0.5 °C to 1–1.5 °C), check water pressure (should be 1.5–2 bar when cold), or address the boiler output.

The house isn't heating even though the controller displays correct values

A tricky case. The controller shows the set and actual temperatures, the pump is running – but the house is cold. Possible causes:

A closed balancing valve on one of the branches – the plumber closed it and forgot to open it.
Thermostatic radiator valve heads not released – after summer, the heads "stick" in the closed position.
The boiler is firing, but at low output – a boiler service parameter is limiting output, or the boiler is in eco mode without the owner's knowledge.
The boiler's clamp-on sensor is poorly attached and measures the ambient temperature instead of the water – the controller thinks the water is hot and shuts off the boiler.

Pump runs even when it shouldn't (warm long pipe runs)

If the pump runs continuously even in summer, when there's no heating, the relay in the controller has likely "welded" (welding contacts). This happens with very frequent switching or with poor-quality controller contacts. Solution: replace the relay or replace the controller's control board. Alternatively, replace the entire controller with a modern one – for example the ADEX Comfort 6 Controller, which has a relay rated for a long service life under normal pump switching.

When to opt for a new controller instead of a repair

Repairing an old controller is sometimes worthwhile, but sometimes it's better to invest in a new one. Rules from practice:

If the controller is older than 15 years and the fault is on the control board – it usually isn't worth repairing.
If the controller is older than 10 years and you want to add weather-compensated control or zoning – also consider replacement.
If spare parts aren't available, or their price exceeds 50% of the price of a new controller – replace it.
If a repair has failed twice in one season – you're fixing symptoms, not the cause.

When choosing a new controller, first read the article How to choose a controller for a boiler room – what to focus on before buying. It's important that the new controller is compatible with your boiler (OpenTherm, 0–10 V, or just On/Off switching), with the type of sensors, and with the range of output voltages for the pump. The installation of a new controller is described in the article Installing a boiler room controller step by step – what you can do yourself and what needs an electrician.

If your system also includes electric underfloor heating or direct-heating appliances, the ADEX TTUV Electric Thermostat may also be suitable for controlling these circuits independently of the boiler circuit.

Preventive maintenance – what to do to prevent faults

An experienced technician will tell you: prevention is cheaper than repair. Here is a specific annual maintenance plan for a boiler room controller:

Every September, before the season: Check the water pressure in the system (1.5–2 bar), manually turn the pump shaft (if it was idle over summer), bleed the radiators, check the controller settings.
Annually: Visual inspection of the wiring – whether the insulation is cracking, whether terminals are oxidized. Clean the terminal blocks of dust with compressed air.
Every 2–3 years: Compare sensor readings with a reference thermometer. Check the outdoor sensor – housing tightness, mounting.
After a storm: Check the function of the controller and pump, see whether the display shows any error codes.

Frequently Asked Questions (FAQ)

The pump hums, but no water flows – what does that mean?

Most often this is a seized shaft after the summer break. Turn off the pump, find the service screw on the side of the head, disconnect the power supply, and manually turn the shaft with a flathead screwdriver. If the shaft doesn't turn even after manual release, the motor may have burnt-out windings – in that case the pump or motor needs to be replaced. Another reason: a clogged filter screen before the pump – clean it.

The controller shows a sensor fault, but the sensor looks undamaged – what should I do?

The sensor fault might not be in the sensor itself, but in the wiring. Check the entire cable route from the sensor to the controller: whether it's pinched, broken, whether the terminals are oxidized, whether the cable passes through a place with high temperature (e.g. next to a hot pipe). Disconnect the sensor and measure the resistance directly at the sensor – and compare it with the table in the manual. If the sensor's resistance is correct and the wiring is fine, the fault could be in the controller's input circuit.

The thermostat is set to a temperature, but the house doesn't heat up even after an hour – is the thermostat broken?

Not necessarily. First check the following: is the thermostat set correctly above the current room temperature? Is the boiler operating (flame not out, no fault)? Is the pump running? Are the thermostatic radiator valve heads open? If everything is OK and the house still isn't heating, only then suspect the thermostat. Isolate the problem by bridging the thermostat terminals at the controller – if the boiler starts, the thermostat is at fault. If not – the problem is elsewhere.

Can I replace a sensor myself without an electrician?

Outdoor and clamp-on sensors are usually powered by low voltage (2–12 V DC) from the controller's bus, so replacing them doesn't pose an electric shock risk. Just switch off the controller, disconnect the sensor wires from the terminals, physically replace the sensor, and connect the new one according to the original wiring. The sensors must be of the same type (the same technical NTC type – e.g. the same resistance at 25 °C and the same temperature coefficient). Both outdoor and clamp-on ADEX sensors are available as spare parts. Replacing the controller itself or working at 230 V must be done by an electrician.

The boiler switches on, but shuts off shortly after – is it a controller or boiler fault?

This usually isn't a controller fault – most often it's the boiler's safety thermostat (temperature limiter), which shuts off the boiler when it overheats. Causes of overheating: the pump isn't working (water isn't moving), a closed circuit, air in the system, a clogged heat exchanger. Check whether the pump is actually running, and whether the water flow is sufficient. If the boiler restarts only after cooling down – the cause is thermal protection, not the controller.

Is it worth repairing an old controller from 2005, or buying a new one?

Rule of thumb: if the repair is simple (sensor replacement, freeing up the pump, battery replacement) – repair it. If the fault is on the controller's electronic board and the controller is older than 12–15 years – buy a new one. Today's controllers offer much better features at a reasonable price: weather compensation curve, pump protection, weekly programming, sensor diagnostics. The energy savings usually pay for the investment within 2–3 years. Read more about choosing a new controller in the article How to choose a controller for a boiler room – what to focus on before buying.

Conclusion – systematically, not randomly

Boiler room controller faults tend to repeat in practice: a seized pump, an oxidized terminal, a cracked outdoor sensor housing, a dead thermostat battery, or a welded relay in the controller. It's not rocket science, but you need to proceed systematically. The biggest mistake I see is "swapping" – the customer replaces the pump, then the controller, then the sensors – and it turns out in the end that the problem was a loose screw connection on the terminal block. Always diagnose first, then repair.

If you're unsure where the problem lies, respect the limits of your knowledge. Work at 230 V belongs to an electrician. Everything else – diagnostics with a multimeter, sensor replacement, bleeding the system, manually freeing the pump – are things a handy homeowner can manage on their own, given the right information. And that's exactly what you've just received.

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