Why a manual thermostat does not turn on heating – causes and solutions

Out of all the faults we encounter with manual thermostats, the non-functioning of the heating is by far the most common reason why customers pick up the phone or visit us in person. It is cold in the apartment, the thermostat is hanging on the wall, the dial is turned all the way to the right – and the heating simply does not work. The problem can be very trivial (wrong installation, boiler main switch turned off) or a bit more complex (burnt fuse, faulty bimetal strip, incorrect calibration). In this article, we will systematically go through all relevant causes – from the simplest to electrical and mechanical, and even system-related – and assign a specific solution to each. Nothing here is theory: each point is based on real-life situations we have seen in hundreds of cases.

How a manual thermostat actually turns on heating – basic principle

Before we get into the causes, we need to understand how a manual (bimetal or capillary) thermostat works at all. Without this basic knowledge, we cannot diagnose the fault correctly.

A classic manual thermostat works on the principle of a bimetallic strip. It consists of two different metals welded into one strip (most commonly brass and invar, or steel and copper), which have different coefficients of thermal expansion. When the room temperature drops below the set value, the bimetallic strip bends in one direction and mechanically closes the electrical contact. This closes the boiler's control circuit (or the floor heating thermostat's relay), the boiler receives a signal and starts heating. When the temperature rises above the set value, the strip bends in the opposite direction, the contact opens and the heating turns off.

This mechanism is on one hand extremely reliable (no electronics, no software), but on the other hand sensitive to mechanical damage, dirt, vibrations and – most importantly – to the correctness of the installation. If any part of this chain is broken or disturbed, the thermostat will simply not switch on.

It is also important to know that a manual thermostat typically switches so-called no-voltage contact (dry contact) – this is a mechanical switch that only closes or breaks the control signal. The thermostat itself does not directly switch the boiler's power supply. This means that if the control circuit is broken somewhere (not the boiler's power line), the boiler will not ignite the burner.

Most common cause #1: Incorrect or incomplete wiring

This is the number one reason in practice – the thermostat is wired incorrectly. You may be surprised how often this happens even in seemingly simple installations. A manual thermostat typically has 2 terminals (for simple models like Avansa TH1) or 3–4 terminals (for models with a power circuit, e.g. for controlling a fan, emergency heating, etc.).

Typical wiring errors

• Swapped wires on L and N terminals: For thermostats that have their own power supply (230 V), it is necessary to observe the phase (L) and the neutral conductor (N). Swapping them does not always damage the thermostat, but switching will be non-functional or intermittent.
• Control wire connected to the terminal for continuous power: On some boilers, there is a terminal for continuous power (for the control panel) and a terminal for the thermostat's control contact. Swapping them will cause the boiler to run continuously, or not at all.
• Wires are not tightly connected: A loose contact "hangs in the air" – the thermostat seems to switch (you hear a click), but the intermittent contact does not cause the boiler to start.
• Too thin a wire used: For thermostat control circuits, the standard is CYKY 2×1.5 mm² or at least 2×0.75 mm². A thinner wire is prone to resistance and can cause problems with long runs.
• Incorrect polarity in DC systems: Rarely, but in some floor heating systems with DC regulation, it is a common problem.

More about the correct procedure can be found in the article Connecting a manual thermostat to a boiler and floor heating, where the step-by-step wiring diagram is described for different types of systems.

Cause #2: Incorrect thermostat location – the thermostat measures the wrong temperature

This is a sneaky cause, because the thermostat technically works correctly – it just thinks the room is warm. The bimetallic strip reacts to the air temperature directly near the thermostat. If the thermostat is placed incorrectly, the measured value does not match the actual room temperature.

Problematic placements in practice

• Above or next to a radiator: Hot air convection heats the thermostat directly – the thermostat thinks the room is 24 °C, while the actual temperature is 18 °C. The heating does not turn on, even though it should.
• On an external wall: In winter, cold penetrates through the wall directly to the bimetallic strip, the thermostat measures a lower temperature than the actual air temperature. The heating runs almost continuously.
• In a draft (near doors, windows): Temperature fluctuations cause irregular switching.
• Behind a curtain or in a cabinet: The air does not move there, the thermostat does not measure a representative room temperature.
• On a sunny side: Direct sunlight heats the white thermostat cover and the bimetallic strip – the thermostat does not turn on the heating, even though the room is still cold.

The standard STN EN ISO 7726 recommends for a room temperature sensor a height of 1.1 to 1.5 m above the floor, on an internal wall, more than 50 cm from the radiator, more than 1 m from windows and doors. If your thermostat does not meet these conditions, it is likely the cause of non-functional or unstable switching.

Cause #3: Poor calibration – the thermostat switches at a different temperature than the scale indicates

An older manual thermostat, or even a new cheap model without factory calibration, may have a deviation between the set value on the scale and the actual switching temperature. This deviation can be ±2 to ±5 °C. In practice, it looks like this: you set 20 °C, but the thermostat switches at an actual temperature of 23 °C – and you feel that the heating does not turn on (you set 20 and the room is 20, but the thermostat still does not switch).

Some models have a hidden adjustment element (trimmer, small screw) for correction. For example, Avansa 2003 has a trimmer on the bottom of the cover that allows shifting the characteristic by ±5 °C. The Euroster Q1 model has an adjustment element accessible after opening the cover.

How to verify calibration? Simply: place a calibrated thermometer (ideally digital with a precision certificate) next to the thermostat, wait 20–30 minutes and compare the values. If they differ by more than 1.5 °C, it is worth doing a calibration. The procedure is described in detail in the article Calibration and adjustment of a manual thermostat.

Cause #4: Wear or damage to the bimetallic strip

The bimetallic strip is a mechanical component with a limited lifespan. Although high-quality thermostats can last 10–15 years without problems, aging material, repeated mechanical stress (too frequent manual turning and unturning) or corrosion can damage the strip. Symptoms:

• The thermostat does not switch even when the dial is turned all the way to maximum (e.g. to 30 °C)
• The thermostat does not switch consistently – sometimes yes, sometimes no
• When the cover is opened, visible deformation, breakage or oxidation of the strip is visible
• The contacts are visibly burned (black, charred areas)

In such a case, the only correct option is to replace the thermostat. Repairing the bimetal is not feasible in everyday practice. The good news is that manual thermostats are affordable – for example, SALUS EUROTEMP RT 10 or Saswell 919 are reliable and affordable models that will last many years in most households without problems.

Cause #5: Problems on the boiler or heating system side

A very important and often overlooked aspect: the problem is not always in the thermostat. The thermostat may be working perfectly, but the heating does not turn on due to problems on the boiler or system side.

What to check on the boiler and system

• Boiler protective functions: Modern gas and electric boilers have a number of protective blocking functions. If the boiler is in an error state (e-code on the display), it ignores the signal from the thermostat. Check the boiler's display.
• Summer/winter switch: Most boilers have a switch or menu to switch between summer (only TUV) and winter (heating + TUV) mode. If the summer mode is set, the heating will not run regardless of the thermostat.
• Minimum boiler thermostat: Some boilers have an adjustable minimum temperature at which it starts – if it is set too high and the system has not yet cooled down, the boiler waits.
• Blocked circulation pump: If the pump does not work, the boiler has overheat protection and either will not ignite the burner or will turn it off after a short time. The heating may start, but it will immediately turn off.
• Pressure relief valves and system pressure: Too low or too high water pressure in the system (the manometer on the boiler shows outside the range of 1–2 bar) will block the boiler.
• Control terminals TA are bridged: On some boilers, the terminals for the thermostat are bridged with a metal bridge from the factory. If it is not removed during installation of the thermostat, the thermostat will never control the boiler – the boiler will run continuously, or not at all (depending on the wiring). This is a very common error during installation.

Cause #6: Oxidation or contamination of thermostat contacts

The electrical contacts in a manual thermostat are small metal plates (bronze, silver or gilded metals) that physically touch when closed. These contacts are exposed to oxidation (rusting, greening), dust and moisture. Oxides on the contacts act as an insulator – the contact seems to close (the bimetal bends, you hear a click), but the electric current does not pass or passes irregularly.

This happens most often in:

• Kitchens and bathrooms – humidity accelerates oxidation
• Old houses with damp walls
• Thermostats that have not been turned for a long time (e.g. summer – months without switching)

Solution: Open the thermostat cover (after turning off the power). Carefully clean the contacts with fine sandpaper P400–P600 or a special contact spray. Never use WD-40 or other hydrocarbon-based oils – they leave residue and damage the contacts in the long run. If the contacts are severely damaged, it is usually more economical to replace the thermostat.

Cause #7: Mechanical blocking of the control element

A mechanical thermostat has a physical dial or lever. Sometimes it happens that:

• The dial is blocked by a foreign object (a piece of paper, furniture pushed against the wall)
• The plastic cover is deformed – crushed, jammed at the limit
• The control dial loses friction and slips back after setting
• The maximum temperature stop (adjustable protection, present for example in hotels or social facilities) is set too low

In hotels or rentals, this stop is quite common – the landlord limits the maximum adjustable temperature to 21–22 °C to save energy. The tenant turns the dial to maximum, but physically cannot set it higher, which can be confusing.

Cause #8: Power supply problem in thermostats with their own electronics

Although manual thermostats are mostly mechanical, some more advanced models (e.g. Saswell 919) have simple electronics (LED indicator, output relay, protective circuit). If these thermostats do not have power (fuse blown, broken 230 V wire), the entire switching fails.

For purely mechanical bimetal thermostats (without their own electronics), this is not relevant – they work without their own power supply, only switching the control circuit.

Cause #9: Difference between thermostat hysteresis and actual need

A manual thermostat does not switch immediately when the temperature drops by 0.1 °C below the set value. It has so-called hysteresis – the temperature must drop by several degrees below the set value before the contact closes. Typical hysteresis of a mechanical thermostat is 1–3 °C, in cheaper models it can be 4–5 °C.

Practical example: You set 20 °C. The thermostat has a hysteresis of 2 °C. The heating turns on only when the temperature drops to 18 °C and turns off when it rises to 20 °C. This is normal and acceptable. A problem arises when the customer expects the heating to start immediately when the temperature drops to 19.5 °C – this simply does not happen.

If you are not satisfied with a large hysteresis, it is an argument in favor of a digital thermostat with an accuracy of ±0.5 °C. This question is discussed in the article Manual vs digital thermostat – which is more cost-effective.

Systematic diagnostic procedure – step by step

Based on everything mentioned above, we present a practical procedure for what to do when the heating does not turn on. We recommend going exactly in this order – from the simplest to the most complex.

Step 1: Check the boiler status (5 minutes)

Look at the boiler. Does it have power? Is there an error code? Is it set to winter mode (radiator symbol, not sun)? Is the water pressure in the range of 1–2 bar? If you find a problem here, fix it first.

Step 2: Bridging TA terminals (2 minutes)

This is the fastest test: Turn off the power. Disconnect the wires from the thermostat from the TA terminals on the boiler. Connect both TA terminals with a short wire (bridge). Turn on the power. If the boiler immediately starts heating – the problem is in the thermostat or wiring, not in the boiler. If the boiler still does not start – the problem is in the boiler.

Step 3: Check thermostat wiring

Open the thermostat terminal cover. Are the wires connected to the correct terminals (according to the diagram in the manual)? Are they tightly connected? Is it the correct type of wire? If you see a mistake, fix it.

Step 4: Test thermostat function

Turn the dial to maximum (e.g. 30 °C). If the room temperature is lower, the thermostat should immediately switch on. You can verify this with a multimeter (measuring continuity/resistance) on the thermostat terminals in the closed and open state. In the closed state, the resistance must be zero (0 Ω), in the open state, it must be infinite.

Step 5: Check location and calibration

Place a calibrated thermometer next to the thermostat. Wait 20 minutes. Compare the values. If the deviation is more than 2 °C, check whether the thermostat is exposed to false heat sources (radiator, sun) or cold (external wall). If the location is correct, perform calibration with a trimmer.

Step 6: Physical check inside the thermostat

Open the thermostat cover (after turning off the power). Look at the bimetallic strip – is it straight, undamaged? Look at the contacts – are they clean, without oxidation, without visible burning? If you see damage, replace the thermostat.

When is it time to replace the thermostat?

A manual thermostat is a relatively cheap device, so in the case of a serious fault, it is usually more economical to replace it than to repair it. We recommend replacement if:

• The bimetallic strip is visibly damaged or deformed
• The contacts are heavily oxidized or burned
• The thermostat is more than 10–12 years old and the problems recur
• Calibration is out of trimmer range (deviation greater than ±5 °C)
• The dial turns freely without resistance and the position does not change with the change in resistance of the strip

When choosing a new thermostat, we recommend reading the article How to choose a manual thermostat for your home, where key parameters are compared. You can find the full overview of available models in the category manual thermostats.

Preventive maintenance – how to avoid problems

A manual thermostat requires almost no maintenance, but a few simple steps will extend its life and reliability:

• Once a year (before the heating season) turn the dial through the full range: From minimum to maximum and back. This "exercises" the bimetallic strip and contacts after the summer break, removes initial oxidation from the contacts.
• Keep the thermostat freely accessible: Do not block it with furniture, do not cover it with curtains.
• Never spray cleaning agents near the thermostat when cleaning the room: Aerosols and cleaning fumes damage the contacts.
• Check the tightness of the terminals once every 2–3 years: Thermal expansion and contraction can loosen the connections.
• Do not exceed the maximum allowable load on the contacts: If you want to control a load higher than the rated current of the thermostat (typically 8–16 A), it is necessary to use an external relay.

More tips on economical use of the thermostat can be found in the article How to save on heating even with a manual thermostat.

Special cases: floor heating and electric systems

For electric floor heating, some differences apply that can be the cause of non-functionality:

• The thermostat for floor heating must have an output relay of sufficient capacity: Electric floor heating typically has a power of 100–200 W/m², in a standard room of 15–20 m² this can be 1,500–3,000 W = 7–13 A. A manual thermostat intended for a relay output boiler (only a few mA through the control circuit) cannot switch such a load directly!
• Protective thermostat for the floor (floor sensor): Some systems have a double thermostat – room (manual) and floor sensor. If the sensor evaluates that the floor is already warmer than the set maximum (usually 28–35 °C), it blocks further heating regardless of the room thermostat.
• Triac regulators vs. relays: Some floor heating regulators use smooth regulation (triac), not just on/off. A manual bimetal thermostat may not be compatible with them.

A detailed description of wiring for different heating configurations can be found in the article Connecting a manual thermostat to a boiler and floor heating.

Most frequently asked questions (FAQ)

The thermostat makes a click, but the heating does not start – what does it mean?

The sound of the click indicates that the bimetallic strip is moving and mechanically "jumping" the contact. The problem is therefore not in the bimetal, but likely in the electrical part: oxidized contacts (the contact sounds, but does not conduct current), broken wire between the thermostat and the boiler, incorrect terminals, or a problem on the boiler side (error state, summer mode, TA terminals bridged, which were forgotten to remove). Perform a TA terminal bridging test on the boiler – if the boiler starts, the problem is in the thermostat or wiring.

The manual thermostat does not turn on the heating, even though it is turned to maximum. Is it broken?

Not necessarily. First, check whether the thermostat is located in a warm place (above a radiator, on a sunny side) – it thinks it is warm in the room, even though it is not. Next, compare the measured temperature next to the thermostat with a calibrated thermometer. If the thermostat shows 3–5 °C higher than reality, calibration or relocation is needed. If that does not help, check the contacts with a multimeter and replace the thermostat if necessary.

When does the thermostat turn off the heating after it is turned on?

A manual bimetal thermostat turns off the heating when the room temperature rises to the set value. Due to hysteresis (typically 1–3 °C for mechanical thermostats), the turn-off happens at the set temperature, and the turn-on happens only when the temperature drops by that hysteresis. So if you set 21 °C and the hysteresis is 2 °C, the heating turns off at 21 °C and turns on again at 19 °C. This is normal behavior – it is not a fault.

Can I connect a manual thermostat myself, or do I need to call an electrician?

Law No. 124/2006 Coll. and Ordinance No. 508/2009 Coll. in Slovakia require that electrical installations under 230 V be performed by a qualified person (an electrician with the appropriate qualification). A manual thermostat is connected to the 230 V network (the control circuit does not rise, but the thermostat's own electronics does). For safety and insurance validity, we recommend that the connection be done by an electrician, or at least have the completed installation checked. If you are electrically skilled and only want to make a simple bridging of two control circuit wires (TA) without interference in the power part, many people do it themselves – but you do so at your own risk.

Why does the heating not turn on in the morning, but works normally in the evening?

A classic sign of incorrect thermostat placement. In the morning, the sun shines on the wall or window where the thermostat is – it heats the cover and the strip. The thermostat thinks it is warm, the heating does not turn on. In the evening, the sun does not heat the wall and the thermostat works correctly. Solution: move the thermostat to an internal wall, away from windows and direct sunlight. Alternatively, some thermostats have a sealed sensor with thermal insulation from the cover (e.g. Avansa 2003), which reduces this effect.

The thermostat worked for years, now it suddenly does not work. What changed?

The most common causes of sudden failure after a long period of functioning: (1) Summer break – contacts oxidized; (2) Repair or service of the boiler – the technician may have left a TA bridge or changed the boiler setting by mistake; (3) New furniture added to the wall – the thermostat was covered; (4) A fuse in the distribution board tripped and was not reset; (5) A wire became loose from the terminal – thermal expansion after years. Go through all the diagnostic steps described above systematically.

Conclusion: Diagnosis is a logical process, not a coincidence

If a manual thermostat does not turn on the heating, the cause is always somewhere in the chain: boiler → wiring → thermostat → location → calibration → mechanics. Systematically going through this chain from the boiler towards the thermostat usually identifies the problem in 15–20 minutes without any special tools (a multimeter and a calibrated thermometer are sufficient). The vast majority of cases we encounter in practice are resolved in the first two diagnostic steps: the problem is on the boiler (summer mode, error code, forgotten bridging bridge on TA) or in the thermostat wiring (loose wire, swapped terminals).

If you have come to the conclusion after diagnosis that the thermostat is mechanically or electrically damaged and you need a new one, take a look at our overview in the category manual thermostats. More related topics can also be found in the articles Installation of a manual thermostat step by step, Common faults of manual thermostats and how to fix them and Common questions about manual thermostats, where we cover further practical scenarios from real customer cases.