The Heatmiser Slimline-B V3 is a battery-powered programmable room thermostat from the Heatmiser Battery Series. It is designed to control home heating by sensing room temperature and switching the heating system on or off based on the selected schedule and target temperature.
The official manual covers installation, battery setup, clock settings, comfort levels, keypad locking, temperature control, holiday mode, frost protection, optional features, recalibration, factory reset, wiring diagrams, and Heatmiser support information.
What Is a Programmable Room Thermostat?
A programmable room thermostat combines two controls in one device:
| Function | Purpose |
| Programmer | Sets heating times based on daily routines |
| Room thermostat | Measures room temperature and switches heating on or off |
The manual explains that increasing the thermostat temperature does not make the room heat faster. Heating speed depends on the heating system design. A better approach is to set the lowest comfortable temperature and allow the thermostat to manage the heating automatically.
The manual also notes that the thermostat needs free airflow. It should not be covered by curtains, blocked by furniture, or installed near heat sources such as electric fires, televisions, wall lamps, or table lamps.
Key Features
The Heatmiser Slimline-B V3 includes:
- Battery-powered operation
- Surface-mounted design
- Programmable heating schedules
- Weekday/weekend, 7-day, and 24-hour programming
- Four comfort levels: Wake, Leave, Return, Sleep
- Manual temperature override
- Temperature hold
- Holiday mode
- Frost protection mode
- Keypad lock
- Battery replacement indicator
- Optional feature settings
- Recalibration option
- Factory reset
- Switched live and volt-free wiring options
Parts and Controls
The thermostat has a front LCD screen and control buttons below the display.
Main Controls
| Button | Function |
| Power button | Turns thermostat on/off or enables frost mode |
| Clock button | Sets clock and comfort level programming |
| H button | Used for temperature hold and confirms time-related settings |
| A button | Confirms settings and returns to the main display |
| Down button | Reduces temperature, time, or setting values |
| Up button | Increases temperature, time, or setting values |
LCD Display Icons and Indicators
| Display Item | Meaning |
| Room Temp | Shows current room temperature mode |
| Day indicator | Displays current day |
| Frost icon | Shows frost protection mode |
| Flame icon | Shows heating demand; flashes during optimum start |
| Hold For | Shows active temperature hold and remaining time |
| Set | Shows settings or set points are being changed |
| Holiday icon | Shows holiday mode |
| Current temperature | Displays current room temperature |
| Program cycle indicator | Shows Wake, Leave, Return, or Sleep during programming |
| Battery icon | Shows batteries need replacement |
| Keypad lock icon | Shows keypad lock is active |
| °C / °F | Shows temperature unit |
| Clock | Displays time in 24-hour format |
Product Specifications
| Specification | Detail |
| Product model | Heatmiser Slimline-B V3 |
| Product type | Battery programmable room thermostat |
| Series | Battery Series |
| Mounting type | Surface mounted |
| Battery type | 2 x AAA batteries |
| Temperature display | Room temperature and set temperature |
| Time format | 24-hour clock |
| Programming modes | 5/2 day, 7 day, 24 hour |
| Comfort levels | Wake, Leave, Return, Sleep |
| Temperature format | °C or °F |
| Switching differential | 1°C to 3°C |
| Output delay | 00 to 15 minutes |
| Up/down temperature limit | 00°C to 10°C |
| Optimum start | 00 to 03 hours |
| Wiring terminals | NC, C, NO |
| Compatible wiring styles | Switched live, valve systems, volt-free, combi boiler systems |
How to Install the Heatmiser Slimline-B V3
The Slimline-B V3 is designed for surface mounting.
Installation Steps
- Separate the thermostat front
Use a small flat-head terminal driver in the slots on the bottom face of the thermostat to separate the front half from the back plate. - Mark wall fixing points
Use the back plate as a positioning template and mark two holes on the wall. - Drill and insert wall plugs
Drill the marked holes and insert wall plugs. - Wire the thermostat
Terminate the thermostat using the wiring diagrams shown in the manual. - Fix the back plate
Screw the back plate securely to the wall and fit the terminal cover. - Reattach the thermostat front
Clip the front half of the thermostat back onto the back plate.
Installation Do’s and Don’ts
| Do | Don’t |
| Mount the thermostat at eye level | Do not install near a direct heat source |
| Read the full instructions before use | Do not press hard on the LCD screen |
| Follow the wiring diagrams carefully | Do not block airflow around the thermostat |
The manual states that wiring should only be completed by a qualified electrician and must comply with local installation regulations.
Installing the Batteries
The thermostat is supplied with 2 x AAA batteries.
Battery Installation Steps
- Push and release the battery compartment door on the bottom face of the thermostat.
- Remove the battery holder.
- Insert the two AAA batteries.
- Make sure each battery follows the correct + / – polarity.
- Push the battery holder back into the thermostat until it is fully secured.
Replacing the Batteries
The thermostat shows a flashing battery icon when the batteries need replacement.
Important note: the batteries must be replaced within 1 minute of removal to retain the current clock and comfort level settings.
Setting the Clock
To set the clock:
- Turn the thermostat on.
- Press the Clock key twice.
- Use Up/Down to set the hours.
- Press H to confirm.
- Use Up/Down to set the minutes.
- Press H to confirm.
- Use Up/Down to set the day of the week.
- Press A to confirm and return to the main display.
Temperature Display
The Slimline-B V3 shows two key temperature values:
| Display | Meaning |
| Room Temperature | The current room temperature measured by the thermostat |
| Set Temperature | The target temperature the thermostat is trying to reach |
Comfort Levels and Programming
The thermostat supports three programming options:
| Program Mode | Description |
| 5/2 Day Programming | Weekdays share one schedule, weekends can have another |
| 7 Day Programming | Each day can have a different schedule |
| 24 Hour Programming | Same schedule applies every day |
Default Comfort Levels
| Period | Time | Temperature |
| Wake | 08:00 | 21°C |
| Leave | 09:30 | 16°C |
| Return | 16:30 | 22°C |
| Sleep | 23:00 | 17°C |
Unused comfort levels can be set to –:–, and the thermostat will ignore them.
How to Program Comfort Levels
- Press the Clock key once.
- Use Up/Down to set the required Wake time.
- Press H to confirm.
- Use Up/Down to set the Wake temperature.
- Press H to confirm.
- Repeat the same steps for Leave, Return, and Sleep.
- Set unused periods to –:–.
- Press A to confirm and return to the main display.
In 7-day mode, each day can be programmed independently. In weekday/weekend mode, the thermostat shows Monday to Friday first, then Saturday and Sunday for weekend settings.
Locking the Thermostat
The keypad lock prevents unwanted changes.
Lock or Unlock the Thermostat
- Press and hold A + Down together for 10 seconds.
- The lock symbol appears when locked.
- Repeat the same step to unlock.
The keypad lock icon only appears while the lock is active.
Temperature Control
To manually adjust the set temperature:
- Press Up or Down.
- The screen shows SET and the desired temperature.
- Use Up/Down to change the set value.
- Press A to confirm.
This manual override stays active until the next programmed comfort level.
Temperature Hold
Temperature Hold lets users override the current schedule for a selected duration.
How to Set Temperature Hold
- Press H to start temperature hold.
- Use Up/Down to enter the required hold time.
- Press H to confirm.
- Use Up/Down to enter the required hold temperature.
- Press A to confirm.
The display shows Hold For and the remaining time. When the countdown ends, the thermostat returns to the normal program.
Cancel Temperature Hold
Repeat the same steps and reduce the hold time to 00:00.
Holiday Mode
Holiday mode keeps the thermostat at the frost set temperature while users are away.
How Holiday Mode Works
- The thermostat maintains the frost temperature during the holiday period.
- It returns to the programmed schedule when the holiday ends.
- The holiday period starts at 00:00 the next day.
Example: if a 2-day holiday is set on Friday, Saturday counts as day one, and the thermostat returns to the schedule at 00:00 on Monday.
How to Set Holiday Mode
- Press H three times until the suitcase icon appears.
- Use Up/Down to enter the number of holiday days.
- Press A to confirm.
To cancel holiday mode, reduce the holiday duration to 00 days.
Frost Protection Mode
Pressing the Power button once places the thermostat into frost protection mode.
In frost mode:
- The frost icon appears.
- Heating only turns on if room temperature drops below the frost set temperature.
- The flame icon appears if heating turns on.
- Pressing the Power button again cancels frost mode.
Power On and Off
The heating is active when the flame icon appears. If the flame icon is not displayed, there is no current heating demand, but the thermostat remains active.
Turn the Thermostat Off Completely
Press and hold the Power button.
The display and heating output turn off completely.
Turn the Thermostat Back On
Press the Power button once.
Optional Features
The Slimline-B V3 includes optional settings that usually do not need adjustment.
| Feature | Description | Settings |
| 01 | Temperature format | 00 = °C, 01 = °F |
| 02 | Switching differential | 01°C to 03°C |
| 03 | Frost mode | 00 = Disabled, 01 = Enabled |
| 04 | Output delay | 00 to 15 minutes |
| 05 | Up/down temperature limit | 00°C to 10°C |
| 06 | Not used | Not used |
| 07 | Optimum start | 00 to 03 hours |
| 08 | Rate of change | Minutes to raise temperature by 1°C |
| 09 | Program mode | 5/2 day, 7 day, 24 hour |
| 10 | Thermostat mode | Manual or programmable |
How to Adjust Optional Settings
- Press and hold the Power button to turn the thermostat off.
- Press and hold the Clock key until the optional setting screen appears.
- Use the Clock key to cycle through features.
- Use Up/Down to change the setting value.
- Press A to confirm.
- Press Power once to turn the thermostat back on.
Re-Calibrating the Thermostat
Recalibration adjusts the displayed temperature reading.
Recalibration Steps
- Press and hold Power to turn the thermostat off.
- Press and hold Power + Down together until the temperature appears.
- Use Up/Down to configure the new temperature.
- Press A to confirm.
- Press Power once to turn the thermostat back on.
Factory Reset
Factory reset restores the thermostat to default settings.
Factory Reset Steps
- Press and hold Power to turn the thermostat off.
- Press and hold Power + Up together until the LCD powers up.
- All icons appear on screen.
- When the icons disappear, the thermostat has reset successfully.
- Press Power once to turn the thermostat back on.
Wiring Diagram
The manual includes two wiring diagrams for the Slimline-B V3.
Wiring Options
| Wiring Diagram | Use Case |
| Switched Live & Valve Systems | Uses permanent live from boiler or fused spur and switched live to boiler or valve |
| Volt-Free & Combi Boiler Systems | Uses boiler room thermostat connections such as LS and LR |
The wiring diagrams show NC, C, and NO terminals. For combi boiler wiring, the manual notes that LS and LR may be labelled differently, but they are normally the boiler’s room thermostat connections.
Troubleshooting Guide
| Problem | Possible Cause | What to Do |
| Battery icon flashes | Batteries need replacement | Replace both AAA batteries |
| Clock or schedule is lost after battery change | Batteries were removed for more than 1 minute | Reset clock and comfort levels |
| Heating does not turn on | Set temperature may already be reached | Check whether the flame icon is displayed |
| Buttons do not respond | Keypad lock may be active | Hold A + Down for 10 seconds |
| Thermostat does not follow schedule | Comfort levels may be set incorrectly | Recheck Wake, Leave, Return, and Sleep settings |
| Temperature seems inaccurate | Calibration may need adjustment | Use the recalibration steps |
| Thermostat does not switch boiler | Wiring issue or wrong boiler terminals | Check wiring diagram and boiler manual |
| Heating stays off in frost mode | Room temperature is above frost set temperature | Normal operation unless the room drops below frost setting |
Support and Contact
Heatmiser provides support resources, FAQs, PDF documents, videos, and technical specifications.
Heatmiser UK Ltd
Support phone: +44 (0)1254 669090
Website: www.heatmiser.com
Twitter/X: @heatmiseruk
Facebook: facebook.com/thermostatsAddress:
Heatmiser UK Ltd
Units 1-5 Hurstwood Court, Mercer Way
Shadsworth Business Park, Blackburn
Lancashire, BB1 2QU
United Kingdom