Thermostats
Outside Sensor (weather compensation)
In short
An outside sensor measures the outside air. A boiler with an outside sensor input reads it, and sets the heating's flow temperature by the weather. Its terminals: 1, 2.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
An OUTSIDE SENSOR measures the OUTSIDE AIR. A boiler with an outside sensor input reads it, and sets the heating's flow temperature by the weather. That is WEATHER COMPENSATION.
Colder outside, hotter radiators. Milder outside, cooler radiators: the boiler then runs cooler for longer, and CONDENSES more of the time. That is when a condensing boiler saves the most gas.
Inside the white housing is a THERMISTOR: a resistor whose resistance falls as it warms. The boiler reads that resistance. The sensor has no supply, no switch and no display.
It sets HOW HOT the heating runs. A timer and a room thermostat (stat) still decide WHEN. Ideal's kit says "A timer should be fitted".
Many boilers also stop the heating on a warm day: a SUMMER CUT-OFF. This simulator has none.
In England a new gas combi in an existing home needs one extra energy-saving measure. Weather compensation is one of the four (Boiler Plus; Approved Document L, paragraph 6.2).
DrawWarm's own design, wired the way the makers' sensors are. Product and brand names belong to their owners; DrawWarm is not affiliated with any manufacturer.
Terminals
| Terminal | What it is for |
|---|---|
| 1 | One core of the sensor's two-core cable. It goes to one of the boiler's outside sensor terminals, printed OS on DrawWarm's combi. It goes either way round: it has no POLARITY. |
| 2 | The other core, to the other outside sensor terminal. It is EXTRA-LOW VOLTAGE: never a mains conductor. |
What the meter reads
Its two cores on the DrawWarm Combi's two OS terminals: the combi COMPENSATES. Its screen shows OUTSIDE, and the temperature the sensor reads.
In this simulator the heating's flow follows a straight line: 30 °C at +20 °C outside, 50 °C at 5 °C, and 70 °C at -10 °C. Beyond those ends it holds 30 or 70.
On site the installer sets its two ends.
The hot water is never compensated: the tap's water heats as before.
Move Outside on the Conditions card, and the flow figure on the combi's screen follows it. The Outside row shows while a sensor is on the canvas.
This sensor's resistance is about 59 kilohms at -10 °C, and 34 at 0 °C. It is about 26 at 5 °C, 13 at 20 °C and 10 at 25 °C.
In this simulator the meter's ohms setting reads it across 1 and 2, dead. The figure follows Outside on the Conditions card.
Where it connects
Lessons that use it
- Weather compensation
Intermediate · Protection and extras
Practise it in the simulator.
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