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, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
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. A timer should be fitted as well.
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
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
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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