Thermostats
Pipe Thermostat (frost protection)
In short
A pipe thermostat (pipe stat) is a thermostat strapped to a pipe. It feels the water in the pipe through the copper. Its terminals: 2, 4, E, 1.
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
A PIPE THERMOSTAT (pipe stat) is a thermostat strapped to a pipe. It feels the water in the pipe through the copper.
In FROST PROTECTION it is wired IN SERIES with a frost thermostat (frost stat). Makers differ on where it goes: on the boiler's RETURN pipe, or on the pipe furthest from the heated area. Follow the boiler maker.
On a freezing night the frost thermostat calls for heat. The pipe thermostat lets that call through only while the pipe is cold. Once the water in the pipe is warm, it stops the call. So the boiler is not run for hours to warm a cold garage or loft: the pipes are already safe.
It is a CHANGEOVER switch worked by temperature, and it needs no supply of its own.
Its dial is printed 20 to 90 °C, and it comes set at 25 °C. The setting is the number over the small triangle below the dial, and on the tag in the corner.
Its terminals are under a cover. To wire it, right-click it and choose Take the cover off, or click it with the Wire Tool.
It is DrawWarm's own design. Product and brand names belong to their owners; DrawWarm is not affiliated with any manufacturer.
Terminals
| Terminal | What it is for |
|---|---|
| 2 | The CALL, normally open (NO): joined to 1 while the pipe is colder than the setting. It goes on to the heating valve's brown, at block 5 of an S-Plan. |
| 4 | SATISFIED, normally closed (NC): joined to 1 while the pipe is warm. It is not used for frost protection: leave it empty. |
| E | The earth (E), for the metal strap and the plate on the pipe. Land the earth here, from block 3. |
| 1 | The common (COM). In a frost kit the call of the frost thermostat (frost stat) comes in here. |
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.
It feels the Pipe row on the Conditions card. The pipe colder than the setting: 1 to 2 is made, and 1 to 4 is open. The pipe warm: 1 to 4 is made, and 1 to 2 is open. Never both.
In this simulator it changes over 4 degrees either side of its setting. That is an 8 degree DIFFERENTIAL.
Set at 25 °C, it calls until the pipe reaches 29 °C. Then it stays satisfied until the pipe falls to 21 °C. So it does not keep switching on and off at the setting.
A dead test, isolated, with the cover off: continuity from 1 to 2 while the pipe is cold, and from 1 to 4 while it is warm.
Live, in series with a frost thermostat (frost stat): the frost path puts 230 volts (230 V) on 2 only while BOTH call. The unheated room is cold AND the pipe is cold.
On an S-Plan its 2 shares block 5 with the call of the room thermostat. So 2 also reads 230 V whenever the room thermostat calls.
In the Sandbox the pipe warms while a boiler fires with its water moving. It cools again when the boiler stops, and the pipe thermostat calls again.
Where it connects
Lessons that use it
- Frost and pipe stats together
Intermediate · Protection and extras
Practise it in the simulator.
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