Volt-free or mains-switched? How to tell from the back of the thermostat

What volt-free means

A thermostat (stat) is a switch worked by temperature. The question is where its switch gets the live it switches.

• VOLT-FREE: nothing inside the unit feeds the switch. While it calls, it joins its COMMON (COM) to its call contact, and gives out whatever you connect to the common: 230 V, a much smaller voltage, or nothing at all. Its other name is POTENTIAL-FREE.
• MAINS-SWITCHED: the switch is fed from the unit’s own live (L), inside it. Its output gives out 230 V with nothing more to wire.

Why have volt-free at all? Because the same volt-free stat can switch 230 V on one system and 24 volts (24 V) on another. The boiler or the system decides the voltage, not the stat.

Volt-free does not mean safe to touch. On a 230 V system, the common carries the mains live you feed it.

How to tell from the terminal map

The map is printed on the back-plate, inside the cover or in the installation sheet. Read it before you wire, and look for three things.

1. Does the unit have L and N at all? A digital stat on batteries usually has neither: only COM, normally open (NO) and perhaps normally closed (NC). With no supply of its own, its switch can only be volt-free.
2. If it has L and N, does the switch have its own COM? A mains-powered stat or receiver with L, N and a separate COM has a volt-free switch. Its L and N feed only its electronics.
3. Are there outputs with no common at all? Then the switch is fed from L inside: mains-switched.

NO is the CALL: the contact that closes when heat is wanted. NC is closed at rest, and some sheets call it the satisfied contact. Makers print their own numbers beside them, so land each wire by the printed function, never by the number alone.

A mechanical stat marked L, a switched live and N is another kind again. Its L is the way in for the demand, and its switched live is the call. Its N feeds a tiny heater inside it, the ACCELERATOR.

Two units that look alike can be wired differently. Read the map of the one you are fitting, every time. See how to read the back of a programmer.

Wiring a volt-free stat on a 230 V system

On a 230 V S-Plan or Y-Plan, feed the common with the heating demand, so the programmer still decides when. On a wiring centre numbered the common way:

• COM from block 4, the programmer’s central heating ON (CH ON);
• NO, the call, to block 5, on to the heating valve;
• NC left empty, unless the diagram for the job uses it.

That is the same pair a mains room stat makes: the demand in on block 4, the call out on block 5. Only the names of the screws change. See the S-Plan with a digital room stat.

A programmable thermostat keeps its own time. Its maker’s diagram says where its common is fed.

A programmer or receiver with volt-free commons needs a live on them too. On a 230 V system the installer fits a LINK, a short sleeved conductor from L to the common, where the maker’s sheet shows one. Without it, the screen says ON and nothing comes out.

On a boiler’s low-voltage input

Some boilers run their room-thermostat input at EXTRA-LOW VOLTAGE, such as 24 V. There the boiler feeds the contact itself. It gives a pair of terminals, a feed out and a return, often joined from the factory by a link.

Take the link out and put the volt-free contact across the pair: COM on one, NO on the other. The contact then carries only the boiler’s own voltage. Check the contact is rated for it.

Never put mains on a low-voltage input. One maker’s combi, for example, takes a 230 V switched live on one terminal and a low-voltage control on another. Its instructions warn that mains voltage on the wrong terminals may destroy the electronics. So a mains-switched output, or a volt-free common linked to L, never goes on a low-voltage input. Read the boiler’s manual for the input it has, every time.

Proving it with the meter

Isolate, lock off and prove dead before any cover comes off. See safe isolation in five steps. The readings below are live tests: only for a competent person, or a learner under the supervision of one, with test leads made to the HSE’s guidance note GS38.

On a 230 V system, read each terminal to a neutral you know is good:

• COM: about 230 V while the programmer allows the heating.
• NO: about 230 V while the stat calls, 0 V once it is satisfied.

Read the common first. A volt-free stat gives out nothing that you did not put in.

On a boiler’s extra-low voltage input, a reading to the mains neutral tells you nothing. Read where the boiler’s manual says.

A battery stat keeps switching with the mains isolated. So, with the circuit proved dead and the stat’s wires off its terminals, the meter’s continuity setting across COM and NO shows the call: closed while it calls, open once it is satisfied. A mains-powered unit cannot be tested that way: isolated, its relay rests on its rest contact. Never test continuity on a live circuit.

Sources

• The Electricity at Work Regulations 1989, regulations 14 (live work) and 16 (competence).
• HSE guidance note GS38, Electrical test equipment for use on low voltage electrical systems.
• The makers’ installation instructions for each thermostat, receiver and boiler: they give the terminal names, the link and the voltage of each input.