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Under Construction

This page is still being written, and has not had any form of review or comment.

Do not rely on any information presented here.

--John Rumm 03:56, 18 November 2007 (GMT)

This article discusses some of the many and varied ways in which thermostats are wired into domestic central heating systems.

<cautionary statements about mains voltages etc>

What does a thermostat actually do?

A thermostat is a simply a temperature controlled switch. In its simplest form the switch will default to closed or "On" until the stat reaches a preset temperature. Once it is warm enough, it opens or turns "off".

The temperature at which this happens is usually selected by a user adjustable control.

Do I need one

Yes. If you dont have one, the boiler will keep cycling when the programmer switches it on, even through a hot summer, thus wasting fuel and adding wear and tear to the boiler.

TRVs alone don't maintain a fully stable temperature, so again heat will be wasted when temps rise above what's required for comfort.

Cover heating system interlock, TRVs etc.

How many should I have

Most systems need one room thermostat. Large houses will be split into several zones, and require one thermostat per zone.

It is possible to find more than one thermostat per zone in a minority of cases, but this is not usually needed.

When underfloor heating is fitted each UFH circuit will need its own thermostat to regulate the circulating water temperature.

There should also be a hot water cylinder thermostat. However there are systems without one in service, and these operate reasonably well, with the extra energy waste being small. Without one the hot water may become hot enough to scald though.

Discuss cylinder stats, multiple zones, UFH etc.

Types of thermostat


Bimetal stats contain a bimetallic element that bends with temperature change. This is a purely mechanical control method. The inherent hysteresis in a mechanical room stat is mostly cancelled by an inbuilt resistor known as an accelerator resistor or compensator resistor. The resistor requires a neutral connection to work; without this the room temperature will swing up and down by a few degrees, which is not a satisfactory way to run CH. These thermostats can hold room temp to within half to one degree. Being mechanical they are pretty reliable long term, though not perfect.

diagrams and piccies of trad Honywell style stat

Electronic Programmable

Electronic thermostats use a thermistor sensor plus some electronics. These can maintain a slightly closer temperature control, to within a quarter to half a degree C.

The main advantage of electronic stats is that they're programmable. The set temp can be different at differing times. By setting the temp to minimum at some time the system can also effectively be switched off then. In other words these stats act as controllers as well as thermostats.

The downsides of electronic thermostats are:

  • less long term reliability
  • cease working when the battery runs out
  • can be complex to operate

The advantages are:

  • settings can be locked
  • can act as controller as well as thermostat
  • can set different temps at different times - though whether this is useful IRL depends on personal preferences
  • can maintain closer temperature setting by a fraction of a degree

Wireless or wired

Wireless thermostats avoid the need to run a mains wire to the stat, and are battery powered. They stop working when the batteries run flat. They also can be moved from one room to another.

Tamperproof stats

There are 2 types of tamperproof stat:

  • most programmable stats can be locked
  • Tamperproof bimetal stats have the control knob accessed only after removing screws

Thermostat features

Cover why there are a number of variations and explain things like:

  • Anticipator / Compensator heaters
  • Changeover contacts
  • Frost protection
  • Aux switching and contacts
  • Optimising stats

What does the thermostat control

  • Connecting to a boiler
  • Controlling a zone valve

How should I wire it

  • Type of cable
  • Earthing or lack of
  • what to do with spare neutrals

What else can I control with a thermostat

  • other apps including switching electric heating loads directly
  • cooling loads
  • over temperature alarms and sensors.