Thermostat

This article discusses some of the many 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 basically a temperature controlled switch. Typically used to turn on the heating, and keep it running until the stat reaches a preset temperature. Once it is warm enough, it opens turns the heating off, and the periodically runs the heating as required to maintain the preset temperature.

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

Do I need one

Yes most heating systems will need at least one. If you don't have one, there is no way to turn the heating completely off once the desired temperature is reached. (the building regs refer to this as a "boiler interlock").

But I have Thermostatic Radiator Valves, do I still need a stat?

Yes even with TRVs the stat is needed to stop the boiler from running where there is no need for it to do so.

How many should I have

Most systems need at least one room thermostat (larger houses may have the heating split into several "zones", and each of these will require a thermostat). If your heating system also stores hot water, then the hot water cylinder will also need a cylinder stat.

Systems fitted with Under Floor Heating may have additional stats to control the UFH heating zones.

Types of thermostat

There are two broad categories of stat in use, mechanical and electronic.

Mechanical

Mechanical stats usually use a bimetallic element that bends with temperature change. This is used to make or break a switch that controls the "call for heat."

Being mechanical they are pretty reliable long term, and easy to understand. They typically control the temperature to an accuracy of half a degree. They lack the extra features of electronic programmable stats, such as ability to lock the temp control, and change temp settings through the day.

Compensation / acceleration

Mechanical CH stats include a tiny compensation heater (a small resistor eating less than a watt) that comes on when the stat switches "on," heating the thermostat very slightly. The purpose of this is to reduce the hysteresis of the bimetal stat, which without compensation is around 3 degrees C.

This resistor requires a neutral connection to work; without this connection the room temperature would swing up and down by a few degrees, which is not satisfactory.

(Its also possible to miswire a bimetal stat so that the compensator is on at the wrong times. If this happens, a wide range between switch on and switch off temps will be seen)

Mechanical stats are usually designed for operation in mains voltage control systems. The 240v compensation resistor won't work on a lower voltage.

Electronic

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.

Rather than employing a rotary knob and temperature control, they will usually have a LCD readout.

The main advantage of many electronic stats is that they're programmable. The preset temp can be set to automatically switch to different levels at specific times of the day. Programmable stats can in effect act as programmers as well as stats. (on combi boiler systems where there is no need for a cylinder stat, then the system programmer can be dispensed with altogether)

The downsides of electronic thermostats are:

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

The advantages are:

• settings can be locked
• can act as controller as well as thermostat
• can set different temps at different times of the day
• can maintain better control of the temperature, making for better comfort.

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

These are stats which once preset, can't easily be altered by casual users.

• Many programmable stats can be "locked" to achieve this
• Tamperproof bimetal stats have the control knob accessed only after removing screws

Thermostat features

Changeover contacts

Some thermostats have 1 pole 1 way switching. These switch on when heat is needed, and off when not. These can't therefore control air conditioning for example (i.e. turn something "on" when the room is warmer than a preset temperature)

Two way switching stats, switch either on or off as temp rises, depending on where you connect the wires. Thus they can control heating or cooling equipment.

Frost protection

Frost protection is a 5 degree C setting designed to avoid pipes freezing. Many bimetal stats can be turned down to 5C rather than switching the system totally off. Electronic stats normally incorporate frost protectino.

Aux switching and contacts

Some stats have additional contacts available for switching extra equipment along with the heating.

Proportional / Modulating stats

These are more sophisticated stats that are able to either report the actual temperature back to the boiler, or indicate the amount of heating required. These are often bespoke items that need matching to specific boilers that understand how to use this type of stat, and are often included in "weather compensating" thermostats. This can allow for more efficent operation of a boiler by adjusting the flow temperature of the boiler water to match the actual needs. Hence the heatin will run hotter on cold days, rather than just longer.

Optimising stats

These are programmable stats that will attempt to more accurately meet the changing temperature demands of the stat as it switches to each new temperature setting throughout the day. To do this they can turn the heating on before the demanded time to ensure that the temperature requested is met at the start of the time.

Hence if you say have the heating set to tick over at 15 degrees over night, but then want it to be 21 degrees between 7 and 9am. Assuming it is not particularly cold over night, a non optimising stat will wait until 7am before calling for heat. This may mean the desired temperature is not actually reached until sometime after 7am. The optimising stat however will turn the heating on some time before 7 to endure it is already 21 degrees by the time 7am arrives.

Some optimising stats will attempt to automatically learn the response of the house, others may need to be set manually to adjust for the typical lag of the building.

What does the thermostat control

Depending on the type of stat, and the type of zoning used, the stat may connect to one of more of:

• The boiler
• A zone valve
• A pump

How should I wire it

Type of cable

Fixed house wiring should be implemented in flat T&E cable:

• 1mm2 T&E where no neutral is needed
• 1mm2 3&E where a neutral is required
• Larger cable is also fine

Earthing or lack of

Earthing of the stat is usually needed where metal parts are touchable. No earth connection is required for double insulated stats. This is typical for battery operated programmable stats. (If an earth wire is present simply sleeve it and "park" it in a spare unused terminal). Note is it not acceptable to re-purpose am unused earth wire for some other purpose!

What to do with spare neutrals

If neutral is not needed (ie with battery powered electronic stats), it is simply not connected. Put the stray wire end into a lone screw connector to stop it touching things it shouldn't.

Some stats that don't need a neutral have a parking position for a neutral wire. This can be used instead of the above. If your stat doesn't have this, don't connect N to an unused connector position unless it is specifically marked for the purpse, or spitzensparken and fusenpoppen may result, and this may kill the thermostat's relay.

What else can I control with a thermostat

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