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.
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 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.
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
Cover why there are a number of variations and explain things like:
Anticipator / Compensator heaters
Bimetal room stats use a compensator resistor to enable them to maintain a steady temperature. This mostly cancels out the inherent hysteresis of bimetal stats. This inbuilt resistor is also known as an accelerator resistor or compensation resistor.
This resistor requires a neutral connection to work; without this connection the room temperature will swing up and down by a few degrees, which is not satisfactory.
It is 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.
Hot water stats don't require such temperature accuracy, so don't incorporate a compensation resistor and don't need a neutral connection.
The power consumption of these compensation resistors is a fraction of a watt for part of the time only. The annual run cost is less than 10p, much less than electronic controller batteries.
Some thermostats have 1 pole 1 way switching. These switch on when heat is needed, and off when not. These cant therefore control air conditioning.
2 way switching stats switch either on or off as temp rises, depending on where you conect the wires. Thus they can control heating or cooling equipment.
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
Instead of switching on and off, these stats modulate the boiler as set temp is approached. With a compatible modulating boiler these can reduce energy to some degree.
What does the thermostat control
- Connecting to a boiler
- Controlling a zone valve
How should I wire it
Type of 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 conection is needed if no metalwork is touchable.
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.
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