Difference between revisions of "Thermostat"

Revision as of 19:39, 8 March 2012

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

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 rely one some form of bimetallic element that bends or otherwise changes shape with temperature change. This is used to make or break a switch that can be used to enable or disable the "call for heat"

Being mechanical they are pretty reliable long term, and easy to understand. However they can only usually control the temperature to an accuracy of a couple of degrees.

Compensation / acceleration

Most mechanical stats also include a small compensatory heater (i.e. just a small resistor) that is connected when the stat switches "on". This helps heat the stat itself, making it better able to sense the air temperature without needing to wait for the air to bring the whole stat up to ambient temperature first. This makes the stat more responsive and improves control.

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 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.

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

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

Cylinder stats will often have a pre-attached length of heat resistant flex attached, and this should be connected to the fiuxed wiring at a wiring centre or junction box.

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

@@ Line 1: / Line 1: @@
-{| border="1" cellpadding="6" cellspacing="0" style="width:500px; text-align:center;  background:yellow"
+This article discusses some of the many ways in which thermostats are wired into domestic central heating systems.
-|-
-| '''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.
---[[User:John Rumm|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.
@@ Line 19: / Line 6: @@
 ==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".
+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 temperature at which this happens is usually selected by a user adjustable control.
+The preset 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.
+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?'''
-TRVs alone don't maintain a fully stable temperature, so again heat will be wasted when temps rise above what's required for comfort.
+Yes even with TRVs the stat is needed to stop the boiler from running where there is no need for it to do so.
-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.
+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.
-It is possible to find more than one thermostat per zone in a minority of cases, but this is not usually needed.
+Systems fitted with Under Floor Heating may have additional stats to control the UFH heating zones.
-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.
+==Types of thermostat==
+There are two broad categories of stat in use, mechanical and electronic.
+===Mechanical===
+Mechanical stats usually rely one some form of bimetallic element that bends or otherwise changes shape with temperature change. This is used to make or break a switch that can be used to enable or disable the "call for heat"
-Discuss cylinder stats, multiple zones, UFH etc.
+Being mechanical they are pretty reliable long term, and easy to understand. However they can only usually control the temperature to an accuracy of a couple of degrees.
-==Types of thermostat==
+'''Compensation / acceleration'''
-===Bimetal===
+Most mechanical stats also include a small compensatory heater (i.e. just a small resistor) that is connected when the stat switches "on". This helps heat the stat itself, making it better able to sense the air temperature without needing to wait for the air to bring the whole stat up to ambient temperature first. This makes the stat more responsive and improves control.
-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.
+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 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)
-diagrams and piccies of trad Honywell style stat
+Mechanical stats are usually designed for operation in mains voltage control systems.
-===Electronic Programmable===
+===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.
-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.
+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:
@@ Line 65: / Line 57: @@
 * 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 set different temps at different times of the day
-* can maintain closer temperature setting by a fraction of a degree
+* 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.
-===Tamperproof stats===
+* Many programmable stats can be "locked" to achieve this
-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===
-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.
 ===Changeover contacts===
-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.
+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)
-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.
+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.
-===Aux switching and contacts===
 ===Optimising stats===
-===Modulating===
+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.
-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.
+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:
-* Connecting to a boiler
+* The boiler
-* Controlling a zone valve
+* 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
+Cylinder stats will often have a pre-attached length of heat resistant [[flex]] attached, and this should be connected to the fiuxed wiring at a wiring centre or junction box.
 ===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.
+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===
+===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, or spitzensparken and fusenpoppen may result, and this may kill the thermostat's relay.
+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==