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The wall thermostat
The most obvious part of any heating or cooling system is the wall thermostat. It senses the room temperature so it can turn on the heat or cool.
They can be extremely simple or the most complicated part of the system.
It is usually located approximately 55 inches off the floor. It should not be located where it can be affected by sunlight, heat sources such as coffee pots or lighting, drafts from open doors or near any supply air outlets. It should also not be located in a dead air space such as a closet. Hallways are commonly used.
They can be extremely simple or the most complicated part of the system.
It is usually located approximately 55 inches off the floor. It should not be located where it can be affected by sunlight, heat sources such as coffee pots or lighting, drafts from open doors or near any supply air outlets. It should also not be located in a dead air space such as a closet. Hallways are commonly used.
How to tell if the thermostat is calling for heat.
Below is a video on mechanical thermostats. These thermostats have no digital readouts and are electromechanical rather than electronic. The video covers how to make sure the thermostat is calling for heat before calling for service.
If the thermostat is electronic, the video below covers how to make sure the thermostat is calling for heat.
How the mechanical thermostat works
The mechanical thermostat is the oldest way to control space temperature.
The cheapest and most accurate way to control temperature is by use of the bimetal strip.
Because different metals expand or contract at different rates, if I were to bond 2 different metals together to make a bar then change the temperature of the bar, the bar will bend. When the bar bends, that action can open or close a switch. Coiling the bimetal as in the pics below will increase the sensitivity of the bimetal. Switches used in these thermostats can be open contacts snap action switches using a magnet to open or close the switch or older ones used sealed mercury switches.
Below is a video on how the mechanical thermostat operates.
The cheapest and most accurate way to control temperature is by use of the bimetal strip.
Because different metals expand or contract at different rates, if I were to bond 2 different metals together to make a bar then change the temperature of the bar, the bar will bend. When the bar bends, that action can open or close a switch. Coiling the bimetal as in the pics below will increase the sensitivity of the bimetal. Switches used in these thermostats can be open contacts snap action switches using a magnet to open or close the switch or older ones used sealed mercury switches.
Below is a video on how the mechanical thermostat operates.
Thermostat lag and overshoot
A mechanical thermostat, as all things mechanical, resists movement. The bimetal in the thermostat does not move immediately upon a change in temperature. This resistance causes the thermostat to lag behind the actual temperature by approximately 2 degreesF or 1 degreeC. The effect of this is the thermostat that is set at 70F will drop to 68F before turning on the heat. Once the heat comes on, the thermostat contacts will not open until 72F.
The drop below set temp is called lag. The point at which the contacts open is called overshoot.
The difference between on and off is 4F. This amount of temperature difference can be felt by the occupants. The perception is of too hot then too cold. The video below covers lag and overshoot.
The drop below set temp is called lag. The point at which the contacts open is called overshoot.
The difference between on and off is 4F. This amount of temperature difference can be felt by the occupants. The perception is of too hot then too cold. The video below covers lag and overshoot.
Above on the left is one type of anticipator.
The small silver wire around the white disc is the anticipator heater. The silver dial is the adjustment for the anticipator.
The anticipator should produce a specific amount of heat to operate properly. Because the power used by the furnace control circuit varies with the different types of controls, the power going through the anticipator and thus the amount heat produced by the anticipator varies. In order to produce the proper amount of heat for the anticipator to work properly, we adjust the amount of wire exposed. Higher amp draw of the circuit needs less wire exposed to get the same heat. The numbers on the dial are equal to the amp draw of the circuit. Set the pointer to the proper number and the anticipator should work properly.
The video below covers how to check amp draw of the control circuit and set the anticipator.
The small silver wire around the white disc is the anticipator heater. The silver dial is the adjustment for the anticipator.
The anticipator should produce a specific amount of heat to operate properly. Because the power used by the furnace control circuit varies with the different types of controls, the power going through the anticipator and thus the amount heat produced by the anticipator varies. In order to produce the proper amount of heat for the anticipator to work properly, we adjust the amount of wire exposed. Higher amp draw of the circuit needs less wire exposed to get the same heat. The numbers on the dial are equal to the amp draw of the circuit. Set the pointer to the proper number and the anticipator should work properly.
The video below covers how to check amp draw of the control circuit and set the anticipator.
If you do not have an amp meter to check control circuit, the video below may help to set it properly.
If your thermostat is electronic, anticipation is controlled by the cycling control inside the thermostat.