Many technicians, including myself, consider the compressor as the heart of the air conditioning system. It pumps the refrigerant through the system as the heart pumps blood through the body. If the compressor is the heart, then undoubtedly the thermostat must be considered the brain of a system. It tells the system to heat, cool, or ventilate, and exactly how much of that to do.
There are many types of thermostats on the market today. Some of them are very simple and others look like something from another planet. Price determines their intricacy, and they can have microprocessor circuitry. These mini-computers can be programmed to adjust or change temperatures automatically during the week. They can be programmed to shut the system down if the structure is to be closed and vacant for a while. They turn themselves on automatically prior to the occupants returning in order to pre-cool the structure. These along with other advantages can be found in the new technology being brought into the thermostat manufacturing.
In Fig. 11-1 the popular round one is, in my opinion, number one of the workhorses of thermostats. It is a bimetal control that uses a mercury switch. When the bimetal flexes in either direction, the mercury switch closes in the way the mercury tips. In this way, both heating and cooling can be accomplished through the same mercury switch. With the addition of multiple mercury switches, staging of heating and cooling is made possible. The thermostat shown in Fig. 11-1 is only a switch. This switch mounts to a sub-base. It is the sub-base that contains the specific control circuitry. Table 11-1 lists some of the more popular round sub-bases giving part number and function. Honeywell is about the largest manufacturer of thermostats. They manufacture them for other companies and place the other companies names on the face of the thermostat, in essence they are Honeywell products. There are other manufacturers however producing their own products that work along the same line as do the Honeywell products.
Fig. 11-1. Honeywell T87F thermostat.
Table 11-1. Sub-base Applications with the T87F Thermostat.
In Fig. 11-2 a sub-base is shown for a round thermostat. Note the indexing lines on the sub-base. It is very important to plumb this type of thermostat when installing. Vibration of the wall can also change this adjustment. Ordinary use with a customer turning it on and off swinging the levers can throw the sub-base out of alignment. If this happens, the calibration of the thermostat is wrong. This means that the desired temperature settings will not be accomplished, due to mis-alignment to the sub-base. A small wrench tied to a string makes a good plumb-bob.
Fig. 11-2. Sub-base.
You can see that the mounting surface has to be stable in order to use a mercury-switch type thermostat. Any vibration or quiver in the wall can cause short-cycling of the condensing unit. A door slam shouldn’t cause a vibration that would cause a mercury switch to act, but there are many cases where this does happen. Some are found in mobile homes, modular homes, and in certain cases where aluminum studding for the wall has been used. Be aware that there are other options to you for a thermostat selection.
Figure 11-3 shows a bimetal thermostat with a magnetic assist. This type of thermostat would be more appropriate to an area with slight vibration.
Fig. 11-3. Bi-metal thermostat.
Another type of thermostat that can be used in this situation is the bellows type. It is a gas-filled bellows that expands and contracts with heating and cooling. This movement actuates the switching device. Many of these thermostats are manufactured to use line voltage.
If you have a situation where there is a vibration in a wall that is causing erratic thermostat operation, it might be more cost effective to change the thermostat style than to pull wiring through a wall in order to change the thermostat location. Thermostats are built in different shapes. They can be round, square, or rectangular, but remember that they all perform the same job.
The last one that can be used is the electronic thermostat. It uses electronic switches with thermistors. The basic theory is that no energy flows when resistance maintains equal potential in the circuit. An imbalance caused by the heating or cooling of a thermistor changes resistive value and allows a flow of current. Of course the cost of this unit is very high. In most cases, installation also takes time due to the fact that additional wiring is usually required.
In my opinion the thermostat and the expansion valves are the most unjustly accused components of the system. They are both very dependable and are sometimes falsely condemned. Remember the thermostat is nothing more than a series of switches that open and close telling the system what to do.
The thermostat is the center of operation. It receives wires from the evaporator section and from the condenser section. Number 18 wire is usually used. It can be 18/2, which designates 18-gauge wire, two conductors. There is a difference in price between 18/2 and 18/7; however, in the event of a wire breaking or shorting, it is a lot cheaper and faster for you to have a couple of extra conductors pulled through the structure. In fact there may come a time that another control is to be added to the condensing unit or evaporator unit, the wire will be available to carry another circuit. This is the reason I suggest the use of 18/7 thermostat wire. Each of the conductors insulation has a different color. This makes the mating of different circuits easier.
Reviewing Chapter 5, remember that the relay has a common wire to it. In the case of a 24-volt control circuit, each relay has T1 from the transformer wired to it. This means that each relay’s holding coil only needs the T2 of the transformer to energize its electromagnet coils.
R. This is the terminal that accommodates the T2 wire from the thermostat. When this voltage passes through the thermostat switch, a relay/relays will energize. In certain applications where there might be two separate control circuits, such as those found in some fossil fuel units, a separate transformer is used in each system. One would be for the cooling that would route through the R1 terminal, and one perhaps a different voltage through R2 If both heating and cooling controls are of the same voltage, a jumper wire can be placed between R1 and R2. Some thermostat manufacturers use V (voltage) or RC in place of the R. It is the same terminal and the common from the transformer is affixed to it.
Y. This is the terminal that is used to energize the condensing unit, of course in the cooling mode. Depending upon the application, there might be a few stages to cooling, thus Y1, Y2, Y3 terminals indicating first, second, and third stage of cooling.
W. This terminal is used to energize the heating. As with the cooling, the heating mode might have more than one stage, thus, W1, W2, W3, would be used. This would indicate three stages of heating.
G. This terminal is used for the evaporator fan. In some cases it might be indicated with an F.
C. This is a terminal where T1 of the transformer is attached. The reason is usually for illumination of a pilot light. This could show when a compressor is in operation, or when a filter is clogged, or any other type of 24 volt signaling device used. In some thermostats, the C is used for cooling if there isn’t a Y terminal.
O. This is an auxiliary terminal that will energize when in the cooling mode.
B. This is an auxiliary terminal that will energize when in the heating mode.
In Fig. 11-2, a round sub-base is shown with the terminal identification letters. Although the coding might be different depending on the manufacturer, the functions are the same. One more thing, after the wire is hooked to the sub-base plug, the hole where the wire comes through the wall should be sealed in order to prevent erratic thermostat operation due to hot or cold drafts that might be flowing in the wall. A cotton ball or something similar can be used.
This word when used in relation to the thermostat refers to the span of degrees of temperature between the off cycle and the operating cycle. The ultimate differential is two degrees. In the mechanical type of thermostats, this is sometimes difficult to achieve. If a thermostat is set to maintain 76 degrees F., in a conditioned space, it would have to cycle on at 77 degrees F. and cycle off at 75 degrees F. thus giving you a two-degree differential. This can be attained with electronic thermostats but is quite difficult to achieve with the mechanical types. Some thermostats might have a very wide differential, causing the conditioned space to become too hot or too cold before it cycles on. Many thermostats can be calibrated, some have a fixed differential. Try to adjust to the two degree optimum.
In Fig. 11-4, the plumb-bob is being used to level a round sub-base. Notice the alignment marks used. In Fig. 11-5 the rectangular sub-base is shown. This and the square type use a bubble-level to accomplish the leveling. There are two important things I want to mention at this point. First, be careful of marking up the customer’s wall around the thermostat. Grease or dirt from your hands can mar the wall. Second, with some dry-wall construction, it can be difficult to get a secure mount for the sub-base. The result is a larger hole than the round thermostat will cover. A piece called a designer, or decorator plate can be purchased at the parts supplier. This commonly called goof, or oops or idiot, plate is nothing more than a larger circular piece of plastic that can be used to fasten to a different section of the wall where the screw holes have never been, then attach the sub-base to this plate.
Fig. 11-4. Leveling sub-base with plumb bob.
Fig. 11-5. Leveling rectangular sub-base with bubble-level.
Don’t let this word scare you. I’m going to touch lightly upon this type of controlling for those technicians that might come in contact with it. Up till now, the only means that were used for control were electrical, in the form of a switching device, or a bellows type that causes pressure exertion with expansion of the gas. The medium used in pneumatic controls is compressed air. Why is it used? Good question. Remember that for every 50 feet of wire run, the resistance factor dictates that the wire size be increased.
Where the control circuitry must be lengthy, compressed air lines can be more economical to install and maintain. A common air compressor is used to drive the entire system. There could be many units in an office building that will be controlled with the air pressure from the common air compressor. The air is transported through the building using either plastic or copper tubing as a conveyance. The air pressure finally operates pneumatic actuators that open and close dampers, or open and close switches. The switch is activated by air pressure. The PE (air pressure operates an electrical switch) switch can operate anything electrical as does the standard electromagnetic switch. The PE stands for pneumatic electric. The reverse of this is the EP switch. This EP (electric pneumatic) performs the exact opposite action. An electric switch controls the air flow. Those are the two main types of switches found in the pneumatics. Figure 11-6 shows a typical air compressor assembly used in the pneumatic control system. These systems are manufactured by Honeywell, Johnson, Robertshaw, and Barber-Colman to name some. The pneumatic thermostat as shown in Fig. 11-7 still uses the bimetal principle to bleed air from the source. By bleeding the air, pressure is not sent into the vessel that leads to either the pneumatic motor or the PE switch.
Fig. 11-6. Air compressor system to supply air pressure in a pneumatic control circuit.
Fig. 11-7. Pneumatic thermostat. 114
In Fig. 11-8 a pneumatic actuator is shown. This device converts the air pressure by the use of a piston to a mechanical force in the push rod. This actuator can operate a valve, dampers, etc. Figure 11-9 illustrates a valve with a pneumatic actuator. Valves such as these are also made to operate with electric motors.
Fig. 11-8. Pneumatic motor.
Fig. 11-9. Pneumatic operated valve.
Alright, the next time someone mentions pneumatic controls to you, this basic knowledge will be helpful to you in understanding … if you didn’t up till now.
This is another type of thermostat that you may encounter. It is basically used in a commercial application where too many people turn the temperature up and down all the time. Figure 11-10 is the basic accustat. The principle of its workings are extremely simple. A vial of mercury similar to a mercury stick thermometer is used across a set of open contacts. The mercury completes the circuit when it reaches both contacts. These are non-adjustable and are purchased at specific temperature ratings. If the owner wants 76 degrees F. in his establishment, that’s it. If he wants to change that, he will need a new accustat sensor. These accustats come in different models from the basic single-stage cooling to a two-stage cooling, two-stage heating with an automatic changeover that needs no attention to maintain a set temperature regardless of the season. An embossed numeral on the glass envelope of the sensor designates the temperature of the switch closing.
Fig. 11-10. Accustat, with cover removed.
These are used to keep units from cycling on in cold weather or to place hot water or steam into a water tower during cold weather operations. It is a sensing device for piloting another control that is set to ambient temperature.
