HVAC Unit 39 Controls

What year is R-22 totally banned from production?

2020

What rpm do modern compressor turn at?

3600 rpm

How many amps does a 40VA transformer provide?

40VA / 24V = 1.666A which rounds to 1.7A.

What is the sensing element in modern thermostats?

A thermistor.

Which type of control works better, electronic or electromechanical?

Electronic circuit boards can monitor high voltage while electromechanical cannot.

Why are controls operated with 24V instead of 230V?

For safety and convenience.

The three main components that are controlled by A/C controls

Indoor Fan, Compressor, Outdoor Fan.

Why do some contactors only have one set of contacts and the other connection is a straight bar?

It provides a small amount of current to a crankcase heater to keep the compressor warm during off cycle.

Describe the compressor contactor.

It starts and stops the compressor and outdoor fan. It is a power-consuming device due to the magnetic coil. It received power through the room thermostat that energizes the coil and closes the contacts to start the compressor and fan.

Describe a fan relay.

It starts and stops the indoor fan. It is a power-consuming device due to the magnetic coil. It received power through the room thermostat that energizes the coil and closes the contacts to start the fan.

What does a low pressure control do?

It stops the compressor from pulling the suction pressure below a predetermined point such as when the system has a leak and loses all or part of its charge. It prevents air from being drawn into the system.

What does a high pressure control do?

It stops the compressor when a high pressure condition exists such as the condenser fan not running.

What type of safety control is used with modern compressors?

Most only have a motor temperature control that senses the temperature of the windings and opens when there is a problem. Some systems also have a low-pressure control to stop system in case of total loss of refrigerant.

If a system has a TXV, what type of motor does the compressor have?

Motors with capacitor-start, capacitor-run. These motors needed a high starting torque due to the TXV not equalizing the high and low pressure sides during the off cycle.

The two main types of functional controls

Operating and safety controls.

What type of systems use a PSC motor?

PSC motors have little starting torque and used with systems that equalize the pressures between high and low sides during the off cycle. These systems are usually fixed bore and never TXV. A PSC does not have a start capacitor.

What are the components of a starting system?

Potential starting relay, start capacitor, run capactor.

When the compressor is running, what other devices should also be running on a typical system?

The outdoor and indoor fans should always be running when the compressor is operating.

If a 40VA transformer is only providing 18 volts, what is the problem?

The transformer is pulling more amps than it is rated. 40VA / 18V = 2.222 amps. With this higher amperage, the transformer will get hot.

Are the compressor and outdoor fan wired in series or in parallel?

They are normally wired in parallel.

What do internal motor protection do?

They sense the actual motor temperature and stop the motor when it is too hot.

Describe a high-pressure control

Usually modular type, permanently installed and nonadjustable.