Category Archives: Commercial HVAC

DDC Control Loop

DDC Control Loop

Direct Digital Controls are nearly everywhere in commercial and some industrial processes. We have several articles here at High Performance HVAC which describe the basics of DDC and how DDC works. There is also a page here concerning DDC where we describe the process of DDC including the DDC control loop. We wanted to make the process a little simpler for the layman audience and to make it more helpful for the new HVAC technicians and Control Technicians to learn the process in simpler terms. In the basic control loop we have input, processing, and output.

HVAC DDC Control Loop

DDC Control Loop – Input

The control loop begins with the input sensor or switch. A device that measures something such as temperature, humidity, carbon dioxide, carbon monoxide, or even a switch that closes or opens can be the input signal for DDC.  An example of a switch input would be a fire detector alarm or dry contacts closing on a current transformer that senses a motor is energized.  The temperature sensor and other sensors are analog and are configured by the installation crew and start-up technician. These analog signals are scaled for the medium they are sensing and the either send a 4 to 20 milli-amp signal, or a DC signal of 0 to 10 or 2 to 10 volts. These input devices are connected directly to the controller for next process in the DDC Control Loop.

DDC Control Loop – Processing

output device ddc control loop

Actuator for Chilled Water – the actuator modulates from 0% to 100% based on the program and the input from the input device(s). This is floating control.

The input signal is received by the controller and the controller processes this signal based on the program. Depending on the program will depend on the control response to the output device. The output signal can be the output signal to a variable frequency drive (analog) or to dry contacts (open or close the contacts) or many other devices. The control response can be either

  • Floating Control e.g. an actuator controlling an air damper or a chilled water valve where the CFM’s are controlled or the GPM is regulated for the chilled water.
  • Two-Position control e.g. a hot water valve that is either open or closed
  • Proportional is typically abbreviated by a P and is a response sent out by the controller to the output device
  • Proportional plus Integral is typically abbreviated by PI and is a response sent out by the controller to the output device
  • Proportional plus Integral plus Derivative is typically abbreviated as PID and is a response sent out by the controller to the output device.

It is out of the scope of this article to explain P or PI or PID as it is somewhat complex however when enabled these control responses can smooth out motors and flow until a steady state is achieved. The closest way to describe PID is to compare it to a thermostat anticipator where the anticipator eliminates or reduces temperature swing by optimizing how the system is controlled.

DDC Control Loop – Output

We now have Input and Processing and finally we need Output to an Output device. This can be either an analog signal or a binary signal where we turn something on or off or modulate something based on the input and the program in the processor of the controller. A good example of this is a VAV box where the tenant is too cold and adjusts the thermostat. The thermostat is the input device and is going to tell the controller it is too cold based on the current set point. The controller processes the request and sends a signal to the VAV Box damper to close a little for reduced airflow while at the same time sending a signal to the hot water valve actuator to modulate open. Depending on the set up the actuator and valve may be floating or two-position but either way the valve opens and hot water is introduced to the hot water coil. The tenant immediately feels the room getting warmer and is satisfied. Some DDC thermostats are non-adjustable and only sense temperature. The temperature set point is controlled by the building manager or engineer in that case.

In conclusion, we learned that the DDC Control Loop involves Input, Processing, and Output and involves sensors or dry contacts, a controller, and output devices.

How Does an Expansion Tank Work? HVAC Hydronics

how does an expansion tank works.

The difference between a steel expansion tank and a bladder type expansion tank.

Expansion Tank Basics – Types

What is an expansion and how does it work? I get that question all the time even though I have written an extensive article on expansion tanks.  The two most common expansion tanks you will see in HVAC applications include the bladder type expansion tank and the steel type expansion tank.  The steel type expansion tank needs some basic maintenance from time to time to ensure it doesn’t become waterlogged or overfilled which would defeat the purpose of having an expansion tank. For more on the basic maintenance of a steel expansion tank and how to properly fill it check out this article.

How Does an Expansion Tank Work

How does an expansion tank work and why do you need one in your boiler or chilled water system? If you have a hot water boiler system (not a low pressure steam boiler – there is a huge difference between the two) or a chilled water system (not many chilled water systems in residential but I have seen them) then you have a closed loop piping system that runs to either convectors, radiators, or a coil or two somewhere in the system where the main heat exchange process takes place. Inside this piping is water (sometimes a water/glycol mix but that is mainly in commercial applications) and when water heats and cools down it expands and contracts. The piping system along with the parts of the boiler or chiller that have water cannot handle this expansion and contraction with breaking or busting open. Therefore in every hydronic system there is an expansion tank to handle this expansion and contraction. An expansion tank is basically a shock absorber that absorbs this expansion and contraction of the volume of the water.

High Performance HVAC Air Conditioning and Heating Systems

How Does an Expansion Tank Work


Replace Air Conditioner Compressor | HVAC Refrigeration

Replace Air Conditioner Compressor Decision | HVAC Refrigeration Question

Replace Air Conditioner Compressor | HVAC Refrigeration – Question – Richard – I am an HVAC technician and have read all of your articles on refrigeration and compressors. Great information and it has helped to hone my skills and offer better service for my customers. Thanks! I was wondering what your experience is with replacing compressors. How do you arrive at the decision to replace a compressor technically speaking of course? I look forward to reading your answer. Brett

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Unit Heater Mechanical Room Equipment Protection

Unit Heater Mechanical Room

How Water Unit Heater in a mechanical room

Unit Heater Mechanical Room Equipment Protection – Mechanical rooms have very important equipment that needs to be protected. Often mechanical rooms are not conditioned and subject to outside temperatures and freezing conditions that can freeze piping inside the mechanical. For that reason mechanical rooms often have unit heaters located there to protect piping and other things that cannot be frozen or subject to freezing conditions. The components and sequence of operation are fairly simple. The main components in a hot water unit heater include a fan, a valve, and of course the piping and coils along with the cabinet.

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Disconnect and Electric Motor Starter with Overloads

Disconnect and Electric Motor Starter is used for turning the power off and on the an electric motor used in HVAC applications. It also has overloads built into the starter so if one of the windings begin to pull too many amps the overloads open the circuit and the motor stops operating. Motor starters can be used for single phase and 3 phase motors and offer a manual way to turn the motor off or disconnect the electricity to the motor to prevent it from starting for maintenance or repair of the motor or peripheral systems. Control wiring can be integrated into the motor starter for automatic control and or monitoring. Monitoring usually occurs with auxiliary contacts or status contacts near the starter. Some solid state motor starters will also detect and stop a motor if it is turning backwards. It is important when troubleshooting motor problems that the technician understand the motor starter and the limitations of the motor starter.

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