HVAC Posts Archives - JDB Engineering, Inc.

May 08 2018

The VRF Checklist

By Kevin Angle, PE

If you’re anything like me, you are constantly making lists, whether it be a grocery list, materials list, or to-do list to make sure everything gets done. In my opinion, lists are one of those “necessary evils.” They can seem aggravating at times because, let’s face it, the entire list rarely gets checked off!

As VRF systems are continuing to grow in the marketplace across the globe, as well as here in the United States, it is perhaps time to create a checklist for VRF systems. These systems are being installed in numerous buildings throughout the country. In many cases, manufacturers are directly designing these systems for contractors to install. While this may be an effective approach for some projects, there are significant benefits to having an engineer, working with the manufacturer, to review the design criteria, design the system, and develop sealed and signed construction documents in accordance with the building codes.

A Checklist for VRF Systems

By no means is this list inclusive, but it can be viewed as a baseline to incorporate the fundamental elements required for your VRF system to meet those baseline expectations, and you can “check the boxes.”

■ Ventilation

Couple VRF system with a ventilation system, whether it be natural or mechanical. Natural ventilation would typically include operable windows in accordance with building code requirements. Mechanical ventilation can be accomplished through various methods, including direct mechanical ventilation to the space or common ventilation ducted to each air handler or fan coil unit.

■ Refrigerant Piping Location

Do not locate refrigerant piping in an elevator, enclosed public stairway, stair landing, or means of egress. Install refrigerant piping a minimum of 7.25 feet above the finished floor.

■ ASHRAE Compliance

Comply with ASHRAE 15 and ASHRAE 34, as both standards are referenced in the International Mechanical Code. If you are utilizing a VRF system and are not aware of these standards, you should get up to speed quickly. ASHRAE 15 was put in place around 1930, while VRF systems were first installed around 1980. Therefore, some of the information within ASHRAE 15 does not specifically address VRF systems. In future updates, however, I predict this standard will address VRF systems. Until then we must comply with the current standard.

The ASHRAE standard considers the ability of people to respond to potential exposure to refrigerants, which are classified within ASHRAE 34 with an RCL (Refrigeration Concentration Limit) designation. The standard relates to a leak of refrigerant and its ability to displace oxygen and affect the personal safety of occupants. A leak may occur in any space, and the entire amount of refrigerant within the system or circuit should be considered. An example would be R-410A, which has a maximum RCL of 26 lbs. per 1000 ft3 of room volume for occupied spaces. Most manufacturers calculate the pounds of refrigerant based on the line lengths of refrigerant piping. Smaller rooms with refrigerant-based evaporators may pose issues when considering RCL; therefore, each space volume should be evaluated to make sure the refrigerant concentration limit is not exceeded.

There are some strategies to yield refrigeration concentrations below the limits, like utilizing door undercuts or transfer grilles to increase the volume of rooms. In my opinion, these approaches tend to be ineffective and should not be used, as most refrigerants are heavier than air, and transferred air will not be effective in dispersing the refrigerant.

Many hotel brand standards and medical facility standards require that designers subtract the bed and other furniture in the rooms from the total volume, as the refrigerant will disperse around the bed. Certain school districts do not allow designers to utilize the full height of a room, as the student breathing zones are lower than full height. These items must be considered by designers, who in turn must utilize their engineering judgement to protect the occupants.

■ System Selection

Select the appropriate system type. There are multiple types of VRF systems in the marketplace, including the following air-cooled outdoor units:

Heat pumps
Heat pumps with heat recovery
Heat pumps with auxiliary heating or hyper-heat

Heat pump systems are traditionally rated at 47° F and heating capacity is reduced below 47° F. Most manufacturers’ heat pump VRF systems can only supply 60% of heating capacity at 5° F. In climate zones with winter design temps below 47° F, the hyper-heat option or auxiliary heating methods should be implemented to provide adequate heating capacity. Heat recovery options provide the system the ability to simultaneously heat and cool for temperature controls zones that have differing needs.

■ Line Lengths

Verify that line lengths of refrigerant are within manufacturers’ requirements. Limitations of the refrigerant line lengths vary among manufacturers.

■ Qualifications

Engage a qualified engineer and contractor with the experience and certification to design and install VRF systems.

In summary, VRF systems have many advantages over traditional HVAC systems in certain applications, and I predict they will continue to increase their market share within the HVAC industry. However, using proper precaution when designing and installing VRF systems is critical to providing a system that is both code-compliant and highly effective.

Considering VRF as an option on your next project? Interested in our AIA CES program on VRF applications? Contact:

Kevin G. Angle, PE
Mechanical Engineer
717.434.1562

Thomas E. Leary, PE
Executive Vice President
717.434.1555

AIA CES Program – Real-World Applications for VRF Systems

VRF Infographic

When do Variant Refrigerant Flow (VRF) Systems Make Sense?

Feb 28 2018

VRF Infographic

JDB Engineering just published a new infographic about Variable Refrigerant Flow (VRF) systems: overview, applications, sustainability, ideal building types, advantages, and disadvantages.

Check it out:

Interested to learn more about VRF Systems? Contact Timothy A. Warren, PE, LEED AP, president of JDB Engineering, Inc.

Also check out Kevin Angle’s blogs, When Do Variable Refrigerant Flow (VRF) Systems Make Sense? and The VRF Checklist as well as our AIA LU/HSW-approved training program, Real-World Applications for VRF Systems.

Jan 16 2018

Electrical Considerations for Variable Frequency Drives (VFD)

By Craig G. Malesic, LC, PMP, EIT
with Brian E. Pitzer, PE and Rich M. Lindemon, PE, LEED AP

Variable Frequency Drives (VFD) are commonplace in the design of mechanical systems. Where “VFD” is the most common terminology used, they are also called adjustable frequency drives, variable speed drives, and AC drives. Their primary use is to adjust the speed and/or torque in an electro-mechanical system by varying input frequency and voltage.

So when should VFD be employed as part of a mechanical design? That’s a topic for a future post! However, the intent of this post is to share some of the common electrical design considerations used in VFD selection.

What’s in a Name?
The term “VFD” commonly refers to two different pieces of equipment, depending on the application. Technically speaking, a VFD consists of a Variable Frequency Controller and Operator Interface. It has connections for standard input power (60 or 50 Hz Sine Wave), variable frequency output power (square wave), and external controls (sensors, building automation controls, building management systems). For the sake of this post, we’ll call this a “Stand Alone VFD.” Another common configuration is a “Stand Alone VFD” coupled with an overcurrent protective device (OCPD), such as a circuit breaker or fuses. We’ll call this a “Combination VFD.”

Although the industry uses the same name for both configurations, there are two slightly different systems. Similarly, we use the term “Motor Starter” for the contactors and coils used to connect the motor to the power sources, and “Combination Motor Starter” to describe a “Motor Starter” coupled with a fused disconnect.

How are VFDs Sized?
The sizing of the equipment is straightforward. VFDs are chosen by voltage, phase, and horsepower (HP). Next, it is important to use the FLA (full load amps) of the motor to finalize the VFD size. Once this is selected, a few other considerations need to be factored in.

Horsepower
An oversized VFD can be used for a smaller HP motor. The advantage of this application is that if the motor requirements increase, the VFD may be large enough to still handle the increased load. The disadvantages for this are that the VFD will be more expensive, and the feeder to the VFD needs to be sized to match the VFD, not the motor, again increasing the initial cost.

Constant Torque or Variable Torque
Centrifugal equipment, such as fans or pumps, most often utilizes variable torque drives. This allows the best access for fine-tuning control for energy savings and proper sequence operation. Constant torque VFDs are used for applications requiring a constant level of torque at all speeds. Examples of this include conveyors, positive displacement pumps, punch presses, and extruders.

Control Method
VFDs are often used to control variable motorized operations as part of building mechanical process or HVAC systems. These operations may include start/stop, change speed, constant speed, limits, ramping, forward/reverse, or energy conservation. The commands to perform these operations are communicated to the VFD through some form of operator interface – the simplest of these being local manual control such as an external switch, push button, or keypad. However, other more sophisticated methods include automatic control from an external process control signal. These methods may include remote control signals generated as digital or analog inputs, digital/relay, or analog outputs from some control device associated with the equipment tasked to provide variable control to the motor. Examples of these signals are a voltage range (0 – 10 Vdc), current range (0 – 20 mA) from a temperature sensor or pressure transducer, or a set of dry contacts on a control relay. Other control methods include multiple motors on a single drive, lead/follow, closed loop/PID, and cascade control card (fixed and variable stages).

A building automation system (BAS) may be used to enable/disable the VFD control of a motorized process through a host of information including schedules, energy optimization, and electrical demand limiting. A direct digital controller (DDC) associated with the process equipment manages the necessary information obtained from various control devices wired into it (points) and enables/disables the VFD, based upon the BAS programming. The DDC may also send a reference signal command that may correspond to a motor speed required to maintain a control set point. A serial communications protocol may be used to allow a DDC controller to communicate directly with a VFD over a local area network (LAN) or Bus (RS-485 connection). Much more sophisticated functions and information management are made possible through this form of communication. Some other communication methods include Ethernet protocols such as BACNet and LonWorks, which form more of a Global Network of multiple control systems hosted by a PC-based graphic user interface (GUI).

Overcurrent Protection
Either circuit breakers or fuses can be used as the overcurrent protection for a VFD. Fuses are less likely to have nuisance outages than circuit breakers, and are a better option for circuits that have a high-fault current. However, fuses can drop out one leg of a three-phase circuit and cause damage to a motor if not properly equipped with phase-loss detection. Circuit breakers can easily be reset if tripped and do not require spares to be stored, which can be costly.

Enclosures
Providing the proper enclosure for the surrounding environment is essential. There are standard configurations known as NEMA (National Electrical Manufacturers Association) configurations that dictate what environment for which an enclosure should be rated. NEMA 1 is for standard locations. Most indoor applications will be in a NEMA 1 enclosure. NEMA 3R is for wet locations. VFDs installed outside, or in wet locations inside, will be in a NEMA 3R enclosure.

Line and Load Reactors
Line Reactors are used to help protect the VFD from harmful disturbances (surges, spikes, transients, etc.) produced on the distribution system and reduce the harmonics produced by the VFD from bouncing back onto the distribution system. They are placed in series with the VFD on the line side of the VFD.

Load Reactors are used on the load side of the VFD to protect the motor and wiring where the distance between the VFD and motor is very long. High voltage is created by noise spikes and high frequency waves. The insulation of the cables is not rated for the high voltage spikes and over time will deteriorate. A good rule of thumb is for distances more than 100 feet, a Load Reactor should be installed.

VFD Cable
VFDs expose the load-side cables to high voltage, intense heat due to corona discharge, and electromagnetic interference. Standard PVC-insulated single wire conductors are not designed to withstand these effects and can actually create more problems. The cross-linked polyethylene insulation on VFD cables withstands the intense heat created by the corona discharge. Shielding within the cable prevents the EMI from escaping and being picked up on other equipment in the area. Cable geometry and grounding reduce the high voltage wave effect created by the variable frequency drive. VFD cables should be used on the load-side of the VFD, from the VFD all the way to the motor.

In the end, mechanical systems do not work without control and power. VFDs offer a great way to dial in the operation of your mechanical system, but if they do not function properly with the control and power requirements, they are more of a boat anchor. These electrical considerations will allow a VFD to make your system work as the mechanical engineer intended.

Have questions about VFDs? Contact us to discuss your questions and challenges.

Craig G. Malesic, LC, PMP, EIT – Email or 717.434.1558
Brian E. Pitzer, PE – Email or 717.434.1550
Rich M. Lindemon, PE, LEED AP – Email or 717.434.1563

Electrical Safety in the Workplace

Dec 14 2017

Putting the “V” in “HVAC”: Ventilation

By Richard M. Lindemon, PE, LEED AP

Most people are familiar with the abbreviation “H-V-A-C,” whether in full detail or nothing beyond reciting the four letters in succession. Sometimes people try to pronounce it as a two-syllable acronym, “H-VAC,” which may be as divisive as you can get among those in the trade! Still, others will offer that to be technically complete you should add an “/R” to it for Refrigeration, but let’s just set that aside for now.

Basically, the abbreviation quite literally stands for “Heating, Ventilation, and Air Conditioning.” For the most part, people are familiar with, and easily grasp the concepts of, heating and cooling the indoor environment simply from a comfortable space temperature management standpoint. Moreover, humidity control is also widely understood as a key component of air conditioning from an indoor environmental comfort point of view. That would generally take care of the “H” and the “AC,” but what about that “V” in between? Among the people who know that the “V” stands for “ventilation,” how many of them know what “ventilation” really is? It is likely the most misunderstood element of the abbreviation itself.

Try asking anybody this simple question – “What is ventilation?” – and you will get more answers than you thought possible. Simply stated, “ventilation” is “fresh air” – nothing more, nothing less. The term “fresh” is loosely used here because you can have extensive debates on what constitutes “fresh” air, but we’ll just accept the notion that “fresh air” is the air outside as opposed to that within a room or other enclosed space.

So, what do we need to do with this fresh outside air that somehow became the “V” in HVAC? We need to bring it inside, that’s what we need to do with it. Why? Because it’s good for you. When I was a child, my mother was always telling me, “Go play outside. The fresh air is good for you.” She probably knew more about the “V” than even she realized. So, to avoid throwing this respect for authority to the wayside, our building and construction codes have also convinced us that fresh air is good for us, because they have mandated that buildings must incorporate a minimum amount of fresh outdoor air for proper ventilation of occupied spaces. The primary purpose is to protect building occupant health by controlling the quality of indoor air from the effects of airborne contaminants, which can be damaging. Additionally, ventilation is required to protect the building itself from the harmful effects of excessive heat and humidity.

Methods for bringing fresh outdoor air into the building environment include “natural” ventilation and “mechanical” ventilation. Natural ventilation is essentially the effect of opening a window or door to let fresh air enter freely into the building. It can be effective, but very unpredictable as natural air movement is dependent on many parameters – both naturally and humanly controlled – that must be met consistently. We all know that Mother Nature is rarely consistent, and humans can be unreliable at times as well.

Mechanical ventilation uses fans or blowers to force movement of air to and from occupied spaces. This equipment is either dedicated to providing ventilation or can be part of a heating, cooling, and air-conditioning system that serves the spaces to be ventilated. In either method, the fresh outdoor air brings with it the outside temperature and humidity conditions that conflict with the indoor environmental conditions we are trying to establish with our “H” and “AC.” To bring the outside conditions in line with our indoor conditions, we must provide additional mechanical energy to heat, cool, and control humidity of the outdoor air required for ventilation – in addition to offsetting the building and space loads. This creates additional cost for larger capacity HVAC equipment as well as increased energy consumption. The conditioning of outdoor air for ventilation can represent 20% to 50% of a building’s thermal load, depending on the quantity of air required and the seasonal thermal conditions.

Overlooking the “V” when planning a building project can not only jeopardize the health of your building and its occupants, but can also lead to the surprise of additional life cycle costs associated with purchasing, supporting, and operating your HVAC systems.

Got a question about the “V”? Or even the “H” or “AC”? We’ve got you covered! Reach out to Rich Lindemon via email or 717.434.1563.

Bigger Isn’t Always Better! Why Oversizing Packaged DX Rooftop Equipment Isn’t a Good Idea

Oct 31 2017

Bigger Isn’t Always Better! Why Oversizing Packaged DX HVAC Equipment Isn’t a Good Idea

By Rich Lindemon, PE, LEED AP

It is very easy to fall into the trap of thinking that bigger is better when you consider something that you like or enjoy or that benefits you and those around you. That super-sized value meal with the extra-large fries and drink must be better than the regular sized portions. An extra 100 horsepower out of your car’s engine can make you go faster whenever you want. Who wouldn’t want a 70-inch TV on the wall of their family room, right? But that line of thinking doesn’t always apply favorably to all things, and it’s probably easy to come up with several of them rather quickly. One that is often overlooked is thinking that a bigger air conditioner must be better because it would have the ability to condition a space quicker and easier, and never fall short on capacity. There are several flaws with this way of thinking, which demonstrate that in the case of oversizing packaged direct expansion (DX) HVAC equipment – bigger is NOT better.

Before getting into the reasons why oversizing DX HVAC equipment is a bad practice, we should start by noting that all HVAC equipment should be sized using proper heating and cooling load calculations. There are several methods for doing this, but bypassing this critical design step in lieu of a “Rule of Thumb” method or a generic load-per-square-foot will usually get you off to a bad start. An accurate load calculation will allow you to select the correct size HVAC unit. Once you know your load demand, you need to focus on why you are designing an air conditioning system in the first place – usually simply for comfort. Comfort results from properly controlling the temperature, relative humidity, and air circulation uniformly and consistently through the space. In most cases, we concentrate primarily on regulating the temperature because we want it cooler when we feel uncomfortably warm. But we also feel uncomfortable when the humidity levels are too high. This heightened sensation of heat causes the desire to try to compensate by lowering the set point on the thermostat. This in turn causes the unit to overcool the space, leading to a more uncomfortable environment. A right-sized HVAC system that is properly matched to the calculated load will operate far more efficiently than an oversized unit. An air conditioning system that doesn’t run efficiently can unnecessarily drive up energy costs, going against today’s focus on creating a sustainable environment.

When an air conditioner runs, it does two jobs. It lowers the temperature of the space while also removing moisture from the air. To do the second task, it must run for a long enough time to allow the water to condense out of the airstream. Oversized air conditioners typically do not run long enough to completely dehumidify because they satisfy the space temperature and then shut off. As the space temperature rises again the unit will start running, but then quickly shut off when the space temperature setpoint is reached. This is called short-cycling and leads to another problem with an oversized air conditioner. As it short-cycles frequently – “bang on, bang off” – it takes far more initial energy to start the unit as it does to maintain continuous operation, leading to increased wear and tear on the components, particularly the compressor. An oversized short-cycling packaged DX air conditioning unit will not only fail to satisfy all the required comfort conditions, but it will ultimately fail completely long before reaching its life expectancy.

While sacrificing human comfort with an oversized packaged DX HVAC system, you may also be causing inadvertent damage in other areas as well. These include the building structure, personal health, and lifecycle costs. Limiting the ability to properly condition the space and control moisture can lead to environmental issues like the development of destructive organic growth that can damage your building structure. Indoor air quality can suffer greatly, leading to all sorts of potential health risks. Ultimately, you will pay more to purchase, install, operate, and repair an oversized HVAC unit that will also fail prematurely.

Now, about that supersized value meal…

JDB Engineering offers a full complement of HVAC services. Learn more here. Questions about this post? Reach out to Rich Lindemon, PE, LEED AP.

The VRF Checklist

A Checklist for VRF Systems

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