In HVAC Hydronics, What's Not Directly Measured is Not Directly Controlled
“Life is difficult” is the first sentence from the book The Road Less Travelled by M. Scott Peck. In the same manner, this article starts out, “Flow control is difficult.” If it was not difficult, there would not have been so much debate about it over the years. There would not have been so many engineering opinions about it over the years. There would not have been so many product developments over the years that promise to be the answer to proper flow control in an HVAC hydronic system.
However, all that being said, flow control need not be as difficult as it is sometimes perceived to be. Keep it simple is a good way to begin thinking through the design of any HVAC hydronic flow control system and piping design. One simple thing to keep in mind: What is not directly measured is not directly controlled.
Some have suggested it’s possible to have a self-balancing HVAC hydronic installation when a system is “properly designed.” That is a great idea; however, is it defendable in court to define “properly designed” self-balancing hydronic HVAC systems? Who would be the determining entity to define a “properly designed” self-balancing system? Pause to ponder before jumping on engineering marketing concepts that may not be as defendable when things go wrong.
So, where does any good engineering decision-making process start? With marketing material from a product manufacturer? With just the experience and we’ve-always-done-it-that-way approach? Would it be prudent to have a discussion with the building owner on what the criteria options are and start with a design-intent discussion to determine what the top priority is? Is it lowest first cost, energy, or flow control that ultimately gives building occupants the most comfort and indoor air quality too?
The start of any building HVAC system design begins with understanding the architecture and functionality of the building. The building is simply an enclosure for humans and/or processes to stay out of the elements and be able to function in a safe, comfortable, and essential environment to be productive.
That being said, the focus on the hydronic HVAC flow control really must begin with a systems decision and then work down to the product decision. There are way too many system strategies to list in this article, but, suffice it to say, it is essential to understand the system before talking about the flow control. More importantly, it’s essential to discuss the design intent of the entire building. Is the building one that is optimizing energy or comfort, and can each of these variables be optimized at the same time over all part-load conditions?
Taking time at the start of a project to understand the hydronic HVAC system selection process is important. In the HVAC Systems and Equipment ASHRAE Handbook, this is the focus of the chapter entitled “HVAC System Analysis and Selection.” That alone is key to flow control. This is the first chapter, thus it’s implied it’s pretty important to analyze and make a system selection decision prior to getting into the weeds of looking at the flow control product selection decisions and how to specify those products. Understanding the goal is key to the system selection. This goal should be agreed upon and discussed with the owner of the structure who will likely be operating and maintaining the building and paying for the energy of the cooling and heating equipment and distribution pumps that all work more efficiently when properly maintained. In addition, the owner likely is the one who also pays the salaries of the occupants who work more effectively when they are comfortable. Setting the design intent of the hydronic HVAC system and its components is essential at the beginning of the process.
For sure, system and constructability constraints are important to discuss and acknowledge. Constraints that are “difficult” are not the same as those that are “impossible.” The question is, what is the cost/benefit of eliminating or minimizing the constraints? Whether or not a building uses a distributed or central HVAC system, mechanical strategies are important to discuss upfront. Those decisions alone will impact the hydronic HVAC system’s flow control product decision.
Is flow control the goal, or is it some other measurable variable? Is it possible to control what isn’t directly measured? What variables are truly measured when hydronic fluid “flow” control is what is desired? Is it flow of the fluid volume or flow of energy? Can there be more fluid flow but less energy flow? Can there be less fluid flow but more energy flow? Is it the energy in the fluid flow that matters or the total energy in the hydronic system that matters when the pumps, chillers, boilers, and other components, like heat exchangers and even pipe insulation, are considered as part of the first cost and energy cost analysis? Is it possible to optimize a system for fluid flow control without optimizing energy flow and do a good job with the engineering of the HVAC hydronic system?
In effect, it has been said that there are four basic methods of flow control in HVAC hydronic systems. These are on/off control, throttling, bypass, and variable flow. Each has a time and a place depending on the system type, size, and degree of optimization desired of energy and human comfort as well as which is most important for the end user.
One way to consider flow control is that it is a process of controlling the pressure in an HVAC hydronic system so as to provide the intended comfort within the building while optimizing the energy used. With this goal, the flow is not controlled; the pressure is, yet this is not even the main goal of the flow control. The main goal is comfort for the human occupants. Why? Because humans who are comfortable will work more efficiently and effectively optimizing their energy.
One challenge in HVAC hydronic systems flow control is the fact that they are not usually constant volume systems, and, thereby, the flow requirements change based on the demand of the comfort zone for the human occupants. A thermostat, which is an indirect way to “measure” human comfort, is modulating a valve to throttle the flow to a coil in a terminal unit with the intent of controlling the flow in order to provide comfort in the space. Is that a direct measurement of the human comfort? Not at all, yet that is what the flow control is intended to do.
Now take that to the larger overall system level of flow control through the boiler or chiller and heat exchangers and pumps. The goal is to optimize each of those components by using a combination of piping layout strategies, balancing valves, two- or three-way control valves, differential pressure controllers, and/or pressure independent control valves for not only peak heating and cooling load demands but for each and every part-load demand throughout the day and season of the year. How hard can that really be? Just keep it simple? Is it even possible to optimize energy and human comfort in all part-load and peak conditions simultaneously?
As with any design, the place to begin is at the end. The goal is human comfort these days more than ever. For sure, indoor air quality and energy are important; however, the cost/benefit of human comfort is becoming more clearly understood. One of the largest costs in business is the labor cost. Why wouldn’t it be the most important goal of the design of every HVAC hydronic system flow control strategy?
Each piece of equipment in an HVAC system has an efficiency profile. This is not always readily available from a manufacturer, but it’s worth asking for to understand the equipment’s part-load efficiencies. There are times when the best flow control may be to stop the flow. Stop the flow in one piece of equipment by using multiple pieces of modular equipment to be able to better match the load of the demand for heating and cooling. Sometimes, multiple chillers, boilers, pumps, heat exchangers, etc. that are better matched to part-load conditions and equipment optimization may be the better design. At the same time, there is built-in redundancy in the system for maintenance of the equipment while still maintaining the building in operation.
It’s always good to evaluate whether it’s worthwhile to use complicated variable flow pumping and piping arrangements if constant flow equipment with variable temperature control would be a better solution and give better comfort in the spaces where the human occupants work. This is worth a discussion in a system selection brainstorming session at the early stages of a project.
Rising energy costs have been the focus for many years with ASHRAE 90.1 being the benchmark standard. Sometimes too much focus on energy can overshadow the importance of ASHRAE 62 regarding ventilation and for sure overshadow ASHRAE 55 for human comfort. With rising labor costs, owners may start to realize that energy is not really the highest cost of business and start challenging engineers to do a better job with flow control that optimizes the human comfort of their human resource labor force.
It’s generally not a good idea to make the flow control the design objective in and of itself, which can easily happen when too much complexity is incorporated in the perception of energy optimization. Care should be taken to fully understand and document the part-load performance of a building and of each component in the HVAC hydronic system.
Start at the end. What is the ultimate vision and goal of the flow control in the hydronic HVAC system? With testing, adjusting, balancing, and commissioning (TABCx), there is a verification goal to ensure the flow control in the design intent is indeed occurring. The documentation requires some means of measurement. Either the measurement is an integral part of the products used for flow control or part of a system design that has ports provided for external measurement or both. Understanding how the verification is going to happen is a good place to start the design process of the HVAC hydronic system piping layout and product selection.
One aspect of HVAC hydronic systems balancing is the fact that water treatment and fouling of piping and components and equipment over time will impact the balancing and performance of the flow control strategy. How this reality is accounted for in the system design is important to consider. There are now more piping products on the market, such as PEX piping, that are not as subject to fouling as traditional piping materials; however, that is a topic of another article, yet it’s relevant to flow control and performance.
What exactly are flow control valves? There are so many types of valves and technologies on the market, that it’s indeed a challenge for an engineer to keep up with. It’s essential to look at the system first before looking at the flow control valve and other system components. Does it matter if it is a primary-secondary system, a constant volume pumping system, a variable volume system, or a one-pass system in a small building? Does the system design affect the flow control valve product selection decision?
There are always technological developments influencing flow control valve products. Some of the products integrate multiple functions into one product, incorporating what used to be and still can be.
Is this good or bad? Can a multifunctional product really be as good as a product that is engineered and optimized for one function? Those are challenges engineers who specify hydronic flow control products and write specifications have. In effect, the flow control of a system is only as good as the worst product accuracy allowed to be installed in the system. In other words, what is the weakest link in the system? It doesn’t matter how good every other component is or how much money was spent if the one flow control valve that is the heart of the system is not optimized in quality, accuracy, and dependability to perform consistently over time to give reliable control.
Pressure independent control (PIC) valves, Delta-P valves, flow limiting valves, or whatever the newest buzzword product label may be, all have a purpose and a place. It’s essential to understand the applications before arbitrarily deciding on an entire hydronic HVAC system piping and major product selection strategy just to utilize and specify flow control valve technology.
It’s always good to get multiple perspectives on any HVAC product design. For sure, some products have been engineered to give a competitive advantage, and those products should indeed be considered without concern for the competition that attempts to control project costs or profitability of a manufacturer or local sales rep agency. On the other hand, new product technology needs to be confirmed with a high degree of confidence on behalf of a facility owner, and specifying a product by proprietary name or performance can put an end user in a position of not getting what a sales pitch may promise.
A good product performance specification must have a high degree of accountability of performance on behalf of the end user/facility owner. The cliché “You get what you pay for” is not always true when overpriced, unique products under-deliver from what marketing hype may promise. The “buyer beware” cliché is part of healthy new technology specifications as well, and engineers should be cautious and use some good Missouri “show me” discretion in product selection.
Is “flow control” the right term? Flow control is the main system fluid that flows through the chiller, boiler, or heat exchanger. There will likely be different flow control requirements and goals than the flow control of the fluid that flows through the end terminal units, such as VAV box coils, radiators, chilled beams, fan coil units, etc. There may indeed be two different flow control strategies depending on which part of the system is being considered.
The flow control in the mains may be more energy-focused, whereas the flow control in the branches may be more comfort-focused. The flow control in the mains may indeed be by closed-loop direct measurement to meet some ASHRAE 90.1 requirements, whereas the flow control in the branches will likely be by open-loop indirect control with a thermostat on a wall controlling a control valve to maintain a temperature in a space within a certain range to meet ASHRAE 55 requirements. Both need to be understood and have a defendable control strategy and product selection decisions.
In today’s world of online education and research, it is much easier than ever before to get a lot of information quickly. The challenge in having all these resources at quick access for continued education in making the right hydronic HVAC flow control system and product selection decision is that it can become overwhelming. There are so many opinions, experiences, and testimonials as well as marketing messages about products that seem too good to be true. However, the great thing about all of this information is that engineers excel at taking a lot of ideas and options and applying fundamental engineering decision-making processes to make an appropriate decision based on the criteria being considered.
Beyond the online resources, there are also great educational resources in the ASHRAE handbooks, Engineered Systems, and other HVAC trade publications. In particular, a good place to start is in the HVAC Systems and Equipment ASHRAE Handbook in the “Hydronic Heating and Cooling” chapter and the “Valves” chapter. The chapters that talk about any of the water-side products, like chillers, boilers, pumps, heat exchangers, and coils, explain the evaluation, selection, and performance of these products with regard to optimization at both full- and part-load performance.
There is no doubt substantial education also comes from product manufacturers and sales rep agency personnel. Reading all of these materials at the early stages of a project is like having a brainstorming session with a multitude of engineers and others in the industry who have opinions and experiences. The challenge is to sort out the bias in each resource. The challenge is to sort through the engineering facts from marketing hype that is sometimes disguised as engineering facts.
The good thing is the laws of physics and affinity laws are constant. The fact is what can’t be measured directly can’t be controlled directly, and once there is separation of direct measurement from the desired controlled variable, there is additional error in the control loop that needs to be understood. There is enough error in direct closed-loop systems that it makes no sense to introduce additional errors in a control systems loop if the criteria is considered important enough to the owner to invest in engineers to make the decisions as to the right system and products to install.
The design of any system and product these days truly must take into consideration maintainability. If a system or product can’t be maintained, the entire design intent will not be sustainable. This maintainability must not only occur from the product and system design but also the accessibility to each and every component in the system. This should be a part of the discussion with the owner as well as the architect in one meeting. This is an owner investment that may challenge the architect to provide access panels in ceilings or walls that they may not want to have them in. The architect may need to challenge his or her own creativity in designing access panels out of materials instead of using premanufactured access panels.
The solution to flow control, as with any engineering challenge, will vary depending of the variables and the desired end results. What is controlled may or may not be what is ultimately perceived as being controlled. Controlling human comfort through multiple levels of control devices, such as thermostats, balancing devices, control valves, and control strategies, is for sure not a closed-loop control system. Controlling the flow in an HVAC hydronic system can be simple or it can be made complex depending on the design intent.
Making flow control in and of itself the end objective is not really the goal of any HVAC hydronic system. The flow control is a way to attempt to optimize energy and human comfort. It is good to always look back at the end of every design and re-evaluate whether or not the “keep it simple” approach was taken.