In the HVAC industry, it’s easy to feel like everything is becoming more advanced and more complicated. Building automation systems (BASs) are more capable. Building load calculation software and system modeling software are adopting automation and AI. Hydronic systems have become more flexible. And yet we continue to see the same problems when it’s time to implement what we designed.

Sometimes a system looks good on paper but needs constant adjustment to stay functional. Issues such as low delta-T, unstable control and pumps not operating the way they are intended are signs of dysfunction. Other times, facility managers override controls just to keep the building operating.

 None of this is new. As technology advances and as we continue to learn and grow, the fundamentals we learned in school can help us avoid these real-world problems. 

The fundamentals have not changed. The energy balance we learned in thermodynamics still drives system performance. Flow and temperature differential still define capacity. BASs still rely on inputs and outputs. Those parts of hydronic design are simple and always have been. 

What has changed is how often we move away from those fundamentals. We might depart from the fundamentals to improve on an established best practice, to optimize a system, or add flexibility. Other times, we start solving problems that don’t need to exist.

The fundamentals still apply

Every hydronic system comes back to a few fundamentals at its foundation: the correct flow, the correct temperature difference, the correct pressure drop through the loop, and the correct controls to respond to system dynamics. These four elements show up in every system, regardless of how complex it is. 

Whether it’s a simple boiler loop or a campus chilled water system, the same rules apply. If the flow is too high, delta-T drops. If delta-T collapses, pump energy goes up and efficiency is dinged. If control inputs are not stable, the system pumps start chasing themselves. 

In practice, however, we don’t always remember the fundamentals when we are designing. We forget the difference between mixing and diverting flows, and we neglect the need to bypass the minimum chiller or pump flow. We select temperature reset based on arbitrary points versus what makes sense for the process. Then we rely on the BAS contractor to sort it out. 

That’s where problems start to build. 

When issues arise

Most issues don’t show up on day one, and definitely not from the information noted on the drawings alone. What we put on the drawings are only surface level; as long as there are basic directions to follow, a maximum head loss of 4 feet per 100 feet and flow velocity of 8 feet per second to prevent erosion, essentially anyone can size chilled water or hot water piping and put size labels on a piping drawing.

Numerous online calculators are available to size pipe based on flow rates. Sounds easy, right? But engineers still make mistakes during design, and how hydronics work still seems to be a mystery to many. 

Hydronic systems require a large range of operating conditions. A building may go from 100 tons of cooling in peak summer conditions, to 20 tons of cooling in shoulder seasons, to zero tons in winter. All these conditions need to be considered during design, as does the way water moves through pumps, coils, control valves and equipment. Systems start to fail when engineers only think about hydronic system theory and ignore its real-world applications.

Hydronics and BAS are the same conversation

Hydronic design and building automation are not separate scopes. They are two parts of the same system. To be effective, both need to be understood. 

The drawings define what should happen, and the sequence of operations defines how it happens. If the system is built on solid fundamentals, controls can stay straightforward. The sequence matches how the system behaves naturally. Control inputs are stable, and outputs are predictable. 

Put another way, what is written in a control sequence is what is seen through a BAS. However, when the fundamentals are not implemented, the controls start to compensate. 

Most often this shows up as additional control logic to stabilize pump control, tighter control loops to manage temperature swings, and overrides enabled to prevent short cycling. At this point, the system still runs, but it’s harder to understand. 

A real project example

On one project, my team and I were asked to help sort out a hydronic system that wasn’t performing. The design intent was straightforward: variable primary chilled water with supply temperature reset and differential pressure control. 

On paper, it checked all the boxes of a well-designed system. But in operation, it didn’t.

The plant was running at high flow most of the time. Delta-T was consistently low, and pumps were ramping up and down to maintain differential pressure, but the signal itself wasn’t stable. The operator had already started overriding parts of the sequence to prevent the spaces from drifting and to keep the building comfortable.

Nothing was technically wrong on paper or in isolation. The equipment was sized and installed correctly. The BAS was operational, and the control sequences were written with accurate intent, but the system wasn’t behaving as a “system.” 

When we started breaking it down during problem-solving, most of the issues were traced back to fundamentals: 

• The differential pressure sensors weren’t located where they represented the critical path;

• Control valves were oversized so they were not operating in a controllable range; 

• The supply temperature reset was too aggressive, which drove higher flow and pushed the system out of balance. 

We realized the BAS was doing exactly what it was told to do; it just didn’t have stable inputs to support the control strategy. We adjusted sensor locations, revisited setpoints so the system could operate within a stable range, and adjusted parts of the sequence so it matched how the system actually behaved rather than how we expected it to behave. 

Once we realized and aligned the fundamentals, system flow came down, delta-T improved and pump operation stabilized. The operator stopped needing to implement overrides to make the system function. The system did not change in any major way; it just started following the fundamental rules of hydronics.

Simplicity is a design choice

Simple systems perform better over time while still being capable of high performance. They tend to rely on fewer assumptions. And they’ve been proven to operate and function across the world, on countless buildings and systems, across decades and generations of experiences. 

If you don’t know where to start with hydronic fundamentals, numerous resources are available for designers. For example, Bell and Gossett has many articles and design guides that have been developed over several decades, which are proven design concepts for hydronic systems. Endless companies have used these design guides to create their own standards, schematics and details for hydronic systems. 

If you’re new to hydronics or unfamiliar with systems such as primary-secondary or variable primary, I recommend reading through their design guides, which can be found for free online.

Often missed

There’s no secret to hydronic design, and yet there are items I continuously see getting missed by young engineers that end up being a request for information and sometimes result in a change order. When I work with emerging professionals, I try to give them a list of items to check and make sure those items make it to the drawings or specifications.

• For every pump, sketch the flow from the pump outlet to the farthest coil or device served in the system and back to the pumps, and account for all the pressure drops along the way. Include the piping friction losses, valves, coils and control valves.

• Control valve pressure drop is often ignored. Typical pressure drops are around 5 PSIG, or 12 feet of head on the pump.

• Remember what controls the speed of the pump: the differential pressure sensor. The sensor is typically located two-thirds to three-quarters downstream of the longest loop on the system to ensure the farthest point in the system receives flow. 

• Remember the system’s minimum flow. All equipment has a minimum flow that is typically acceptable for operation, including chillers, boilers and pumps. In a primary-secondary system, minimum flow through a chiller or boiler is not a concern. But pumps on these systems, as well as on variable-primary systems, also have minimum flow requirements. 

Ensure there’s a way in the system to bypass this minimum flow, whether it’s a modulating control valve on the supply and return piping mains, three-way diverting valves on coils or terminal devices, or even an end-of-main bypass with a circuit setter. I have seen all these methods used to maintain the system’s minimum flow; it’s important to remember the need to bypass this flow.

When we rely on hydronic design fundamentals instead of overcomplicating systems, we create designs that are more stable, more efficient and easier to operate. Simplicity, backed by experience, is what allows these systems to perform the way they were intended.

Dale Garfield, EMBA, PE, is a senior mechanical engineer and team lead at EUA, specializing in HVAC system design and controls integration. With a passion for mentorship and a knack for continuous improvement, Garfield helps bridge the gap between design intent and real-world performance. He’s also an advocate for continuous learning and collaborative problem-solving within small, high-performing teams. Garfield is a U.S. Marine Corps veteran and holds an Executive MBA from the University of Wisconsin-Oshkosh and a Bachelor’s degree in Architectural Engineering from the Milwaukee School of Engineering.