All HVAC installations should try to reach two fundamental objectives. First, they should deliver the specified comfort level. Second, they should reach the first objective using a minimum amount of energy, thereby maximizing efficiency and minimizing costs. In theory, current building-management-system technology can help achieve these objectives, but, in practice, even the most sophisticated control system can have problems that lead to reduced comfort and higher operating costs. In many cases, these problems are found in hydronic systems.

Symptoms of an Unbalanced System

A hydronic problem usually reveals itself when building occupants report an indoor climate issue. Indoor climate problems are quite common. In fact, two out of three buildings experience indoor climate issues. Common indicators of hydronic problems include:

• A space that is too hot in one area and too cold in another.

• A difficult initial startup that takes substantially longer in some rooms following an off-hours setback.

• Installed power that is not deliverable at intermediate and/or high loads.

• Higher-than-expected energy costs.

• Room-temperature fluctuations or room temperatures that do not reach the required set point at high loads.

• Control-valve and actuator maintenance problems.

• Poor system delta-T.

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To solve temperature issues, occupants often compensate by using space heaters, opening windows, and adjusting thermostat settings. Maintenance managers sometimes attempt to correct problems by installing larger pumps, resizing components, changing off-hour setback and initial startup times, and/or adjusting flow in system mains, branch lines, and circuits.

Such “fixes” typically are costly and ineffective. They serve as a bandage and ignore the root of the problem. By only addressing a symptom, they can create a larger problem. Tenants who were comfortable previously may begin to complain.

Engineers typically design HVAC systems with extra capacity to meet heating and cooling needs, but the challenge is using that energy in the most efficient manner possible. The key to HVAC-system effectiveness and efficiency, as well as the solution to these symptoms, often resides in a hydronic system. This article will discuss the three key conditions to achieving perfect hydronic control:

• Design flow at terminal units.

• Differential pressure across control valves.

• Flow compatibility.

Key No. 1: Design Flow at Terminal Units

The first key to hydronic control is design-flow availability. Design flow must be available at all terminal units under design conditions.

Simply indicating design flows on drawings is not sufficient. For design flows to be obtained, they must be measured and adjusted. In theory, it is possible to obtain correct flows by sizing the plant carefully, but installation does not always match design. Some oversizing occurs because components must be selected from existing commercial product lines. For example, control valves with exactly the required valve capacity typically are not available on the market. As a result, most control valves are sized incorrectly. Total opening of control valves cannot be avoided in many situations, such as during startup or large disturbances, when some thermostats are set at minimum/maximum value, or coils have been undersized. In these cases — and when balancing valves are not in place — overflow will occur in some circuits.

Additionally, some components are not known during design because the contractor selects them at a later stage. Original plant designs commonly are modified during installation. Hydronic balancing enables required flows to be obtained in actual installations and compensates for incorrect sizing.

Without balancing, initial circuits may experience overflow, which creates underflow in other circuits. To avoid occupant complaints, pump head usually is increased, sometimes increasing overflow and reducing underflow. The delta-T in circuits with overflow then becomes poor.