Achieving perfect hydronic control

Published:  10 April, 2008

Tour and Andersson's
One of keys to successful balancing is ensuring that the differential pressure across control valves does not vary too much, especially in variable-flow systems. Differential-pressure-control valves such as T&A’s STAP valves sized according to the system, meet this requirement.

Jean Christophe Carette shares the secrets of successful hydronic balancing.

Hydronic balancing is a necessity for good control of a waterborne system. In theory, control technology can satisfy the most demanding requirements for indoor climate and operating costs. However, in practice not even the most sophisticated controllers perform as promised. As a result, comfort can be compromised, and operational costs are higher than expected.

Careful attention to three important conditions will ensure the stable and accurate control of a waterborne system.

1. The design flow must be available at all terminals.

2. The differential pressure across the control valves must not vary too much.

3. Flows must be compatible at system interfaces.

Design flow

Dealing with the first condition, if the design flow is not available at all terminals, some parts of the building will be too hot and others too cold. In addition, installed power will not be deliverable at intermediate and/or high load. There can also be a long delay before the desired room temperatures are obtained at start-up after each setback, and energy costs can be higher than expected.

The power transmitted by a terminal unit depends on the supply water temperature and the water flow. These parameters are controlled to obtain the required room temperatures. Control is only possible if the required water flows are available, and for this to happen the required water flows must be measured and adjusted. This is why hydronic balancing is essential.

The big question is, how?

Is it, for instance, possible to obtain a correct flow distribution by sizing the plant carefully? The answer in theory is yes, but not necessarily in practice. Production units, pipes, pumps and terminals are designed to cover the maximum need. If a link in the chain is not properly sized, the others will not function to optimum performance. As a result, the desired indoor climate will not be obtained and the comfort will be compromised.

At the design stage, the characteristics of some components are unknown, because the contractor will specify them at a later stage. It is then necessary to modify the original plant design to take into account the installation as built, which frequently differs from the initial design. Balancing the system provides the opportunity to verify that the design and installation are correctly executed and will detect and correct most malfunctions of clogging, air and hydronic faults to ensure the design flow is available at all terminals.

Hydronic balancing prevents overflow in some circuits causing underflows in others, detects the degree of pump oversize and generally verifies that the plant works as intended.

Differential pressure

If the condition that the differential pressure across the controls valves must not vary too much is not met, there are several common problems.

• Continuous oscillation at room temperature.

• Maintenance problems with control valves and actuators due to fatigue from hunting.

• Room temperatures not reaching the required set point at low loads.

• Higher energy costs than expected due to unfavourable control settings to avoid instability.

Variable-flow systems are becoming more and more popular, mainly because of their benefits compared to constant flow systems. Pumping costs are reduced, the return temperature is minimised in heating systems, and the return temperature is maximised in cooling systems. There is one major disadvantage, however, in that the differential pressures in the plant may vary considerably during operation — which is a condition to be avoided.

By ensuring the differential pressure across the control valves does not vary significantly, the negative impact on the system function and performance can be reduced and even avoided — by using an efficient hydronic design.

One important measure of hydronic design quality is the circuit characteristic, translated as the relationship between the control signal and the resulting thermal power from the coil, determining the controllability of the system.

Simply by looking at the compound of the circuit characteristic, it is apparent that a low valve authority will make the circuit characteristic curve unfavourable.

This is why the second condition must be fulfilled. Too much variation in differential pressure across a control valve leads to low authority, distorted circuit characteristic and poor control. In addition, large variations in differential pressure will lead to interactivity between circuits, making control even more difficult.

Compatible flows at system interfaces

Flows must be compatible at system interfaces.

The incompatibility of flows at system interfaces can result in the design supply temperature being either too low (heating) or too high (cooling), the installed power is not transmittable when required (especially at high load) and there being a long time before all rooms reach the correct temperature at start-up after each setback.

In many systems, the installed power exceeds the maximum required by 50%, yet the distribution circuits do not receive enough.

The power produced by the boilers and chillers simply does not reach the heating or cooling circuits.

This problem can be particularly common in systems with several chillers or where boilers are working in sequence. The reason is usually a lack of compatibility at the interfaces between production and distribution. In most systems, production and distribution circuits are in direct contact with each other, which can cause serious and often mysterious disturbances unless adequate measures are taken to avoid them.

To avoid interactivity problems, a bypass line between production and distribution is the best solution. However, solving interactivity problems in this way may lead to compatibility problems if correct measures are not taken. Over-sizing the plant, adding extra boilers or chillers, increasing the pump head or changing the set point will only aggravate the problems or, possibly, solve them at a very high and unnecessary cost. The correct solution is simply to balance the flows on each side of every interface, which will give the best operation and the lowest costs.

When designing a waterborne system, the indoor climate is of paramount concern for all those within the building. To avoid the many problems of inadequate temperature and high energy costs, hydronic balancing is essential. As explained above, designing a system that complies with the three hydronic conditions will result in stable and accurate control for the best possible indoor climate with minimum energy expenditure.

Jean Cristophe Carrette is heat of Tour & Andersson’s hydronic college.

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