Balancing and Control of Small Terminal Units

​​​​​​​​​​​​At IMI Hydronic Engineering, we believe in always using the right tool for the job. That is why we provide several solutions for balancing and control of small terminal units. There is not one solution that fits all hydronic systems. To achieve a fully controllable hydronic system and thereby the desired indoor climate at a minimum of energy use, you have to choose the hydronic solution that best fits the system conditions.

We have gained experience from servicing more than 100.000 heating and cooling plants around the
world. Whatever the requirements of your hydronic systems, we can help you find the best solution.
Why balance?
Before we move into the different solut​ions for control of small terminal units let us agree on the
following;
Every hydronic system – and every terminal within a hydronic system - operates more effectively
when balanced correctly.
Here’s why.
With the best possible hydronic control comes the best possible indoor climate at the lowest possible
energy cost. You achieve this by meeting three key conditions:

  • ​The design flow must be available at all terminals.
  • The differential pressure across control valves must not vary too much.
  • Flows must be compatible at system interfaces.


To read more about the three key conditions for perfect control, ​click here​

The best way of meeting these conditions is to balance the system.
What control?
There are two basic reasons for you to control a hydronic system; to improve comfort and minimise
energy costs.
The kind of control that is right for you depends on a number of factors like;

  • Comfort requirements​
  • Energy use requirements
  • Controllability of the hydronic system
  • Cost of control equipment
  • Cost of installation and commissioning

​​

With so many variables, this is often far from an easy choice. There are several theoretical formulas to
calculate the “Control difficulty” but the parameters in the formulas are quite difficult to establish. That
is why we have developed a chart that sets out a basic rule of thumb;




Choose on/off control, where the control accuracy demands are not prohibitive, and the terminal unit

in question is rather slow in response.
Choose modulating control, where Hydronic control accuracy demands are tough and harder to
control.
Circuit characteristic - think linear
The efficiency of a modulating control system depends on the controllability of the hydronic system.
The circuit characteristic is one important measure of the hydronic design quality and thereby the
controllability.
The circuit characteristic is the relationship the control signal (u) to a control valve and the resulting
thermal power (P) from the unit.
The steeper the circuit characteristic curve, the higher the risk of control instability. A more linear
shape will result in greater control, and therefore, greater comfort.



How can we ensure a linear curve?
  • By using a control valve with the correct valve characteristic adapted to the terminal characteristic and a good linear actuator.
  • A hydronic design and balancing that minimise the impact of valve authority.

Which solution to use?
So you’re convinced of the importance of balancing your hydronic system, you’ve figured if you need
on/off or modulating control and you have been introduced to our range of terminal valves.
The last step is finding the right control and balancing valves for your terminal units. The charts below
make that choice a simple one.



For on/off control
STAD + TBV-C Normal solution for most system using on/off control STAP + TBV-C Simplifies the balancing procedure and minimises the risk of noise

For modulating control with pressure independent modules
STAP + TBV-CM Normal solution for most systems using modulating control STAD + TBV-CM When acceptable valve authority can be obtained without dp control

For modulating control with pressure independent control valves
TBV-CMP A more costly solution that ensures favourable circuit characteristic and simplifies
commissioning.
Proven control
We specialise in controllability. To us control means to know and to know you need to measure.
Measuring is key to obtain a fully controllable hydronic system.
Whichever solution go opt for, you will be able to:

  • Verify the flow (i.e. check that the design flow is available at each terminal)
  • Receive full system documentation (the balancing report)
  • ​Perform trouble-shooting whenever necessary.

We call it Proven control.​
Balancing and Control of Small Terminal Units