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Zone control with change-over function

For fancoils or cooling beams, the so-called two-pipe connection with change-over (C/O) function is sometimes used. This feature ensures that the common register heats, cools or shuts down the room according to the energy needs of the room and the available medium in the piping. The main advantage of a two-pipe connection is the saving of investment costs: instead of separate pipes for heating and cooling water, there are only one pipes in the building that distribute either hot or cold water, and there is only one exchanger or other heat exchange surface in the rooms. However, the system has several limitations that can fundamentally affect the comfort of the building. In addition, there is a risk that, if incorrectly designed or installed, the entire system will be functional only to a limited extent and will become a source of annoyance and disputes between professions. Let's look at the basic principles of the connection function with change-over and the most common mistakes in the project and implementation.

The difference between a four-pipe and a two-pipe connection
In the case of a four-pipe connection (Fig. 1 a), heating and cooling water are available (of course, if the sources are in operation - boiler room, cooling unit, or heat pump). It is therefore possible for part of the zone controllers to heat and at the same time cool part as required. This situation occurs mainly during the transition period, ie in spring and autumn, especially in the morning and morning. At that time, the rooms on the east side have heat gains from the sun such that they no longer require cooling, while the west façade is still heating.

However, in the case of a two-pipe connection (Fig. 1 b), there is either heating or cooling water in a single pipe. The heating or cooling mode must therefore be determined centrally for the entire building and the heat or cold source is activated accordingly.

Fig. 1 a) four-pipe connection, b) two-pipe connection with change-over thermostat

How to decide whether to heat or cool a building?
Whatever algorithm we choose, it will be wrong in some respects. There will probably always be a room that needs the opposite mode. In short, it is necessary to accept that the required temperatures in 100% of rooms cannot be observed - this is a tax on the lower acquisition and operating costs of a system with change-over.

Switching sources can be automatic or manual. When switching manually, the building operator decides on the outside temperature, the expected weather development, the clouds that affect the heat gains on the facades, or other factors, such as the priorities of some rooms. For automatic switching, algorithms similar to those for district heating operation are usually used, ie "if the average outdoor temperature in the last 72 hours falls below ...", which may not, however, reflect the real need in the rooms. Each room can be loaded differently (heat gains from technology and people), the difference between the corner director's glass office facing north and the accounting office on the south side of the building, etc. For communicative zone controllers it is possible to collect room temperatures, heating or cooling requirements, if the control system allows it, and calculate from these values ​​whether the building needs more heating or cooling energy. Requirements can be weighted (area or importance of the room, etc.). The exact algorithm should be determined by the technologist, with the fact that specific values ​​of parameters can be fine-tuned during test operation.

If possible, the control system (SCADA) should also be able to set the mode manually. For example, the mode selection might look like this:

  • Heating - the building is permanently in heating mode
  • Cooling - the building is permanently in cooling mode
  • Transition period - switching between heating and cooling takes place according to a time program, user-adjustable (eg 9 - 18 h cooling, 18 - 9 h heating)
  • Auto - heating or cooling mode is automatically changed according to the outdoor temperature with an adjustable value of the summer - winter transition and hysteresis (for winter - summer return).

The current mode information (binary value, usually False=heating, True=cooling) is also used to write to the zone controllers if communicative controllers are used.

Resources department
The source of hot and cold water can be, for example, a gas boiler and a cooling unit, a heat pump, or a combination of these sources, where the heat pump provides cold water and heating at an outdoor temperature above a certain limit. At lower outdoor temperatures, it then switches to the boiler. The branch with two-pipe distribution therefore switches to either the heating or cooling water source. The two circuits should be mixed as little as possible, in addition it is advisable to observe a delay between closing one source and opening the other so that energy is not wasted unnecessarily and hot water residues do not enter the cooling unit. At the same time, the circuits must be hydronically designed so that even with the temporary complete closure of both sources, the feed pump on the branch to the zones, if installed, has suitable operating conditions. The length of the delay is determined by the technologist: as long as the media is not directly mixed between sources, 1 minute is enough; if the entire circuit is to cool down, it is usually set to 30 minutes or more. In some states, there are detailed rules (energy codes) that determine the conditions for change-over: eg the US Building Energy Codes Program of 2018 stipulates that the heating time for switching must be at least 4 hours and the outdoor temperature hysteresis greater than or equal to 15 ° F (ca. 8 K).

It is important that the switching technologist selects the appropriate control element. Ideal are intermediate flange dampers, eg Belimo D6...W, which the manufacturer explicitly specifies "for connection and disconnection of heat and cold sources". We usually see them in pipes with a diameter of DN100 and larger. On the contrary, we consider three-way taps to be unsuitable, which hydraulically connect the heating and cooling sources when moving between the positions and it is not possible to observe the delay when changing the mode. In addition, the taps show a leak, especially with a pressure difference between the two systems.

Change-over thermostat
Each controller in the room must have information on what energy is available to control the valve accordingly. There is a fundamental difference between communicative and non-communicative regulation.

If the controllers are connected to the bus and allow the change-over signal to be transmitted over it, the master controller simply sends this information centrally to all controllers. This is an ideal situation, as there is no need to purchase and install C/O thermostats, their wiring and any related errors.If the controllers are connected to the bus and allow the change-over signal to be transmitted over it, the master controller simply sends this information centrally to all controllers. This is an ideal situation, as there is no need to purchase and install C/O thermostats, their wiring and any related errors.

For autonomous controllers, it is necessary to connect a change-over thermostat, which measures the water temperature on the supply pipe and transmits this information to the controller. It is best to use a special C/O thermostat, eg Siemens RYT182. It has fixed switching limits: at water temperatures above 30°C (± 4 K) it switches to heating mode, when the water temperature drops below 19°C (± 4 K) it switches to cooling mode.

Conventional contact thermostats are not so suitable, because they tend to be larger and harder to mount in tight conditions in ceilings. The biggest problem, however, is the possibility of setting the limit temperature for switching, because if the setting element is outside the housing, it is tempting to manipulate, and if it is inside, it is not available for service and inspection. Improperly set limit value is fatal for the correct function of the control - if the room is overheated, the controller wants to cool down, believes that there is cold water in the pipe and opens the valve, it makes the situation even worse. The same applies to cooling.

Sometimes one common C/O thermostat is used for multiple zone controllers. If the controller manufacturer allows it, the C/O inputs of the controller can all be connected to the thermostat contacts in parallel, otherwise an isolating relay must be used. Always consult the controller manufacturer for this connection. When connecting in parallel, the inputs must be connected in the same way with regard to polarity. While a common C/O thermostat reduces the number of thermostats in a building, it increases the demands on cabling and proper connection.

Instead of a thermostat, a (attached) temperature sensor is sometimes used, which has several advantages:

  • is usually cheaper,
  • measures the water temperature continuously and if this temperature can be read on the controller display, we have another diagnostic element,
  • if the controller is installed in a room, the sensor can be checked by simply measuring the resistance at the terminals at the controller without having to enter the ceilings (unless we check the installation) - this is not possible with the thermostat, if we want to check its function, we must induce or simulate temperature change average,
  • it is no longer possible to set the limit value on it in principle, and thus the possibility of incorrect setting is eliminated,
  • it does not have a changeover contact, but two wires, interchangeable with passive resistance sensors, so there is no risk of incorrect connection.

However, the controller must support a resistance temperature sensor for C/O at its input.

Location of C/O thermostat or sensorLocation of C/O thermostat or sensorAbove all, the thermostat must be installed in such a way that it can correctly measure the supply water temperature. The problem occurs when mounting on plastic pipes. While brass has a thermal conductivity of 100 Wm-1K-1, PP-RCT is 0.24 Wm-1K-1, three orders of magnitude less. The measurement and control designer should require readiness in the requirements for other professions in the form that the pipeline designer determines the place where the C/O thermostat or sensor can be mounted. Well thermostats are not used for small pipe clearances, for higher prices and more demanding installation.

When determining the installation location, we must be aware of how the pipeline is routed and where the medium actually flows so that the measurement makes sense.

Fig. 2 - Location of the change-over thermostat

Permanent flow is ensured only in place a) or d), and only if there is a permanent short circuit (with limited flow) at the end of the pipeline. In the branches to the individual rooms, the water only flows when the valve is open! With the valve closed, the thermostats installed in places b) or c) have stagnant water, which cannot provide true information about the current temperature of the medium in the main branch. Mounting a thermostat or C/O sensor on the tap is therefore not suitable.

Unfortunately, it is not always possible to access the main branch with the thermostat cable. Therefore, some controllers (Siemens RDG100) have a so-called flushing function, which opens the valve even when it should be closed, in two-hour intervals for an adjustable time of 1...5 min. This should ensure sufficient water circulation in the main pipe and in the branch. Then installation in place b) is permissible. Position c) has the disadvantage that the measured water is already cooled or heated in the heat exchange body, and therefore its temperature is closer to room temperature. We therefore try to avoid this place at all costs.

It would probably not even occur to anyone to install a C/O thermostat on a common return, so this variant is missing in the picture. In this context, only pay attention to the possible confusion between the supply and return pipes. At the time of mounting the thermostats, not all parts of the piping may be properly labeled.

Even with regard to the inconvenience of installing and servicing C/O thermostats or sensors, it is only possible to recommend the use of communicative controllers that receive the C/O signal via the bus. The above-mentioned risks and difficulties are thus completely eliminated.

Logic in the zone controller
The decision logic in the room controller is in principle relatively simple. The room temperature controller has two sequences at the output - heating and cooling. The valve for which the water temperature in the pipe is suitable is then fed to the valve.

Fig. 3 - Change-over logic in the zone controller

This means that when heating is required (the required room temperature is greater than the measured temperature), the valve opens the fan coil or ceiling beam only if there is heating water in the pipe (or if the heating mode is active). If cooling mode is currently active, the valve remains closed. Similarly, cooling mode must be active when cooling is required. If the controller has a display, it is useful to indicate to the user either the current valve status (heating/cooling /off) or the current mode (heating/cooling). For some controllers (eg Domat FCR010) it is possible to select the indication method during commissioning.

This basic function can be supplemented by a delay in switching between modes (approx. 30 minutes). This must be taken into account during commissioning and tests, because when the state of the thermostat or the bus C/O signal changes, then apparently nothing happens for some time.

User training
An important part of the whole process of commissioning and handing over the change-over system is the training of users, both the operator - the house technician, and the people who live in heated or cooled rooms. In particular, the domestic engineer should understand the algorithms for switching sources between heating and cooling modes and be able to switch modes manually and set limits if these parameters are available. It is necessary to explain to the users of the rooms that the building is always either heated or cooled, never both at the same time, and that some rooms have to adapt to the current mode, so it will not always be possible to keep the set temperatures in them. It is unnecessary to set the desired temperature to the extreme limits by the controller - if the building is in the opposite mode than a certain room needs, its zone controller cannot contribute to increasing comfort.

It is perhaps not necessary to add that the regulation must take place even in the transitional period, when the rules for the choice of mode play a crucial role for the economy of operation and comfort. Only with a sufficient exchange of information between the technology company, the control system supplier and the building operator can the change-over system, however with some limitations, work as economically as possible while achieving the maximum possible comfort.