Block Air Conditioner / Fan Coil

This function block controls a fan coil unit or a split air conditioner. It receives the control value of a controller at input SG and derives the fan speed from it, operates the valves in the correct order and provides the protective functions of the unit: minimum run time and minimum idle time, valve lead time, fan overrun, changeover lock between heating and cooling, condensate monitor, frost protection and window contact.

The control value is usually supplied by the Block Room Controller block. If the parameter "Fan coil connected" is set there, the controller gets four additional outputs SG, KU, SH and BA that match the first four inputs of this block exactly - same abbreviations, same order, so they can be wired straight across. The air conditioner is then operated in the Block Room Controller control element; this block has no control element of its own.

The block also works without a room controller. In that case the control value comes from another source, or the unit is only switched by hand through input SH.

Fan speeds

The fan speed results from the control value at input SG. A threshold in percent is set for each speed; the highest threshold that is exceeded determines the speed. Typical values are 10, 40 and 70 %.

The hysteresis only takes effect when switching back down: switching up happens immediately as soon as the threshold is exceeded. Switching back down only happens once the control value falls below the threshold of the running speed by the hysteresis. Without this hysteresis the fan would audibly cycle at every threshold.

In addition, the minimum run time per speed and the minimum idle time apply. Switching up to a higher speed always happens immediately; switching down or off only once the minimum run time has elapsed. After switching off, the unit restarts at the earliest after the minimum idle time. A missing enable, an open window, a responding condensate monitor and frost protection, in contrast, take effect immediately, without any minimum time.

Input SH specifies a fixed speed by hand and overrides the automatic mode. Input SB and the operating mode limit the speed at the top.

The parameter "Speed switching" defines how outputs V1 to V3 behave. Changeover means that only the output of the running speed is set, which is the normal case for a speed selector switch. Stepped means that at speed 3 the outputs V1, V2 and V3 are all set.

Valves and fan

Valve and fan are not switched at the same time but in a fixed order:

Starting in heating mode: the valve opens first, the fan only starts after the valve lead time. Without this delay it blows cold air into the room — the most common complaint with fan coil units.
Starting in cooling mode: the lead time does not apply here, valve and fan start together.
Switching off: the valve closes immediately, the fan keeps running at the lowest speed for the overrun time and dissipates the remaining heat or cold.

The valve outputs VH and VK provide 100 as long as the valve is to be open, otherwise 0. This allows both a 1-bit valve and a continuous valve to be driven.

Heating and cooling

The parameter "Pipe system" defines how the unit is connected hydraulically:

4-pipe: separate valves for heating and cooling. Output VH drives the heating valve, VK the cooling valve. Output UM is provided as well and indicates the mode.
2-pipe: one shared circuit, the direction is given by the changeover valve at output UM (0 = heating, 1 = cooling). So that no warm water reaches the cooling mode, the changeover only happens once valve and fan have stopped. The unit shuts down for that by itself.

After each change the mode remains in place for the changeover lock. Without it the system oscillates between heating and cooling during the transitional season. Fifteen minutes and more are common.

Which mode applies is decided by input KU in the "Automatic" operating mode. With several units on one 2-pipe circuit this input is best driven by a shared, central changeover, so that one unit does not heat while the neighbouring unit cools.

Operating mode

Input BA is the user's manual intervention. It is usually driven by the output of the same name on the room controller and set there in the control element or by a switching time:

0 Automatic: mode from KU, speed from SG or SH respectively. This is the default and the usual behaviour.
1 Heating and 2 Cooling: the mode is forced. The changeover lock still applies, and so does the condensate monitor.
3 Fan only: the valves stay closed, the fan runs according to SG or SH, at least at speed 1. There is no valve lead time here.
4 Off: the valves close, the fan runs out after the overrun time. Frost protection still remains active.

Protective functions

Condensate monitor. If input KW responds, this affects only the cooling mode — heating continues unchanged. Depending on the parameter, cooling is blocked completely or the speed is limited to 1. Output SR is set as long as cooling cannot work because of this.

Window contact. With an open window the unit is switched off. Optionally frost protection remains active.

Frost protection. If the room temperature at input RT falls below the set threshold, the heating valve opens and the fan runs at speed 1 — regardless of the enable, the window contact, the "Off" operating mode and the minimum idle time. Output SR is set while this happens. Without a linked input RT there is no frost protection.

Operating hours

Output BT counts the hours during which the fan has been running. The counter is stored and survives a restart of the controller. It is written once an hour so that the storage medium is spared.

Inputs
SG	Control value	Control value 0 to 100 % of the controller. It determines the fan speed. The input has to be linked; without a link the block calculates with 0 and the unit only runs on a manual command.
KU	Heating/cooling	Optional. 0 selects heating, other than 0 selects cooling. The input is only evaluated in the automatic operating mode. Not linked: the unit heats.
SH	Speed manual	Optional. 0 leaves the speed to the automatic mode, 1 to 3 specify a fixed speed. Higher values are limited to the configured number of speeds. Not linked: automatic.
BA	Operating mode	Optional. 0 = automatic, 1 = heating, 2 = cooling, 3 = fan only, 4 = off. Unknown values count as automatic. Not linked: automatic.
RT	Room temperature	Optional. Room temperature in °C, used exclusively for frost protection. Not linked: there is no frost protection, even if it is switched on.
SB	Speed limit	Optional. Highest permitted speed, for example at night because of the noise. The value 0 means no limit. Not linked: no limit.
KW	Condensate monitor	Optional. Other than 0 blocks cooling or limits the speed, depending on the parameter; heating is not affected. The input can be inverted. Not linked: condensate is never reported.
FE	Window contact	Optional. Other than 0 counts as an open window, the behaviour is set by the parameter. The input can be inverted. Not linked: the window counts as closed.
FR	Enable	Optional. Other than 0 enables, 0 switches the unit off immediately — without minimum run time, but with fan overrun. Not linked: the block counts as enabled.
Outputs
VS	Fan speed	Running speed as a value: 0 = off, 1 to 3 = speed. During the valve lead time this is 0, during the overrun it is 1.
VH	Valve heating	100 as long as the heating valve is to be open, otherwise 0.
VK	Valve cooling	100 as long as the cooling valve is to be open, otherwise 0.
UM	Changeover valve	0 = heating, 1 = cooling. In a 2-pipe system it is only switched once valve and fan have stopped.
SS	Info status	State as a bit mask: 1 = fan running 2 = heating 4 = cooling 8 = manual operation or operating mode other than automatic 16 = condensate 32 = frost protection active 64 = window open 128 = changeover lock running 256 = valve lead time running 512 = fan overrun running
SR	Fault	1 as long as a protective function intervenes — cooling blocked because of condensate, or active frost protection. Which one it is can be read from the bit mask at output SS.
BT	Operating hours	Running time of the fan in hours. The value therefore only changes once an hour.
V1 to V3	Fan speed 1 to 3	One output per speed. The number follows the parameter "Number of fan speeds"; speeds that do not exist have no output.
Parameters
Number of fan speeds		1 to 3. Determines how many speed outputs the block has.
Speed switching		Changeover: only the output of the running speed is set. Stepped: all outputs up to the running speed are set.
Threshold speed 1 to 3		Control value in percent from which the respective speed is used. Typical values: 10, 40 and 70 %.
Hysteresis		By this amount the control value has to fall below the threshold before switching back down. The hysteresis has no effect when switching up.
Minimum run time per speed		A speed stays in place at least this long before switching down or off.
Minimum idle time		After switching off, the unit stays off at least this long before it starts again.
Fan overrun		After the valve has closed, the fan keeps running at the lowest speed for this long.
Valve lead time		In heating mode the valve opens first; only after this time does the fan start. The time has no effect in cooling mode.
Pipe system		4-pipe: separate valves. 2-pipe: one valve, the direction is given by output UM.
Changeover lock heating/cooling		The mode stays in place at least this long after a change.
On condensate		Block cooling, or limit the speed to 1. Heating is not affected in either case.
With an open window		Everything off, or frost protection stays active.
Frost protection		Switches frost protection on. It needs input RT.
Frost protection temperature		Below this room temperature the unit heats.
Speed limit when absent		Highest speed in the presence states absent and holiday. 0 means no limit.
Invert window contact input		For contacts that provide a 1 in the closed state.
Invert condensate monitor input		For monitors that provide a 1 in the good state.

See also general parameters of all function blocks.