Grundfos MIXIT Guida rapida alla configurazione

MIXIT integration Guide

Application guide JAN2022

page 1 
 
Integration of MIXIT with BMS systems 
 
Introduction............................................................................................................................................... 2 
1 Alternative option for interfacing MIXIT to a dedicated controller .................................................. 3 
Application specific defaults...................................................................................................................... 4 
Configuration of setpoints and bus control............................................................................................... 5 
MIXIT with Air Handling Units ................................................................................................................... 6 
1 MIXIT AHU control of discharge air temperature (heating coil application)..................................... 6 
2 MIXIT AHU control of hot water temperature (radiator heating application).................................. 7 
3 Integration of MIXIT in AHU control with MIXIT configured for the heating coil application........... 8 
3.1 AHU sequencing with safety functions for heating coil ............................................................ 8 
3.2 Signal diagram of AHU............................................................................................................. 12 
3.3 Signal diagram of AHU with MIXIT configured with the heating coil application ................... 14 
4 Integration of MIXIT in AHU control with MIXIT configured for the radiator heating application. 16 
MIXIT with radiators and underfloor heating.......................................................................................... 20 
1 The build in underfloor overheat protection function.................................................................... 20 
2 Signal flow diagram for the radiator and underfloor heating applications..................................... 21 
Interfacing MIXIT to the BMS using analogue signals ............................................................................. 22 
 
 

page 2 
 
1INTRODUCTION 
This document describes how to integrate MIXIT into an automation system via either the Modbus or the 
BACnet fieldbus protocols. Examples in this document and especially drawings mention specific control and 
status datapoints referring to BACnet objects. In those cases, tables are listed where the corresponding 
registers in the Modbus profile can be seen. The document is only concerned with the most relevant 
datapoints for start/stop, setpoints and sensors. The full list of datapoints are available in the functional 
profiles for the product at https://product-selection.grundfos.com/,or www.grundfos-eica.com search for 
MIXIT, click on a MIXIT product and browse to documentation. 
The appropriate way of integrating MIXIT with the automation system depends on the use case. Three use 
cases are sketched in Figure 1 
1. Interface to a controller that is programmable and has a Modbus RTU or BACnet MS/TP interface. 
2. Interface to a dedicated controller that is not programmable to the extend it can interface to 
MIXIT’s fieldbus profile. 
3. Interface to a programmable controller that only supports analogue I/O. 
Use case 1 is the common situation for large systems. 
Use case 2 is sometimes used in smaller buildings. 
Use case 3 can be encountered if the integration task you face is for retrofit with an older BMS system. 
If facing use case 1, sections 3 and 4 are of relevance if integrating with an air handling unit or sections 3 
and 5 are of relevance if integrating with a radiator system or a floor heating system. In addition to these 
sections, if facing use case 2, then section 1.1 is also of relevance. If use case 3 is faced please consult 
section 6. 
 
Figure 1: Use cases for BMS integration, the field level reflects the case for an air handling unit (AHU). 
The corresponding use cases with MIXIT is shown in Figure 2. The difference between integrating MIXIT into 
a BMS system compared to a traditional valve is that MIXIT shall receive a start/stop signal and a temperature 
setpoint instead of a valve position, that many datapoints are available via the MIXIT fieldbus profile that 
relates to integrated sensors and the connected pump, and that the valve position of MIXIT cannot be 
controlled directly. 

page 3

It is possible to send start/stop signal and the setpoint to MIXIT via the BACnet or Modbus or on an analogue

interface. If the analogue interface is used for sending signals to MIXIT it is still possible to read datapoints

from the BACnet or Modbus fieldbuses.

Use case 2 with the dedicated controller can be a problem as it is designed to control the valve position

directly. However, sometimes the temperature setpoint and on/off signals are available in the bus profile of

the dedicated controller. They should then be transferred to MIXIT via the programmable controller at the

automation level.

Figure 2: Use cases for BMS integration with MIXIT, the field level reflects the case for an air handling unit (AHU).

1.1 Alternative option for interfacing MIXIT to a dedicated controller

If MIXIT cannot be connected at the automation level in use case 2, for example because no spare analogue

outputs are available or there is no BACnet MS/TP or Modbus RTU fieldbus master available at automation

level, there is one additional way of interfacing MIXIT to the dedicated controller.

The analogue valve opening setpoint from the dedicated controller may be interpreted as a temperature

setpoint because of the following steady state relation that is true for any mixing loop:

        

where

: The secondary supply temperature

: The return temperature

: The primary supply temperature

: The differential pressure across the valve

: The secondary flow

󰇛 󰇜: A nonlinear function

page 4

By a closed valve         whereby   , and when the valve is

fully open for most mixing loops           whereby  



Therefore, setting up MIXIT to interpret the signal from the dedicated controller as a temperature setpoint

may make sense. For example, if the analogue control signal from the dedicated controller is 0-10 Volts and

the supply temperature is normally around 70 deg C and the minimum return temperature from the coil is

15 deg. C the configuration of MIXIT can be made via the Grundfos GO app as shown in Figure 3.

Some caution is to be made though. This solution cascades the control loop of the dedicated controller with

the temperature control loop of MIXIT. The tuning of the dedicated control shall be slower than that of the

MIXIT controller otherwise there is a risk that the overall control will oscillate and therefore the

recommended option is to transfer the setpoints from the dedicated controller to MIXIT via the automation

layer as shown in Figure 2.

The source for the start/stop signal that shall be given at a digital input on MIXIT depends on the dedicated

controller model.

Figure 3: Configuration of analogue input as temperature setpoint.

2APPLICATION SPECIFIC DEFAULTS

In this document there are reference to the build in application specific safety functions in MIXIT: Frost

protection, coil preheat and floor overheat protection. When configuring MIXIT the first time these functions

are enabled but they can be turned off via Grundfos GO. Table 1 gives an overview. The safety functions are

described in detail in sections 4.3.1.1, 4.3.1.2 and 5.1.

page 5

Application

feature

Factory default status

Heating coil

Coil preheat

Frost protection

Default temperature setpoint

40 deg. C

Floor heating

Floor overheat protection

Default temperature setpoint

40 deg. C

Radiator heating

Default temperature setpoint

40 deg. C

Cooling applications

Frost protection

Default setpoint

15 deg. C

Table 1: Application specific factory default values.

3CONFIGURATION OF SETPOINTS AND BUS CONTROL

The field bus profile for MIXIT is made such that for the commands that can be given on an output

object/register, there is a corresponding input object/register for reading back that the command was

registered. The integration of MIXIT via the field bus protocols can only work if:

•MIXIT is set for bus control, otherwise it will not react on commands and setpoints. The bus control

state is not persistent if the communication fails or MIXIT experience at power cycle. Therefore,

there must be a cyclic checking and setting in case it has fallen back to local control.

•The correct setpoint chain is configured. For BUS control this is a persistent configuration and shall

only be set once.

The configurations are described by Table 2.

Action

BACnet

Modbus

Persistent setting

Object for

setting

Object for

status

setting

status

Config. set-

point from

fieldbus

MV,0 = 3

MV,0

00117 = 2

00117

Yes

Set setpoint

from fieldbus

AO,0

AI,0

00102

00311

Set bus control

state

BO,0 = 1

BI,0

00101.0 = 1

00201.0

Config. default

setpoint

AV,6

00118

Yes

Set start/stop

BO,1

BI,2

00101.1

00201.3

Table 2: Configuration and setting of remote state.

If MIXIT experience a communication failure, it will use the default setpoint which is factory configured at

40 deg. C for heating applications and 15 deg. C. for cooling applications. Therefore, it is a good idea to

configure with a value meaningful for the application.

Notice! The default setpoint (BACnet: AV,6 / Modbus: 00118) is a persistent datapoint and shall not be

used for regular/cyclic setpoint control. It is a fall back setpoint in case bus communication fails.

Notice! Single bits in Modbus register 00101 cannot be addressed even though the description of the

their desired values.

page 6

4MIXIT WITH AIR HANDLING UNITS

The objectives of using an air handling unit (AHU) are multiple:

•Supplying fresh air to a building zone.

•Discharging dirty air from a building zone.

•Pressurizing the building zone.

•Heating and cooling the fresh air supplied to the zone.

•Dehumidification and humidification of the fresh air supplied to the zone.

The heating and cooling of the fresh air happens by a combination of heat recovery and use of a heating

coil or a cooling coil. The AHU controller manages the local setpoints, sequencing and safety functions of

the AHU.

When MIXIT is used with AHUs it shall run a temperature control loop for either the discharge air

temperature from the coil or the fluid temperature flowing towards the coil. It is up to the integrator to

choose between these two options as follows:

•The selected MIXIT application is the Heating coil application: MIXIT controls the discharge air

temperature leaving the coil.

•The selected MIXIT application is the Radiator heating application: MIXIT controls the fluid

temperature flowing towards the coil.

The difference between the two are briefly described below. In either case local setpoints, sequencing and

safety functions can be managed by the AHU controller.

The application selection is done via the Grundfos GO app.

4.1 MIXIT AHU control of discharge air temperature (heating coil application)

Figure 4 shows the simplified cascaded control loops associated with the AHU. The control loop for the air

temperature which MIXIT runs when the heating coil application is selected is highlighted by the green box.

Figure 4: Simplified AHU control loops with MIXIT.

The air temperature needs to be feed to MIXIT via an analogue signal. The air temperature is also used by

the AHU and the signal can be shared as shown in figure 3.

By method 1 the signal is shared via a signal splitter, all signals are analogue. By method 2 the analogue

signal is fed to MIXIT and the AHU controller reads the air temperature value from MIXIT’s fieldbus profile.

By method 3 the sensor is connected to the AHU controller which feeds the signal to MIXIT via the field bus.

In the current release of MIXIT there is no datapoint for this in the fieldbus profile it will be available in a

future release.

page 7

Figure 5: Ways of sharing the air temperature signal with MIXIT.

Integrating MIXIT where it controls the air temperature has the following benefits for heating coil control:

•MIXIT has a build in algorithm that can detect a frost risk at the coil and protect against coil

damage.

•MIXIT can run a coil preheat routine when the AHU is started op.

This way of integrating is described in section 4.3 together with the setpoint configuration in section 3.

4.2 MIXIT AHU control of hot water temperature (radiator heating application)

Figure 6 shows an alternative way of cascading the control loops when MIXIT is integrated with the AHU. In

that figure, the AHU controller gives the setpoint for the water temperature that is supplied to the coil.

MIXIT controls therefore the water temperature (TF) out of the mixing loop according to the setpoint. The

AHU controller is responsible for setting the appropriate water temperature setpoint, such that the air

temperature leaving the coil is at its appropriate setpoint for TA.

Figure 6: Alternative AHU control loops with MIXIT.

The drawbacks of this way of integration are, since MIXIT does not have access to the air temperature at

the coil:

•AHU controller needs to handle frost protection of the coil via the water temperature setpoint for

MIXIT.

•AHU controller needs to control coil preheat via the water temperature setpoint for MIXIT.

method 2

method 3

(in future MIXIT release)

method 1

analogue

MIXIT

AHU

controller

Signal

splitter

Air temp

sensor

analogue

field bus

Air temp

sensor MIXIT AHU

controller

field bus MIXIT

Air temp

sensor AHU

controller

page 8

The advantage is that MIXIT does not need connection of additional sensor signals. This way of integrating

is described in section 4.4 together with the setpoint configuration in section 3.

4.3 Integration of MIXIT in AHU control with MIXIT configured for the heating coil

application

Here some basic topics of interfacing MIXIT with AHU control are covered. It is a prerequisite that MIXIT is

configured for heating coil application to follow the below guides.

Notice! When MIXIT is configured with the heating coil application, it will assume control of the air

temperature leaving the heating coil, and it needs an air temperature sensor connected, e.g. see Figure 5 in

section 4.1.

4.3.1 AHU sequencing with safety functions for heating coil

This section describes the start and control sequence when MIXIT is integrated with the AHU controller.

Figure 7 shows the state diagram of MIXIT with the integrated coil preheat and frost protection functions

enabled. The shown states are as follows:

READY: If a start signal is not given MIXIT is in the READY state. The pump is stopped, and the valve is

closed.

PREHEAT: When a start signal is given the PREHEAT state is entered, e.g. see section 4.3.1.2.

TEMPERATURE CONTROL: When the coil has been preheated the air temperature after the coil is

controlled according to the setpoint. MIXIT signals this at the fieldbus by setting BI,1=1.

FROST: The frost state is entered if a frost risk at the coil is signaled or it is detected by the integrated frost

protection function. Both conditions have to be false for MIXIT to enter the TEMPERATURE CONTROL state

again, e.g. see section 4.3.1.1.

STOP: When the start signal is removed MIXIT initializes for next start via the STOP state. After the STOP

state MIXIT always enters READY.

Figure 7: State machine for MIXIT heating coil application.

The data points associated with the state transitions are listed in Table 3.

page 9

Action

BACnet

Modbus

Persistent setting?

Object for

setting

Object for

status

setting

status

Set bus control

state

BO,0 = 1

BI,0

00101.0 = 1

00201.0

Set start

BO,1 = 1

BI,2

00101.1 = 1

00201.3

Set stop

BO,1 = 0

BI,2

00101.1 = 0

00201.3

Set frost risk/

Frost risk detected

BO,4 = 1

BI,7

00101.7

00204.4

Temperature

control Enabled

BI,1

00201.4

Table 3: Data points associated with the state machines in Figure 7 and Figure 8.

In situations where the coil preheat function is not needed and has been disabled, the state diagram is as in

Figure 8. The data points associated with the state transitions are the same as above.

Figure 8: State machine for MIXIT with the preheat function disabled.

Even simpler, if the internal frost protection function has also been disabled, the only thing that can bring

MIXIT to the frost state is either a signal on the fieldbus (BACnet: BO,4/Modbus 00101.7) or a signal at a

digital input from a frost thermostat.

Figure 9 shows how to interface to MIXIT, when the coil preheat function in MIXIT is enabled. The example

in the figure is based on BACnet objects but the corresponding Modbus registers are listed in Table 2 and

Table 3. For example, as shown in Figure 9, BI,2 is read back to check that the start command BO,1 was

received by MIXIT. It is good practice always to do this, because if a command was for example not

registered, it shall be resent.

Notice! The control loop in MIXIT runs each 1 second asynchronous with the fieldbus communication.

Therefore, up to 1 second may elapse before a command takes effect.

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