Overview

Systemair SAVE VSR 300 (residential, 300 m3/h) and SAVE VTC 700 M (commercial, 700 m3/h) are heat recovery ventilation units with Modbus RTU capability.

Wiring and Parameters

Connection: 3-wire RS485 (A, B, GND) from the SAVE service board RS485 terminal to the KNX Modbus gateway (MDT SCN-MIG.01 or Weinzierl 5013). Wire type: Belden 9841 (120-ohm characteristic impedance shielded twisted pair). Termination: 120-ohm resistor on the SAVE terminal and on the gateway end if gateway is at the end of the bus. Modbus RTU parameters: 9600 baud, 8 data bits, no parity, 1 stop bit (8N1). Default unit ID: 1. To confirm or change: on SAVE front panel, hold right-arrow button for 5 seconds → service menu → Communication → Modbus address.

Full Register Map

4001 (supply fan speed rpm, read), 4002 (extract fan speed rpm, read), 4003 (supply air temp, int16, 0.1 degrees C, read), 4004 (extract air temp, read), 4005 (outdoor air temp, read), 4006 (exhaust air temp, read), 4007 (operation mode, int16: 0=off, 1=auto, 2=speed1 low, 3=speed2 medium, 4=speed3/boost, read/write), 4008 (bypass damper: 0=closed, 1=open, read only), 4009 (defrost active: 0=no, 1=yes, read only), 4010 (boost request: write 1=activate boost for 60 minutes), 4016 (heat recovery efficiency %, read).

MDT Gateway Configuration

MDT SCN-MIG.01 gateway: configure poll group A (registers 4003-4006 every 60s for temperatures), poll group B (register 4007 every 10s for mode). Write object for register 4007 (mode) and 4010 (boost). KNX group scheme: 10/1/1 (supply temp), 10/1/2 (outdoor temp), 10/1/3 (extract temp), 10/2/1 (boost trigger DPT 1.001), 10/2/2 (operation mode DPT 5.010).

CO2 Boost and Night Setback

CO2 boost trigger: Elsner KNX CO2-T-UP in living room sends ppm as DPT 9.008. ETS6 logic comparator: when ppm > 1000 → write 10/2/1 = ON (boost, register 4010=1). When ppm < 800 for 10 minutes → write 10/2/1 = OFF (register 4007=1 auto). Night setback: KNX time switch (02:00 → mode 2 speed1, 06:00 → mode 1 auto). Bypass control: register 4008 is read-only — SAVE firmware controls bypass based on outdoor vs indoor temperature crossover automatically; KNX only monitors status.