Register reads return zero even though the heater is running. Why?

Register 317 (power) reports zero if the safety temperature limiter or thermostat has cut out, even when relay contacts are closed. Check the tank temperature against the configured limit in register 640. If the limit is reached, the heater disables and load drops to zero until the tank cools.

The gateway connects on port 502 but values appear stale or never update.

The ASKOHEAT+ updates its Modbus data every second and considers a connection interrupted after 5,000 milliseconds of silence. Set your polling interval to no faster than one second and ensure no firewall or managed switch is blocking TCP port 502 between the ModbusCloud Gateway and the heater.

Writing the power setpoint has no effect on the heater output.

Register 319 (setmaxpower) is an energy-manager setpoint. If the ASKOHEAT+ has not had a Modbus command within the last 60 seconds, it clears this setpoint automatically for safety. Ensure the ModbusCloud Gateway writes the value at least once per minute, and confirm the device is in energy-manager mode via the web interface.

Is it safe to write large watt values to the power setpoint register?

Use caution when writing to register 319. The ASKOHEAT+ will attempt to match the requested load, but setting a value beyond the heater's rated capacity or above the configured temperature limit can trigger the safety limiter, potentially disrupting legionella protection schedules. Always verify the written value does not exceed the device's rated power.

Free Modbus template: Askoma ASKOHEAT+

The Askoma ASKOHEAT+ is a Power-to-Heat immersion heater designed for direct installation in hot-water storage tanks and buffer tanks, converting surplus PV or grid energy into stored heat. This ModbusCloud template reads the device over Modbus TCP and exposes actual heater load, tank temperature from the integrated PT1000 sensor, the configured safety temperature limit, and the energy-manager power setpoint via a readable and writable register at address 319. Use it to integrate the ASKOHEAT+ into automated solar self-consumption workflows from your ModbusCloud portal.

Name	Address	Function	Type	Unit	Access
setmaxpower Power setpoint written by energy manager, in watts.	319	FC16	int16	W	W
getmaxpower Current power setpoint readback, in watts.	319	FC04	int16	W	R
power Actual electrical heater load currently consumed, in watts.	317	FC04	uint16	W	R
temp Water temperature at the integrated PT1000 sensor, degrees Celsius.	325	FC04	float32	°C	R
limittemp Configured safety temperature limit for the storage tank.	640	FC04	uint16	°C	R