Vaillant VRT 392 Thermostat (wired) not working

[xvvjohn]

Hello, my Vaillant VRT 392 Thermostat just stopped working, it's completely dead, I read another thread suggesting changing batteries, but this model does not use batteries, it's wired to a Vaillant control center VR65 with eBUS, my boiler is Vaillant ecoTEC plus 624, anyone can help identify the problem will be great appreciated, thanks.

 

[Gasmk1]

Are you sure it hasn’t got batteries ebus is just an information cable doesn’t carry any power shows you where the go in these instructions https://norcalgas.files.wordpress.com/2016/11/427-instructions.pdf

 

[xvvjohn]

Yes, I am 100% sure about that. it's model VRT 392, NOT VRT 392f.

 

[Chuck]

Are you sure it hasn’t got batteries ebus is just an information cable doesn’t carry any power shows you where the go in these instructions https://norcalgas.files.wordpress.com/2016/11/427-instructions.pdf

Strictly speaking, eBus can, and is, used to supply up to 100mA to power to devices. Its signalling levels ("0" = ~10V, "1" = ~20V) were selected to allow devices to derive a power supply from the signal by using a simple filter/regulator circuit.

If I were the OP, I'd use a multimeter to check the eBus signal arriving at the thermostat was in the range 9–24V. If it is, the thermostat is likely a gonna. If it's not then the cable, connections or boiler are the suspects.

If the OP has the right skills, tools and time on their hands, they might want look at the board carefully for dry joints and/or swollen capacitors. These can sometimes be fixed.

 

[xvvjohn]

Strictly speaking, eBus can, and is, used to supply up to 100mA to power to devices. Its signalling levels ("0" = ~10V, "1" = ~20V) were selected to allow devices to derive a power supply from the signal by using a simple filter/regulator circuit.

If I were the OP, I'd use a multimeter to check the eBus signal arriving at the thermostat was in the range 9–24V. If it is, the thermostat is likely a gonna. If it's not then the cable, connections or boiler are the suspects.

If the OP has the right skills, tools and time on their hands, they might want look at the board carefully for dry joints and/or swollen capacitors. These can sometimes be fixed.

thanks for the tip,

the signal arriving at the thermostat is 1.97V, and the signal from the eBUS connecter on Vaillant control center VR65 is 1.97V as well, which should be 24V.
On the PCB board of the Vaillant control center VR65, there is a green light flashing slowly, according to the troubleshooting of the VR65, it says 'LED flashes slowly - Communication error in eBUS protocol to the boiler'.
seems to me like no DIY zone.

 

[Chuck]

the signal arriving at the thermostat is 1.97V, and the signal from the eBUS connecter on Vaillant control center VR65 is 1.97V as well, which should be 24V.

One possibility is that the thermostat is faulty and drawing too much current, which is overloading the eBus and pulling the bus voltage down.

Most people would check the thermostat by substitution. If you're an electronics geek, you can find the eBus specification on line and with the aid of an oscilloscope and a dummy load you'll be able to figure out whether the stat the boiler (or the cabling) is at fault.

Someone more familiar with this specific setup than I am may recognise "1.97V" and be able to tie it to a stock fault.

 

[Deleted member 120897]

Could be the thermostat, could be the vr65 or the boilers pcb (although unlikely). You could ask on diynot.com as there’s a person there who works for Vaillant will likely know the answer.

 

[Basher]

Surely the 1.97V indicates a problem!
I would expect somewhere between 9V and 24V depending on what it's doing and how the multimeter behaves.
If you disconnect the thermostat and measure the eBus voltage, is it more normal then?


PS and how about some static handling precautions for that pcb?