When using the hot water the boiler heats the water fine, but after 30 seconds the gas goes out, the water coold slightly then the gas re-ignites and the water heats - this cycle is then repeated, I also hear a humming sound coming from what seems like the heat exchanger - I recently drained the system. I wonder if air has got into the heat exchanger part of the system ?? it seems like the circuit boils the water - and it over heats - turning off the gas - is there a way to bleed this part of the system if air gets in ??

Rob, October 2005

I had the same problem with my 828e. It turned out to be a problem with the plate heat exchaanger which apparently had become scaled (limescale etc).
It has now been replaced and works fine with no temp fluctuation/variation as before.

Apparently this is a fairly common problem with the Vaillants.

Rizwan Ahmed, May 2008

you may have drained some muck into the plate heat exchanger remove it and flush it through that would be my 1st option...

d.., October 2005