Not sure if it's the same as you but I get a one off loud fog horn sound when flushing the toilet or running a tap.

It only occurs after DHW heating, not always though.

The Mitsubishi service engineer suggested a "System Shock Arrestor".

Just noticed I have lost my pressure and had to recharge. I have a thought it’s the mains water pressure that’s too high. 

@david999 Recharge what? Your circulating fluid to your emitters? If yes, then something odd would be happening as the circulation system is normally pressurised to approx. 1.5 bar and then both of the valves (one on either side of your pressure vessel) are shut off so you then have a closed system. Higher main pressure should not then influence this circuit at all. Have I mis-understood your question perhaps? Regards, Toodles.

@toodles a bit confused toodles, no idea what’s blowing off the expansion vessel of the tundish but it was loud.  I checked my pressure on the gauge at the filling loop and it was zero so I filled it back up but over did it and let a bit off. It came out the pipe outside and no antifreeze in it, but could be just because I just filled it up and the loss could just be me draining air off the other day when I first heard it blow off. Am I correct that the tundish is just for the cylinder and no antifreeze in that side and it could be my supply pressure creeping up.

@david999 I think the more knowledgable need to answer this one as I am not sure I understand what is happening. Don’t want to cause you more problems, I expect some other readers will know what is going on. Regards, Toodles.

@toodles the installer coming out tomorrow toodles. I was curious if the tundish routinely blew off or if it’s a fault but looks like a fault or they wouldn’t come out I guess. Due a service soon so hopefully the samsung authorised blokes sign it all off as satisfactory.  My installation guarantee expires in November so need to ensure all is well.  I’m learning the system but a bit to go yet. 

if someone else replies could they clarify two other points please.

1. is the expansion vessel with the filling loop only for the radiator side.

2. Is the tundish only for the domestic supply in the cylinder, the stored water. It must be the mains pressure that pushes it out and the tundish protects the cylinder?

3. Blowing off at the tundish will not lower the system pressure in the rads?   I think it was me bleeding air and getting the two mixed up. 

if anybody has a thermal solar, what’s their outputs, I’m getting about 40c at the minute for a 20 tube system

Heating engineers out today and my fault is the expansion vessel for the dhw cylinder.  It started making this noise like a foghorn on Saturday and has gotten worse, but tied it down to when hot water is run, but it only sounds for a few seconds. A thing to check if you have a bicycle pump with a gauge is the pressure in the vessel, it has a standard car valve on it.  Failing that its knackered and you need a new one.  Hope that helps someone in the future.