Erratic hot water on new Vaillant EcoTec Green IQ 843, can a TMV or S.40 state be the issue?

[apprentisorcier]

I had a new Combi boiler (a Vaillant EcoTec exclusive green IQ 843) installed a few days ago replacing a previous system one. My flat has 2 bathrooms on two floors with 2 showers and 1 bath.

Heating works, and hot water mostly works as well but there's something that kicks in when we shower that is not good.

After a few minutes using the shower (say, 4-5) a vibration/buzzing noise starts being audible through the pipes, and the boiler goes from outputting water at the configured 65C down to a value that constantly swings between 48C and 54C, resulting in perceivable changes of the water temperature coming down the shower.

At some point in one case the boiler completely stopped outputting hot water, and the display showed 17C with the tap icon constantly on (meaning "draw-off mode with no burner") as far as I can see.

I've noticed that the boiler live monitor constantly reports state S40, which is "comfort protection active", and according to the maintenance manual (https://www.vaillant.co.uk/download...nce-instructions0020193968-03-pdf-1413944.pdf) should be a temporary mode (but for us it's constantly on).

Our installers suggested that there might be a TMV "somewhere" that might be malfunctioning, but we couldn't find one and before starting to open floorboards and break walls/ceilings I would want to make sure there is not a more plausible explanation. Someone advised me to turn the cold feed to the boiler off and try to open the hot water taps, saying if I got water out it would mean I have a TMV, but none of the taps/showers actually get any water when opening the HW tap in that case.

Does anybody have any idea what might be involved here?

 

[SimonG]

If all the other hot taps are fine then I would say shower. Might get away with a new cartridge.

 

[Riley]

Yeah if all other outlets are fine than it can only really be the ahower

 

[apprentisorcier]

I wasn't clear. This happens the same way on at least two outlets in two separate bathrooms on two separate floors (the shower upstairs and the bathtub downstairs).

 

[Riley]

Is this when running them simultaneously

 

[apprentisorcier]

Nope, this happens with just one running. It doesn't start immediately, the shower (or the bath) runs for a while (up to a few minutes) normally, then the vibration starts and the temperature starts dropping.

 

[oz-plumber]

It may be that the boiler hasn't been commissioned properly.
It may be struggling to get to minimum modulation to maintain the DHW at a constant temperature.

It could also be a sensor or a dozen other things.

 

[Chalked]

IQs have a very specific commissioning procedure. Takes me nearly an hour to do one. If it’s not done properly, or there is a fault. It will go to comfort mode.
Comfort mode used to mean hot water pre heat. But now it’s like a limp home mode on a car.
Get the installer back.

 

[Steve Bell]

Hi
I am having a similar problem with a 2015 Vaillant Ecotec Plus. Intermittent fault with DHW on bath or shower (CH seems OK) causing water to go luke warm and S.40 code. I have to switch off electrical supply to reset. The boiler was last serviced on 12/11/20 has extended warranty but Vaillant will charge £98 to come out and it might be working ok. I would really appreciate any thoughts on this