Vaillant TurboMax Plus Combi DHW Too Hot

That doesn't answer the question. Min BP needs to be checked by locking the boiler in min output using test mode, not by slowing a tap down. One millibar out will make a considerable difference on one of these. Rough and ready tests are unhelpful in this type of situation. Asking your engineer to check the min heat input should give them a big hint!
Of course if you are trying to do this yourself you must realise that we cannot give out gas safety related advice.

The OP can ( should ) only do what he is allowed to do. So he cannot measure the burner pressure.

But just because the boiler is modulating does not mean that it is modulating down to the appropriate heat input to give the selected temperature output. It is modulating down but obviously at an appropriate gas rate for a higher and fixed temperature.

We don't get information on how boiler PCBs actually work.

On that vintage of boiler it was common for the applied burner pressure to be calculated according to the reading obtained from the flow thermistor. That enabled a rapid heat up from cold which applying just a calculated value would not be able to do.

Tony

wannocks said:

That doesn't answer the question.

Click to expand...

Hi Wannocks,

Sorry, I must avoid brevity. I should have added ‘if not, I will ask him to’ after I’ll ask the engineer if he has done this…

You should see from my other posts that I’m not trying to do this myself. I’m trying to gather info/opinions so I can have an informed discussion with the ‘central heating engineer’ fixing (attempting to) this.
Thanks for the comment on a millibar out making a significant difference. This may help with my discussion with the engineer.

Sticking with your comment on BP – would this lead to the HW always being at 65C no matter where the DHW dial is set or the HW being, say 10C, hotter than set on the DHW dial?

Thanks.

The boiler varies the heat input by applying a variable voltage to the modulation coil of the gas valve.

The effect of that is to vary the burner gas pressure ( BP ) which can be measured by an RGI. But in your case we already know that its too high for the required power input.

Its not very important in this instance to know exactly what it is, apart from being too high.

What you can measure yourself is the voltage applied to the mod coil. But as we know that its too high then its not important either in this case. Usually varies from about 6 v at minimum BP to about 12 v at maximum BP.

But it should be a feedback loop to obtain the set output DHW temperature. In your case that is not happening as its being varied but at the higher level than what has been set.

Tony

There is no DHW thermistor on a Turbomax, so DHW temperature (DHW heat input) is regulated by referencing the F&R thermistors and the Aquasensor. From your graph there is only 10C difference between F&R temps and the Gas valve is cycling. This indicates the heat input is too great at the lowest requirement, so it turns off and waits till the heat is dissipated. There is also a good chance the maximum heat input is set too high as well. At 15 l/min a 837 will sit at 70-75C flow temperature all day with the hot water at 50-53C and never turn off. The gas valve needs to be checked for correct settings on all THREE settings. A RGI (who understands this) needs to do this as mentioned many times before.

Problem solved.

The 'heating engineer' booked a Vaillant engineer who fixed it quickly. He said the gas pressure was a bit off (heating engineer should have found this!) and replaced the circuit board.

Thanks for all the helpful comments all.

We knew that from getting too hot.

Your engineer might have measured the burner pressures and realised they were too high but not known why.

Oddly, I am dealing with a different make of boiler with a similar problem!

Tony