Vaillant Turbomax 242e Hot Water Running Cold at High Rate

[sidepipe]

My mother-in-law has a Turbomax 242e, about 11 years old ( green control knobs. ) The initial symptom that was noticed was that her shower began to run cold ( a thermostatic mixer job with a single flow/temp control. ) She has a service contract with Southern Electric, and at first they told her that it was a faulty shower.

However, I went round and did some very simple diagnosis, and found that when any of the hot taps is turned above a certain point, the water begins to run cold, to the point where it doesn't even feel luke warm. This starts to happen at roughly 11 litres/minute AFAICT. Obviously with the shower not having a flow rate control, this is a problem. We temporarily "fixed" the problem by turning the mains stop-cock right down, and hey presto the shower became very hot ( if a little on the slow side! )

Gas people out again, twice. Still insisting its the shower, but they have now changed the DHW heat exchanger AND the flow diverter valve. Now they haven't a clue, and are saying that she needs a powerflush. Given that at around 7l/min the water is red hot, and at 11 its stone cold, I can't see that having an effect ( but then I am not a boiler engineer! )

Anyhow, I've noticed that the gas burner doesn't seem to modulate with the water flow rate - I assume that it should? ( Though I would have thought that the "engineer" would have noticed this! ) I haven't checked for voltage changes on the input to the gas valve or anything yet, because I don't know what should happen, or what is used to sense the water flow rate etc, so frankly don't know what I'm looking for yet! Any pointers would be appreciated!

Thanks.

 

[sidepipe]

Just following up my own posts - I did some ( very inaccurate ) measurements. At roughly 9l/min, the rise at the tap is about 30 degrees. At around 13l/m, which is pretty much fully open, this drops to only 18 degrees. The spec says 35 at 10l/min, so given the inaccuracy of the readings it may not be far off. So I guess that it's possible that the water pressure has incresed since last winter and there may really be nothing wrong! If anyone has one of these and could tell me if the burner visibly modulates with flow rate it might help avoid me chasing red herrings.

 

[sidepipe]

Sorry to bump, but does anyone have one of these or know if the burner should VISIBLY increase in output with DHW speed, so that I have some idea whether there is a problem or not. I would assume that it should, but never saw it when it was working! The spec says that the output varies between 9kW and 29kW, which I am pretty sure would mean a visible change in the burner, so I'm 90% sure, but it would be nice to have confirmation before I start moaning at the service company!

If the central heating is on, you turn the hot water on slowly and the burner output seems to increase slightly ( from the level it was when the heating is up to temp. ) Increasing the water speed seems to then make no visible difference....

Thanks.

 

[twgas]

The modulation is controlled by temperature, so, if the water does not get up to temp there will be no modulation.
You need to measure the flowrate and temp rise fairly accurately.

 

[Bunnyman]

Classic case of a thermostatic mixer shower failure lmao!

Untitled

• Bunnyman
• 26 Mar 2010
• 499