Vaillant Turbomax DHW temp. problem

[andrewpreece]

The Turbomax 828 allows control of CH and DHW temperature from two knobs on the front panel. I had set my DHW to about 50C to allow me to wash my hands in the hot water continuously, but lately I've felt the water is too hot to keep my hands immersed.

I tried adjusting the front panel DHW temperature control knob, but it has no effect. The DHW temperature is shown as 66C on the LCD display, rising to maybe 71C when the boiler has been fired up for 30 seconds. The CH temperature OTOH is still controllable via its own front panel knob.

Even turning the DHW knob fully ACW has no effect. This problem is not a show-stopper, as I can live with it, but obviously I'd rather know what is going on.

OK, it's probably something like a new pcb needed in which case it's hand-deep-in-pocket time, but I'm wondering whether anyone out there has experienced this problem, and has found the answer? Also, if I call up the Vaillant technical helpline, will they have access to technical bulletins detailing common problems with the Turbomax series, and will they speak to me ( non-trade )?

As a first level diagnostic test, the potentiometer attached to the DHW temperature knob appears OK, it behaves electrically in exactly the same way as the CH potentiometer knob. Also, the temperature sense thermistor must be working as it picks up the high DHW temperature and sends it to the LCD display.

There are no fault codes to speak of and the mode and diagnostic displays offer no obvious clue to any malfunction. Perhaps I'm imagining the fault!

cheers,

Andy.

[andrewpreece]

Hello Ed,

Thanks for replying and for the info. The situation is more complicated than I thought, and I'm beginning to question my memory now! I tried again today to investigate, and found that fully ACW the hot water knob will get me water at 65C, whereas at fully CW, the temperature reaches

76C (after 30 secs ). This means the combi is responding to demand, but I'm perplexed as I swear I remember being able to dial up 50C last year. A range of about 10C for DHW temperature control doesn't seem very large.

I also got the Vaillant into diagnostic mode and dialled up the modulator current, which I presume corresponds to gas modulation. I found that turning the DHW knob ACW, when demanding HW from a tap, caused the modulator current to drop like a stone, though it springs back a bit, so this confirms the DHW knob really is having an effect.

I shall just leave the DHW knob fully ACW for the time being and live with water at 65C, as that's an improvement on the recent situation (>70C). I am wondering if the groundwater temperature effects things? I thought the Vaillant delivered water at absolute temperatures, not relative. I last adjusted it in december, when things were cooler. Also, I wonder if I am confusing memories about the CH temp control, which shows demand ( as opposed to actual ) with DHW in my memory.....

Finally, I tested the combi this time with a small cloakroom basin hot water tap which didn't exist when the combi went in, and I wonder if a higher flow rate would give different results? ( shouldn't matter though as I rarely turn a hot water tap on full except for the bath ).

thanks anyway,

Andy.

[Ed Sirett]

Well you have gone through all the things I would have done to check it out. There is only one more test I can think of which is does the "Hot Standby mode" on/off function work. I.e. If you turn the DHW knob fuly clock wise the green light comes on if you turn it full anti-clock it goes off? There are slight detents at the top and bottom of the range but I don't think these correspond to extra switches just the hi/lo end of the "pot".

I suppose there might be a poor connection between the pot. in the PCB? Anyway it all looks like a new PCB.

See if geoff at cetltd.com does them, he lists a Turbomax board but I don't know whether this is for a Turbomax or Turbomax+ (your one). The price is around half the £150 that Vaillant would want. The part number he quotes is not the one I have listed for the Turbomax+.

[Ed Sirett]

AFIK the display shows the temperature of the primary water as it exits the DHW exchanger ('sardine can'). This is the only NTC sensor in the box. Since a little water always flows through the primary side [1] of the 'can' this shows the primary flow temp in CH mode and approx HW delivery temp in HW mode.

Even if the flow rate is very poor the HW temp should not exceed the maximum specified by the DHW temp knob, if necessary the gas will be cut. The range should be something like 45-75C.

Perhaps there are some fixed or preset resistors on the pcb near the pot. pot which have gone awry.

Does the Hot-Standby-Selection feature work?

[1] That's how the bypass circuit is implemented.

[andrewpreece]

Just a correction to my last post, the warmstart works ok, but although the diagnostics said it was at 48C, in fact it is variable via the DHW knob from

30-60C ( diagnostic display varies instantaneously with the HW knob position, so that's obviously a demand figure ), showing the DHW pot works OK and that info at least is getting to the display. The book says the warmstart temp can be varied between 40C and 65C however, so I'm not sure it's still in kilter. I must say that the diagnostics function is a great help on this boiler, it may not tell everything but I'd be messing around with voltmeters and thermometers without it.

Andy.

[andrewpreece]

Interesting, since my DHW knob is very nearly fully ACW to give 46C ( not that far from the knod position you used, though I get an eventual display reading of 63C ), and I'd agree with your subjective interpretation of what

45C feels like! I had a final fling at understanding things today, and I can say that the Vaillant Turbomax contains three temperature sensors:-

one on the loop on the input to the burner, one on the loop on the exit of the burner, and one on the loop as it comes out of the heat-exchanger ( sardine can ).

That last sensor doesn't measure the DHW temperature directly, but it's close, as I reckon Vaillant are using 44004 series thermistors as sensors ( about 2250 Ohms at 25C, more like 550 Ohms at 60C ), and I've measured the resistance of this sensor and comparing it to its calibration curve, it agrees well with the actual water temp.

What I didn't know is that the lcd display only dislays one measured temperature, and it takes that from the sensor coming out of the burner, so that's the hottest part of the loop, generally 20C hotter than the tap water. I expected the sensor on the sardine can to talk to the electronics and modulate the burner down when it sensed the water was up to the demanded temperature, but it has no measurable effect. I even frigged the sensor with a parallel resistor to try and fool the electronics into thinking the DHW was way too hot, but no effect. So, I reckon, unless I'm paranoid, that the feedback loop involving that sensor has failed and I'm getting a kind of open loop reduced control from the other sensor(s).

I shall live with it, it's not a problem ( though the water had risen to 58C until I figured out I had a problem, and that IS painful )! That's where mixer taps are handy. The CH works on the sensor coming out of the burner loop, and that feedback circuit is separate, so it's spot on.

thanks,

Andy.

[Ed Sirett]

I was doing a Landords' today on a site with a 824e and I was able to run the hot tap with water at around 45C (unpleasant but not impossible to hold hand under for an extended period) the DHW knob was about 10% up from the bottom and the LCD was showing mid 50s. Did not check any parameters though.