Vaillant TURBOMAX VUW 242E (switching on and off all the time)

11 Nov 2016
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Hi all,

Hope someone can help me with this. I have a Vaillant Turbomax VUW 242 (early model without the display) connected to a Nest thermostaat.

The problem I am having is the following; Let's say I put the Nest to 22 degrees C...I can see the heatlink firing up the boiler and both the heating and the circulation pump on the boiler start. Heating continues untill the desired temperature on the boiler is reached and then switches off, leaving only the circulation pump going. So far so good. But then the problem starts...from then on the boiler will fire up for 2 min..then up, stop...etc...This untill the Nest senses that the room temperature is 22 C and switches off the boiler all together.

When I asked the technician he said this was normal behaviour ?! According to him every time the boiler temperature drops below the set temperature (on the boiler itself) it will fire up again ?!

I understand that this should be happening, but not for every 2 or 3 degrees it drops below the set temperature. I can only imagine the wear and tear on this thing powering up and down every 2 to 3 minutes not to mention the cost in gas ?

Does anybody else have this experience ? Is this really normally behaviour ?

Also does anybody know what the "normal" CH water temperature setting would be on the boiler itself the lower green dial

Thanks for any help,
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Usually it is best to keep the heating on the highest setting, you can check whether the boiler is operating correctly by turning this control down while the boiler is on (central heating only) and when the boiler goes out, .Immediately turn the control back to full. The boiler should go through a short time delay before lighting to prevent recycling
Hi Spraggo,

Thanks for your help ! So I did as you explained, put the heating on the highest setting (at the boiler), and then turn it completely off/on. And indeed it takes a short delay before it fires ups gain. So thanks for that.

But then I went a bit further in my testing. Seeing as I think it is a waste to heat rooms that are not in use (spare room, bedroom, bathroom, hall) I tend to turn the radiators off and only heat the rooms that are in use (basically the living room). So just as a test I switched on all radiators. No more problem ?! The boiler keeps heating in one go until the desired heating of 21 degrees is reached on the Nest ! OK, so I turn off radiators in the bedroom and spare room again. Boiler starts doing it's 3 min on 3 min off cycling again ?! Why ? Note that the living room radiators are closest to the boiler and the bedrooms are furthest away. Am I forced to heat the complete appartement ?

Thanks for any advice,
When you have turned off several radiators the amount of heat the boiler is putting into the water is more than the remaiining radiators can disipate into the room(s). Hence the water returning to the boiler becomes too hot and the boiler senses this and shuts itself down until the water circulating round the radiators has cooled down.
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Hi bernardgreen,

Thanks for the response. That brings up one more question. Seeing as it is only a smallish apartment (90sq m2) Which would be beter; Having the boiler only heat the living room radiators with the 3min on/off intervals until it reaches the desired 21 degrees. Or heating all the rooms and have the boiler burning all the time until the 21 degrees is reached, but this time in all the rooms ?

Personally I would heat all the rooms as it removes the need for the boiler to cycle on and off due to over heating which ( in some boilers ) increases wear and tear. Having a warm bedroom is nice.

Also heating all rooms reduces the risk of condensation that could occur in un-heated rooms.
your boiler is not modulating get an experienced engineer in that knows how to check
hi ianmcd,

Could you elaborate ? I don't think this boiler can modulate ? Or am I wrong ?
Well that solves the problem then I guess, because as far as I know the Nest 2nd gen does not support modulation.
Shouldn't make any difference? It's only a posh switch, it has nothing to do with how the boiler runs (or modulates)
The nest looks lovely but is just a basic stat control really, all smoke and mirror to fool customers. Anyway that won't affect the boiler, it has its own built in modulation independent of nest
Actually the latest Nest is a lot more than a basic stat.

It can control the boiler flow temperature directly using opentherm protocol. It accepts regional weather information from the web to create a kind of weather compensation.

But only with those models that accept an opentherm input like Intergas and Viessmann for example.

Sadly not Vaillant currently in the UK.

Even if the OP's boiler was modulating correctly then the minimum heat output is still likely to be more than the radiators can dissipate!

Even if the OP's boiler was modulating correctly then the minimum heat output is still likely to be more than the radiators can dissipate!
Which sadly is the all too often problem that comes with combi boilers. A large burner is needed to provide enough heat for "instant" hot water but then that burner is too large for the much smaller amount of heat needed to keep the house warm. A burner can only be modulated down so far before the combustion becomes unstable and possibly unsafe.

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