Discuss Vaillant Sustain 28kw Combi Heat Input in the Gas Engineers Forum area at Plumbers Forums


Gas Engineer
Could anyone tell me whether the Vaillant Sustain heat input is noted in gross or net please.

I was sure it was NET but someone has said it's gross which makes no sense at all. I work all my gas rates to NET so if I'm wrong my gas rates will be way out.

I've included the data badge.


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Will be Net as it would be a 25kw combi
That's what I thought because it would not be a 28kw boiler now days with a gross figure. I was worried I'd got my figures all wrong.....
If you mean the fuel input then that is the gross heating value, that's what you are paying for.
Yours, I think is a "28kw" and is limited to give 18.5kw to 20.1kw output to CH but gives 28kw output to HW (combi mode), it gives a DHW flowrate of 11.6LPm @ a dT of 35C which equals a output of 28.33kw, the fuel input, depending on whether partially condensing or not, probably not in DHW combi mode, might be ~ 32/33kw or approx 2.9/3.0m3/hr of N.gas. The sustain apparently gives the same CH output in the 24kw, the 28kw & the 34kw models but gives DHW outputs of 24.4kw, 28.5kw & 34.6kw.


  • Vaillant ecotec-sustain-combi-installation-and-maintenance-instructions-2055538.pdf
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