Vaillant Ecotec 937 - Warmstart and F.81/F.91 error

6 Mar 2006
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United Kingdom
Hi all,

Could somebody explain to me exactly what the "Warmstart" feature on the Vaillant Ecotec 937 boiler does, and what the two storage tanks do if the feature is turned off? My basic understanding is that the Warmstart feature maintains the water in the storage tank at temperature, so hot water is instant, and so it can maintain a higher flow rate for longer, by combining the stored hot water with direct incoming heated water.

Is that right? And, if that is right - does that mean that, if Warmstart is off, the two tanks essentially do nothing (and so I might as well have gone for an 837 instead?)

Also - does anyone have any experience of Warmstart causing F.81/F.91 error codes?

Background - we had a new 937 installed about 3 weeks ago, and we keep getting F.81/F.91 errors - Vaillant have been out and changed the PCB, which fixed it for about 10 days, but we got another one late last week. The Vaillant Engineer has been out today, couldn't find anything obviously wrong, but has told me that if it happens again they will come out and change the pump completely (it's a 2-man job, and he didn't have the part with him today anyway). I'm happy with this approach - but he also recommended that I turn Warmstart off, and implied that this might solve the problem, which I don't feel overly happy with - even if Warmstart isn't the greatest feature in the world, it is a feature of the boiler, and it should work without causing error codes. He implied that the Warmstart feature doesn't do that much anyway - so I'm trying to work out exactly what it does do.


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From what I seem to remember this is a store pump fault, or the board that controls this.

Seems one of the most unreliable aspects of this boiler.

They seem to have loads with this fault, one would think they could come up with an answer by now :rolleyes:
I think that the warmstart only heats up the plate heat exchanger inside the boiler to give a slightly quicker response for DHW delivery.

IF thats correct then I could then go on to say the storage facility is to give the ability to deliver an enhanced DHW flow rate for a limited time of about 10 min.

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Tony's right but only for the 837.

937 warmstart heats the backpack so you have immediate hot water and keeps it topped up.

Edit: Just looked up the fault codes and they appear to be to do with the temperature sensors/pump associated with the backpack.

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