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See my earlier post (below) as you have not yet provided the info:- With the Vaillant controls fitted you can have different max kW values for CH (d.0) and HW (d.77). However, this will not make any difference in your situation as you have effectively down rated the HW via d.0 to 11kW already.

When checking again, if you can provide a few examples of d.40/d.41 during initial phase, at turn off & then during cycling? Please also note the S.xx code displayed after it has switched off which may be useful!? It might also be worth trying return sensor regulation (change d.17 to 1) which...

Did you make a note of the D.41 values? I'm particulary interested in the delta T (i.e. the difference between D.40 and D.41) on the initial burn phase and what it was at the point it switched off at 6min 54 seconds in to better understand what the boiler was doing? This information will also...

So the above restated as (MM:SS): WAIT --- BURN 00:00 --- 06:54 03:13 --- 00:03 02:52 --- 00:13 02:46 --- 00:15 02:44 --- 00:11 02:52 --- 00:07 02:44 --- 00:10 During the WAIT times, did you notice what it displayed on the LCD? (S.7, S.8 or S.53?) You mentioned you have D.0 set...

Are they MM:SS? i.e. The boiler fired for 10min 7seconds (ON) then switched OFF for 10mins 10seconds (for the first line)? or something else??? What does the 6min 54sec refer to? :?

There are 171 days in your first range and 175 in your second so when worked out on a like-for-like you have actually "saved" 33.9%! I agree, I am 99% sure the boiler has reduced usage; due to differences in my two different controllers the CH time(s) on/off were not 'identical' in my...

I asked Vaillant if I could re-use my original 230v cables to which they replied yes! with the following provisio; The vast majority of my re-used cabling was already run seperately, there's maybe 1-2m max where it's likely in the same trunking and I've had no issues with eBus signals.

As a reference, I've used 34.2% less gas between Nov-Feb this year (with a modulating controller (Vaillant VRT392) and condensing boiler (EcoTEC 418)) verses last winter when I had a 80% (SEDBUK rating, 15yr old / not serviced in at least 5 yrs) non-condensing boiler with a Danfoss TP7000 room...

:shock: You remind me of an ex-colleague that always tried to take credit for other people's work! (It didn't win him many friends...) :roll: OP, glad you got it sorted in the end! :)

The above (modified) layout meets the results in green and the OP's symptoms (shutting off when in CH only mode) but still doesn't explain the result in red?

I have a feeling that reverse psychology isn't going to work here SS! :wink:

Dan, It's good to see nice neat plumbing on here once in a while! :) - I'm intrigued to know two things: 1) Do you work from a pre-drawn plan when installing something like this? 2) What are the red/white manifold's for at the bottom, UFH still to be connected?

Perhaps he was infering that it's 2% better than the OP's previous TPI controller (I don't know by how much but arn't TPI's more efficient than a standard on/off version)? (see below)

Yes, IF that's what you have!? :cry: Time to do something I agree with, however, as DIA says above and your earlier tests proved, the flow and return at the boiler side are incorrect! (the boiler's 'flow' is connected to the return when pumped). Hmm, this thread is now 21+ pages yet it...

I'm using the end product to clean the tank, not making the stuff!!! :lol: From my (German) Vaillant boiler installation manual: I agree the chemical companies have a vested interest, however I think there are valid reasons... not forgetting that our (treated) mains water is likely to...

... as recommended by Salamander, Sentinal and others! :wink: Do you know of a reason I shouldn't use it? (It's currently sitting in the F&E, killing off the bacteria for a bit before I rinse it out and re-fill) The brown stuff was like wet cardboard, breakable into smaller pieces but...

It's still on sale, it's the £25 I mentioned above for 500ml of Fernox AF-10! :-) I've now cleaned it with my £1 bleach (not sure how Fernox can justify their cost!?); the water was still black after I left it overnight to settle, not sure if the black was magnetite or bacteria/mould/fungas...

The root cause sounds like a high moisture content in the air! :wink: by introducing the water softner, you could effectively be creating a 'fridge' in that space, lowering the ambient temperature (especially if you have icy cold mains water like ours). The lower ambient temperature increases...