Vaillant Return Temperature Control

15 May 2022
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The majority of Vaillant boilers are configured to run with flow temperature target temperature control, however, on most models, return target temperature control can also be selected which may give greater condensing and boiler efficiency.
If anyone reading this is using this control, they might care to share their experiencies or anyone who is reasonably confident in changing parameters might try this return temperature control for a at least a few hours or so.
Might be best to try this on systems without TRVs or weather compensation.

If flow/return target temperature control is available check for d.17 (there may be a 0 prefix in front of all these d parameters).
d.17 default setting is 0, flow temperature control, return temperature control, is 1.
Before changing, note the following
d.0, part load, ( I think this should be in Auto for return temperature control)
d.5, Flow (or return) temperature target value.
d.40, flow temperature.
d.41, return temperature

Change d.17 parameter from 0 to 1.
Set d.5 initially, to say 5C lower than the current d.41 return temperature.
Monitor d.5, d40 & d.41 regularly for a hour or so.
When test complete return d.17 to 0 and d.5 to its original target flow temperature.

Edit: d.5 (indication only) is set via the normal target value method in either flow or return target temperature mode.
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