Hi Felicity,
Another new NIBE owner; welcome! Thanks for describing your situation so well. (Note I took the liberty of adding “NIBE” into your post title to help those readers not familiar with the model numbers.)
I have a NIBE system but it’s Ground Source so I personally have no experience of NIBE Defrost cycles and I’m hoping some NIBE ASHP owners will chip in. Defrosting every 20 minutes surely can’t be right though.
Like you, I would not have expected the BT6 (DHW Cylinder Temperature Sensor) to show any change during a Defrost cycle - but another NIBE owner recently reported they have been seeing ‘tank defrosts’ which sound broadly similar (see NIBE ASHP and tank defrosts).
From everything you’ve described, I get the impression that the recent work by your installer has resulted in some sort of fault, perhaps due to reduced flow somewhere. It seems particularly odd that DHW heating isn’t working properly, since that’s much ‘simpler’ than space heating, with shorter pipe runs - albeit generally at a higher temperature - and none of the complexities of WC Curves or Degree Minute control.
I’d recommend double-checking that none of the valves got closed or wiring dislodged during the recent work (though I’d have hoped your installer would have checked that already):
- Does the ‘Charge Pump’ circulation pump cabling look OK? Is that pump running?
- Does the DHW ‘Diverter Valve’ (typically has a red plastic cap) cabling look OK? Can you see the indicator that shows whether that is pointing the right way for DHW or Central Heating?
If your Degree Minutes are getting steadily more negative there’s nothing to be gained by tweaking the Curve or its Offset. The too-frequent Defrosting will be the cause of that.
If you are able to post of a photo of the indoor pipework that would be useful context.
David