Been having some ongoing problems with the digital bath system we installed late last year. Despite 3 engineer visits, and every component being swapped it was still struggling to reach a hot output. About half the time it works fine and you have a hot bath. The other half - it never reaches temperature (red ring never turns white).
So, I decided to try and investigate. I found that by lowering the cold water pressure (turning bathroom taps to full cold) I could coax it to achieve heat. It seems that the system cannot dial down the cold input enough - so that even with full heat output from the combi, it mixes too much cold (we have a 42KW combi so we should have no problems).
So - a cunning fix. I simply turned the in-line tap half off on the cold side. This artificially limits the cold pressure allowing the hot/cold ratio to be hotter. Seems to be working fine now - even with the combi on economy mode (had to be full heat before).
