Would need a lot more detailed info before being able to offer meaningful advice . you may have a leak internally ,or expansion vessel not functioning properly to name the two most obvious ,but there are other possibilities.
First thing , is water being expelled from the copper pipe ,outside ,that is the pressure relief pipework. May be difficult to notice in current weather, and not to be confused with the plastic condensate pipe. Your usual pressure would be around 1 bar cold and up to around 2 bar when system hot. If it rises to 3 bar the prv should open and allow water out the copper pipe ,it would be doing its job. So you need to monitor your pressure gauge and let's know what you observed .water leaks on radiator valves / pipework /joints etc would be a reason for slow pressure loss if its a slow weep. Not always obvious as small amounts can evaporate away so you need to look really closely for any tell tale signs ,even if not wet. That'll do for starter's
Frequent adding of cold water can eventually dilute the chemical Inhibitor that should be in your sealed system ,and will promote corrosion , causing other issues.
*********Would need a lot more detailed info before being able to offer meaningful advice . you may have a leak internally ,or expansion vessel not functioning properly to name the two most obvious ,but there are other possibilities.
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