Running cold water turns combo boiler on…

Yep you read that right. Terrace house, and boiler is in the loft bedroom. I’ve ripped out the bathroom, and I’ve re-plumbed all the pipework in copper. I’ve not mixed any water pipes up, and the hot tap on the bath runs hot, and the cold- cold. When I run the cold bath tap however, the boiler ‘kicks in’ and starts making its noise like it wants to heat water. I’ve tested the tap (is hot passing to cold) and nothing is coming out). I’m at my wits end. Is it an air lock? If so how do I get rid of it? I’ve tried putting my hands on the tap end to force water up but it just splashed everywhere. When using the sink downstairs it doesn’t affect t the boiler, which makes me think it’s dropping pressure somewhere and kicking it in. Do I drain the system and then try again?? Can anyone help here?

Thanks in advance!!