Check the incoming pressure first to see what you have.
It would need to be a pretty high house to take 1.5bar without running over the top. I would expect a 3 storey with tank fitted in the loft to be no more than 10m high (=1 bar). Pressure guages are not always accurate though and there will be a slight drop due to resistances in the pipework. As it will not rise above 1.5bar on the guage, with the heating off open the fill valve and look in the loft. The only place the water can mix is the coil unless someone has made a bolz up of the pipework.