I have a wood burning fire place with Central heating link all of which works well and the thermo switch cuts in and out just fine, so nice warm fire and radiators. However there is water coming from the roof tank overflow after every use but I can't tell if it's a ball valve issue or a heat expansion issue or something else. Do I have to seasonally drain the system ? Could I have an air trap or blockage somewhere. ? Any advice gratefully received. TIA.