Not long had my thermostat replaced everything ok regularly check levels then the other day noticed heater blowing cold not heating up. /popped the bonnet water chamber empty and had only checked it 4 or5 days before. filled it up went for a run checked level was fine, Next morning checked it was low thought it just need topping up as be run through system but it keeps happening. When water topped up the heater warms up so that means thermostat working right ?