Hello. I have an idea icos boiler. Unvented system with water tank in airing cupboard. I have two thermostats. One downstairs hallway and one on the landing hallway on top of stairs. The problem I got is. If I turn the thermostat up downstairs. The thermostat clicks and the flame light comes on. So there is a demand for heat but it won't fire up the boiler. But if I turn up the upstairs thermostat it makes the boiler fire up. Heating the rads upstairs and Also now the downstairs. So to get heating downstairs I got to go upstairs and turn up the thermostat upstairs. Wasn't always like that obviously. I could keep upstairs off and keep bottom on independently. Something's wrong.looks like the upstairs is controlling both. Any ideas?