Our house has dual zone central heating. We are having a problem with downstairs heating wchich will not come on with its thermostat activated. However, if we turn the upstairs thermostat on together with the downstairs the downstairs heating works ok. searching the web it occurs to me that one of the actuators may be faulty.I looks like there are two Danfoss actuators, one for up and one for down. Could a fault with either one be the cause of the problem?