I've a lennox hot water heating system. It's 3 zone with one circulator pump and three zone valves. The heating on the first floor is fine, but there is no heating on the ground floor (and basement) since last week. There are no bleeders, so we drained the zone near the furnace with no luck. I changed the head of the zone valve (ground floor) with not much success. I switched the thermostats and the one on the ground floor worked on the first floor. So they can be excluded.
When the thermostat is turned on, the valve mechanism works. There is a clicking sound but the furnace doesn't fire up. Can someone suggest some remedies before I call professional help?
THanks

The heating on the first floor is fine

If the heat is fine on the first floor then it will work on all the floors if properly repaired. There should be bleeders on your system and if not I suggest you install them since the only other way to bleed is a pressure power bleed and I do not suggest you attempt that but call for service under those circumstances.