I have baseboard forced hot water system with 3 zones. The problem is that the 3rd zone (upstairs) only kicks in when the 1 zone (basement) is working. I changed the thermostat in the 3rd zone but to no avail. I only have heat in the 3rd zone when the 1st zone is working.

Unless the 1 zone is a master zone (IE the other zone is tapped off the basement zone after the zone valve) you have a interesting problem. First make sure the boiler water pressure is high enough to operate the upstairs zone. Usually 15 psi max on the gauge will do it. If that checks OK if not adjust the pressure since no water for some reason is getting to the second floor due to lack of system pressure. Post back with results on the pressure reading for the next step if necessary.

Thanks for your quick response. Pressure checks OK. What is interesting is that the basement zone was the last one to be added to the system about 5 years ago.

With systems like this it is always possible to have many different items that can cause the same problem. Did you check the master zone thing? Does the upsstairs zone open when its thermostat calls for heat?

Yes, but only if the basement zone is active. If the basement zone is off, the upstairs doesn't open.

If the basement zone has to be working before the upstairs zone will open as you mentioned then I would suspect a wiring or control problem. These type of items have to be checked ON Site since we cannot see your system from here. If you do not have the capability to test the electric part of your control system the you will have to hire local for on site service.

Thanks, this gives me a good direction to follow.