I'm troubleshooting my heating system and I think a zone valve is stuck closed. Thermostat is calling for heat but the hot water pipes (hot water baseboard heat) are not heating up. I have four zones and the others are heating just fine. I replaced the thermostat so have narrowed it down to zone valves.

Most zone valves will allow you to open them manually so open it up and see if it gets hot. Move the lever to open.

Manual Lever
The V8043 normally closed valves can be opened manually by moving the manual lever slowly and firmly to the MAN. OPEN position and pushing up to the stop. The stop permits the valve to be locked in the open position. The valve returns to automatic position when it is energized.

Read it here

http://www.alpinehomeair.com/related/Honeywell_Hydronic_Zone_Valve_InstallationIntructi ons.PDF