Taco Zone Valves:




This is the Taco zone valve. The head on yours may be blue green instead of gold or brown. The top (#1) terminal is the power to the zone valve
The middle (#2) terminal is the common terminal.
The button (#3) terminal is the end switch.   24 volts to 1 and 2 opens the zone valve.
If the thermostat is turned all the way up and you do not have heat in that zone. test to see if you have 24 volts at terminals 1 and 2.  
   


If  you do not have power to 1 and 2 the problem you are having is not with the zone valve, but before you move on make sure that you have turned your thermostat all the way up, This means as high as it possibly goes. then if there is no power at the zone valve the problem is not with the zone valve.
If you do have 24 volts at terminals 1 and 2 and have waited at least 2 minutes, check the lever


Pull down on the lever, if the lever goes all the way down with no resistance, the problem you are having is not with the zone valve. If there is resistance, and you have 24 volts at terminals 1 and 2 and you have waited at least 2 minutes, the power head and or the whole zone valve is defective and needs to be replaced,
Danger Removing a hot power head can be dangerous.












Make sure The power to the zone valve is off, and has been off for at lease 2 minutes. Just turning the power switch off does not mean the power to the zone valve is off. Remove the top (#1) wire if you are not sure. Make sure you wait 2 minutes before you proceed. If there is enough slack in the wires, grab the head and turn it 1/8th of a turn clockwise, and lift, put the new head in by doing the reverse, Then move the wires to the new head. If the new head burns out soon after you have replaced it, you will need to replace the whole zone valve.

Why do I keep saying wait 2 minutes? A Taco zone valve is designed to open very slowly, by heating wax that expands and pushes a plunger that opens the zone valve. It takes about 2 minutes for the wax to fully expand. and the same to contract, removing the head while the wax is hot and under pressure can cause the plunger to pop off, the flying  plunger can hurt you, and the hot wax can cause a serous burn.

So what is the problem, if I do not have power to terminals 1 and 2? If you do not have power to 1 and 2 either the transformer is bad or your thermostat is bad.

 

So what is the problem, If the lever does go down with no resistance and you have 24 volts at terminals 1 and 2? 1- your boiler is not running, 2- Your circulator is not running. 3- your zone is air bound 4- your zone is frozen 5- you have a broken pipe under a slab. There are other reasons. Thanks to masterplumber.net for this info.