This is the Taco zone valve body, sometimes this part of the zone valve fails, The 2 problems I have run in to are, one the part of the zone valve that is pushed by the head gets stuck and will not move freely, this will cause the power head to pop or burn out. Two the part that the power head moves, does not go all the way back up when the zone valve is no longer calling for heat, causing you to have heat when you do not want it, in both cases the whole zone valve needs to be replaced.
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. For further help with this type of problem,