The Amtrol expansion tank on my boiler needs to be replaced but the installers did not include an isolation ball valve. I can close the returns, but the water will still drain down from the upper floors of the house unless I can close the zone valves. The zone valves can be put in the "always open" position by pushing the latch towards the pipe, but is there a way to put the zone valves into a forced-closed position? If the system electrical is turned off when the thermostat is not calling for heat, will the valves be, and remain, in the closed state?
P.S. Here are pics of one that was replaced with a new one. It is not really designed to be user-serviceable. The outer shell is riveted in place. I do not see a hefty spring.
Best Answer
TL;DR:
Taco zone-valve (wax piston actator):
Zone valves with a synchronous motor
Further to Tkw's answer, heres the inside of a zone valve. Note the two springs that close the valve when the motor is not powered.
With some zone valves, where the manual-opening lever is located you can peek in and see the toothed segment that is moved by the motor's gear (which typically turns at only 3.5 rpm)
What you show in your updated question looks like a Taco zone-valve
The bottom cast-metal part is the actual valve.
The top part, in a plastic enclosure, is just the Zone-Valve Power-Head. This is a separately purchased part and is designed to be replaced without disturbing the plumbing.
Once the power head is separated from the bronze valve body, the valve will be shut and there is no way for any of your heating controls to open the valve.
If you look at the cutaway diagram in the manufacturers brochure you'll see there is a very strong spring in the base.
That spring holds the valve closed when the power-head is disconnected.
If you want to be 100% sure the valve remains closed regardless of what your controls are doing, just pull the power-head off.
Though just disconnecting the wiring (or just wire #1) ought to be enough.