One of my Condo accounts is 56 stories high and the only
“shut off’s” are at the boilers and at the roof level upfeed/down feed system.
What most “helpers” fail to realize is you honestly DO NOT want a positive shut off on a hot water heating system.
Think about it . A person not knowing that water can freeze if not moving (a helper for instance) would close a valve as to prevent the excessive heat that could happen with non electric zone valves or a failed aquastat/thermostat.
This in one of the reasons that the better zone control valves will have a minimum fail safe of 40 degrees to maintain piping integrity.
If you went to a plumbing supply and looked into the hydronic manual valves you would see a small hole in the valve disc/cylinder when it is in the “closed” position.
Personally if I were you id wait till the warmer months and ask for a shut down at the source then have non electric zone valves installed where you can modulate the exact temperature requirements you desire.
Even the best isolation valves will fail when used in larger heating applications as normally they are installed and forgotten about and the expansion and contraction does take its toll on the valves integrity.
You could also have only a branch valve that is common on heating/cooling systems with a fan coil and these normally have a washer type globe valve used for throttling and again the integrity is not always the best.
There are certain valves that do have a fail safe built in that we use that fail in the open position to protect against freeze ups.
Why not ask your building Supt. to either repair/replace your valve at the earliest opportunity and don’t take a chance of flooding out the place or scalding yourself.