Cast iron baseboard radiators work well only at 79 degrees (at thermostat) or above

ak3366

Hello, a plumber recently installed 9 cast iron baseboard radiators (9” tall) in the entire apartment, including bathrooms. This replaced copper baseboards. All cast iron baseboard radiators heat up equally well when the thermostat is set to 79 or higher. However, if the thermostat is set to 78 or lower, only 4 out of 9 heat up well. The other 5 are slightly warm. The 5 slightly warm radiators are located BOTH on internal and external walls. For example, in the same bedroom, one of the 3’ radiators on an internal wall fully heats to 180 degrees up while the other 3' radiator in the same room on a different internal wall is barely warm. All radiators are brand new, made by OCS Industries and located on the SAME floor. I have Williamson Hot Water Gas Boiler.
This is a single zone system with only one thermostat. It is a 20-unit building where each apartment has its own boiler. Therefore, my unit has a dedicated boiler. The boiler is on the roof, my apartment is on the top floor, beneath the boiler room. As far I could tell, pipes are not run on the same level, but rather go up and down within the walls for each radiator. Does the issue have to do with pipes going up and down the walls for each radiator? And if that is indeed the case and air somehow gets trapped in pipes because of that, what’s strange is why raising the thermostat temp to 79 fixes the issue? I also noticed that prior to the CI installation, the old copper radiators also had issues of uneven radiator heating. Thanks much for your advice.

kcopp

Makes me wonder if the T-stat is off kilter somehow.... turning it up should make no difference as it is just a switch. What type of T-stat?

ak3366

It's a Honeywell digital thermostat, bought 3 yrs ago. It's odd that in the same room, say at 75 T-stat, one radiator is 180 degrees while the other one is cold. something may be with the piping?

BobC

Maybe they are on separate loops? Has air been bled from everything?

Bob

JUGHNE

Post some pictures of your boiler and all the piping around it.
Several shots from a distance so we can see everything.

Jamie Hall

Sounds like air locks... as @JUGHNE said, pictures. And, if possible, a sketch?

ak3366

ak3366

Here are the pictures. thanks

Paul48

Sounds like a circulation issue to me. What turning the temperature up, does, is allow for more time. What I can't tell from the photos is, if it is a split loop.

kcopp

Not really helpful. Need to see the piping to the rads and from one to the next.