Monoflow or Gravity?

Good morning Wallies, In the past week I looked at two hot water systems, both under performing. I am unsure if they are gravity or monoflow. The piping is of two inch or larger black fittings, with t's that have half inch or three quarter inch takeoffs on the bull of the T.Under the radiators (some free standing, most recessed convectors) is a valve on one side. Most likely still with an orifice plate inside. Secondly, is Danfoss the proffered TRV for tight spaces? Remote sensor would be a must to properly sense air temp. I was pleased to see one system still using the expansion tank up in the ceiling!A side question, can you pump away from the old school expansion tanks, or is that a no-no? (Been rusty on the rules since they are scarce in this part of the woods.)    Thanks in advance.