Here's one. This cast iron mono-flo tee I'm guessing was installed in the mid-fifties to "try" and heat a cold porch. As I was evaluating the system for a boiler replacement I noticed them. I said to the HO " I guess this room stays pretty cold eh?" and he replied "yeh, how'd ya know that?". This was on an original open gravity system that someone recently introduced a pump to but couldn't get the house above 50.
If they all go from supply piping to return piping,
then they shouldn't need monoflow tees. Monoflow tees are used to create a pressure difference between two tees in the same loop. If the piping and circulator are sized properly, then there should be a pressure difference between the supply and return piping, causing flow through all of the Modines. Hope this helps.
Do black iron monoflow tees exist? Looked at a job with several large modines all branched off a common supply loop and dumped into a comon return. The only way this could work would be w/ monflow tees in the 2" iron piping. Anyone seen these before?