Converting Old Finned Baseboard to Panel Radiators on a One Pipe System?! Please help....

the monoflo tees divide the flow between the main and the emitter. 2 plain tees with a valve to throttle the flow in the main will do the same thing.

a boiler replacement may have changed the circulator or zone valves or other things so that the monoflo loop no longer has enough flow. if there is not enough flow in a monoflo loop the water will mostly only follow the main and won't divide to the branches to the emitters.

typically it takes more flow to make the monoflo work properly than it takes just to move the heat load of the zone so people not familiar with monoflo usually don't give it enough flow

I am trying to understand some things you are saying: "Last fall I converted 3 old metal finned baseboard heaters in my basement to the (pex universe.com) beacon-morris type panel radiators that have a 2" inlet and outlet on the bottom."

Do the radiators have a 2" opening? The specifications for that radiator do not support this statement. It appears that the bottom openings are on the side of the panel radiators and they look like 3/4".

There is also a bottom connection that is designed for that bypass valve arrangement you indicated. Those openings are not even close to 2".

Can you betteer explain what exactly is 2" in your system or radiator

Next, you explain that you have a one-pipe system. That usually indicates that there is one main pipe for the system, or for each zone, that starts at the boiler supply and ends at the boiler return. Each radiator, baseboard, convector, or other heat emitter has a standard tee from the main pipe to the supply side of the heat emitter, and the return side of the emitter is connected back to that same pipe using a MonoFlo® tee or another brand of diverter tee.

Then you describe that you are connecting your new panel radiators without a diverter tee and show a drawing of a piping design that is limited in scope. It shows only the PEX connections to something that looks like a supply and return, which could be a larger-diameter pipe, but it does not show how that piping is connected to the boiler. That makes it a little difficult to understand the complete system.

A picture taken from farther back would be helpful. It is a little like a DIY auto-repair post that shows only a close-up of one part and does not show enough of the surrounding system to tell the whole story.

This reminds me of the post about the DIY auto repair where the person wanted to know why his tire was not getting full of air. "I have been putting air in this valve for over 30 minutes and my tire just won't fill up with air. can someone tell me what I'm doing wrong? "

Several different posts about what the pressure is the compressor set? Are you using a Bicycle hand pump? Is the compressor plugged in? and a lot of other helpful comments.

Then someone asked the DIY to take the picture of the car from farther back.

This tells a different story.

@bradmoulton, welcome to Heating Help! I've combined your discussion on that older thread with this one here to prevent confusion.

That makes more sense. now you have several zones of what we call a series loop. There re typically 4 standard piping methods.

Two pipe old gravity heating systems were connected as a parallel piping system pre 1950

Newer two pipe gravity and some circulator systems used a parallel reverse return design late 1940 thru 1960

MonoFlo or one pipe using convectors and diverter tees was popular in the 1950

In the 1960s (and a long ago as the 1940s once circulators were becoming popular) the series loop became very popular as the most economical hydronic heating system to install. here are three of the 4 different systems

Now that we have established the system you have lets look at what you are going to do with it.

This will work however if you reduce the pipe from 3/4" to 1/2" yu will restrict the entire loop to what a 1/2" pipe capacity can carry which is a lot less BTUs