I am trying to add radiant floor heat to a old room that had electric baseboard heater. I currently heat the rest of my house with a pump driven boiler with radiant heat. I would like to tap into this existing system. But because of the allowable space in the new room. I am only going to be able to use 1/2 pex tubing. The rest of my system is running on a 3/4 copper pipe. I want to cut out a section of this 3/4 pipe. Run it into the 1/2 inch pex and back into the 3/4 pipe returning to boiler. Is this going to work or am I going to have to try something else?

I have to ask this question first. Is the rest of your house baseboard hot water heat?

Yes

Here comes the problem. Most if not all baseboard heat systems use 170 to 190 degree water. That is way to high of a temperature to use for in floor radiant and can and will cause major problems. Hot foot is one of them along with the fact that just in case you were going to use carpet the carpet will start to break down and put out some real nasty chemicals. If you are not going to use carpet it can also crack tiles and shrink wood at those in floor temperatures.

There is also a hot water flow problem. When you shrink down from 3/4 to 1/2 you slow the flow rate of the loop. Any additional baseboard heaters after this 1/2 inch pipe will suffer in two ways. Ist is the lower flow from pipe size and second the temperature will be dropped by the heat discharge/loss created by the new section or PEX and those combined will throw the loop out of kilter or in other words the other areas of the loop/zone will be cooler than it was before. This can also be a major problem.

The cure is to Isloate this zone from the other zones in the house. Running a plate type heat exchanger or other temperature lowering device so that the water temperature will be within specs for in floor heat. It will also allow you to have the infloor heat on its own separate zone along with its own thermostat.

There might be other cures but I am not there to see exactly what you have. This will not be the cheap project it once was but using this design or others will assure that the original system as well as the new infloor heat works well and does not cause any trouble. Good luck with your project.