Adding anything besides pure water to a radiant system opens another discussion. The reason for doing this is corrosion resistance or freeze protection.
Ethylene or propylene stop corrosion and provide freeze protection, but contamination of the drinking water is an issue. Usually, there is a fill regulator connected to the domestic water system that automatically fills and maintains the system pressure. This is a possible path into the drinking water side if the domestic pressure is lost. Never use ethylene glycol if there is any cross connection to the house water system. Propylene glycol is not poisonous and can be used in this way, but it's still not advisable without an approved backflow preventer.
Freeze protection is not usually a concern and so another additive can be used. I use a product called Formula 10 corrosion inhibitor. This is a one time dose that does not wear out and is non toxic. It also helps the pumps start after a seasonal shutdown.
The reason we put in an inhibitor is because oxygen gets into the system. This does not occur in closed loop systems with metal radiant tubing. But it does occur with PEX or other non metalic tubing. Even oxygen barrier PEX tubing. The term for this is oxygen permeation and means oxygen can pass through the non metalic tube wall into the system. This oxygen finds the iron parts like the pump, the expansion tank, the boiler heat exchanger, or other, and causes corrosion. In the worst cases it severley damages the system.
Since all modern systems have, or should have, PEX tubing, they all must have a good additive. The best for non freezing applications is Formula 10 with 1 quart per 50 gallons water, the best if you are concerned about freezing is propylene glycol in a 30-50% solution with water. Propylene comes in pre-mixed or pure forms. Pure would mix 50-50 and pre would go in straight. Then add enough water through the fill system to bring the pressure to 12 PSI. Maintain the connection through the fill regulator to make up losses as the air bleeds out. Of course, all of the additives go in after the system has been thoroughly flushed and tested. Once a year take a small sample of the propylene in a cup and put in in the freezer for a test. Be sure to have valves in all appropriate places for servicing the boiler, the expansion tank, the pump, etc. This way you'll lose the least when you open the system up to fix something.
By the way, the fill valve and expansion tank should connect to the system at the same point. This point should be a tee with a valve so you can completely isolate these items for testing and service.
When widespread use of plastic or rubber tubing in radiant systems became common, back in the early '80s, there was no understanding of the oxygen permeation problem and tubing had no oxygen barrier. Tubing manufacturers were competing and there were a lot of tubing types on the market. Installers had no knowledge of all the problems they might be getting into. There is a parallel to the thermal solar industry where many saw an opportunity and jumped in, but few really understood it or have spent the time to figure it out.
When copper or steel was the tubing of choice, back in the '50s and '60s, this oxygen problem never occured. So the industry never had to deal with it. That's why you might see an old closed loop radiator system and boiler from the '40s that is still clean inside. Closed loop used to mean no corrosion concerns. Not now.
Even today, inhibitors seem, to many, like an option that is unnecessary, especially with the new oxygen barrier tubing. Or they just don't think of it. Some try to use only potable rated equipment in the radiant system. This isn't the best solution either.
