Most modern boiler systems are sealed units which need to have their water replaced or topped up due to evaporation or air loss. Newer boilers will include an automatic hydronic feeder that maintains the correct level. A 30/40% propylene glycol/water solution is recommended to prolong boiler system life and prevent freezing if the system is turned off or goes offline during cold weather.
Automatic hydronic feeders can also introduce air into the system, which can dilute the water and glycol mixture. Part of your scheduled maintenance should include testing the level of glycol and adjusting as needed along with pH.