The major advantage of the hydronic system is the low cost of operation, it is very efficient and because the liquid remains in the system in a continuous loop the heat effectively transferred to the room.
Cast Iron boilers are a cost effective method to heat the liquid, however, those boilers need to be replaced every couple of years (depending on the amount of use of the system). Also, Calcium build up in the boiler tank make its operation less effective and efficient over time.
Hydronic systems are normally quite expansive to install. Closed loop hydronic systems are normally filled with water and 25% glycol. The boiler heats the liquid in the system and a single or a series of pumps transfer the hot liquid through the system to a series of tubes that are laid in the floor.
The system requires a series of valves that control the flow of the hot liquid. These valves require regular maintenance as Calcium in the water builds up around the seals in the valves.
Since the system requires several mechanic components, it also requires to be located out of site and thereby requires floor space. This limitation may make the system less attractive in many applications.
Normally the PEX tubing which is laid on the floor can be as much as an 2 inches thick, one must consider the consequences of raising the floor, as it affects other flooring surfaces, height of doors and the need to remove and reinstall the baseboards.
When choosing between the various warm floor systems, you must also consider the weight of the cement compound that would need to be placed on the subfloor and ensure that the flooring is sufficiently strong to support the extra weight.