Hardwood floors can be used over a hydronic heating system if extra time and care is used during the installation process. Wood floors are what we call hydroscopic. That means the wood reacts to water, in much the same way a sponge does. If the wood is dry and goes into a wet environment, then the wood floor will adsorb moister and expand. Likewise, if the wood is too "wet" and is installed into a dry environment, it will dry and shrink.
Wood floors continuously move, just like the door jams in your home. In the summer they expand due to the increased humidity in the air and become harder to shut And, in the winter the humidity is typically lower, and the doors shrink, becoming easier to close.
Wood floors will also experience this seasonal change in dimensions. However, there are some "tricks to the trade" that will help minimize these swings.
Use a wood that is kiln dried. This helps to ensure the wood's moisture content is the same on the inside as it is on the outside.
A quarter-sawn wood is better than a plane-sawn wood. Plane sawn woods tend to "grow" or expand in width, while a quarter-sawn wood will tend to expand more in thickness. This helps reduce visual cracking and gapping.
The wood should be 7% - 10% in moisture content. This may require an acclimation period in order for the wood to reach this level. Sometimes, it is best to have the radiant system installed before the wood. This will help accelerate the acclimation process.
The wood floor should not be higher than 4% in moisture content than the floor it is being installed onto. This will allow for the subfloor and hardwood floor to move and react as a single unit. Otherwise, moisture maybe able to travel from layer to layer.