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Forced Air generally increase the exchange of new air from outside. Individual rooms will tend to be above or below ambient pressure which causes an exchange of air with the outside. Carefully balanced systems can minimize the effect, but homes are rarely built with this in mind.

You can get a related effect with thermostats which raise and lower the house’s temperature over the day as air expands with increased temperature.

>>Retric+(OP)
That is definitely an issue that is out there. Sadly cheap builders try and get away with a single return per floor in a house which is simply not enough. My point is mostly that forced air versus hydronic isn't what causes a difference in dryness of air. It largely comes down to the quality of the install of each and the tightness of the home. Hydronic is typically more expensive and tends to be found it better built homes. My house is an exception to that rule though, I am still working on sealing up air leaks from where all the pipes ran for our baseboard heat.