Heating will encourage a higher wet content in the air, resulting in rust, hence the need for that dehumidifier.
Even without heating my dehumidifier can collect a litre a day.
I'm a bit mystified by this. Are you saying your dehumidifier collects more water when you artificially heat your garage?
Relative humidity will decrease as you heat the air, not increase. And RH will increase as the temperature goes down. Absolute humidity, if it's a sealed space, will not change but the ability of the moisture to condense, and thus settle and cause corrosion, will be reduced as temperature rises in comparison to the outside air.
There could be an effect from air pressure here i.e. warm air is less dense so as the air in your garage is heated the cooler denser air from outside tries to get in. But I don't think this would be significant.
The reason why the air is so dry in an airliner is that air with an already low humidity (at high altitude) is drawn in and then heated, reducing RH to the extent of drying our skin and making eyes sore.
Signed, mystified non-meteorologist.
