The main limitation is the vertical distance the wick can pull the liquid up. Usually these heat pipes are less than a few inches high or are used where there is no gravity, such as in space. Type 2 Heat pipes can be controlled using valves. Under the cold end and above the hot end is a reservoir sized large enough to collect all the liquid. The valve is used to admit liquid to the hot end when heat transfer is needed.
Water is often the liquid of choice as it has a high latent heat of vaporization, a desirable trait.
One problem with water is that the vapor pressure is less than 1 atmosphere at room temperature. Other fluids are refrigerant gases and fuel gases. Butane, or pentane if you can find it, is preferred when using flexible lines as the pressures are lower. Propane is preferred when solid lines are used as the pressures are higher. Filling a heat pipe using the pressurized gases is easier than with water or other room temperature liquids as evacuation is not needed.