Can natural gas appliances run on Hydrogen?
Pure hydrogen, as opposed to hydrocarbons, degrades metals.  $\ce {H_2}$ molecules are tiny, so they wedge themselves into the relatively large metallic crystal structures and form hydrides.  This is the entire reason nickel-metal hydride (NiMH) batteries work - the hydrogen is stored in an iron-hydride matrix. An appliance that might last 20 or 30 years burning natural gas might not make it past ten years burning $\ce{H_2}$, and a failure may take the form of a dangerous leak.