You are making Rube Goldberg jealous. Usually, steam boiler capacity is measured in sq.ft. steam, or lb/hr. steam. Steam modulators are sized and built differently, than an HD zone valve. With condensate open to atmosphere and a full port valve, swung fully open. The steam chest, (that you will need to figure out the size of), will not hold steam at the pressure needed to stay at a high temp. The boiler water being superheated will change state rapidly, in the boiler, due to reduction of pressure, blowing water into the coils. Then, all you will have is a water heater producing steam at atmospheric or slightly above, until valves are closed. Kind of like how 17th century Savory and Newcomen boilers for steam engines worked. Basically, you have a one pipe steam system open to atmosphere when valves are open. You're building a carpet cleaning steam genny, with add on de-superheaters. You would be better off coming off a steam header to a modulator on each coil, with a small steam trap and return on each coil, an equalizer and Hartford Loop. All this helps in maintaining reasonable pressure equilibrium in the boiler and somewhat of a balanced system. The Loop helps in keeping the water above the heaters, if piped correctly. Control the boiler with a pressure switch. Stick a McDonnel Miller low water sensor in the jacket, coupled to a 24 volt auto feeder and done. Modulators can be controlled by PIDS, PODS, WOGs, Floppy Dishes, SSSSSRs, Thermos-couples, WEFEs, Iprods and all kind of esoteric lights and other junk that attract and enthrall poindexters, like Pavlov's dogs to a bell. Just so you know. The riser is usually, minimum 24" above the water line. It helps in getting dry steam to the coils. Ensure that domestic heater elements won't fail at the fusite at superheat temps. Elements for low pressure steamers are built much differently. Even though the elements are covered by a metal plate, God forbid the fusite blows out spewing superheated water. You did a fine job with BTU, kilowatt, change of state, etc.. Brew on and good luck with the venture.
