Sq ft of steam to btu conversion

If you have any data

on the air handling unit, that would help the overall picture.



The issue with these hybrid systems is pressure- an air handler typically needs 2 psi and sometimes 5 psi in the coil to meet ratings, to reduce freezing potential and to clear the traps of condensate.  The control valve in this case "may" (I say "may" because I am not there), control to a discharge temperature controller or a space temperature controller. It probably modulates.



The church (radiation) valve may (again "may" and stating a common application), serve both a control function (on, off, more, less), but more likely in my experience, serves a pressure reducing function. This is because steam radiators need often a pound of pressure or less, much less than the air unit coil.



So here you have it, a coil needing higher pressure, probably. A group of steam radiators needing low pressure, likely. 



In the hybrid systems I have seen, the radiation master control valve just modulated to maintain a certain pressure downstream of the valve, say 12 ounces, whatever, regardless of what the upstream pressure was. The valve normally does not care about temperature in the space, anywhere for that matter, just pressure.



Such a pressure controlling valve would be smaller in size than an

on-off valve, for example. ("Compared to what?" you may well ask.) I guess line size, reducing two sizes from the main pipe size versus one size or full line size with no reduction at all.



Downstream of the valve, the radiators would draw and condense thus reducing pressure which in turn causes the valve to modulate open. The radiators may also have individual thermostatic valves, pneumatic valves or electric valves. May even have none at all! But I suspect that the radiation valve controls to pressure. I may be wrong but it seems most common for so large a system.



If so, you might see a pressure controller, probably on a pigtail siphon loop, a few feet or more on the radiation side of the valve.



Does this help? I figure by understanding or finding out the principles of operation, the selection of the valve can be attuned to your needs.



If you have a controls contractor, they are responsible for sizing the valve, based on desired flow rate (Cv) and total pressure drop.



From your 1900 EDR number, I get about 475 pounds per hour steam draw. As an EXAMPLE ONLY (full disclaimer here), that flow rate dropping from 5 psig to 3 psig will have a Cv (flow rate at 1.0 psi differential), of about 21.5. A valve with this Cv rating more or less will get you to the right size. The valve size will vary with type, so I cannot give you a size.