Sensible heat required to heat space is be calculated by the formula
Q = 1.08 x cfm x dT (where dT is the temperature difference, in
0F, of entering and leaving air.
This is the amount of heat you have to supply from hot water and heat capacity of hot water is calculated by
Q = mCpdT1 Here m is mass flow rate of water, Cp is specific heat and dT
1 is difference between entering and leaving temperatures of hot water. (here you have to calculate mass flow rate of water required by using
UAdTm formula used for heat exchangers.
Finally steam consumption can be calculated by equating heat required to raise the water temperature to
m1H where H is latent heat of steam at working pressure.
For some idea on steam heaters download this.
This is basic outline and upon further discussion with suppliers you will get better idea.
Regards,