I thought given that usually deltaT is bounded, (ex.Return Air at 60F and Supply air at 85F ) and air is usually at 35 % rh during heating, and specific heat is constant, that maybe the outcome as to cfm/Btu is generally within typical range.
Decide what temperature supply air you need, and what temperature you will control the space you're heating. I guess you've already computed a heat load since KW of heater is a given...
Example: 1 kW heat load, room temp desired 70°F, supply air temp desired 120°F. That gives you a delta-T of 50°F. It's all sensible heat, so the air flow will be:
Yes, the heat you're adding only raises the dry bulb temperature (with corresponding drop in RH -- but dew point remains unchanged). Walk youself through the process on a psychrometric chart, it's a big help in understanding.
Heating adds no moisture to the air, nor does it remove any moisture from the air. It only changes temperature.
One more quick note -- the 1.08 btu/°F/cfm will change with both humidity and pressure changes. It's valid at sea level and 50% RH.
However, it doesn't change much. For purposes of this discussion, it's best left alone. After all, we're talking about cfm of air, which nobody can measure to 3-digit accuracy in most building applications.
As pressure (and thus density of the air) rises, the 1.08 factor will go up. As humidity (and thus specific heat of the air) rises, the 1.08 factor will go up.
Post-script :
If you are installing duct heaters, you need a minimum of 50 cfm per KW of heat. Other wise your ductheaters will overheat. So there is a minimum, or lower limit, you could work off of !
I mean when you go into free cooling on a standard configuration, would your rule of thumb be applicable? If you go to a dual duct/dual economizer, what is the relevance of the rule of thumb for heating? It goes to the idea of the energy use being dependent on the delta T and the outside air portion. If a rule of thumb worked, then the economizer and relief dampers would not need to modulate. As a result, area served per ton can vary over 200%.