[Greenbuilding] Sizing Thermal Mass

[Eli]

Here is another one of my thermal modeling mysteries.   I have of attempting to model accurately the impact of various thermal masses.  

What determines the temperature swing likely from a thermal mass.  

A thermal mass stores the heat determined by Specific Heat x Density to come up with Heat Capacity (BTU/sf F).  For of given temperature change in the mass, I can accurately determine how much energy is stored or released.  However, what is more a mystery to me is how to size mass.  How do I determine how much will the mass will actually rise.  In theory, any mass could store any amount of heat if you just increase the temperature accordingly.  However, in the real world, it will only go up so far.  I am trying to get beyond rules of thumb for thermal predictions.  

If I come up with an average solar gain or a maximum solar gain for a building per regional climate data I have an energy source.  Heat losses in the building depend on the outdoor temperature.  I can use the average heat load using Degree Day concept.  Some of my solar energy will be consumed immediately to warm house during a cold sunny day.  I can subtract that amount from the heat gain to determine the heat that is to be stored.  However, if I do not have enough mass, the temperature will rise and increase the rate of heat loss.  If I take the heat gain on sunny day and subtract the estimated daytime heat loss, I can come up with energy that is intended to be stored in mass.  If I can deliver all that surplus heat to my mass, I can accurately determine the temperature swing of mass.  However, I am uncertain about predicting the efficiency of this process.  If the heat is not absorbed by the mass, the air temperature will rise during the day to increase the heat losses and therefore, reduce the available energy to be stored.  What temperature swing is likely to be successful in a mass. When I assume a temperature swing for mass in my spreadsheet I get dramatically different results.  

What determines the temperature swing likely from a thermal mass.  

Eli Fishpaw