Hi John,

Isn't it "simply" a matter of the derating coil heat transfer and
pumping efficiencies, based on the percentage of glycol concentration?
Of course, deriving the derating factors is the not so simple part...
and probably the general solution would be to define a parametric
relationship between the glycol concentration and the coil and pumping
efficiencies.

Glycol manufacturer's data should have plenty of info on the derating
factors, perhaps even a parametric equation that can be utilized in BDL.

Well that was the easy part... hope it helps!

Brandon Nichols, PE