Hi John- Wouldn't it be nice if ASHRAE had just defined unloading curves like they should have? Anyway, I'll take a stab at this... It would've been nice for the reviewer to site his or her source. I think the document being referred to is this: http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/cac_tp_101105.pdf Cutting straight to the issue: The TRACE default 90.1-2007 heatpump shuts off at the minimum operating temperature of 40 degrees F and switches to backup heat. There's a lot of ways you could create custom biquadratic curves so that between 40 and 17 you have a "series" arrangement - but that won't solve the problem (because the loading curves would need to change from project to project anyway, given that there are no defined unloading curves!) Here's what would work: 1) Copy the 90.1-2007 equipment in the TRACE library and reset the minimum operating temperature to 17 degrees 2) Use this as your new heat pump 3) Run the simulation with this new equipment 4) From the equipment energy consumption report, get the total kw-hr in heating mode for the heatpump 5) Same report - Get the total KW-hr of the auxiliary electric resistance heating 6) Go into the visualizer and get the total KBtu-s on the heating coil for the heatpump systems 7) Take the total Kbtu-s divided by the total kW-hrs (from 4&5) and that is your current HSPF (It needs to equal to or higher than the HSPF in 6.8.1) 8) If the HSPF is lower than required by 6.8.1, adjust your efficiency of your heat pump (or alternatively of your Electric resistance equipment) and reiterate until you get an HSPF equal to or greater than that in 6.8.1. This is textbook "Thermodynamically similar" - so that should work. Obviously, it is best to get the HSPF to be EXACTLY the same as specified to maximize your energy in the baseline building. Have a great weekend Bob Fassbender Energy-models.com On Fri, Aug 3, 2012 at 3:06 PM, wrote: