I have two units installed with a closed loop (3000') system, DSH and backup resistive heat coils. The units are 13 years old. The P040 is ducted to the lower floor and basement (~3500 sq ft) and the P026 is ducted to the upper floor only( ~1800 sq ft). In the winter, when temps are below 20 degrees F, the P040 runs nonstop and uses a lot of resistive heat, mainly during the night. This is the way the system has always performed. The P026 cycles periodically during these times but never calls for resistive heat. I've tried lowering the set points to 69 degrees lower and 71 degrees upper, but cannot find a way to reduce the amount of resistive heat usage during the winter months. During these times the DSH will occasionally reach high limit. Questions: Would turning the DSH off help reduce the resistive heat usage? Is there a way to determine if the systems are undersized?