Second Floor at 76, First Floor at 66, One Filter Grille

Both, and they compound - but the split is primarily a distribution problem, not an insulation problem.

Thermal: the first floor is served by long runs of compressed R-4 flex in a 25°F attic, the second floor by short runs. Long, thin, compressed runs lose both heat and volume. So the second floor gets nearly full delivery and the first floor gets a cooler, smaller one. That alone tilts the building warm on top.

Pressure/airflow:

• 0.95 in. w.c. at the supply plenum with a clean filter is high. High static means the blower is being throttled, and every duct in the system delivers less than its design airflow. The compressed elbows are a large part of that.
• One return grille on the second floor plus a panned joist cavity for the first floor means return capacity is lopsided. The second floor returns easily; the first floor return path is a joist cavity that is almost certainly leaky and undersized.
• Two closed bedroom doors turn those rooms into pressurized supply-only boxes; the air leaves through the envelope instead of returning, which further unbalances the system.
• 210 CFM25 to outdoors is a big leak into a 25°F attic. Supply-side leakage reduces delivered capacity; return-side leakage pulls cold attic air into the airstream. Either way the equipment's output is being spent outside the conditioned volume.

The thermostat is the amplifier. It is on the cold first floor, so it runs long, and every extra minute of runtime dumps more heat into the already-warm second floor. The system is doing what it was told; it is reading the wrong room and delivering to the wrong one.

1. Seal the ducts, starting with the 210 CFM25 to outdoors. Highest return per dollar, and it is not just energy - a duct system leaking 210 CFM to the attic is actively changing the pressure balance of the house, which affects everything downstream including the combustion appliance zone. Seal with mastic, not tape; replace the panned joist return path with a properly sized duct.
2. Fix the static pressure. Correct the three compressed elbows, verify the filter grille is not undersized for the blower setting, and confirm total external static is in the blower's rated range. If the blower has been set high to compensate for a bad duct system, it is drawing more watts to deliver the same or less air.
3. Fix the return path from the first floor. Enlarge it and, ideally, add a return in the first-floor living area. Undercut or transfer-grille the two bedrooms so closed doors stop pressurizing rooms.
4. Then balance. Adjust dampers (or add them) so more supply air reaches the first floor now that the system can actually deliver it.
5. Only then consider the equipment or the thermostat. Relocating the thermostat to a representative location, or adding zoning, comes after the duct system works. Zoning a leaky, high-static system makes it worse, not better.

The discipline here is refusing to skip to step 5 because the equipment is the visible object. The duct system is the actual cause and steps 1–3 are the ones that move the number.

Continuous fan circulates air through the same unbalanced distribution system that produced the problem. If the first floor is already getting less supply volume and a worse return path, mixing the air harder does not move more heat there - it just mixes the same distribution losses more times per hour.

What continuous fan actually does in this house:

• It runs the blower through the ducts 24/7 while the attic is 25°F. Every hour the fan runs without a call, the long compressed first-floor runs lose heat to the attic and the leaky return pulls cold attic air into the airstream. Standby duct loss goes up, not down.
• It keeps the pressure imbalance active. With 210 CFM25 to outdoors, continuous operation means the envelope sees that imbalance continuously rather than only during burner cycles. In a house with atmospheric appliances or a naturally drafted water heater, that is a safety consideration, not just a comfort one.
• It can make the split worse, because the short second-floor runs deliver almost undiluted air back to the second floor, reinforcing the warm-upstairs pattern.
• It raises blower energy at a static pressure of 0.95 in. w.c., which is an expensive way to move less air than design.

What the owner should hear: continuous fan is a reasonable strategy after the ducts are sealed, the static is corrected, and the return path is balanced - at which point it does what she wanted. Before that, it is a workaround that pays for itself in the wrong currency. In the meantime, opening the two bedroom doors is free and helps.