I still hear this during audits: shut a few registers to cut costs. In a 2,100 sq ft 1998 home with a PSC blower, closing 3 of 12 supplies bumped external static from 0.4 to 0.9 in. w.c. and increased blower draw by about 120 W — over a season, does that net out as savings or a penalty, and why?
Penalty in most PSC setups, @OP: closing registers spikes static, leaks more to attics/basements, and the blower draws more while moving less air — like pinching a straw; ECMs behave differently. Easiest check: log blower watts and temp split with everything open vs 3 closed, and if you must balance, use trunk dampers not registers. Is that 0.9 in. w.c. still under the unit’s nameplate max?
In my tests on late-90s PSCs, closing 2–3 supplies drove static to about 0.8–0.9, dropped airflow enough to hit heat-limit on long calls and upped duct leakage, so net was a penalty. If a client insists, I set a 0.5 in. w.c. cap with a cheap manometer and tweak trunk dampers like @csmith mentioned — did you log temp rise/ΔT along with the extra 120 W?
But quick tip from the field: if you’re going to close a few, put a manometer on and reopen each register lever until external static is ≤0.5 in. w.c. and the furnace ΔT sits in the nameplate range — if you can’t hit both, you’re paying more for less air — @jordan_m88 did your runtimes stretch when static was about 0.9?
Seen this bite more in cooling: closing a few grilles on PSC can nudge the coil toward icing and stretch runtimes. I leave grilles open and tweak the branch takeoff damper a quarter-turn while keeping ΔT about 16–20°F and compressor amps flat, @dave47 — can you log liquid line temp to make sure the coil isn’t starved?
At “120 W” extra and 0.9 in. w.c. on a PSC, it’s a net penalty — tiny fan kWh plus longer runtimes from pressurized rooms and leakage. If you’re going to shut a few, keep closed-room pressure under about 3 Pa with a transfer grille or bigger undercut and verify the coil/furnace stays in nameplate ΔT. @jordan_m88 did you log fan-hours on that 2,100 ft² case to ballpark the seasonal hit?
Closing “3 of 12” on a PSC and jumping to 0.9 in. w.c. is almost certainly a penalty; , you’re off the fan curve and driving more duct leakage and room pressure imbalances. If you’ve gotta trim, leave grilles open and throttle at the branch takeoff while watching external static back near about 0.4 and the furnace ΔT stay on the nameplate, or better, add a return in the tight rooms. Curious, @jordan_m88 — have you logged room-to-hallway pressure with doors shut to see how much that stretches runtimes?