Bedroom Comfort Complaint After a Furnace Replacement
After a furnace replacement in a 2,400 sq ft ranch, the homeowner complains that the bedrooms at the end of the trunk are 5°F colder than the living room. The contractor installed a 100,000 BTU/hr 96% furnace with a 4-ton blower, reused the existing 20 x 8 trunk and 6-inch branch ducts (eight supplies), and set the blower to the highest tap. Measured total external static is 0.95 in. w.c.; the temperature rise is 52°F; the farthest bedroom supply reads 220 fpm at a 6-inch register while the nearest reads 480 fpm. There is one 16 x 25 filter in a central return, and the return static is 0.35 in. w.c. No commissioning report was left.
1. Is the furnace delivering the expected amount of air? Support your answer with numbers.
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Output: 100,000 x 0.96 = 96,000 BTU/hr.
Airflow implied by the rise: CFM = 96,000 / (1.08 x 52) = about 1,710 CFM. A 4-ton blower is nominally 1,600 CFM, so the unit is moving at or slightly above nominal, consistent with the highest speed tap.
So the answer is yes in total volume, but the number hides the real problem. Total external static is 0.95 in. w.c., which is very high, and 0.35 in. w.c. of that is on the return side alone. That means the blower is operating near the end of its curve, and the delivered airflow is bought at the cost of high velocity and noise. Total airflow being adequate does not mean the air is reaching the right rooms - the 220 versus 480 fpm spread at identical 6-inch registers shows the trunk and branch layout is doing the dividing, not a balance damper.
2. Diagnose the room imbalance. What are the candidate causes, and how would you separate them?
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Candidates:
- Undersized or overextended trunk. A 20 x 8 trunk carrying roughly 1,700 CFM is at high velocity, and static drops along its length; the end runs see the least pressure.
- Unbalanced branches. No dampers or all dampers wide open at takeoffs means the nearest supplies take the air.
- Return restriction. A 0.35 in. w.c. return static from a single 16 x 25 filter and central return is a large penalty and may also pressurize bedrooms with doors closed.
- Register and grille resistance at the far end, including long flex runs with sharp bends.
How to separate them:
- Measure static at the start and end of the trunk. A large drop confirms trunk sizing or velocity as the cause.
- Measure branch velocities at each takeoff, not just at the register, to separate duct loss from register loss.
- Do a door-open versus door-closed room pressure test. If closed doors drive bedroom pressure up and flow down, the return path is the problem.
- Record filter pressure drop separately; a loaded filter can account for much of the return static.
Only after these readings should the remedy be chosen: balancing dampers, trunk enlargement, added return path, or a lower blower tap plus duct correction.
3. What documentation should have been left at commissioning, and what should you record now to support a remedy?
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What should have been left: equipment model and serial numbers, blower speed and tap, filter size and MERV with measured pressure drop, total external static split into supply and return, temperature rise, gas supply and manifold pressure, combustion analysis, room-by-room airflow or velocity, room temperatures, and a sketch of the duct layout with damper positions. A signed commissioning sheet with the measured values and the date is the minimum.
What to record now:
- The same list, taken as a baseline before any change.
- Static at the trunk start, trunk end, and at each branch takeoff.
- Door-open and door-closed room pressures and supply velocities for every room.
- Filter pressure drop new and loaded.
- The 220 versus 480 fpm spread documented at each register so the before/after comparison is defensible.
The missing commissioning report is itself a finding worth noting: without baseline data, the contractor cannot show the system was ever balanced, and the homeowner has no way to judge whether a proposed duct repair actually worked. Record the deficiency and the remedy plan together, with target values for each room.