The Cold Bonus Room and the Upsizing Proposal
A 2,400 sq ft two-storey house has an 80,000 Btu/h two-stage furnace and a 2.5-ton A/C. The trunk runs through an unheated crawlspace; branch ducts are 6 in. flex with sharp 90-degree bends, and the only return is a central hallway grille. The bonus room over the garage is 4°C colder than the rest of the house, and the furnace runs long cycles on mild days. The contractor proposes a 100,000 Btu/h furnace and a 3-ton A/C, saying “more capacity will push air to the far rooms.” No room-by-room load calculation or duct design has been provided.
1. What is wrong with the proposed upsizing, and what design data must be produced before selecting equipment?
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Upsizing is a guess that can make the problem worse. The symptoms point to air distribution and envelope losses, not insufficient nominal capacity. A larger furnace may produce higher static pressure, more noise, shorter cycling and greater temperature swings, while a larger A/C may cool the house without removing enough moisture. Neither change fixes a kinked flex duct, an undersized return, or a bonus room with more exposed envelope.
Before equipment is selected, require a room-by-room heating and cooling load calculation using an accepted method, with outdoor design conditions and actual envelope areas, insulation, windows and infiltration. Require an equipment selection based on that load, a duct design based on available static pressure and room airflows, and a return-air design. The contractor must show that the existing or proposed duct system can deliver the required airflow at the selected fan setting. If the design cannot be produced, the equipment proposal should not be accepted.
2. What field measurements would separate an envelope problem from a duct and airflow problem?
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Collect data that distinguish heat loss from air delivery:
- Static pressure across the blower and at key duct locations, compared with the equipment’s rated external static pressure.
- Temperature rise across the furnace and compare it with the nameplate range.
- Airflow at supply and return grilles, especially in the bonus room and the central return.
- Room-by-room temperature and pressure differences with the air handler running and with the door closed.
- Duct leakage and visual inspection of disconnected, crushed or uninsulated ducts in the crawlspace.
- Blower-door test and thermal imaging to identify envelope leakage or insulation gaps in the bonus room.
If the bonus room has adequate airflow but remains cold, the envelope is the likely cause. If airflow is low and static pressure is high, the duct system is the likely cause. If both are marginal, the remedy may need both envelope work and duct correction.
3. Recommend a compliant corrective scope, including duct and pressure considerations, and explain why the original proposal should be rejected.
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Reject the upsizing as unsupported. A compliant scope should begin with sealing and insulating the crawlspace ducts, replacing sharp flex bends with smooth fittings, eliminating unnecessary flex length, and adding a properly sized return path from the bonus room or a transfer path that does not compromise privacy or fire separation. Balance the system with dampers and verify room airflows. If the load calculation shows the existing equipment is adequate, retain it; if it is not, select equipment from the calculated load rather than from floor area.
Verify static pressure after modifications, confirm temperature rise is within the nameplate range, and check that the HRV and exhaust systems still balance. The bonus room may also need envelope improvements - air sealing, insulation or window upgrades - because duct fixes cannot overcome a missing insulation layer. The final test is room-by-room temperature and airflow under design conditions, not a larger nameplate.