The 1920s Bungalow That Won't Warm Up
A homeowner in a cold climate (7,000 heating degree days) complains that the second floor of a 1920s brick bungalow stays 5°F colder than the first floor even after replacing the furnace with a 96% AFUE unit. The house has 2x4 walls with original plaster, a vented unconditioned attic with R-11 batts, single-pane double-hung windows, and a full basement. Blower door testing yields 4,800 CFM50 on a 1,600 sq ft home (approximately 2,100 sq ft conditioned including the finished basement). The homeowner wants the contractor to simply add a second furnace zone. Infrared imaging shows the second-floor ceiling is cold near the exterior walls and the attic hatch is dramatically cold. The homeowner has a limited budget and wants the biggest comfort improvement per dollar.
1. Calculate the approximate air changes per hour at 50 Pa (ACH50) and interpret what this means for the home's shell performance and the second-floor comfort complaint.
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ACH50 = CFM50 × 60 / conditioned volume. Assuming 8 ft ceilings over 2,100 sq ft, volume ≈ 16,800 cu ft. ACH50 = 4,800 × 60 / 16,800 ≈ 17.1 ACH50.
This is very leaky - typical targets are 3–5 ACH50 for a weatherized home and under 3 for new construction. The stack effect in a leaky two-story home drives cold outdoor air in at the lower levels and pulls conditioned warm air out through the attic. That convective loop is a major reason the second floor feels cold: warm air escapes through the attic hatch and ceiling penetrations before it can heat the upstairs rooms.
So the comfort complaint is partly a distribution problem but substantially an air-sealing and insulation problem. Adding a second furnace zone without addressing the shell would spend money heating air that immediately escapes.
2. The contractor must choose between (a) a second furnace zone, (b) air sealing plus attic insulation upgrades, or (c) window replacement. Recommend the priority order and justify with building science reasoning and cost-effectiveness.
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Priority order: (b) air sealing and attic insulation first, then (a) zone control if comfort issues remain, then (c) windows last.
Reasoning: Air sealing the attic plane (hatch, top plates, penetrations) directly reduces the stack-driven heat loss that makes the second floor cold. Attic insulation from R-11 to R-49 or better addresses the cold ceiling seen on infrared. These are the highest comfort-per-dollar measures because they attack both the convective loop and conductive loss.
A second zone treats the symptom: it delivers more heat to a space that is losing it faster than it can be replaced, and it does nothing for the attic heat loss.
Windows are the lowest priority: single-pane windows are a real loss, but replacement cost per unit of savings is very high compared with air sealing and insulation, and window air leakage is often better addressed with weatherstripping and storm windows. The contractor should sequence work so air sealing precedes insulation (never insulate over an unsealed attic plane), then reassess comfort before recommending zoning.
3. What combustion safety and ventilation considerations must be checked before and after tightening this home, and why?
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Before tightening: perform a combustion appliance zone (CAZ) worst-case depressurization test. The home has a 96% AFUE furnace (likely direct-vent or Category IV) but may also have a natural-draft water heater in the basement. Tightening a leaky home reduces the incidental infiltration that currently supplies combustion air and dilutes spillage. If a natural-draft appliance is present, worst-case depressurization could exceed the appliance's draft margin, causing spillage of flue gases.
After tightening: re-test CAZ depressurization, check for spillage at the draft hood, and measure CO in the flue and ambient air. If the home becomes tight enough that natural-draft appliances cannot vent safely, the contractor must address it - options include sealing the CAZ from the house, adding a combustion air duct sized to the appliance, or replacing the appliance with a direct-vent or power-vented unit.
Ventilation: after air sealing, the home may drop below roughly 0.35 ACH natural, which is a common threshold for recommending mechanical ventilation. The contractor should measure or estimate post-retrofit infiltration and, if needed, specify a balanced ventilation strategy (e.g., an HRV/ERV) to control moisture and indoor air quality. Tightening without ventilation planning can create moisture and pollutant problems.