Cold Upstairs and Sweating Windows: Sizing Question
A 1,900 sq ft two-story house built in 1992 has a single-stage 80% AFUE furnace rated 90,000 BTU/hr input in the basement, one centrally located return, and a single first-floor thermostat. On a 12°F night the homeowner reports 68°F downstairs, 61°F in the upstairs bedrooms, and heavy condensation on the second-floor double-pane windows. Indoor relative humidity at the thermostat is 48%; outdoor RH is 55%. A contractor has bid a 120,000 BTU/hr two-stage replacement on the grounds that the existing furnace is 'too small.' Measured external static pressure is 0.75 in. w.c., and the nameplate temperature rise range is 35-65°F. The homeowner wants the upstairs fixed before the holidays.
1. What does the window condensation tell you about the building envelope and the moisture source, and how does it bear on the furnace-sizing question?
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Condensation forms when the interior glass surface drops below the indoor dew point. At 68°F and 48% RH the dew point is roughly 49°F; a double-pane unit with a cold frame, a failed edge seal, or a poorly insulated rough opening can easily fall below that on a 12°F night.
That makes this a surface-temperature and humidity problem, not a capacity problem. An indoor RH of 48% at 12°F outdoor is high for a cold climate; many homes begin condensing on glass somewhere around 30-35% at those temperatures. Likely sources are a humidifier set high, uncovered basement soil or a damp crawlspace, dryer or bath fans dumping indoors, or a tight house with no controlled ventilation.
A larger furnace cannot fix this. Raising supply air temperature changes the glass surface only slightly while increasing short cycling and duct losses, and it does nothing about the 7°F room-to-room split. The condensation is telling you to look at air sealing, insulation at the rim joist and window frames, and ventilation/humidity control before touching equipment.
2. Using the numbers given, evaluate the 120,000 BTU/hr upsizing proposal.
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Existing output is about 90,000 x 0.80 = 72,000 BTU/hr, or roughly 38 BTU/hr per sq ft of input for a 1992 house with insulation. That is already generous, so the claim that the furnace is 'too small' is not supported by any load calculation in the bid.
The proposal adds 30,000 BTU/hr of input. Consequences:
- A bigger unit on the same duct system will not fix a 7°F floor-to-floor split; stratification is a distribution and envelope issue.
- Static pressure is already 0.75 in. w.c., above a typical 0.5 in. w.c. rating, so the existing blower is working against restrictive ducts. Pushing more air through them raises static further, increases duct leakage, and can trip the high limit.
- Because the temperature rise depends on both input and airflow, the 35-65°F range tells us nothing until the actual rise and blower speed are recorded.
- Short cycling from oversizing reduces mixing and typically makes the upstairs colder, not warmer.
3. What diagnostic sequence would you run before recommending any equipment change, and what would each test rule in or out?
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Sequence from the whole house inward:
- Room-by-room load calculation - establishes the actual design load and gives a defensible size. Rules out 'too small' if the load is near or below 72,000 output.
- Blower door and duct leakage test - separates envelope losses from duct losses. High duct leakage to the attic or crawlspace can explain both the cold rooms and the high static.
- Temperature rise and static pressure - supply and return separately, plus filter pressure drop, with the blower speed recorded. Low rise with high static suggests restricted ducts; high rise suggests low airflow.
- Room-by-room airflow and pressure balance - supply velocities, door undercuts, return path, and closed-door room pressures. This separates a duct-sizing problem from a return-path problem.
- Humidity and surface temperature survey - hygrometer in each zone and an IR thermometer on glass and frames to confirm where the dew point is being reached.
Only after these results should equipment or duct changes be proposed. The likely remedy set is air sealing, return-path improvement, duct correction, and humidity control - not a bigger furnace.