Combustion Safety and Ventilation
Combustion fundamentals
Combustion appliances include furnaces, boilers, water heaters, and ranges. Complete combustion produces CO2 and water vapor; incomplete combustion produces CO, a dangerous poison. Know that draft is created by the vent connector and chimney, and that atmospheric appliances rely on the house not being depressurized. Backdrafting occurs when the house is depressurized (by exhaust fans, dryers, or the blower door) and combustion gases spill into the living space.
Testing
Combustion safety testing includes: spillage testing at the draft hood, draft measurement, CO measurement in the flue and ambient air, and worst-case depressurization testing. Know the order: set up worst case (all exhaust fans on, doors closed), then test. Ambient CO limits are commonly cited around 9 ppm; flue CO varies by appliance. A combustion analyzer also computes steady-state efficiency from O2 and stack temperature.
Ventilation
Houses need controlled ventilation, especially after air sealing. Know ASHRAE 62.2 as the residential ventilation standard and that exhaust-only, supply-only, and balanced (HRV/ERV) systems exist. An HRV recovers sensible heat; an ERV also recovers moisture.
Traps
A classic trap: assuming a natural draft water heater is safe because it has a vent. Another: testing combustion before establishing worst case. Also watch for confusing CO with CO2, and for forgetting that tighter houses need mechanical ventilation.
Rule of thumb
ASHRAE 62.2 ventilation roughly equals 0.03 CFM per sq ft of floor area plus 7.5 CFM per occupant, with adjustments for infiltration credit.
Energy Modeling
Purpose and inputs
Energy modeling predicts savings from proposed improvements. Understand the inputs: envelope areas and R-values, window U-factor and SHGC, air leakage (ACH50 or natural), HVAC efficiencies, thermostat schedules, and climate data. Models compare a baseline to a proposed case. The output is annual energy use and cost, plus savings.
Methods
Know the difference between simple payback (cost ÷ annual savings) and life-cycle cost, and between prescriptive and performance paths. Whole-house modeling tools use hourly simulation; simpler tools use degree-day methods. Understand that savings estimates depend heavily on the baseline assumptions and on interactions: air sealing reduces both conduction losses and infiltration, but can change pressure and combustion safety.
Traps
A common trap is double-counting savings: you cannot claim full duct-sealing savings and full furnace-replacement savings if both affect the same load. Another is ignoring interaction between envelope and equipment: after air sealing, the existing furnace may be oversized. Also watch for using nameplate efficiency instead of measured or derated efficiency.
Worked example
If a proposed measure saves 1,500 kWh per year at $0.14 per kWh, annual savings are $210. If the installed cost is $2,100, simple payback is 10 years. Compare that with a measure saving $420 per year at $1,680 cost, which pays back in 4 years.
Reporting
Models should state assumptions, weather data, and uncertainty. Savings should be presented as estimates, not guarantees.
Work Scope and Client Reporting
Building the scope
The work scope translates assessment findings into prioritized actions. Understand the hierarchy: health and safety first (combustion safety, moisture, electrical), then building shell (air sealing, insulation), then HVAC and ventilation, then renewables. A scope should be specific, measurable, and tied to the diagnostic results. Include quantities, locations, and specifications.
Client communication
Reports should be clear, jargon-free, and prioritized. Explain what was found, what it means, what is recommended, and the expected cost and savings. Distinguish must-do safety items from optional improvements. Set expectations about savings estimates and about interactions between measures.
Traps
Questions often test whether you put safety first: a scope that ignores a backdrafting water heater to focus on insulation is wrong. Another trap is promising exact savings or failing to document baseline conditions. Watch for scopes that list measures without quantities or that skip ventilation after air sealing.
Documentation
Record test results (CFM50, duct leakage, CO readings), photos, and assumptions. A good report lets another contractor reproduce the work. Include a clear statement of what is not included.
Worked example
A homeowner wants new windows. The assessment finds high air leakage at the rim joist and attic. A responsible scope recommends air sealing and attic insulation first, explains the cost and expected savings, and notes that windows can follow if comfort issues remain, rather than leading with the most expensive measure.