Building Envelope Interaction
How the building envelope interacts with heating systems including depressurization effects, make-up air requirements, combustion appliance zone testing, and worst-case draft analysis.
- Explain how building depressurization affects combustion appliance performance
- Determine make-up air requirements for exhaust devices
- Perform Combustion Appliance Zone (CAZ) depressurization testing
- Apply worst-case draft analysis to heating system assessment
Lección 1
Depressurization & Combustion Appliances
The Pressure-Draft Connection
Every naturally vented combustion appliance (atmospheric furnace, standard water heater, fireplace) relies on the building maintaining near-neutral or slightly positive pressure in the combustion appliance zone. The natural draft that carries combustion gases up the chimney is a relatively weak force - typically only -1 to -5 Pascals. If the building is depressurized by more than this amount, the draft reverses and combustion gases spill into the living space.
This is the fundamental interaction between the building envelope and the heating system. Everything that affects building pressure - air sealing, exhaust fans, duct leakage, HVAC system operation - potentially affects the safety of combustion appliances.
Sources of Depressurization
Any device or condition that removes air from the building without providing replacement air creates depressurization:
Exhaust fans - Bathroom fans (50-110 CFM), kitchen range hoods (100-1,200 CFM), laundry dryers (100-200 CFM). A tight home with a 600 CFM range hood can reach -20 Pa or more of depressurization.
HVAC duct leakage - Supply duct leaks in unconditioned spaces (attic, crawlspace) remove conditioned air from the building and dump it outside the envelope. Return duct leaks draw in unconditioned air but do not directly depressurize the building. The net effect of supply-dominant leakage is building depressurization.
Fireplaces and wood stoves - An open fireplace can draw 200-500 CFM of room air. Even with the damper closed, a masonry chimney can draw 25-100 CFM through leakage.
Stack effect - In winter, warm air exits at the top and cold air enters at the bottom. If more leakage area exists at the top than the bottom, the building may have net depressurization at lower levels where combustion appliances are typically located.
The Tighter Building Problem
Modern energy efficiency standards require tighter building envelopes (3-5 ACH50 per IECC). While this saves energy, it reduces the natural air exchange that provides combustion and makeup air. In a leaky old house (15-20 ACH50), there is always enough air leaking in to replace what exhaust fans remove. In a tight house (3 ACH50), running a 300 CFM range hood can quickly depressurize the building to dangerous levels.
Leaky Building (15 ACH50)
Natural infiltration: High - plenty of makeup air
300 CFM range hood effect: -2 to -3 Pa
Risk to combustion: Low - draft easily maintained
Energy efficiency: Poor - massive heating bills
Tight Building (3 ACH50)
Natural infiltration: Low - limited makeup air
300 CFM range hood effect: -10 to -20 Pa
Risk to combustion: High - can overpower draft
Energy efficiency: Excellent - low heating bills
Natural draft in a chimney is only -1 to -5 Pa. Exhaust fans, duct leakage, and fireplaces can depressurize the building more than this, reversing draft and causing combustion gas spillage. Tighter buildings are more susceptible to depressurization because there is less natural infiltration to replace exhausted air. BPI professionals must test combustion safety under worst-case depressurization conditions.