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Building Science Fundamentals

18. Natural / Mechanical Ventilation

Natural ventilation in homes occurs by air leakage from:

1. Electrical and plumbing penetrations
2. Recessed canned lights
3. Leaky doors and windows
4. The stack effect
5. Leakage around attic hatches

All the sources of air leaks energy auditors find and identify in homes is a source of natural ventilation for a homeowner, which is good.  What is bad is that this leakage enters and leave uncontrollably and can be poor air quality. This is why we want to seal a home a tight as possible, the bring in mechanical ventilation where we know and control how much fresh, outside air is coming into a home.  Build it tight, then ventilate it right.

Mechanical ventilation can be in several different forms:

1. Exhaust fans.  We can go a step further than just turning on an exhaust fan (which acts like a mini-blower door exhausting house air outside, and bringing in outside air through air leaks) and install a motion sensor activated exhaust fan, or a timer onto the exhaust fan to help take human error (or human lack-of-use) out of the equation.  Exhaust fans are rated with a CFM flow rate.
2. Whole home exhaust fan.  This is the same idea as an exhaust fan or a blower door, where we depressurize a home and air is forced to enter in through air leakage points.  In the case of a whole house exhaust fan though, homeowners should open their windows to let fresh outside air in while the fan is running.  These can be used as a home cooling system as well in climates that have temperate weather or seasons.  Whole home exhaust fans are rated with a CFM flow rate.
3. Fresh air duct with dampener on the return plenum.  This is a low end option that works well and qualifies a home for Energy Star Certification.  Every time the AHU kicks on, a small 4" duct brings in fresh outside air on the return side.  The down side is that this air is not conditioned or heated at all, so energy costs go slightly up because the AHU has to work harder.
4. ERV or HRV is a great unit that brings in fresh outside air from a dedicated duct, but the beauty of it is that this unconditioned outside air is precooled or heated by transferring it's line with a dedicated exhaust line from the inside out of the house, which IS conditioned or heated already.  It's like the first shampoo, when washing your hair twice.  These systems are designed to run 24-7 and only add $95 a year to operate and to the heating and cooling costs.

This ASHRAE Standard 62.2 references ventilation requirements and calculations for homes and is a BPI standard you will need to know for the BPI exam.  

Next Section

1. 1a. Basic terms and definitions
   
   1. Understand airflow in buildings / ducts: CFM, CFM50, CFM25, ACHn, ACH50, FPM
      
   2. Understand equipment efficiencies: AFUE, SSE, SEER, EER, HSPF
   3. Understand power and energy: watts, BTU/hr, ton of refrigeration  watt-hours, BTU, therm, decatherm
   4. Understand effective leakage area
      
   5. Understand area weighted R-Value
      
   6. Understand baseload / seasonal energy use
   7. Understand driving forces (including natural and mechanical: Pressure, temperature, moisture differential
   8. Understand behavior of radiation: emissivity, reflectivity, absorbtivity
   9. Understand thermal resistance / transmittance: R and U Values; including conversions
   10. Understand latent / Sensible heat: evaporation, condensation / specific heat, heat capacity
       
   11. Understand total equivalent length
       
   12. Understand basics of dehumidification / Humidification as well as measurement equipment
       
   13. Understand and convert Pressure units: Inches of Water Column (iwc), Pascal (Pa)
       
   14. Understand, identify thermal bridges
       
   15. Understand pressure boundary 
       
   16. Understand/define stack effect 
       
   17. Understand and define exfiltration and infiltration 
       
   18. Natural / mechanical ventilation 
       
   19. Understand net free area 
       
   20. Understand input / output capacity 
       
   21. Understand peak electrical demand 
       
   22. Understand permeability and perm rating 
       
   23. Understand standby loss 
       
   24. IAQ (indoor air quality): moisture, CO, dust

1b. Principals of energy, air & moisture

1. Thermodynamics: conduction, convection, radiation, ΔT including air movement due to temperature gradients
   
2. Factors that affect insulation performance: density, installation, moisture
   
3. House pressurization/depressurization by various forces
   
4. Heat gain / loss: internal, solar, heat transmission, air leakage 
   
5. Power and energy: BTU content of fuels, capacity of combustion appliances and electrical appliances 
   
6. Moisture transport mechanisms: bulk water, air leakage, diffusion, capillary action 
   
7. Identify areas of highest relative humidity 
   
8. Principles of combustion: combustion analysis, CO 

1c. Combustion science

1. Combustion analysis: oxygen, flue-gas temperature, carbon monoxide 
   
2. Carbon Monoxide (CO) testing of combustion appliances 
   
3. Basics of: Combustion appliance venting, draft, and combustion air including identification of proper sizing/vent tables 
   
4. Understand combustion safety issues: Combustion air, draft, worst case / baseline depressurization, spillage, backdrafting, unvented combustion appliances