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Building Science Fundamentals

13. Understand and Convert Pressure Units: Inches of Water Column (IWC), Pascal (Pa)

This is an important unit of measurement during your energy audits for:

  1. Blower door readings
  2. Pressure pan readings
  3. Room pressures
  4. Dominant duct leakage
  5. Draft pressure for combustion appliances
  6. Static pressure (the only test listed here reported in IWC, so you will to know the conversion here and for the test)

1 Pascal (Pa) is a unit of pressure and equal to about the weight of one post-it-note laying on your hand.  A common mistake is thinking that pressure is a flow rate and you can measure airflow with the same tools... that is wrong my friend but you are close.  To get a flow rate from a pressure reading simply multiply your pressure reading by the area your flow is coming from (i.e. the duct area).  A Pascal is measured by energy auditors with a manometer.
Inches of water column is a term more familiar to the HVAC folks.  It is also expressed as inH2O.  It is also a unit of pressure, just as a Pa is or PSI, they are all the same thing.  The definition of an IWC is a unit of pressure measurement required to lift a column of water.  

Converting Pa to IWC

1 IWC = 248 Pa OR 1 Pa = 1/248.84 IWC 


Convert 100 IWC to Pa.

Answer: so if you want to know how many IWC 50 Pa is, divide:
100 / 248.84 = 0.402 Pa (a very small number)

or

0.004 x Pa = IWC

Convert 113 Pa to IWC

Answer: so if you measure a static pressure of 113 Pa on the return side, convert it to IWC by multiplying:
0.004 x 113 = 0.452 IWC (more common conversion than IWC to Pa)

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