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BPI Written Exam - Section 1 Building Science Fundamentals

3. Basics of: Combustion Appliance Venting, Draft, and Combustion Air Including Identification of Proper Sizing/Vent Tables

Combustion Appliance Venting

Atmospherically vented appliances use the surrounding air to mix with combustion gases for a clean burn.  On the other hand, power vented appliances have a dedicated pipe to the outside that draws air into the combustion chamber.  


Homes with atmospherically vented appliances have several potential health and safety issues we need to address as part of our energy audit and they are:
  1. Is the room large enough for the size of the water heater or furnace.
  2. Is there enough ventilation if the room is not large enough so the gas appliance can draw air from another room that is connected by vents.
  3. If there are vents, are their two vents and located 12-inches from the ceiling and 12-inches from the floor.
  4. Is the slope of the flue correct


Below are guidelines for atmospherically vented flue pipes.
  1. The flue must be sloped upwards 1/4 an inch every 1 feet
  2. No horizontal pipe lines

Draft

Draft is a naturally occurring process (think stack affect) that vents CO and combustion by-products from a gas appliance through the flue pipe and to the outside.  The combustion by-products exit the appliance hot, and so they are carried up the flue stack.  Upon appliance start-up however, the combustion gases still need time to warm-up, and this is the highest risk for those gases to back draft and enter into our living space.  

That is why we test the draft pressure and spillage of a combustion appliance WITHIN THE FIRST 60 SECONDS OF APPLIANCE START-UP.  If the appliance fails to pass the spillage and draft test after the first 60 seconds of start-up, the appliance fails.  Remember, both of these tests are done under worst case conditions first, we want the house to fail while we are there to inform the homeowner then, not weeks later when no one is aware.

Here are some important details you need to know about draft pressure for the BPI test.

  1. You should measure draft 1-2 feet above the draft diverter
  2. Draft and spillage are measured together, within the first 60 seconds of turning the appliance on from a cold start
  3. Start measuring draft at the smallest appliance first if two appliances share a common flue pipe or chimney exhaust
  4. You need to seal the hole you drill for the draft test with metal tape or fire rated caulk
  5. Measure draft and spillage under worst case conditions first, then if the appliances fails, test under natural conditions
  6. Measuring draft only needs to be done on atmospherically vented appliances, power vented or fan assisted furnaces or water heaters do not need a draft test

Below are the BPI Standards for the draft test.  The hotter the temperature outside, the easier the appliance should draft on its own so the lower the BPI Standard is.  To pass the draft test, you need to be at the Minimum Draft Pressure  OR MORE NEGATIVE.  So let's say it's 80 degrees outside, our minimum draft pressure will be:

(80/40)-2.75 = 2 - 2.75 = -0.75 Pa OR MORE NEGATIVE

Now let's say the temperature is 20 degrees:

(20/40) - 2.75 = 0.5 - 2.75 = -1.75 Pa OR MORE NEGATIVE

So as it gets colder outside, the harder it is for the appliance to naturally draft and the minimum draft pressure decreases, or gets more difficult to pass.

Combustion Air Sizing

This section on combustion air room sizing is only for atmospherically vented appliances which draw air from inside the house, sealed combustion units draw air directly from the outside and you do not need to worry about proper room sizing.

All atmospherically drafted appliances require combustion air from the room where the appliance is located.  In other words, the appliance needs to breath in fresh air from the room to:

  1. Draft properly, this is another possibility for failed draft or spillage tests along with a strong negative pressure from worst case
  2. To mix oxygen with the combustion gases for the combustion process to take place, that is why too little oxygen can cause incomplete combustion and high CO readings.

If a room is "too small" for the appliance, the appliance may not draft properly.  An easy test to see if a room is "too small" is to take a draft pressure, then open a window in the same room as the appliance and take another draft pressure... if opening the windows decreases the draft pressure (makes it more negative), more combustion air may be needed.  

BPI Standards for combustion air supply is 50 cubic feet of air per 1,000 BTU.  You will need two pieces of information to check if your room is big enough for the combustion appliance.

  1. The volume of the room.  You can find the cubic feet or volume of a room by multiplying the length and width of the room by the ceiling height.  
  2. The BTU input rating of the gas appliance.  You need to look at the name plate info of the combustion appliance to get it's BTU rating, then you can compare the gas appliance to the room size and what the minimum room size should be.


There must be two combustion vents, one 12-inches from the ceiling and one 12-inches from the floor in the room of the combustion appliance.

Let's do an example.

What is the volume required for a 40,000 BTU gas water heater in a garage 20x30x10 feet?

  1. Our volume is 20x30x10 = 6,000 cubic feet
  2. Find the minimum room size with our conversion factor, 50 cubic feet of combustion air per 1,000 BTU or 50 cubic feet per 1 MBTU
  3. First let's convert our BTUs into MBTUs by taking our appliance input, a 40,000 BTU water heater / 1,000 BTU = 40 MBTU.  
  4. Now we plug our volume and converted input into the formula... 6,000 cubic feet / 40 MBTU = 150 cubic feet/MBTU or 150 cubic feet per 1,000 BTUs.
  5. The lowest our answer can be before we are required to add combustion air is 50 cubic feet per 1,000 BTU.  
  6. We are at 150 cubic feet, above the minimum of 50, so we DO NOT need outside combustion air for this garage.


Let's do another example, but add a 100,000 BTU furnace in the same garage as our 40,000 BTU water heater.

  1. The volume is the same, 6,000 cubic feet.
  2. Our BTU input is now 100,000 BTU + 40,000 BTU = 140,000 BTU
  3. Let's convert our BTUs into MBTUs by taking our appliance input, a 140,000 BTU / 1,000 BTU = 140 MBTU.  
  4. Now we plug our volume and converted input into the formula... 6,000 cubic feet / 140 MBTU = 42.9 cubic feet/MBTU or 42.9 cubic feet per 1,000 BTUs.
  5. The lowest our answer can be before we are required to add combustion air is 50 cubic feet per 1,000 BTU.  
  6. We are at 42.9 cubic feet, below the minimum of 50, so we NEED outside combustion air for this garage.


Let's keep going, but take off the interior garage door so our area now includes part of the house, another 400 sq ft of house with 10 ft ceilings.

  1. The volume is different, if we take 400 sq ft x 10 ft ceilings = 4,000 cubic feet.  
  2. 4,000 cubic feet of house + 6,000 cubic feet of garage = 10,000 cubic feet total volume
  3. Our BTU input is the same at 140,000 BTU
  4. Let's convert our BTUs into MBTUs by taking our appliance input, a 140,000 BTU / 1,000 BTU = 140 MBTU.  
  5. Now we plug our volume and converted input into the formula... 10,000 cubic feet / 140 MBTU = 71.4 cubic feet/MBTU or 71.4 cubic feet per 1,000 BTUs.
  6. The lowest our answer can be before we are required to add combustion air is 50 cubic feet per 1,000 BTU.  
  7. We are at 71.4 cubic feet, above the minimum of 50, so we DO NOT need outside combustion air for this garage.


(You can also size a room if you are designing a new home with an atmospherically drafted water heater but that is less common for energy auditors as we deal mainly with existing homes.)

That is the reason why it is code to have upper and lower vents in a garage with an atmospherically drafted water heater for example (something you can check for although I have only found one home that only had one high vent in a garage and we installed a new low one).  

Next Section

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