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RED Calc Free Help Depressurization (DTL)
Depressurization Analysis Tool User Guide

Depressurization Analysis
Red Calc Tool User Guide

What this tool can do for you

The Depressurization Analysis "solve all" tool allows you to solve for "Building leakage @ 50Pa" (DTL), "Exhaust flow" of exhausting appliances, or "Depressurization" (combustion appliance depressurization limit).

The primary purpose of this tool is to allow you to determine if vented combustion appliances will vent properly from a house while all the appliances that exhaust air (bathroom and kitchen exhaust fans, clothes dryers, etc.) are operating.

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Source: EPA

Calculated values

  • Depressurization - negative pressure created by operation of exhaust fans and other appliances that exhaust air.
  • Building leakage @ 50Pa - sometimes referred to as the Depressurization Tightness Limit or DTL.
  • Exhaust flow - fans and other appliances that exhaust air, such as clothes dryers.

Tips

  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • This is a "solve-all" tool. Select the radio button to the left of the label for which you wish to solve. This label will turn to blue and the input box will disappear.
  • The most significant result you can obtain from this tool is the "Building leakage @ 50 Pa", often referred to as the Depressurization Tightness Limit (DTL). Knowing the DTL before weatherization can be vital in planning for ventilation and knowing if existing combustion appliances will need to be replaced. A number of weatherization programs use this calculated DTL value as a planning/analysis threshold for weatherization work.
  • The tighter a weatherization crew makes a house (lower and lower CFM50), the greater the magnitude of the negative pressure created by a given total CFM of the exhaust appliances.
  • All input values must be entered as positive numbers.
  • Some users of this tool have reported good results when using it to determine the flow of an exhaust appliance. For example, they have used the Depressurization Analysis tool to find the exhaust flow of a kitchen range hood with the following procedure:
    • Set the Depressurization Analysis tool to solve for "Exhaust Flow".
    • Set up the house in blower-door-test condition.
    • Perform a whole-building blower door test to determine the CFM50 of the house. Enter this value in the Depressurization Analysis tool in the "Building leakage @ 50Pa" input box. Let's assume this value is 1405 CFM50.
    • With the blower door off and plugged, turn on the kitchen range hood fan and measure the negative house-to-outdoor pressure created by the fan. Enter this negative pressure (as a positive number) in the "Depressurization" input box of the Depressurization Analysis tool. Let's assume this value is -5 pascals. The tool will solve for the "Exhaust flow" of the kitchen range hood. The range hood Exhaust flow result is 300 CFM (141 L/s).
    • In order to verify this value, use your blower door to create the same Exhaust flow rate, for this example, 300 CFM. This known blower door flow should create the same house depressurization as the kitchen range hood fan did, for this example, -5 pascals.
    • Note: Accuracy of this procedure increases on calm days (lack of wind) and when the depressurization created by the exhaust appliance is significant. We recommend turning on the Uncertainty feature for the tool (see "Preferences" in the tools menus) when using this procedure.

Inputs and field measurements

  • Pressure exponent - you may not solve for this value. This value characterizes the leakage of the dwelling. We have used the default value of 0.67, a reasonable value for post-weatherization conditions. We recommend you use this value unless you have performed a multi-point blower door test yielding the actual pressure exponent for the dwelling. Be aware that as a house is tightened, the pressure exponent value changes because the characteristics of the holes in the envelope are altered.
Because this is a "solve-all" tool, each of the values below will be treated as inputs unless you select one as your result by checking the radio button to the left of the label.
  • Building leakage @50Pa - as an input, this is the building leakage rate at 50 pascals during a standard blower door test or the estimated value of a post-weatherization blower door test. When selected as the result, this is often referred to as the Depressurization Tightness Limit (DTL) and serves as a screening threshold for safe combustion appliance venting. Many weatherization programs use the DTL as a tightening limit for their air sealing activities.
  • Exhaust flow - as an input, this is the sum of all of the exhaust flow rates of exhaust fans and other exhausting appliances, such as clothes dryers. If it is not possible to measure the exhaust airflow, do your best to estimate it. Supply fans and other appliances that supply air mechanically to the dwelling, should not be included. When selected as the result, this is the limit to the "Exhaust flow" sum, based on the "Building leakage @ 50Pa" and the limiting "Depressurization" value.
  • Depressurization - as an input, this is the limit to depressurization in the dwelling or combustion appliance zone (CAZ). This limit is usually based on the vented combustion appliance that is most vulnerable to back drafting. When selected as the result, you are determining the magnitude of the dwelling or combustion appliance zone depressurization. This might be done to determine if a particular vented combustion appliance is likely to experience venting problems.

Background

Solving for "Building leakage @ 50Pa" (DTL CFM50) is the most useful way to use this tool. To solve for this, you must select the "Building leakage @ 50 Pa" radio button and enter values for all the other variables. For example, assume the 400 CFM (188.78 L/s) total for "Exhaust flow" is made up of a bathroom fan of 80 CFM (37.76 L/s), a kitchen fan of 120 CFM (56.63 L/s) and a vented clothes dryer of 200 CFM (94.39 L/s). Assume the combustion appliance "Depressurization" limit of -2 (enter as a positive number) pascals is your weatherization program's appliance depressurization limit for a conventionally vented gas water heater in the combustion appliance zone (CAZ). The standard pressure exponent of 0.67 is entered as the default. The resulting "Building leakage @ 50Pa" or DTL is 3500. This means that if this house, with a total of 400 CFM flow from exhaust appliances, is tightened to 3500 CFM50, a negative pressure of -2 pascals will be created in the combustion appliance zone. Tightening the house even more will result in a negative pressure of even greater magnitude.

A gas-fired range/oven or other unvented combustion appliance is not affected by negative pressures in a house because they are not vented or coupled to the outdoors, so the indoor-outdoor pressure difference is irrelevant for unvented combustion appliances.

With this solve-all tool, you can also solve for "Exhaust flow", the sum of the actual exhaust rate for all exhaust appliances operating simultaneously. To do this, you must select "Exhaust flow" as your result and enter values for all the other inputs. This solution allows you to determine the maximum exhaust flow rate for a house with a given CFM50 and "Depressurization" limit.

Finally, you can solve for "Depressurization". If you know the actual "Exhaust flow" of exhaust appliances and the "Building leakage @50Pa of the house, you can determine the resulting "Depressurization". This routine helps you determine if the combustion appliances are in danger of back drafting when all the exhaust appliances are operating simultaneously for a given house tightness level. Table 1, below, lists suggested combustion appliance depressurization limits.

Combusiton Appliance Depressurization Limits

References

Related tools

  • Pitot Tube Airflow: With this tool, a pitot tube, and a digital manometer, you can determine the airflow of a ducted range hood, a clothes dryer, or other appliance will an accessible duct.
  • Box Airflow: Determine the flow of an exhaust fan with this tool, a cardboard box, and your digital manometer.
  • ASHRAE 62.2-2013 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
  • ASHRAE 62.2-2010 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.

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