Understanding CAZ Depressurization Testing

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CAZ Depressurization testing is a requirement for most home performance audits. Sealing home envelopes tighter can affect the safe and proper operation of combustion equipment. Tightening the home’s envelope will reduce the air supply needed for combustion. When there is not enough combustion air, equipment may have combustion ventilation problems.

Weatherization and building performance auditors and raters needed a way to evaluate when envelope sealing could potentially cause combustion problems. Their first line of testing is carbon monoxide (CO) monitoring. At any point, if CO levels exceed 35 ppm they quit testing and fail the home. However, in most homes the CO levels are not exceeded and the practitioners utilize testing methods designed to indicate when the conditions for improper combustion or venting are present. It is assumed that if no problem is found during the depressurization testing, that there are no combustion or venting related problems. Thus, it is used by the home performance industry as a safety check.

When one of the three main combustion problem indicators are found – backdraft (see Figure 1), combustion spillage, or flame rollout. Building performance specialists have generally called in HVAC experts to resolve the issue. The HVAC expert will be told that the home failed the depressurization / CAZ test. In many instances there may be a point score provided. The tests commonly used do not evaluate failures at the same level of depressurization. The point scores are based on a pressure differential measurement between the interior of the home and the outside and values depend on the type of appliance. The unit of measure the scores are based on is the Pascal (one Pascal is equivalent to 0.000145 psi.).

Often, the potential performance problem can be with the venting, in the appliance, or it can be related to the way appliances were installed, and/or the operating conditions. Once identified, the cause of a CAZ test failure may be a combination of unrelated factors.

Traditional home performance/weatherization depressurization/CAZ tests simply identify when conditions are present that could cause combustion or venting problems and stop there. Generally, the first person called in to resolve the reported problem is an HVAC technician. Therefore, to be able to identify the proper course of action, HVAC technicians need to be cognizant of building science factors beyond the basic OEM installation requirements for HVAC equipment. HVAC equipment and/or the water heater will usually get the blame for any CO problems in the home. Fire places and fossil fuel appliances (e.g., gas cook tops & ovens, clothes dryers, and auxiliary room heaters) installed without consideration of combustion air requirements are just as likely to be the source of elevated CO levels in the home. Additionally, automobile exhaust in an attached garage can result in measurable CO levels in the occupied space. In the end, the ability to identify the actual source of the elevated CO levels is a stronger position for an HVAC contractor than only being able to provide proof that the HVAC equipment was installed to code. HVAC contractors can become the “go to” companies for whole home performance improvements in the future by developing the capability to resolve whole home and appliance-related CO issues.

When potential problems are identified during a CAZ/Depressurization test, professional HVAC contractors are called in to provide home performance and weatherization practitioners as well as homeowners with the qualified expertise needed to resolve the issue. NFPA 54 procedures require combustion analysis and equipment evaluation competencies that are beyond the capabilities of most home performance and weatherization specialists. All HVAC related equipment evaluations and the related combustion analysis testing should be provided by a professional licensed HVAC Contractor who employs qualified technicians with the appropriate certifications and training.

For more details on CAZ testing as done by home performance practitioners see ACCA’s Technical Reference Bulletin 2012-2 Understanding CAZ Testing by logging onto the ACCA web site. Then: click on the Member Services button, followed by the File Downloads button, followed by the Technical Bulletin Button. Many contractors use the Technical Bulletins as training tools for their technicians.

Below the Technical Bulletin button there is a selection called Customizable Brochures (ComforTools). The ComforTool is designed to be a lead generator. They are based on the material covered in the Technical Bulletins and written for home owner’s consumption. Thus, once the technician has learned about the Material in a Technical Bulletin, there is a leave behind product available for homeowners on the subject.

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