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C.A.Z. testing question...

To my associates who are actually performing combustion analysis on fired equipment, how many of you are performing a worst case Combustion Appliance Zone (depressurization) test as a part of your regular combustion analysis?

This test requires you to create some conditions that might not be the norm, but can occur for certain (depressurization) and can cause products of combustion to be spilled throughout the dwelling.




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  • Tim_Hodgson
    Tim_Hodgson Member Posts: 60
    Not Always

    I am pretty good at calculating my combustion air every time, but I do not stress the air supply unless we have had a problem with odors, moisture, limits,headaches, heat streaks or spillage signs.

     I remember the problem job, where it would smell up the house intermittently in the evening. I would get the call once a week. I would turn on the dryer, range and bath fans trying to make spillage at the draft diverter. No such luck. Two weeks later I figured out it was only when they had two or more fans on and they had left the glass fireplace doors open while burning a fire. If you closed the glass doors, the spillage would cease, open them and it would spill again.

    I do believe that a lack of combustion air is a big problem. Both contractors and homeowners remodel and love hiding the heating equipment behind a wall or door.   We have a pretty temperate climate in Portland. Most older homes have a

    lot of air infiltration and most new, tightly constructed homes have

    sealed combustion equipment.

     We should adjust our equipment when the air supply is at its worst. I will stress the next few jobs and see what results pop up. Although I would hate to stoke up the fireplace in July.

    Best regards,

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Thanks Tim...

    Anyone else care to chime in?


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  • JStar
    JStar Member Posts: 2,752

    At my previous company, I was the main guy doing CAZ testing for a third party energy auditing company. They were very strict.....very, very strict. The entire test could take up to 2 hours for a larger home. First we would measure our "natural" conditions with no other appliance or fan running. Mark those down as our control numbers. Then we would create the "worst case scenario". Dryers, bathroom fans, exhaust hoods running, all doors and windows closed, etc. All of these numbers are referenced against a minimum allowable draft on the chimney at certain outdoor temperatures. Lower temperatures, higher draft. There was also a maximum allowable difference between Natural and Worst-Case. Obviously, induced draft, or power vented appliance could overcome more than just a natural draft appliance.

    The only problems I've ever had involved water heaters. And sometimes, I would have too much draft! UGH! Then what do we do? Nobody seemed to know what the solutions for a problem were, only that there was a problem. Do we drop liners? Convert to power vent equipment? The audit company was weather proofing the houses to a point of almost no air-change, so wouldn't that make the problem worse?

    In the end, I realized that a lot of the testing was convoluted and subjective. The details get lost in the process. Nowadays, I'll do a smoke and mirror test on water heaters, and draft test with a manometer if I suspect a problem...which are pretty few and far between. The answers are usually obvious. IE: You can't put a 12000 CFM exhaust fan in your house without making other accommodations for the appliances.
  • Bob Harper
    Bob Harper Member Posts: 1,040
    when WCDT?

    Mark, I include a WCDT with combustion analysis every time I reline a chimney or install new venting. I install MUA per the IRC then test during the WCDT to ensure it is working or at least not hurting as it sometimes can. In that case, it is usually interzonal competition from unsealed ducts in an adjoining space or grossly unbalanced ducts in the adjoining space, which I correct. I also include at least one low level CO monitor in all my jobs. I incorporate the x2 baro. dampers w/ spill switches on gas WHs and I've done a few post purge timers.

    I install custom powered MUA systems as needed, which get tested to confirm efficacy.

    I've been called in to diagnose several B-vented appliance venting failures. I do an NFPA level II inspection first then WCDT w/ combustion analysis as part of troubleshooting the appliance for possible spillage tripping the high limit switch.

    Those not doing these tests really need to. I find problems in most cases. As houses get weatherized, it will get a lot worse. All the while, the higher efficiency heaters are killing off masonry chimneys at a record pace.  
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