I'm having issues with my boiler, and according to this answer, one of the problems might be that my chimney draft is incorrect.

  • What steps are required to determine if my chimney draft is correct?
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    I will take a pic of news paper and light it at the opening If the smoke goes up the flue it is good If it is not drawn in and smoke in the room it may need cleaning or the home is sealed and a make up air source is needed to allow some fresh outside air in. – Ed Beal Feb 15 '18 at 19:52
  • @EdBeal won't the airflow change with outside temperature? – virtualxtc Apr 23 '18 at 20:49
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    If it is cold outside the could be more draft but this all depends on if there is make up air available or how tight the house is sealed, I have seen homes that had make up air available but if the bathroom fan or stove top fan were running there was not enough draft. – Ed Beal Apr 23 '18 at 22:00

Two months later I finally found the right search string to get an better answer than 'light a stick on fire'.

Whenever preforming a test for safety, you always want to choose a worst case senator.

According to this page, worst case chimney draft conditions happen when the house is fully sealed and all exhausting appliances are activated. Thus one needs to:

  • Shut all windows and doors
  • Turn on all bathroom and kitchen exhaust fans
  • Turn run all appliances that exhaust such as dryers and central vaccumes

The source fails to mention whether or not to fire up other flue based heating sources. However my intuition tells me that heating devices that don't share the same flue as the test should be fired up, but ones that are common to the device being tested should be left off because the added heat might add to the updraft)

Similarly, the source fails to mention what the temperature differential should be between the inside of the house and the outside should be as there will clearly be more of an updraft when the air inside the home is warm, and the outside is very cold. For a furnace / heating stove, this likely won't matter as you'll always be operating under ideal conditions. However with a water heater, you may run into the case where you have a high outside temperature, AC chilled ducting, and short heater run times such that the flu never gets hot enough to create an updraft. This would be your worst case, attempts to run your test when conditions most closely mirror this situation would be preferred, but may make less of a difference than the other items listed above.

According to the reference, once you've sealed your house and started up all venting appliances, your can simply tur on the appliance you want to check for several minutes then cup your hands around the fume hood without touching it and feel the air draft. However, the manuals for professional draft testing equipment say that their tests should be preformed within the first 60 seconds after igniting your appliance. Thus it seems the best next step would be to:

  • Light a long match stick or dense cardboard strip (like a pizza box) and then blow it out so it smolders and smokes
  • Turn on your appliance and place your smoke stick near the vent hood.
  • If the smoke goes up the hood, your vent passes
  • If the smoke appears to be being blown into the house, the you have a down draft, and should repair it (especially if the down draft persists after your device has been on for several minutes).
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  • Turning on all exhaust fans or testing in the middle of summer is not the best idea in my opinion for the same reason that the electrical code allows the larger of the heating or cooling load to be used we don't add both in most cases but you can mark your opinion as an answer I disagree for this reason. – Ed Beal Apr 23 '18 at 22:06
  • @EdBeal the two senairos you mention don't seem at all similar to me. In the case of power draw, there is very little likely hood where the AC and electric heat would be on at the same time. However, if you are having family over for the 4th of July, it seems very likely that one might have all the windows shut with the AC on and all all bathroom fans, the kitchen fan, and the laundry going at once. – virtualxtc Apr 23 '18 at 22:22
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    But as I said the 4th is warm and no need for heat. – Ed Beal Apr 24 '18 at 17:34
  • Your comment (in this thread) seems to be about exhaust fans, and season, not 'heat'. Moreover, as stated in my answer, I agreed that July isn't the time to test a furnace, but one still needs "hot water" on the 4th of July and thus should test a water heater under those conditions. – virtualxtc Apr 25 '18 at 20:49

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