Matt,

I too have had water heaters backdraft when the furnace inducer fan starts up. I have asked HVAC guys about this and the answer that I usually get is the tee is wrong and it should be a wye (when the furnace has an induced draft). However, I have yet to find a specific reference in the installation instructions or the code. The HVAC guys have not been able to give me a code reference either.

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The apostrophe troll.