Both smoke and co detectors are so cheap now. Big box has them as three packs on sale all the time. Personally I never go more than three years on either. Wait for a sale do them all at the same time. Between having pets smoking cigarettes and random home improvement projects they get gunked up fast. I figure I spend less than $50 a year on detectors cheapest peace of mind money can buy.
Just sayin
Have you considered calling your HVAC company to make them aware of what happened? Maybe you’re already working with them to remedy the problem, but if not, I think you should bring it to their attention. You wouldn’t want them doing the same thing in someone else’s house who isn’t so vigilant about up-to-date alarms.
The "computer" in your air handler reads the amp draw on the blower and can tell if the blower is working too hard (drawing high amps). Blower speeds in cool mode are generally higher than in heat mode. You may well have sufficient CFM for heat but not quite enough for cool mode. You could cover that return duct during heating season. If you add an OAK, it would likely solve the problem without covering the duct. If having a return duct in the same room as a stove was an automatic problem, 95% of houses couldnt use a stove.
EDIT: It just occured to me that the giant return duct they cut in your return boot is probably way more CFM than the supply registers in that same room. That WILL create a vacuum in that room where the stove sits. That duct is most assuredly the culprit. Devise a way to cover that for now and then plan on having a duct run to floor registers in the upstairs. Dont leave it. Someone down the line could die.
Mechanical code regarding return air. It sounds like there may be more than one violation here.
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