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Hello Bruce, and welcome to Just Answer. My name is ***** ***** I'll be happy to assist you today. You are correct in calling the EL296V a sealed combustion unit. Improperly sealed return air is an issue with 80% furnaces that are not sealed combustion units. There have been deaths that have occurred from this, so Lennox (and other manufacturers) put this warning in all gas furnace manuals as a release of liability statement. The sealed combustion units will not be effected by improperly sealed return air as long as all seals on the furnace are intact and good. There could be the potential of a problem if one of the seals breaks down, tears, etc. From an Indoor air quality (IAQ) standpoint you would definitely want the return sealed regardless. The filter housing will also have a seal that will prevent leakage, so installing the filter in the closet is an accepted practice.
No problem! The seals are factory installed seals, so no need to worry. However, if there is ever service performed on the internal parts of the gas furnace (would presumably be years down the road) the seals would need to be checked and replaced if cracked or torn.
Yes, the sealed combustion units rely less on the installers for combustion safety than an 80% furnace that would require external combustion air to be pulled into the closet.
The live phone call was an automated response. Please ignore that. Yes, CO is the concern when talking about pulling flu gasses into the return air. I would recommend a low-level CO detector in the house (not the closet unless you add a separate one there) to ensure your mother would be alerted should there ever be a presence of CO in the conditioned space.