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Combustion and Flue Product Safety in Non-Standard

Residential HVAC InstallDave (or another...
Combustion and Flue Product Safety in Non-Standard Residential HVAC InstallDave (or another HVAC person if I'm not able to choose Dave after clicking "Get an Answer),The old furnace my mother is replacing was in a space too small to position the return and filter next to the furnace and also allow the recommended 25" of service area in front of the furnace. Placing the return and filter under the furnace needs to be explored given the very limited horizontal space available in the HVAC closet. The manual for the Lennox filter housing she's looking at shows vertical is an accepted configuration for the filter housing (and presumably Lennox furnaces (she'll probably get the EL296V)). At first read it seems like there might be a safety issue if the installers don't seal the return to the furnace (and presumably filter housing) perfectly...From the Lennox EL296V manual (I'll attach this if given the chance after clicking "Get an Answer"), p. 6... (In a section discussing placing the furnace in a small closet...) "When the furnace is installed so that supply ducts carry air circulated by the furnace to areas outside the space containing the furnace, the return air must be handled by ducts which are sealed to the furnace casing and which terminate outside the space containing the furnace. This is especially important when the furnace is mounted on a platform in a confined space such as a closet or small equipment room. Even a small leak around the base of the unit at the platform or at the return air duct connection can cause a potentially dangerous negative pressure condition."
From p. 10... "Improper installation of the furnace can result in personal injury or death. Combustion and flue products must never be allowed to enter the return air system or air in the living space. Use sheet metal screws and joint tape to seal return air system to furnace. In platform installations with furnace return, the furnace should be sealed airtight to the return air plenum. A door must never be used as a portion of the return air duct system. The base must provide a stable support and an airtight seal to the furnace. Allow absolutely no sagging, cracks, gaps, etc. (Later they sort of repeat this...) Return air can be brought in through the bottom or either side of the furnace installed in an upflow application. If the furnace is installed on a platform with bottom return, make an airtight seal between the bottom of the furnace and the platform to ensure that the furnace operates properly and safely."So it seems like combustion gas and negative pressure are always a safety issue BUT ARE EVEN MORE SO IF THE FURNACE IS MOUNTED ON A PLATFORM AND THE RETURN IS TO THE BASE, NOT A SIDE. What I can't figure from reading the directions is if this applies when combustion air is brought in from the outside (above the roof) and exhaust is expelled outside (above the roof). I think the term for this is "sealed combustion."So my question is... Are the warnings above to super-perfectly seal a return entering the underside of the furnace to the furnace applicable in what I am (maybe incorrectly) calling a "sealed combustion" installation? Or are combustion gasses and negative pressure more of a concern when there is not a PVC pipe bringing combustion air from outside the home AND a pipe exhausting exhaust to the outside of the house? If combustion air/combustion chamber/exhaust are separate from supply and return air I don't see what the need is for all these warnings?Thank you.Bruce
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Answered in 25 minutes by:
9/20/2017
Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 768
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist
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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.

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Customer reply replied 9 months ago
Regan,Thanks for the response. The owner of the HVAC company my mother is working with and his right-hand man are good guys but are new to HVAC. Some of the techs have been around for a while.I think you and I are on the same page but would like to clear one thing up... You wrote "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." Are the "seals on the furnace" you mention seals installed at the factory or are these seals that rely on the installers??? I'm more comfortable if the critical seals in this install are installed at the factory (tried and true) than if critical seals in a more-dangerous-than-usual install (because it'll be up on a platform) must be provided in the field by guys who might have only done "normal" side-return installs.I hope the ??? makes sense. As a reference, the configuration under discussion is Fig. 6 on p. 4 of the Lennox Carbon Clean 16 filter housing install manual. To re-state the ???, does this configuration end up relying less on the installer for safety when it is a sealed combustion unit with PVC in/out of the house than when it is a regular furnace that uses air from inside the house for combustion???Thanks. Sorry to be dense but the manual got me scared of the return to the bottom not being sealed perfectly but I'm hoping it's mostly talking about regular, non-sealed installs.Bruce

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.

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Customer reply replied 9 months ago
OK, last question regarding a safe raised (or any) install...Sounds like return house air through the bottom of a sealed combustion furnace (PVC in and out of the house) raised on a platform is about as safe as a regular furnace on the floor (with return air through the side) with standard maintenance of the factory seals years down the road. Is carbon monoxide the main danger involved in the installer seals the manual is discussing and the factory seals you've mentioned - that is, would a CO detector in the HVAC closet pick up much of the danger being discussed???After your response I'll rate you which I think means pay you. But, seeing this is a health-related question, I might ask it again for a second opinion - so if you see a similarly worded title, please leave it for someone else. Thanks for your input - I feel better about putting a raised furnace in her place!Bruce
Customer reply replied 9 months ago
Not sure if you initiated the "...switch to a live phone call..." offer or if it was auto-initiated. Unless there is something tricky about what the dangers of bad seals are and if a CO detector would likely pick up much of the danger, I'd prefer a fairly straightforward text answer and then rate and wrap this question up. Something along the line of "CO is much of the danger so a CO detector would help" or "There are many dangerous gases beside CO that arise out of bad seals so a CO detector won't help much"???

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.

Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 768
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist
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