I have not seen this before and would like some input. The return air ductwork in the attic, just before the furnace, has a branched-off duct through the roof for fresh outside air. Fresh air is nice, but I am not sure this is proper. In this current configuration, the furnace would be sucking air into the furnace in winter and summer. Info: House 1954, furnace ~2007 with A/C and Medium Efficiency Gas Furnace, Colorado.
I would call that out. Colorado eh? Cold temps could potentially crack the heat exchanger. It also causes the house to be pressurized, loosing heat and pushing moisture into the structure. It is inefficient too.
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I did a couple of modular homes recently, brand new builds, that had a fresh air supply ducted directly into the furnace on both. Neither had filters for the ducting either. Didn’t seem right to me but that’s how the homes were manufactured. Didn’t find much info on the furnace installation, it was some odd brand, but did recommend filtration be added.
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This might add some insight. I am not saying right or wrong, but I have seen it in commercial buildings and some residential.
https://www.contractingbusiness.com/archive/article/20861960/the-many-benefits-of-adding-fresh-air
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Come to think of it, commercial rooftop units do pull air in from outside. Good call Brian.
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I think they have to have a mechanical damper and filter. Damper should close when the unit is not running. I would recommend the @belwood1 review photos and look for a lever in the plenum or return duct. It will look similar to this:
Not just RTU’s, but split systems as well. My memory is returning, I have definitely seen it on split systems, most recently in a retirement home I inspected a couple of months ago.
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I would consider a narrative like this:
The HVAC system had a fresh air intake incorporated into the return air supply ductwork. These fresh air systems should have a mechanical damper and a filtration system which was not visible or not present. Recommend further evaluation for proper configuration and operation by a qualified HVAC contractor.
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This is required and has been for 20+ years, on new builds anyway. Doesn’t affect the heat exchanger as the air is mixing with return air.
If an ERV/HRV is part of the HVAC system, bringing fresh air into the system as pictured above, is not.
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Hi Scott, do you think that is an ERV/HRV or no?
Not Scott, but no, I think not, Brian.
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I disagree. Introduced that close to the furnace it has potential to crack the heat exchanger. In warm climates it’s a non issue but negative 30 degree air or colder the heat exchanger would not like it. It’s still a poor design no matter what part of the country you live. It’s not part of new builds in my area.
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Shouldn’t make sweeping general statements. Not required or done in our area with new builds.
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No. Simply a fresh air intake.
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It’s a poor man ERV/HRV. More info on the topic attached.
hard duct connect.pdf (306.2 KB)
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Up here in Canada, it was very common about 30 or more years ago, to bring outside combustion air into the return air plenum a min of 10’ away from furnace, so as to allow the air to temper a bit before hitting the hot heat exchanger. It then required a 5" diameter spill grille installed in the hot air plenum just above the furnace, which directed air downward towards the combustion port. About 30 years ago, we’ve moved past that and into a separate combustion air vent, independent of the furnace ducting.
I’d be more concerned about that furnace and ducting being exposed to the cold side of the attic.
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Thank you for that link, it certainly added some insight. It appears to be okay to have the fresh air duct, however, there is no damper and no insulation so that an issue apparently.
BUT WAIT! There’s more! I’m concerned about the proximity of the fresh air intake on the roof. It is about 8 feet from the wood-burning chimney and furnace/water heater combustion gas vent. I know it’s a 10 foot rule for swamp coolers to vents so I would imagine this would apply.
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Although I think its a dumb idea and code here in Michigan, it works just fine.
I have seen the first foot frost up near the rim joist but it doesn’t harm the heat exchanger.
The requirement for us is a 4" intake. At 60 CFM on an 80,000 BTU furnace with 1200 CFM running through the system, it doesn’t hurt anything.
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Many years ago, I had that system set up and the result after a few years was a discoloured light grey carpet at perimeter walls due to the resulting positive pressure from such system. Also as Walter mentioned in a previous post:
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