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I recently moved into an older house with a natural gas-burning forced hot air furnace. There are no records of the ducts being cleaned, and based on the layer of black grime I can see, they are quite dirty.

It seems that having the ducts cleaned would increase the efficiency of my system (by improving airflow), but is this true? If so, what is the impact?

I assume that hiring someone to clean the ducts would cost at a minimum $200. My monthly heating bill is about $70.

  • Excellent question, and surely one that has been studied. Someone at ASHRAE should know – EnergyNumbers Jan 4 at 21:27
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    This may not be an issue regarding your ducts, but I'm assuming the black grime would be carbon deposits resulting from improper burning of methane (natural gas). Chimneys from fireplaces that burned coal or wood (which had a higher rate of carbon deposition than fires from gas) would need to be swept clean to reduce the accumulation of carbon to minimize the potential for uncontrolled chimney fires resulting from the accumulated carbon (char) catching fire. – Fred Jan 6 at 2:21
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I doubt it. I recently bought a century home (renovated 30 years ago) where I cleaned the ducts too. There was no noticeable change in costs. Cleaning helps keep the air fresh, and your furnace filter might last longer, or at least be more effective for longer. Plus mould spores, etc.

Be very careful on whom you choose. This is one of those "industries" with a bunch of fly-by-night companies, like roofers and driveway tarers. Check out reviews on the company first. Mine was ~$300. There were offers for < $100, but the reviews showed the broke things and wouldn't come back and fix things, stopped answering the phone or returning messages, etc.

Most reputable companies around here will scope it with fibre optics before and after, to show off their work.

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That's a great question! The US Environmental Protection Agency (EPA) says the main reason to clean is eliminating mold, critters & other toxins, and since your gunk is dark colored it could well have mold spores so I'd vote yes. (They include lots of advice on finding, hiring & checking contractors' work, too, so it's useful as well as authoritative!)

As to efficiency, cleaning is just the first step. Unless there's enough gunk to partly block flow the increase after cleaning will be tough to discern. There may actually be a reduction if the gunk was helping seal gaps & removing it lets the gaps reopen. However, dirty, gunky, possibly moldy residue increases dirt & decreases air quality, so I'd clean for sure.

The bigger issue regarding efficiency is leakage. According to the Energy Star program's reasearch, in typical homes about 20% of conditioned air is lost through poorly constructed ducts, so fixing that can go a long way to improving efficiency. Their Duct Sealing brochure gives a few more details & has some helpful diagrams.

Re your estimate, figuring out which ducts have problems & fixing them is time consuming & often entails some repair/replacement work in very small spaces and/or opening up walls. So, it's not that cheap. Quotes in Sring 2018 in our deep-south, 2nd tier city for cleaning, reconnecting, resealing, replacing a section with sheet metal (much better airflow & easier to clean mold out of- especially important here in the humid South- but more expensive than floppy tubes), and insulating the whole shebang in our single-story, 1600 sf home came to about $1300 for labor & parts and was expected to take 2 guys 1.5 - 2 days to complete.

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