Duct cleaning - high flow for one week - why?

[BrianE22]

Hopefully this won't get red flagged, I'm just curious why cleaning the air ducts in my home would significantly increase the flow but for only a week or so. It's happened twice now. What I've thought was:

1) If the cleaning gets down to bare metal then maybe the friction is dramatically lower than it is with a film of dust on the walls.

2) There's a choke point somewhere where the dust build up is significant compared to the cross section of the duct.

Any other thoughts? From an engineering standpoint I find it an interesting puzzle.

 

[IRstuff]

I can't see either as being plausible; the typical duct NECKS DOWN to 6x12 at the registers and are usually larger elsewhere, so it's unlikely any dust would make THAT much difference to the flow. Likewise, there would need to be monstrous dust bunnies to actually choke the flow, and even then they would most certainly get blown to the registers and be visually apparent.

How did you measure the "significantly" increased flow?

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[btrueblood]

Did they remove the dust filters during cleaning, and then come back a week later and put the dirty filters back in?

 

[LittleInch]

Probably better in the HVAC forum, but never mind.

Cleaning - what sort of cleaning?
Any actual measurements or just "feel"
Air ducts feeding cold air? warm air?
Any filters in there?

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[EdStainless]

Or did they knock a lot of dust loose that they didn't actually remove.
So that the new air filter offered less restriction for a while until it fouled.

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[BrianE22]

The measurement of the airflow was mostly subjective. The high flow was much louder and caused the blinds to blow around quite a bit more. Also, the house heated and cooled quicker.

The first cleaning, about 10 years ago, was done by removing the duct covers (output and return) and positioning the vacuum hose just inside the ducts. There may have been other steps but I don't remember them. The cleaning I just did about 2 months ago was a bit different - he blocked off the return ducts and I don't know where he mounted the vacuum - somewhere downstairs near the furnace.

I installed a new furnace and AC just after the recent cleaning. I assumed the initial high flow was because the new furnace/AC was a bit bigger. After a week or so the flow had decreased like it did after the cleaning I had done 10 years previous. The new flow with the larger furnace/AC is higher than the flow I had with the original furnace/AC.

Air is and was cold during AC and warm during heating. There is a filter in the main return duct to the furnace. In the past changing the filters didn't change the air flow noticeably.

I'll pull the new filter and see if there is a chunk of dust in it.

I assumed the great flow after cleaning was normal but I didn't find anything on google. It's more a curiosity thing right now - I'm o.k. with the amount of heating and cooling I'm getting.

 

[IRstuff]

Personally, I think most duct cleanings are scams. Just consider that the air handler that's actually there can't blow all the dust out into the house, so it seems a bit dubious that something that fits in the back of mini van could do better, particularly given the longer hoses involved.

Note that from a pure physics perspective, the surfaces of the ducts have zero velocity, so dust that's stuck to the walls require something that's basically scrubbing the walls to get them out. By that same token, dust that's stuck to the walls isn't going anywhere, so let sleeping dust bunnies lie?

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[LittleInch]

I suspect the filter myself. As a scientific experiment try and get a water column U bend either side to monitor dp. You only need a short piece of tubing sealed in as the wc should be inches.

When the filter is really new the flow increases,picks up dust and fibres that have laid dormant, slowly clogs the filter just enough to reduce flow, dust etc goes back to where it doesn't move and the system stabilises.

Remove the filter for a short time and see what happens.





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cvg]

suggest a possibility that mounting the vacuum / installing the new furnace / or the cleaning process itself may have loosened a duct that has now started to leak. even small leaks in ducts can cause significant reduction in air flow.

 

[hacksaw]

when you test the individual vents with temporary filters, or actual particulate tests, you'll discover that most of the debris comes from the room contents, not the HVAC.