Cycling Losses for Gas Furnaces

stevehull · 11-23-13, 08:37 AM

Jeff (and others),

Interesting you bring up this case study (above, two posts back) as I have in front of me almost the exact same scenario.

In this home, the thermal mass of the metal duct is huge and it takes almost a minute after the furnace fan comes on for warm air to come out of the registers. The two main ducts are in the crawl space, poorly insulated (maybe R4) and have lots of losses (~120 sq inches cross sectional area each). I could hear them whistling when I scooted under there and can literally feel air blowing out of the duct joints (ducts outside of HVAC space).

I am debating on recommending running flex duct as it is inexpensive and I can get 100 foot lengths with no splices. On the other hand, what I would LIKE to do is to put in duct grade PVC and sleeve insulate it (R10). The problem is cost. The 12 inch diameter PVC is about 1/4 the cost of the flex duct and the installation is a snap with flex duct.

As you know, it is very difficult to clean flex duct of the dust, mildew and other junk that accumulates (even with good pleated filters). PVC and metal is breeze to clean. I really hate flex duct . . .

But cost is cost and I tend to look way down the road in terms of job quality. The HVAC installer is pushing me to suggest flex duct. Me thinks I will put both in the recommendation and allow homeowner to sort it out (and yeah, I have just contributed to the confusion you described above).

Has anyone sleeved flex duct - or seen a product to do this (extra insulation sleeve slid over flex duct to augment duct insulation)? The R value of flex duct is pretty poor, but at least it has very low exfiltration (if installed correctly).

Years ago, there was a product where you could spray foam the INSIDE of metal ducting both to seal up air leakage and put in some degree of insulation. The downside was it collected dust/dirt like MAD, decreased the cross sectional duct area and increased duct resistance to air flow. Haven't see that product in a while . . . .

Thoughts?

Steve

11-23-13, 08:37 AM	#23
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Jeff (and others), Interesting you bring up this case study (above, two posts back) as I have in front of me almost the exact same scenario. In this home, the thermal mass of the metal duct is huge and it takes almost a minute after the furnace fan comes on for warm air to come out of the registers. The two main ducts are in the crawl space, poorly insulated (maybe R4) and have lots of losses (~120 sq inches cross sectional area each). I could hear them whistling when I scooted under there and can literally feel air blowing out of the duct joints (ducts outside of HVAC space). I am debating on recommending running flex duct as it is inexpensive and I can get 100 foot lengths with no splices. On the other hand, what I would LIKE to do is to put in duct grade PVC and sleeve insulate it (R10). The problem is cost. The 12 inch diameter PVC is about 1/4 the cost of the flex duct and the installation is a snap with flex duct. As you know, it is very difficult to clean flex duct of the dust, mildew and other junk that accumulates (even with good pleated filters). PVC and metal is breeze to clean. I really hate flex duct . . . But cost is cost and I tend to look way down the road in terms of job quality. The HVAC installer is pushing me to suggest flex duct. Me thinks I will put both in the recommendation and allow homeowner to sort it out (and yeah, I have just contributed to the confusion you described above). Has anyone sleeved flex duct - or seen a product to do this (extra insulation sleeve slid over flex duct to augment duct insulation)? The R value of flex duct is pretty poor, but at least it has very low exfiltration (if installed correctly). Years ago, there was a product where you could spray foam the INSIDE of metal ducting both to seal up air leakage and put in some degree of insulation. The downside was it collected dust/dirt like MAD, decreased the cross sectional duct area and increased duct resistance to air flow. Haven't see that product in a while . . . . Thoughts? Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

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