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Old 11-23-13, 09:37 AM   #21
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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

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Old 11-23-13, 08:11 PM   #22
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Quote:
Originally Posted by Servicetech View Post
Back to the original topic, it does appear that duct losses are significant in the cycling loss calculations. Not sure hot much the 45 second burn time to heat up the exchanger costs in efficiency, and how much is recovered during the 180 second blower off delay after the burner shuts off. 90%+, 80% and old school pilot light furnaces have different cycling losses.
My furnace runs a blower off delay of 75 seconds and in that 1 minute 15 seconds at the recommended blower speed, it is still putting out 125 degrees of heat through the ductwork when it powers off and my thermometer will actually slowly climb after the airflow stops as the heat in the metal is hotter than the air passing through it and some of that hot slowly rises out of the ductwork but I'd imagine most of it ends up conducting off the surfaces of the ductwork into the basement where I do not need it. I ran the blower for 2 minutes and that seems appropriate but my furnace(and many others) are not adjustable in their blower off delay.

My original thermostat, which was installed from when the house was built in 1985 until 2010 when I swapped it out, used to run the furnace with a 5 minute gas burn time. The new thermostat that I installed runs the gas for about 10 minutes. I can only imagine that cutting the cycles in half has helped a ton but I have a feeling that I still have room for improvement if I could allow for 1 or 2 more degrees in temperature spread before it turns on. I think the key may be to get a thermostat that allows you to adjust the spread or anticipation settings to the maximum comfort/efficiency level compromise for the people living in the house. ..along with making sure that the minimum size furnace is installed in the space.
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Old 11-23-13, 08:40 PM   #23
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Steve, I like the idea of insulated ductwork but I think flex duct is a bad way to go about it. Flex duct is one of the most restrictive ductwork options and a quick search on sites like greenbuildingadvisor shows that it is generally a bad idea. It allows for bad shortcuts such as not installing it stretched to its full stretch, tight 90, 180, or even full U-turns in the flex, bad trunk setups. I've seen other recommendations where rigid ductwork should be used and flex only allowed in the final 10 feet of a run.

It's a frictional loss issue that is the problem even if it is installed properly. The best way to use flex duct but not encounter the issue is to respect maximum duct lengths and be sure that the ductwork is upsized to the appropriate level to be sure that the airflow is correct. The issue with that is unless your flex runs are relatively short, you'll need to make very large diameter runs and on top of that you'll need to factor in the thickness for the insulation and how you plan to fit that flex into the space you are putting it. Once you get to a certain size of flex duct, you might even have R8 flex but you've now got so much surface area of the flex that you've got new problems.

I like this duct grade PVC idea since it is rigid and won't be leaky or have high frictional losses but I don't know what to expect in terms of how to best insulate it. My head says to create a sealed box around it out of rigid foam(beadboard or XPS are cheap) or drywall since it is cheap and stuff it full of cellulose.

As a personal note, if my attic had flex duct in it when it was built(it doesn't), it would probably be R4 so I'd be inclined to replace it with the R8 and since my attic is loose-fill I'd probably baffle around it with drywall or wood and then just loose-fill it to R60. So basically the ductwork would run in a channel that is 2 feet wide and covered 2 feet above the ductwork with cellulose. That is the way you can easily insulate that ductwork if you have the room. In Minnesota, the idea of putting ductwork in an attic is ludicrously stupid because we have winters where we usually hit -20f once or twice and if the ductwork has 130 degree air going through it, we would have a 150 degree difference in the ducts versus the outside. Which is more than the 90 degree with the rest of the houses insulated components. R8 is a hysterical in such a circumstance.

Since I think flex duct is a bad idea and rigid metal doesn't seem appropriate in an attic. How about ductboard? Take the ductboard and then add additional insulation around that, which should be easier since the stuff is square. You could wrap it in EPS, XPS, polyiso, or do the cellulose box route that I suggested above. If you are cheap you could even bust out fiberglass rolls and wrap flex or ductboard in that but no promises on its performance since you can't install fiberglass in an ideal way to get the most R-value by wrapping it around something square or round.
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Old 11-23-13, 08:45 PM   #24
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125 degrees is HOT after 75 second delay, what is the peak tempature while running? Is time delay digital with a board or a manual limit switch? I've not run into any board controlled blowers that don't have a selectable time delay. Manual fan/limit switches are also adjustable in most cases. You could always add a sequencer/time delay relay/fan control center to add extra blower time if you think it would help.

My furnace only gets to 106F after 10 minutes of running. Once the burner cycles off the blower runs for 3 minutes then tempature at vents is about 80f. Furnace is a 44,000BTU 90% with the 1/3HP blower set on the lowest speed.
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Old 11-23-13, 09:01 PM   #25
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Timer based blower on and off time of 75 seconds, not based on temperature. I might be able to modify it electrically which might require swapping a resistor somewhere or something. Either that or maybe setup some kind of relay to hold it on for a certain additional time period after the power is normally cut off. ..I suppose a 90 degree fan switch might not be a bad idea, that would turn it on sooner too and shut off once it drops below 80 degrees to really get the heat out of the ductwork.

After 10 minutes of gas run, it will usually be at 140 degrees(70 degree temp rise). It varies a bit depending on filter age, if any dampers or registers are closed, and fan speed. I have a roommate right now who prefers a warmer temperature than I do so some of the dampers and registers are shut to position the heat where the cold-baby is.
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Old 11-23-13, 09:15 PM   #26
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70 degree rise with 140f discharge from the vent. Add 5-10 degrees for the tempature exiting the furnace, you are proably close to hitting the high limit. Is the furnace a standard 80% induced draft/electronic ignition? Increasing blower speed should get you more efficient operation.

Total fan control can be done by installing one of these:
http://www.amazon.com/ICM251-Fan-Con...m_sbs_indust_3
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Old 11-24-13, 09:57 AM   #27
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MN Renovator

Agree with ALL your points, but need to mention that I am a hired consult on this (hired by the HVAC contractor). As I said, "I hate flex duct"!

That said, I did increase the x sectional area by 15% by using 12 inch diameter flex duct, but exactly as you say, now there is increased surface area for heat loss. We have to use the crawl space (maybe 20" of space) and it is nasty . . .

Now I am leaning towards the PVC ductwork . . . .

To all those that think contractors "rip off" consumers, here I am putting in FAR more time than I am paid, but trying to help out a homeowner. Not trying to be a saint, but I also know quite a few contractors that do the same.

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Old 11-24-13, 12:47 PM   #28
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No way I'd use flex in a crawl space around here. Wouldn't make it a year without a critter tearing it up. Hard pipe only for crawl installs.
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Old 11-24-13, 05:53 PM   #29
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I'm surprised that you professionalistas are venting so much hot air on ducts when hydronics offers so many advantages.



THIS link explains this and other advantages, like the ease of retrofitting a parched air house.


-AC
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Old 11-24-13, 07:18 PM   #30
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Hydronics is practically non-existent in the south due to lack of AC capability.

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