Replies

1. 
   USAnigel | Jan 20, 2009 03:34pm | #1

   Just like driving, any kind of bend slows you down. Tighter the bend slower you go.

   So with the air inside the duct same thing happens and you lose airflow speed and pressure. It can get to the point of no flow. You will note in some houses that some ducts "blow" hard and others next to nothing. This is often down to restrictions in the pipe run like hard bends and long duct runs.

   So yes keep it as smooth as you can.

2. 
   danski0224 | Jan 20, 2009 03:38pm | #2

   Bends are fine as long as the radius is at least equal to the duct diameter.

   Square bends without vanes are very bad. Bends with improperly installed vanes are worse.

3. 
   Clewless1 | Jan 20, 2009 04:59pm | #3

   Generally speaking ... you are very right and as also stated by other responses.

   Smooth, efficient delivery of air depends on smooth transitions throughout. That's why they make elbows to registers w/ curves and not just made out of a box.

   Just read a bit of the Unico thing on line. Not sure much about the ins and outs of it ... left me a bit skeptical in the 'secretive' way they presented it (I did not do a thorough read). Remember there is usually NEVER  a free lunch. There is usually a downside to everything. Smaller ducts and high velocities will have a price somewhere/somehow. Knowing that price will make you a most informed user of the system. Personally, I like to know all the advantages (which salesmen are more than willing to share) AND the disadvantages when I buy something so that when I do (and I usually do) discover the disadvantages, I'll be comfortable with the compromise I've made.

   Damn get me off my soapbox. Sorry.

4. 
   danski0224 | Jan 20, 2009 05:45pm | #4

   Installation of residential high velocity systems is critical for you to have a noise free system.

   You must have the proper number of outlets and there is a minimum length of ducting required from the trunkline to the outlet. The minimum length may dictate some loops in the ducting.

   There is a minimum and maximum distance for the return ducting. 

   If an attic duct system will not be used during the winter, you must plug the outlets or you will get condensation.

   The use of a gasketed spiral pipe system will speed installation and eliminate leaks in your main trunk. Care must be used when installing the duct taps to the outlets so they do not leak.

1. 
   Clewless1 | Jan 21, 2009 05:05pm | #5

   Not sure if I'm reading your comments right. Normally it is critical in ductwork to have lower velocity to be noise free (i.e. low noise). But you say it is critical to make sure we have high velocity to ensure low noise?  Why is this?

   Why do we need MINIMUM trunk lengths?

   I'm not familiar w/ the merits/characteristics of the high volume system so maybe I'm missing some key items here?

   If I have ducts in the attic and they are well insulated ... i.e. w/in the thermal envelope of the house, why would I get condensation in the winter? Generally the house runs relatively dry in the winter anyway (many do), and if the duct is inside the thermal envelope, it should 'theoretically' be no different than if I ran that ductwork in the floor.

1. 
   danski0224 | Jan 21, 2009 05:31pm | #6

   A Unico duct system is a high velocity system, another brand name is Spacepak. There are others. They do not follow the same general rules for low velocity ductwork. A couple of other systems have ductboard plenum boxes with pre-punched holes for the proper number of outlets based on tonnage/cfm.

   The branch ducts have a minimum length to attenuate the noise level, and those same ducts also have a maximum to ensure the outlet velocity is within spec. 

   If the central return that is common with Unico and Spacepak is too close to the fan, there is a lot of noise, hence the minimum length of flex.

   As far as the insulation is concerned, it will not keep the ducts in an unconditioned attic from getting cold: ie, the same temperature as the attic. The warm moist air from the house will rise into the duct system and condense on the duct walls.

   If the attic system is cooling only, and not used in the winter, no air will circulate from the HVAC equipment to dry the duct out. That is not good.

   The only way to have the ductwork in an attic within the thermal envelope of the home is to condition the attic space just like the living space.

   Duct insulation keeps the air inside the ducts from gaining/losing too much heat.

1. 
   Clewless1 | Jan 22, 2009 05:05am | #7

   Hmmm ... well thanks for more info. I remain skeptical .... but admitedly mostly from ignorance at this point.

   If the ducts are covered by the attic insulation, they would be by definition, technically w/in the heated thermal envelope of the house ... even though the attic is not conditioned. The ducts should be warm, not cold ... at least no colder than a duct after a night of stat setback. Of course, if you are heating return air up to heat the space, the RH of that air drops dramatically, so not sure you would have a problem. I see your point, but again, if the ducts are inside the insulation layer (i.e. on the warm side), there shouldn't be a condensation issue that is big.

1. 
   danski0224 | Jan 22, 2009 06:11am | #9

   I have seen very few... make that no... duct installations where the ductwork is in the same insulation area as the home. 

   That doesn't mean that such a situation doesn't exist though.

   In my experience, high velocity duct systems are an air conditioning only retrofit to a home with some type of radiant heat. That means that the duct system is not used in the winter season, so the ducts will (eventually) reach the same temperature as the attic... and the attic temperature in traditional construction is close to/the same as the outdoor temperature.

   The moisture from that heated air is where the condensation comes from- no return air is involved because the forced air system is not being used.

   I know Unico offers hydronic coil modules for their systems.

5. 
   popawheelie | Jan 22, 2009 06:05am | #8

   I have heard brand new work whistle pretty badly. It was in a great room and they had installed one big vent in a wall near the furnace.

   There was just to much air coming out of one vent. It was slab on grade and they just didn't want to run more vents.

    

6. 
   BobChapman | Jan 22, 2009 04:18pm | #10

   I did a DIY of a Unico system; actually two of them, one for second floor and one for third floor.  There are two complete, independent systems in my unconditioned attic.  Each plenum is a  9" steel pipe in a closed loop: this helps balance the airflow so that the end-of-run isn't starved of airflow (since there is no end-of-run in a loop configuration).  I followed all the rules of minimum/maximum runs,  number of outlets per ton, as laid out in the Unico information.  I used a heat-load calculator that I found on-line to calculate the tonnage/outlets in each room. 

   Unico is good about answering questions by phone.

   I also added a supplemental electric heater in one of the systems, since the house steam radiation looked to be inadequate on the third floor.  An easy addition.

   The whole system was easy enough to do, aside from hauling all the parts to the fourth floor (attic), aside from the cramped working space, the dirt from old roofing jobs, etc.  The usual disaaster of working in the attic!

   The system works brilliantly.  All the roooms feel as though they are the same tmeperature, there's no more noise than in other people's homes that I've been in.

   Bob Chapman

1. 
   talkingdog | Jan 22, 2009 07:21pm | #11

   Thanks for the info on Unico. It seems to be popular with DIYers and I have heard that even a first timer can install the ductwork in a week or two.My system is unfortunately not a loop like yours, but rather the main duct is in a U-shaped configuration. Does your system have the advanced controller board?

1. 
   BobChapman | Jan 22, 2009 11:07pm | #12

   It was put in 5 years ago, and I have no idea what the "advanced" controller board is.