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OT: Residential HVAC duct sizing

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Old 01-27-2002, 02:45 PM
  #11
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Kook-Sounds like you are getting some good info from here. Most posts are correct regarding sizing of the supply fan. Ductwork should be sized at .1" s.p. on the supply side and .08" s.p. on the return side. I wouldn't be suprised to see .75" total static once you add the ductwork, elbows, fittings, unit, DIRTY filter, diffuser, etc. For residential applications, I size the equipment using load calculation software. Most contractors use rules of thumb which are good 75% of the times but do not take into account things like the sensible heat ratio of you home, number of people, electronic equipment, glass u-values and shading coefficients, infiltration, etc. All of which play a role in the proper sizing of an HVAC system. There are good contractors out there that will run load calcs for you but that is usually beacuse they have to submit them to the building department in order to get a permit. What ever you decide to do, please do not just double the size of your supply and return duct runs. Your duct system is what is commonly referred to in the HVAC world as ductboard. Typically it is around 1" thick and is taped together. Really makes for a piss poor installation if not done correctly. This is the same stuff that exposes the airstream to the insulation and thus causes both air borne contaminants as well as mold, mildew, and bacteria growth on the inside (and not to mention is a major factor in IAQ litigation). All of which are obviously not good for human consumption. Without a major outlay of cash, you will end up with ductboard. Demand that the ductboard has an anti-microbial coating (black stuff) on the INTERIOR surface and ask the installing contractor if he will do a duct pressure test while you watch. If he knows you are interested in getting a quality installation and are willing to go to the lengths of requesting a pressure test, you may just get a good installation if you are lucky. One last thing, ducts that are sized using constant pressure method like everyone is talking about here typically do not work real well with constant sized ducts. Good airflow distribution is a function of velocity pressure and static regain. You will only get good performance with constant sized ducts with tedious test and balance. Your typical residential contractor will NOT give you tedious test and balance because it takes skill, knowledge, and time. Hope these recommendations help. Once you get a contractors recommendation on the system upgrade, let all of us know what it is and we'll let you know if this guy is on the mark or full of BS.

Mike
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Old 01-27-2002, 03:26 PM
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kook can you tell you found some HVAC folks out here! No fair guys. I was posting at Midnight half s-faced

Big Mike you're right on. Most Mechanical Contractors do not have the experise or time to custom layout every system. Unfortunately, residential units utilize the "it'll work O.K." philosphy. Builders in my area will go out for bid and almost always select low-bidder for a new subdivision. The homes can be from 1800-2400 ft2, and they all get a 4T split system. I guess from their point of view it makes them the most profit, and most folks don't know the difference.
 
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Old 01-27-2002, 03:30 PM
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O' yeah - HP500 you take Q=VA, Big Mike you take wet bulb/dry bulb, and I'll take the mean global stress temperature factor.
 
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Old 01-27-2002, 04:00 PM
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Thanks HP500. That's exactly what I needed. The House should be pretty tight when I finish, I'm installing new insulation in all exterior walls, and the attic, thermal windows, and vinyl siding. The furnace will go in the basement.

You guys really know your s*it.
 
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Old 01-27-2002, 04:20 PM
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Is the furnace in the center of the basement with the ducts going toward each end of the house? A good rule of thumb on 8x? duct is 2 inches of duct for every 6 inch heat run plus 2 inches for duct loss. This works untill you get into large houses with alot of registers. If the addition has a lot of glass and 3 sides are exposed to outside you will have a temperature imbalance with the rest of the house. It will be colder in the winter and hotter in the summer than the main area of the house. A damper system will solve this problem. Adding more heat runs will not-it will only get the room hotter or colder while the system is on and you will have a tremendous temperature swing.
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Old 01-27-2002, 04:29 PM
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I have been in the HVAC business for 25 years for the Univ ot Texas. Call me if you want some advise. 512 657 1104. But I will be busy for the next couple of days, so call in the evening.
There are some factors which many ignore relating to location of outlets and location of return intakes which cause comfort and noise problems.
Also, we do not use any lined ductwork or ductboard as it aggravates IAQ problems.
If the system is desined correctly, the balancing is much easier. I will give you some hints on this matter.
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Old 01-27-2002, 04:43 PM
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Tape together ductwork!!!??? WTF? I have never heard of that!!!!
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Old 01-27-2002, 07:09 PM
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No, the heater is on one end of the house (even I knew the best place was at/near the center). I downloaded some HVAC calculator, but it's got a ****load of input variables.

I'm going on a limb here, but the guy who put it in probably didn't have much, if any, formal education in the area - "learned by experience" - something to be said for that, but it can also be a bad thing if his teacher was showing him all the WRONG ways to do a job.

This has got to be a pretty simple thing - I mean, the system works "OK" now, and was installed by less-than-competent people... how much worse can a engineer (myself) do?

I went through a similar fiasco (sp?) a couple months back - ever since I had the house, I had low water flow (good static pressure, but piss poor pressure when only a single faucet was running) - contractors were telling me to get a pressure riser pump, other contractors were telling me not to get a system like that because the pump would cavitate, blah, blah, blah..... I just did what was intuitive - I had a bigger line installed (was 450' of 3/4", is now 450' of 1.5" - problem solved with common sense). $1000 invested (about what a pressure riser pump setup would cost) and I have all the pressure and flow I could want!

Can't this be resolved with some basic "rule of thumb" guidelines?
 
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Old 01-27-2002, 07:49 PM
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Your ducting is way too small for 13 heat runs. That duct is only good for 7 heat runs. I would use it for the upstairs heat-just remove the downstairs runs and install a new duct for the main floor heat runs. Your return air ducting is probably way too small also. Have a heating company come out and design what you need. Don't let them send out a service man-ask for the ductwork layout man to come look at your system. Poor air flow will destroy your furnace and your a/c.
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Old 01-27-2002, 07:53 PM
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You may be able to simply extend on to your present system for your addition and add a duct booster to get you some extra cfm in that area. A duct booster is a small fan that is installed inside the main duct to boost cfm and is wired into the fan circuit so it only comes on when your main blower is on, they can be had for about $50. Saved a duct job in my place. I am a field troubleshooter for a respected hvac company. Sure we have the knowhow to solve any airflow problem any customer may have, unfourtunatly there is usually only one problem.....money. Far more often than not a major duct refit is not in the cards financially. Hey kook you're not alone, I go into brand new 3-4000 ft homes that have some serious duct problems and the cust thinks I am going to walk in and wave my magic wand and everything is going to be fine.....NOT. But sometimes we can throw a couple of duct boosters in and get some xtra cfm where the cust needs it (typically those huge masters and ensuites) without having to rip their house apart for major $$$$
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