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OT: Residential HVAC duct sizing

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Old 01-26-2002, 08:31 PM
  #1
kook
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Question OT: Residential HVAC duct sizing

For anyone who knows about home HVAC, maybe you I can bounce some thoughts off you:

I think I have a poorly designed duct system. House is 6 years old, I am adding a small addition (350 sq ft), and plan to tap into the existing HVAC supply and return trunks.

I think the HVAC contractor definately [b]undersized[\b] the main supply plenum and the main return, and made up for the poor design by providing a larger than needed oil burner and AC unit. I live in the Mid-Atlantic, house is 1400 sq ft Cape, and I have a friggin' 130K BTU (hot air) oil burner setup! My previous (and much older) house was about the same size, but was using something like an 80K BTU burner, which had to be less efficient than this one.

Dimension of the main supply and return trunks are about 10"x18" - it's not sheet duct work, but it's stuff that looks like rigid foam insulation - about 1" thick. Both the supply and return trunks run the entire length of the house, with 6" flexible, insulated pipes feeding the room registers.

My first thought was to double the size of both the supply and return trunks, I think they are choking the blower motor.

oh, and my AC is a 3 ton unit - oversized for the exiting sq footage of the house IMHO, but seems to run long enough to keep the humidity down in the summer - again, I think the unit was oversized to compensate for the poor ducting.
 
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Old 01-27-2002, 12:44 AM
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Kook - do not randomly remove your supply/return ductwork. 1st, assuming you have a squirrel-cage fan, check the full load amps (FLA) draw against the motor plate data. You should be 85-95% of that rating. Before you go any further, you should know were your at on the units fan curve. You have to maintain the velocity in the ducts to supply your registers. Velocity is determined by the volume of air and the cross-sectional area (duct size). It sounds like you have pre-insulated duct, which is fine. Are your 6" take-off's on 90 degree angles with the main trunk?, this adds considerable static pressure and can cause airflow inbalance, pretty common in residential.

The above amp reading advise assumes your unit is properly charged and your coil is in good shape.

ps. typical design for residential A/C is 1 T(12,000BTU) for every 400 FT2. Feel free to e-mail me after you take some readings if I can help.
 
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Old 01-27-2002, 03:25 AM
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How do you know your furnace is running at 130000 btu? Resi oil burners can be installed with nozzles of different gallonage, eg- A 1gallon per hour nozzle may give you 130000btu but if a .75gph nozzle was installed you may only be running 90 or 100,000. To see if your furnace is running ok you need to do a tempeture rise test, get like a cooking thermometer or some type that you can insert right in to the airstream inside the duct, drill a hole in the duct on the inlet side of the furnace and the outlet side, no closer than about three feet from the furnace itself. Let the furnace run for about 10 min and record the temps on either side of the furn. Then look on the nameplate of the furnace for the tempeture rise data. It will give you a range eg. 55 to 75degrees F is typical, If your furn falls into the stated range your nozzle gallonage is correct for the application, If higher you may have to speed up the fan speed or reduce gph or both. Lower you may have to slow the fan down or increase gph or both. Oh yeah 3 ton A/C is huge for your house. Doug
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Old 01-27-2002, 06:55 AM
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Kook, call me Mr HVAC LOL .... I am in the business only wholesale, but we do a ton of design for residential and commercial projects.

As others have said, if your Oil burner is firing at 130K, then yes it is way oversized. The 3 ton AC unit appears to be oversized as well. Normal homes range in the 600-800 square feet per ton.

Your ductwork is called "fiber duct". It is a very economical duct that when put together is insulated. Normally ducts in homes are sheet metal uninsulated.

Based on your info, your 3ton AC unit needs approximately 1200 CFM (cubic feet per minute) in order to prevent freezing of the cooling coil. Assuming that your coil does not free up when running (Summer time), it is safe to assume that your blower is producing at least this 1200 cfm. I caution you that most older oil furnaces have a very weak blower and in most cases require an upgraded blower when adding AC, so depending on what you have, it may be light.

Your main supply and return ducts sized at 18 x 10 (inside dimensions?) would give you 1000 cfm @ .1" SP. What does this mean? Well your furnace blower produces an amount of air (CFM) at a given SP (static pressure). Normal residential ducts are sized for .1"SP and most furnace blowers can deliver this air up to .5" SP. The static pressure increases as ducts get smaller, filters get plugged etc. As static pressure increases, the demand for more blower HP goes up.

Judging from what you have told, your main ducts should of been at least 20 x 10. One thing you forgot to mention is how many "take offs" are coming off both main ducts. You mentioned 6" round. How many? Each 6" round is equal to about 100 CFM. You should have at least 12 of these 6" round ducts coming off of the supply main. If you have only 8 as an example, you just increased your SP even more because they are an extention of the main.

One last thing, if you have a temperature probe, you can stick it into the return air duct near the furnace and jot down the reading, then stick the probe into the supply air duct (with heat running) and jot down that temperature. The difference between these two temperatures is called the "temperature difference" and that TD must fall in the range of what is listed on the furnace. Usually it is 45-70 degrees. If your TD is beyond the manufactures rating, you are causing damage to the internals of the furnace.

You did not mention if your ductwork decreases in size as it travels down the house. It is necessary to "transition" the duct down after a few of those 6" take offs so that the velocity in the main duct stays "up" enough to push the air down the ducts. If the main duct is 18 x 10 the entire way, the air in the end of the duct has very little velocity and ends up not going anywhere. Think of a garden hose, if you take a garden hose and turn on the water, the water comes shooting out the other end. If you start to poke holes in the hose without downsizing the hose, the water shooting out the end will just trickle. Same thing happens to air in a duct.

Your furnace and AC seem large enough to handle your addition, however your ductwork is not. Doubling the size of the mains will be worse than what you have now.

If you have a chance, list the amount of "take offs" both on the supply and return and we can go from there.
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Old 01-27-2002, 08:49 AM
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First off, thanks for the replies. I started surfing the 'Net to find my answers, but an hour's search turned-up nothing too relavent. 5 minute post on this board has proven to be a much better investment of my time.

OK, here's some more info:

140k input BTU burner, made by Weather King. Blower motor is 1/2 hp.

Nozzle USED to be 1 gph, but I had the burner checked out a few years ago by someone who I had a bit more faith in than the installation contractor - he replaced the nozzle with a 0.85 gph one - said it should make the burner setup more efficient, have longer "on" times. It's been a while, but I think I recall him saying something about the small size of the main trunks, but can't remember much more than that. I *think* he did check the temp at the inlet and at the output, and found it to be within the range spec'd on the furnace - not 100% certain, but I think so.

The 10"x18" main supply and return duct sizing I referenced was outside dimensions - so the inside must be around 8"x16". R value is 4.3, as stated on the foam duct work.

Yes, the size of the supply and return main trunks is the same down their entire length. And as one would intuitively expect, since the supply is the same size all the way down, the take-offs towards the end of the supply line don't "put out" quite as much as those closer to the plenum.

There appear to be 13 take-offs on the supply trunk - all come from the top of the duct, and immediately make a right angle, using a rigid sheet metal piece - the right angle (elbow) transitions have a rectangular input and a 6" round output, to which the flexible ducting is attached. On the register-end of the flexible feed lines is another right angle piece that feeds each register. There are 7 supply lines for the 2nd floor, and 6 for the 1st floor. The first floor of the house is pretty much one large "great room".

On the return side, there looks to be 5 returns - 3 from the 1st floor, and 2 from the 2nd floor. The return registers are 6"x12", but the return lines themselves measure more like 9"x14.5" inside. The returns from the first floor definately draw more than those from up on the 2nd floor.

Supply and return flow on the 2nd floor is what your would expect - significantly less than on the first floor. To compensate for this, I generally close off 2 or 3 registers on the first floor - this seems to keep the 2nd floor at "about" the same temperature as the first floor - I still could use some more mojo upstairs, as it tends to be warmer in the summer than on the first floor.... but I have been hesitant to close off more registers on the first floor, for fear of choking off the blower motor's airflow.

The addition to the house is at first floor level. The floor is 16'x16', and I plan on 10' walls, 6/12 pitch cathedral (sp?) - total volume of room will be about 3700 cubic feet, according to my first-cut calculations. Two (2) 52" ceiling fans will be used in the room, to help maintain even temperature distribution in the room, as I know that the hot air would stagnate up near the ridge, in a cathedral room. The addition will have a significant amount of window area (as does the house). There will be four (4) large skylights, "HP Sun" tint Andersens. All windows to be Andersen, as is the case with the existing house.

My biggest fear it that once I plumb the take-offs for the addition, the flow to the 2nd floor will peter-out to almost nothing. I haven't figured-out how many supply lines I'm going to run into the new room, but I was thinking of around three (3), with one return. There will be a 3'x7' opening between the existing first floor and the addition.

-Mark

[ 01-27-2002: Message edited by: kook ]
 
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Old 01-27-2002, 09:15 AM
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Sounds like you are getting some good advice.

See http://hvac-talk.com/vbb/forumdisplay.php?forumid=1
for more info and discussions.
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Old 01-27-2002, 09:37 AM
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I'd like to throw out a question on the HVAC answer line.

I'm putting together a budget for a home that I'll be rehabbing, and I want to install a brand new HVAC system. I'll probably need extensive duct work as well. The house is roughly 1800 sf, including a 12' X 35' finished attic space. Assuming nothing can be salvaged can you guys give me a ballpark cost to do this? I want to do it right, but not over do it, think builders grade. Any insight would be appreciated.

Regards
Mike Olson
 
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Old 01-27-2002, 09:50 AM
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Mark, your .85 nozzle is more suited for your home (still I believe overkill) but better than the larger nozzle.

With the duct dimensions as OUTSIDE dimensions, your right, the ID is 16 x 8. The 16 x 8 duct would normally be sized for a maximum of 650 CFM. As we discussed, your system needs at least 1200 CFM. The 16 x 8 duct could deliver 1200 CFM however the static in the duct is now .3".

A little lesson on static pressure (since it is Sunday LOL). Your furnace has a blower, this blower is designed to deliver a specified amount of air at a given Static pressure. Most residential blowers deliver the rated CFM at a maximum .5" SP (external). How you come up with the total static pressure is pretty easy, you add up the amount of duct you have, size of the duct, air filter, registers etc and each one of those items has a static pressure. Add them all up and you have your TOTAL STATIC pressure. Your ductwork alone is .3", filter is at least another .1", cooling coil in the plenum (wet not dry) is about .3 to .5" so you can see your TOTAL static is way over the normal .5"

This is why most duct designs are designed at .1" SP maximum because after you add up all the other items in the system that add SP, you max out your furnace blower. Most new residential furnaces are rated at .75 to .85 total SP. Your old oil is more than likely .5 max.

You are correct in stating that if you tap off of your existing system or even redesign the ductwork and STILL tap off of the existing system, your upstairs will suffer big time. Air wants to travel in the path of least resistance and the ducting, elbows etc needed to get to the up stairs is going to be way more than the ducting to the new addition.

I am also suprised to hear that the ductwork is not gradually downsized as it gets near the end. That in alone is a problem.

Don't know that I have any good suggestions for your addition, but hear are a few.

I am not a lover of Zoning ducts but you could modify your ductwork and install a 3 zone damper system to your existing system. Contractor cost on a zoning system like this is around $ 700.00. The zoning system incorporates three dampers (one for the duct to the 2nd floor, one for the existing house 1st floor and finally one for the duct to the new addition) the principal here is that each zone damper would have a thermostat. Normally you would set the 2nd floor higher or lower than the rest of the area when it is not used, same for the other levels. When a zone is not "calling" for heat or cool, the damper closes thus forcing the air to the zones that are "open". As you can see, your ductwork is going to need to be majorly remodeled in order for this to work. If the 2nd floor is "closed", your furnace and air will pump all of the air (most of it at least) into the existing first floor as well as the new addition. If only "ONE" zone is calling, the system incorporates a "BYPASS" damper that allows the excess air to recirculate.

If you decide to not go the zoning route, you could heat the new addition with some electric baseboard heaters around the perimeter and install a mini ductless split type AC unit for cooling. These itmes would have their own independent t-stats and would function seperately from the existing house.


Here is a basic Mini Split application


Here is a little more on zoning


I guess if it were my house, I would go the EBB heater and Mini split route. OR, get a half dozen HVAC contractors to stop by your house and let them give you their recommendations. I would certainly have something in writing about your concerns and how they plan to solve the comfort temperatures in each area.


Jeff
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Old 01-27-2002, 09:58 AM
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Quote:
Originally posted by olysan:
<STRONG>I'd like to throw out a question on the HVAC answer line.

I'm putting together a budget for a home that I'll be rehabbing, and I want to install a brand new HVAC system. I'll probably need extensive duct work as well. The house is roughly 1800 sf, including a 12' X 35' finished attic space. Assuming nothing can be salvaged can you guys give me a ballpark cost to do this? I want to do it right, but not over do it, think builders grade. Any insight would be appreciated.

Regards
Mike Olson</STRONG>
You guys are lucky it is Sunday LOL and I am at work LMAO. I am currently working on the HVAC design of a brand new High School that needs to be done ASAP so unfortunately I am working today because I get a lot done whent he phone doesn't ring.


Mike, depending on your rehab insulating, you may need a 100K furnace with a 2.5 ton AC or possibly a 80K. It will also depend on what efficiency furnace you are talking about. 80% vs 90%. Up here in WI, we sell more 90% furnaces than 80%.

Installed, I would think the entire project would cost about 6K, a little less if it is a 80% furnace. Assuming that the existing house is gutted including the old HVAC system.

Do you have a basement?, if not than usually they install the furnace in the attic. Or crawl space. Is it a ranch? etc....
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Old 01-27-2002, 11:26 AM
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Mike, call me, I offer one stop Alpha drive rebuilding and residential HVAC estimating and installation.
 
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