Help with improving second floor air flow

[joshhighley]

Sorry this is so long. I wanted to give all the info I thought might be useful, and also outline the various options I've thought of, although I'm inexperienced at HVAC. I'll probably call in a pro, but I want to have an idea of what to expect....

I own a 3 year old, one and a half story house that has had problems heating and cooling the second story since I bought the house a year ago. Since it's currently summer here in Kansas City, MO (current temps in low 90s), I'm currently focusing on improving the cooling aspect. The second floor stays about 10 degrees warmer than the first floor, which I keep about 78 degrees.

Since the house is one and a half stories, the second floor basically sits in the attic. The house is a basic rectangle. The stairs to the second floor are in the front middle of the house. At the top of the stairs is a bathroom at the back middle of the house. There is a bedroom to the left (east) and another bedroom to the right. Each bedroom is approx 15'x17'. The door to each bedroom is near the back of the room. Each bedroom has a single supply register at the front of the room. The bathroom also has a single supply register at the back. There are no return registers.

The gas furnace/blower is in a utility closet in the middle of the first floor, under the second floor bathroom. It has a common return on the first floor. It is a downflow furnace to a trunk in the basement that supplies the first floor. There is an 8"x16" vertical trunk that runs up next to the furnace to supply the second floor. This vertical trunk supplies what I think is about an 8"x16" trunk under the second story. This trunk runs about 12' both directions from the intersection. At each end is 14' of 6" flex to the supply register in each bedroom. The bathroom register is also supplied with about 7' of 6" flex from this trunk. All 3 registers are 4"x10".

I'm experimenting with one of the bedrooms before working on the other, since they're basically the same. As a first step, I tried a register booster fan from Home Depot. It sits on top of the supply register and turns on when it detects cold air flow. It hasn't helped at all.

The next step I'm thinking of trying gets a little unusual: Behind the bedroom kneewall, I found an unused 8" flex duct that runs from the back of each bedroom (opposite of the supply register) to a vent in the ceiling of the utility closet, where the air can enter the common return. They never cut a hole in the bedroom floor to put in the register for the duct, though. I'm wondering if completing this return register in each bedroom could improve airflow on the second floor, even though each bedroom would still have only one supply register. Currently, the return air has to go through the bedroom to the door, down the stairs (3 ft wide and not open on the sides), do a 180 degree turn at the bottom, go 13' through the living room to a large open door to the dining room, and then about 5' to the common return. I'm not sure if this is too far for a good return. On the other hand, I could also turn the unused flex into another supply register for each bedroom. I could run a duct off the 8"x16" vertical duct in the utility room that already supplies the second floor. This new duct would go about 8' across the utility room ceiling to where the unused 8" flex is that goes to the back of each bedroom. In addition, I could put an 8" duct booster fan in each of the currently unused flex ducts to help supply or return air. Would any of these options be very helpful? Is one better than the other?

My prefered solution would be to zone my current system. The second story would be a zone, another zone would be the first floor west duct from the furnace (master bedroom, master bath, guest bath), and the third zone would be the east duct (dining, kitchen, living rooms). However, I'm afraid the current duct sizing won't handle only a single zone being active. I could upsize the first floor ducts since they're in the basement. The second floor ducts are mostly inaccessible though. Even if I add two more supply registers, making a total of five second floor supply registers, would the 8"x16" supply duct be too small to handle the full airflow from the blower? I could probably increase the vertical duct size to 8"x22" and then go back to 8x16 after branching off to the duct to the two new supply registers...would that be enough? If it's close, could put bypass dampers in the basement to compensate?

The furnace is a Lennox Merrit Series, model G40DF-36B-090. This is a 3 ton unit with 88000btu heating. Per the engineering specs from Lennox: at high speed, the blower does 1295cfm at 0.50 in.w.g. With a zoned system, I could reduce the blower speed to compensate for the smaller duct capacity of a single zone. This is done on the circuit board and can reduce the blower to 1200cfm, 1055cfm, or 900cfm (all at 0.50 in.w.g). I suspect this would really hurt efficiency, though, and could be a problem if 2 or 3 of the zones are open. Thoughts?

Thanks,

Josh

[t527ed]

if the 8" flex ducts are already drawing return air i would cut return boxes into knee wall as high as you can.
have you tried closing downstairs registers down some to push more air to 2nd floor?

[core]

Josh

The first and most important step is to perform a LOAD calculation. A complete study of the building envelope and construction is what you first need to correct any possible problems. To get this it is recommended that you call a qualified HVAC company in your area. Sure there are some basic things that can possible help like adding returns in each room, zoning, and additional supply ducts. But until you know what the Load calculations are in each room anyone is just guessing. Keep in mind that the rated CFM you see is at a predetermined ESP. Your duct system has its own operating curve, which has to be calculated, and rated to the system fan curve. Also keep in mind you have a furnace, any alterations must be inline with the units "Temperature Rise" across the exchanger.

Your best bet is to call a contractor.

[Gunnery Sergeant Hartman]

3 words...

zoning, zoning, zoning...

[joshhighley]

core - I figured I would have to call a pro eventually, especially if I want to consider retrofitting with a zone system and need all the calculations ran. At this point, I'm looking for some educated guesses for my various options. I'll probably try roughing in the unused flex as a return duct to see if it helps before even considering cutting holes for registers. If the stairway path should be providing adequate return air flow (?), then I might even try roughing in the other supply register.

Yellow Dot - I like those 3 words as well, but it's not an option if the concealed second story ducts can't handle it.

Thanks,

Josh

[Gunnery Sergeant Hartman]

Unconceal them or problem will remain with alot of spent energy trying to fix...

[dash]

Simple test,uf the upstairs can be cooled satisfactorily with the doors left open,It's a return duct/Path problem.

If leaving the doors open doesn't solve the problem,adding a return won't either.

Man J, By ACCA,to determine the supply air needed,and a way to return the air.No return,little or insuffient supply air to a given room.

[core]

Josh

If installing the return duct is an easy job then yes do it. Return air, to me, is more important then supply air clearly a fan can only put out what it gets in. Also you may want to check your supply diffusers on the second floor. Cheap diffusers create excessive drops in pressure. Look for diffusers that have the greatest FREE AREA.

[core]

Originally posted by dash
Simple test,uf the upstairs can be cooled satisfactorily with the doors left open,It's a return duct/Path problem.

If leaving the doors open doesn't solve the problem,adding a return won't either.

Dash

I have to disagree with you here. If your Return air ducts are restricted/ or pressure drop is to high opening the doors is not the same as adding a return in each room nor will it yield the same results.

with all respect;

Core

[joshhighley]

I've been leaving the doors open since that's the only return path.

I think I'll try roughly hooking up the return vent first. I don't want to cut holes in the carpet and floor until I know what works. I can get to the return vent in utility room ceiling from behind the knee wall on the second floor. The new return vent would be very near the access hatch to the knee wall, so I can hook up the flex and pull it through the access door temporarily. I'll put plastic over the access door to seal it up with the flex running through it. It will probably only take about 6' of flex so I won't be out a bunch of money if it doesn't work.

I honestly don't expect the upstairs to ever be cooled/heated very well unless I zone it, though. Per core's advice above, I'll call a pro and ask for their evaluation.

Can anyone venture a guess if the currect ducting could handle the full blower if it's the only zone open? There's duct sizes and lengths in my original post.

[wallynut]

Heat rises,return air is critical for second floor.

[rimek]

RETURN- You need it upstairs to ventilate properly.

[t527ed]

do NOT put returns in floor. you want to pull your return as high as you can on the knee wall. if you do not the cold air from floor supplies will just get sucked back thru returns

[pipedope]

Try setting your fan on the thermostat from auto to on constant air circulation will decrease the temp diffence by half in some cases. Downside will increase humidity in home slightly during off cycle. This is the cheapest thing to try and does not involve cutting holes.

[bmaec]

I also have a 1 1/2 story home with the same problems, I installed ceiling fans upstairs have them running on low setting. You have to have a return path to get good air exchange. I closed some of my supplies to the first floor, forcing more air to second floor and also have booster fan at 2 of the second floor registers. The downstairs room that has the T-stat is one of the rooms were I closed the one of the supplies. I also blocked part of my return air grills downstairs, to pull more warm return air from upstairs, but you have to watch your system very closely if you do this as not to freeze up your evap coil in your inside unit!! The best thing would be to do zoning, but this was the best for me til I can afford to change system out. This has helped alot, my setting on the t-stat is now set higher, my temp between upstairs and downstairs is more balanced. Maybe this would help you.

[core]

Josh

Let us know how you make out. One thing for shure I believe those 6" ducts in the bedrooms are to small for a 15x17 room. If the guy who installed it used a friction of .1" w.c. for the ducts that would mean that the 16x8 would carry about 710CFM and the 6" ducts would carry about 110 CFM @ .1"w.c. friction. A rule of thumb that people use when there not doing a load calculation is 1 CFM for 1 Sq.'. So a 15x17 room would have 255 Sq.' and would need, by the square foot method, 255 CFM. If using .1" W.c. loss you would need a 8.5" duct for one room. This is only a rule of thumb around here in NY, it does not account for solar gain or heat transmitted from unvented attics. It is ony a ballpark number that some guys use.

Good luck

[dash]

Originally posted by core

Originally posted by dash
Simple test,uf the upstairs can be cooled satisfactorily with the doors left open,It's a return duct/Path problem.

If leaving the doors open doesn't solve the problem,adding a return won't either.

Dash

I have to disagree with you here. If your Return air ducts are restricted/ or pressure drop is to high opening the doors is not the same as adding a return in each room nor will it yield the same results.

with all respect;

Core


Sory to disagree,but a "return path"(doors open,is the same as a reyrn in the room,assuming everything else is szed correctly.


Check Man.D,Florida building code,etc.


With resspect,also.

[dash]

1.Location of the return,floor versus high sidewall,has little effect on the perfromacne or comfort.


2.Friction rate ,is not something you <pick,ie can't use .1 FR to design,Friction Rate is determined using Man.D,by ACCA

3.To do it right,get a Pro,they need to do a Man. J,S,and D,and rework of the duct system based on the results.


[Edited by dash on 07-16-2005 at 02:15 PM]

[joshhighley]

Core - I'll keep this forum posted, but what you said worries me a little. I suspected a single supply vent per bedroom wasn't enough. I'm wondering if another supply vent at the back of the bedroom, opposite the door would help a lot. Unfortunately, it would exclude putting in a return vent. The more I think about it, the more a return vent near the ceiling seems to make sense. In my inexperienced HVAC mind, I would think the cold air is coming out of the floor vent, traveling along the floor, through the door, and down the stairs. There's nothing to bring the cool air up to head/shoulder level. Time will tell....

Thanks for all the helpful replies; feel free to keep posting recommendations.

Josh

[dash]

Manual D ,by ACCA,the ANSI approved stanard for duct design of residential systems.


Section 1-4
"Since the return inlet location (high or low) has a negigible effect on the air motion within the room(refer to ACCA Manual T,section 7),the return openings should be placed at positions that are compatable with the location of the equipment and the duct runs."


"The air motion within the occupied zone depend on the performance of the supply air terminal."


The supplies have velocity and create motion in the room,returns do not.

Check your existing returns ,how far away from the return grille do you feel an an air pattern?

Now check a supply,the air pattern of the supply is much greater and creates secondary air patterns!

"The amount of air that is associated with the secondary air pattern is about 10 to 20 times more than than the supply air CFM"


The hot air rises cold falls ,sounds logigal,but it's just "hot air".

[t527ed]

1.Location of the return,floor versus high sidewall,has little effect on the perfromacne or comfort.





if you don't mind crawling around the bedrooms and sleeping on the floor

[dash]

Think of Manual D as Ripleys,





























believe it or not.

[dash]

Originally posted by dash
1.Location of the return,floor versus high sidewall,has little effect on the perfromacne or comfort.


2.Friction rate ,is not something you <pick,ie can't use .1 FR to design,Friction Rate is determined using Man.D,by ACCA

3.To do it right,get a Pro,they need to do a Man. J,S,and D,and rework of the duct system based on the results.


[Edited by dash on 07-16-2005 at 02:15 PM]


Above is what needs to be done.


The floor grilles need to be the type that sends the air stright up ,if they are sending the air along the floor,check the temper ature at floor level and see the difference this causes.


Look at http://www.hartandcooley.com # 210 is the type you need.


It does sound like the grilles and flex duct are bothe too small,have the above done by a Pro to know for sure.

[joshhighley]

An update and question:

I hooked up the 8" return duct like I mentioned in my past posts. It vents into the top of the furnace room with the common return. I ran it through the crawl space door in the bedroom closet and slightly out the closet door. I taped plasic around the crawl space door to stop airflow to behind the knee wall, which is open to the vents in the roof overhang. Basically, it hasn't helped much -- if any. If I close the bedroom door, I can definitely feel the air going into the return duct. If I open the door, though, I can't feel anything. I also removed the supply register vent to get the most air flow possible. Quite a bit of air comes from the register, but it's just not enough in a second floor 14' x 16' bedroom.

After looking at the second floor a little more, I realized the common return on the first floor is directly below the closet I mentioned above. This closet also shares a wall with the hallway at the top of the stairs, so I could add a common return to the second floor without a lot of construction. This would let me add another supply register to each bedroom and a common return if necessary. If I'm able to zone the upstairs, wouldn't this upstairs return help keep the zones more separate?

Also, my furnace (Lennox G40DF-36B-090) has a four speed blower which is set by changing the motor wires on the control board. If a single zone couldn't handle the full blower speed, I was thinking some relays could be wired up to adjust the blower speed depending on how many zones were active. It would take some thought, but I think it could be done with the proper sized relays, although I doubt a HVAC pro would be willing to take it on. My only concern would be if the motor taps are changed while the blower is running. A separate micro-controller might be necessary to govern all of this. I'm a software engineer with a computer science degree so this kinda excites me (pathetic, I know). I realize the blower speeds weren't meant to be changed automatically with relays on the motor taps, so what are your thoughts?

I just changed departments at work, so I haven't had the opportunity to call in a pro for advice -- hopefully I can next week.

Thanks,

Josh

[Gunnery Sergeant Hartman]

With zoning the airflow must be sized to handle the entire full load of the structure.
All that needs to be done is to add a bypass duct large enough to handle the airflow of the largest zone to be bypassed.
All that relay sounds like a mess waiting to happen.
Your going to hurt your brain thinking too much.

[Edited by Yellow Dot on 07-26-2005 at 06:01 PM]

[joshhighley]

My thinking with the relays was that when all the zones are active, the blower could be at full speed. If one or two zones are active it could be at a lower speed -- predetermined by how much the duct work will allow. Yeah, I'll admit it's a hack, but it seems to be a legitimate hack. Basically turning a "set it and forget it" four speed blower into an automatic adjustable speed blower. At the very least, it presents an interesting problem solving excercise to figure out the relays :-)

[core]

Josh

Forget the relays thats old technology, there is a product called "Fan Handler" it will modulate the speed of the fan based on a static setpoint you set.

Core

[trane]

You are not going to help your problem unless you get more supply air upstairs. You have to add a zone damper or manual damper in the 1st floor ducts to force the air upstairs. How much you need upstairs we don't know and maybe the existing supply duct to the upstairs is large enough but your 3 six inch runs is not. Forget the return because it will not help.

[joshhighley]

Trane - I can put an additional register with 8" flex duct to each bedroom without a lot of trouble. Without zoning, though, I'm afraid there will still be too much of a temp difference with the first floor. I'm hoping I can combine the additional registers with zoning and get the result I want.

Core - this fan handler sounds interesting. Can it be used on a blower like mine? If I remember correctly, the blower has four wires. Depending on the desired speed, a certain wire is common, a certain wire is "hot", and the remaining two wires go to two remaining posts.

The relay option would take to many to do it purely mechanically. I'd have to use a controller with inputs from each zone and outputs with relays to connect the motor taps with the appropriate posts on the control board. A fan handler sounds much easier.

Thanks,

Josh

[joshhighley]

One or two people requested I post updates, so here's the latest: I finally got time to call a pro. It was a Premier Lennox Dealer I've used once in the past. The owner himself came out and looked things over.

To recap, the second floor is suppied with a 7"x15" ID rectangle duct that runs vertically next to the downflow furnace in the first floor utility room. There is then a rectangular horizontal trunk between first and second stories that feeds a 6" flex to the bathroom and 6" flex to each of two bedrooms (total of 3 supplies each fed by 6" flex from horizontal trunk).

Unfortunately, he didn't do any Manual calculations. He used a slide rule and said the vertical rectangular duct could supply about 550 cfm (I don't remember exact number, but it was mid 500's). He said each 6" flex could supply 110 cfm. Since there's a closet on the second floor directly above the furnace room, he recommended running 8" flex from the vertical rectangular duct next to the furnace, along the top of the furnace room ceiling, up to the second floor behind the knee walls, and then tee off to two 6" flex ducts. Each 6" flex would then go to a bedroom for an additional supply. He said an 8" flex could supply 240 cfm so could feed two 6" flex ducts. Is this last sentence correct? I knew immediately he was guessing at the static pressure for his calculations.

He also recommended extending the common return in the first floor up through the second floor closet and putting an additional 6"x30" return grill high on a wall the closet shares with the central hallway at the top of the stairs. He said this would help pull the cold air off the second floor in summer and recycle the heat that goes up there in the winter. This *sounds* good since it gets quite hot near the second story ceiling in summer and winter.

I asked a few times about zoning but he didn't say much. He said my blower would move about 1400cfm so I asked about all the bypassed air since the second floor duct would only handle about 550cfm. I was concerned about freezing up the coil in summer or the exchanger getting too hot in winter. He said he could install a relay to reduce the fan speed when AC is called for in only one or two zones. The blower would run at the "heat" speed, which is med-hi, as opposed to current "cool" speed of hi. He didn't really push the zoning and because of too small second floor ducts, I've basically ruled out zoning. But, as he said, the additional supply registers on the second floor are needed whether I zone or not.

Even though he didn't do the calculations that were probably needed, I think I'm going to try the additional supplies and return. For the supplies, about 15' of 8" flex will have two 90 degree bends before the tee to 6" flex. Each 6" flex will be basically straight with a 90 just before the supply registers. One 6" flex will be about 12', the other will probably be about 18' - 20'. Thoughts?

If there's a local pro here willing to do the necessary calculations, I'll obviously pay for your time to visit my home. I'm in Independence, Missouri (suburb of Kansas City for non-locals).

[tinknocker service tech]

trunk handles 550
6 in flex 110
110 times three is 330 leaves 220
two more six in runs from the trunk line is 220, total 550
with out being there and doing a load cal
now if you damper down the down stairs trunk and increase the static the air will flo to the second floor trunk and improve the flo also. take the 8 in flex you found in the attic and make it a return for the second floor and pull some of the heat out. cold air will not over come hot air. you will need to pull it back to the coil where it can be exchanged. before you spend all kinds a monie on this try this simple repair and see where you get

[joshhighley]

I've tried using the 8" flex as a return, but it didn't help. With the doors open, no air was going into the 8" flex return. I think extending the common return could help though, since the air path will be bigger, straighter, and more direct than the flex could be. I tried closing the downstairs registers slightly, but again, it didn't help. As I read many times on this forum, closing downstairs registers doesn't usually help much and can damage the blower. With so few registers upstairs, I think I'd have to force a whole lot of air to make a difference. There's already a good amount of airflow out of the existing upstairs supply registers -- there's just not enough of them.

Thanks,

Josh