Duct Fan Install (HVAC problem)

I here a lot of negatives about duct fans in this forum but let me ask about my particular situation... My house is shaped like a giant "C" the top and the side parts of my house heats and cools properly because there are ducts very near the output of the funace going to these rooms . On the bottom leg of the "C" are the kids rooms and the den, this part of the house is constantly cold in the winter and hot in the summer. At the bottom corner of the "C" is the large return input vent ... the tempeature beyond that point is always a problem. I think its because of the exteremly long 25'-30' foot duct (that ends with four vents in 3 rooms) . The air pressure out of these vents is VERY low (much less than manual J specs). The input of this long run is a plenum box. I intend to put the fan line (near the end) of this long run.

Should n't this help ? I don't think this is a return problem ... ANY IDEAS WELCOME....

 

Not sure just what your home is like. But sounds like the duct should be reworked. As you said on fans in ducts .I have not found one that works. If the duct is wrong then a fan in it dont help. Is your blower on high now??

 

Yes it's on high.... in fact in the front rooms you can hear and feel the air flying out of the vent. I can definitely say this is a poor design because others with the same home design are having the same issue.

If the in line fans increases the cfm in that area I would think that would help at least ?

 

I agree with Ed that these inline fans are mostly just wishfull thinking.
They will increase airflow slightly but their basic design is the cause of their downfall.
The performance curve of propeller fans which these things use favor higher volumes of air at relatively low static pressures.
Their design works best with minimal restriction like portable fans.
Squirrel cage fans like those found in furnaces are the type that are meant to force air under high static pressures.
Unfortunately the physical size of these do not lend themselves to be a easily installed.

In a sense based on ads I've seen for these vent fans it could be considered false advertising to say they will solve the problems they claim..
They will increase airflow but depending on how high the duct static pressure is you may only see a few percent increase.
Based on what you are saying you may have to at least double the airflow to have any effect.
This surely would not happen with a small prop fan.

 

Thank you for your response... you understand I'm trying the easy way out. The last gentleman suggested duct re-work.... Is that what you think ?

 

If its that bad then for sure you need a new duct work setup so it works right. Or might be you need two units in the home. We fine that that is the best way to go in most large homes and 2 story homes

 

Increased duct size to the furthest reaches of your home was something that should have been done when the house was built.
Duct fans are not workable IMO but there may be a few things you can try.
It may be possible to deliver the correct air through the existing ductwork.
When duct sizing calculations are made they take into account how quiet the system should be.
It may be possible to increase the airflow to the far end but at the expense of airflow noise.
If you were to close balance dampers in the areas that are currently warm/cold enough you could try to force more air to the far end.
You don't describe your furnace but along with setting the balance dampers if you have them, increasing the fan speed will help overcome the higher static pressure.
If you can find the specs on your furnace there will be a performance chart that will tell you what your current fan/motor set-up is capable of.
You may have some wiggle room with the current fan to either speed it up or change it out.

 

I'm going to check if the second tech I sent up didn't totally remove the damper thats directly in front of the furnace . I'm thinking if I totally cut off the air to the front of the house it might make a difference ( as you suggested). I'll have to get some type of servo control to bring them damper control down from the attic. I didn't think they could construct a house unless it met the Man. J requirements but I sure do have one. I'll describe the path to the fathest vent:

Starts off front of the furnace making a 180 degree turn in an 18" pipe. then travels 20 feet (flex 18") to a 4 port box (plenum) an 8 inch flex travels 25 feet to a box that send 4 4" flexs to the individual rooms.

A second unit seems awful expensive to install and operate. My house is single story 2500 sq. ft.

 

An 18" pipe can move a fair amount of air if not pinched.
You say that the 18" first travels to a 4 port box(plenum), then in a 8" pipe to another box where it then splits four ways.
What else is connected to the first four port box?

The 8" that feeds the four 4" could be a bottleneck.
One other thing I forgot to mention is that you can do a very rough test to see if the return air entering the furnace fan is adequate.
While the fan is running you could loosen the door to the fan compartment to see if there is a lot of suction there.
If the door is hard to remove and when you have it fully open you hear an increase in airflow you may have return probs as well.

 

Im with GregH check the return and see if its ok .

Starts off front of the furnace making a 180 degree turn in an 18" pipe. then travels 20 feet (flex 18") to a 4 port box (plenum) an 8 inch flex travels 25 feet to a box that send 4--- 4" flexs to the individual rooms. That 8" is good for 200 cfm. The 4-- 4"only lets out about 176 cfm

We dont even carry 4"pipe in the shop. The book say its good for 44 cfm only. We use 6 " even to a small bath . you can always close it down some if you have to.

"I think its because of the exteremly long 25'-30' foot duct (that ends with four vents in 3 rooms) . The air pressure out of these vents is VERY low (much less than manual J specs)."
I cant see how a 4" pipe can take care of a room. You dont say what size AC you have there??? That 18" pipe should carry 2.5 ton at 1000 cfm

 

Details

I had another guy up from a reputable (but expensive) company try to figure out what is wrong . He thinks I need to damper off the whole front of the house (thats warm and cozy) move the thermostat to the back of the house and hope fully that back of the house will get more air volume and velocity with the correct temp. The second guys ripped out the damper that was there, saying it was impeading air flow !!!! I dont think the damper was doing what I think the original designer/builder intened which was to balance the front with the back . When I first went up, the action on the damper was sloppy and the first guy tightened it up and we tried it in the "open and closed " postions with very little if any changes in the flow in the back . I do remember when the damper switch was in the closed position,I heard alot of air noise in the fron registers. The front had the damper system in a fiber duct box. I might have to be reconstuct it... (ugh!)

For $250 I plan to buy a remote termostat unit that I can place anywhere I want. And and damper will be remotely controlled. It will be cheaper (and more efficient) if I use one damper in the duct box but I did think about buying 3ea. 10 inch remote control dampers ($50- $35) which would cover all the branches in the front of the house.

The drawbacks to this plan ( adding the 3 dampers) are (1.) adding the three dampers and learning to use the tape and mastic gunk. (2.) Spending $500 on a plan that might not work.

I think my 58RAV095-16 is big enough for 2600 square feet (is it ?) @ 92000BTU. My gut tells me that all the ducting (all flex) in the back of the house is costing my system a >35% flow degradation penalty.

So here are my questions:

What do yall think ????

 

Is all this duct in the attic ????Or why flex pipe????

 

The notion of adding a damper to force air to another location is a bad fix for what is likely poor duct design.
The remote thermostat will also not solve your problem. All it will do is cause your furnace to run longer and shift your problem to having areas that get enough airflow to over-heat/over-cool.

The solution to your problem could be quite simple. Increase the 8" pipe off the 18" to at least 14" and change the four inch to each room to at least 6".
If you continue persuing a damper solution you might find that the blower on your furnace may not be capable of producing a high enough static pressure to force adequate air through the undersized ducts.

 

To Ed

Yes its in the attic. I guess they thought (flex) was cheaper . A rep from the original builder will be out friday. I don't think he'll do much but I'll ask why they used flex. Everyting I've read abot flex says it should be used at a minimum and that it's in efficient. So what do think about the plan ?

To Greg

Even though it got down to a record 25 degrees last night my furnace was off and on 8 to 10 times an hour. I know that has to be bad. I'm comfortable (65 @night) but my kids ( 55@night ) are freezing !!

Guy number three says that the sensor (thermo unit) should be near the intake and right now it's 30 feet away from it. At the end of the 18 is a plenum (duct box) and out of that you recommend I use a 14" ... should it be pipe ?.

If the duct work gets redesigned the furthest room is still going to almost 60 feet away. Can the furnace do the job ? How many tons is it ?

Many Thanks

 

If you answer my last questions I'll go away !!!!!

 

We don't want you to go away, we want to help you figure out your problem.

I think that even with us giving you our best guess as to what you should do we can not see what you have.
I looked through the posts and maybe I missed it but I can't see if you mentioned where you live. It would be impossible to say if your system is sized properly.
A clue might be to guage the furnace's size compared to the heating load by how long it runs in the coldest weather.
Even that guess at 14" round to supply 4 - 6" ducts to the bedrooms may not be right..
You say you have 18" to a four port plenum then an 8" flex to another plenum where it splits into 4 - 4" ducts.
What else is connected to the first four port plenum off the 18"?
At this point you should have enough volume to supply 4 - 6" ducts.
If there are other branches off this first four port box you may not have enough air to supply the bedrooms.

Regardless of what we come up with here you will still need to find someone who is competant at sizing ductwork.
You may do well to get a sheet metal company who only does sheet metal and preferably does commercial work.

 

Guy #4 just left.

( the person I should have originally brought out) . Said my home was built to have the dual zone option installed . For some reason that option was not installed. It basically will make my house in to a dual zone( back and front) two thermostat single furnace house. He garaunteed that this is the solution that will make the back of the house comfortable. He admits that the front plenum was still originally designed improperly. It didn't help when guy #2 gutted the front plenum box. He said he could have done the dual zone thing (with motorized dampers) for $900 now with the plkenum box destroyed it's going to be another $600.

$1500 to fix the 10 degree difference and my screw up... sounds bearable.

What do you think ?

 

You need to put your faith in someone.
If his diagnosis sounds reasonable then he could be your man.

 

Did he say anything about the 4 " supply pipe you have here for runs to some registers??????

 

No he did not. But before I pay or sign anything I'm going to try to get a performance guarantee. He's supposed to call today ... I'll askabout the 4" .