Air flow issue

[sicilian68]

I recently added onto my cabin and now have an issue with air flow from the heat pump. The house was originally 900 sq. feet with a two ton heat pump..excellent air flow and pressure in the rooms. I added onto the west end of the home for a total of 1350 sq. feet. From the plenum, flex duct was ran approx. 25 ft. to the new addition and the air flow is poor in the three new rooms. A 12" take off was added to the top of the plenum then reduced to a 10" run through the length of the house, and then "Y"s used to branch off with 6" flex runs to the three registers. A return air was added to that end of the house as well. You could barely feel air coming out of the registers. I added a 10" duct fan to try and help, which did some but the air flow still isn't acceptable. I am thinking of replacing the "Y"s with a small plenum (maybe 14x14x24), mounting the 10" duct fan directly to the plenum, and mounting three 6" take offs for the flex duct. I am thinking this should increase the air flow at the registers. Can anyone verify that I'm thinking right or have any other suggestions? Also, does anyone have a 'blueprint' of how to make a small plenum? Thanks in advance.

[skibme]

Rip it all out and restore the original system. Add a second zone for the new work. Nothing is gonna work for you,with what you have.

[TwincamDave]

You never take off from the top of the plenum if it is a verticle A/H,if it's horizontal it could be ok if it's far enough back from the end. You have added 50% more load to the system and haven't improved the system, you can't expect it to perform like it did prior to the addition.

[sicilian68]

skibme: when you say "add a second zone", how would you attach it to the original plenum or system? Or are you saying add a separate heat pump at that end of the house? Thanks.

TwincamDave: the plenum is horizontal and the take off is at the far end. The existing system still works fine (as always). I am just having trouble getting the air to the end of the house farthest away from the unit. The original house was 900 sq. ft, the addition is 450 sq. ft. so that should only be 25% more at which a 2 ton unit shoud endure (according to the company who installed the system).

[kevinnn]

skibme: when you say "add a second zone", how would you attach it to the original plenum or system? Or are you saying add a separate heat pump at that end of the house? Thanks.

TwincamDave: the plenum is horizontal and the take off is at the far end. The existing system still works fine (as always). I am just having trouble getting the air to the end of the house farthest away from the unit. The original house was 900 sq. ft, the addition is 450 sq. ft. so that should only be 25% more at which a 2 ton unit should endure (according to the company who installed the system).

I think he is saying add a separate system. What you are doing is putting a bus on top of a car and then asking why the car does not go the speed that it did when it did not have the bus on it's roof and then thinking that a different type of gas may fix the problem. OK a bus is dramatic but I am just trying to make a point.

[ampulman]

Check your math.

[sicilian68]

ok then...sounds like the general consensus is to install a separate heat pump. Anyone have a suggestion as to what size (tonage) to use with 500 sq. feet or less?

[genduct]

TwincamDave, I am not sure why you don't think it not allowed to takeoff the top?
I understand the D only shows that on the return ( or was that the Canadian Man ?) but I don;t see any reason why that would be a problem.

[kevinnn]

TwincamDave, I am not sure why you don't think it not allowed to takeoff the top?
I understand the D only shows that on the return ( or was that the Canadian Man ?) but I don;t see any reason why that would be a problem.

Taking of the top, that run will experience the bulk of the air with no back pressure to feed the other runs. Top is too much of a path of least resistance, unless that is what you want.

[GssN]

The 2 ton should be ok - a load calc will confirm this. The 12" flex is extreme for the extra room. Size the duct properly (use .08 supply and .06 return) when designing the duct. Dont forget the increased friction of flex running 25'.

This system is more than likely fixable, however, if your going to invest time to guess, reinvest that time into knowing. Lay it out - get a room by room calc done then design the system to meet the needs. Rembember, you dont have to feel air movement to feel comfortable.

Step back, study the plan and fix it (without guessing)

Duke

[sicilian68]

I like the way you are thinking Duke. Put your mind into it and use a little inginuity to get some results. This is the actual setup: the original heat pump with horizontal plenum is above the ceiling in the east side of the house. The plenum has five, six inch flex ducts coming off it to the original part of the house with one return air installed. After the addition, they came off the existing plenum at the top and end of the plenum with one 12" take off, then immediately reducing that to 10" for a 25' run to the west end of the house where the addition is located. The 10" is then reduced to 6" flex duct and is split with "Y"s to the three registers. There has also been a return air installed in the addition. The air flow is ok....but the three added on rooms are about 5-7 degrees cooler/hotter than the rest of the house. So come on...I'm not looking for miracles here, just to get a bit more constant temperature in new rooms. I'm sure someone can offer a productive suggestion. What about the idea of getting rid of the "Y"s and putting in a small plenum? Will that help increase the air flow pressure some? Thanks in advance!!!

[GssN]

Ok, Im only going to guess (but its educated)
It sounds like the new addition is requiring 300 cfm or close to it. The 3 registers are all 6". If the flex to each of these is more than 8' you can prob expect about 80 cfm each. which is a total of 240 cfm. The 10' main feeder is close, but should work - make sure there are long radius bends and as little restriction as possible. I personaly dont like tapping the end of the pleniumn, however done right it still works. There should be a volume damper at the base of the 10" connection at the pleniumn. This duct will have greater velocity and therefore should be adjusted as close to the pleniumn as possible. The 5 6" feeds will give you about 400 to 450 cfm total.
If you do the math you are at the bottom end of where the cfm of a 2 ton unit should be. I would prob consider adding another feed. but you have to see the load to know for sure.

The main 10" should be adjusted to meet the demands of the 3 end ducts. Use a triangle fitting where the 10" ends near the new addition and then tap your feeders from it. the triangle will help with keeping the static up. So yes, get rid of the y's

If when done this still doesnt work to satisfaction, then pull all flex off and make sure the main holes are properly cut to size. I have seen a 6" flex tied into a duct slice before. Going around and using proper seal methods will help as well.

Hope this helps and let me know how the end results come out

Duke

[sicilian68]

Ok, Im only going to guess (but its educated)
It sounds like the new addition is requiring 300 cfm or close to it. The 3 registers are all 6". If the flex to each of these is more than 8' you can prob expect about 80 cfm each.The flex going to them is 6". which is a total of 240 cfm. The 10' main feeder is close, but should work - make sure there are long radius bends and as little restriction as possible. There are no bends, straight shot. I personaly dont like tapping the end of the pleniumn, however done right it still works. There should be a volume damper at the base of the 10" connection at the pleniumn. There is a damper at the plenum takeoff. This duct will have greater velocity and therefore should be adjusted as close to the pleniumn as possible. The 5 6" feeds will give you about 400 to 450 cfm total.
If you do the math you are at the bottom end of where the cfm of a 2 ton unit should be. I would prob consider adding another feed. but you have to see the load to know for sure. Maybe run two flex lines to the new addition instead of the one?
The main 10" should be adjusted to meet the demands of the 3 end ducts. Use a triangle fitting (is that a ductboard triangle?) where the 10" ends near the new addition and then tap your feeders from it. the triangle will help with keeping the static up. So yes, get rid of the y's. This is a great suggestion!!! But wouldn't that be about the same as making or adding a plenum?
If when done this still doesnt work to satisfaction, then pull all flex off and make sure the main holes are properly cut to size. I have seen a 6" flex tied into a duct slice before. Going around and using proper seal methods will help as well.

Hope this helps and let me know how the end results come out

Duke

Duke,
I have added answers and so forth to your reply in red. Thank you so much for the suggestion of a triangle instead of "Y"s. This does make sense as a triangle would help hold up pressure a bit more as well as provide a more equal air flow to all three registers.Thanks for the insight!