another second story duct question

[grizzzlle]

Had a new two stage heat pump installed last year. As part of the install I had new ducting installed to my second floor. We found a path for the new ducting from the basement through a closet to the second floor. After the work started on the install of the new ducting it was discovered there would be problems getting the required size trunk through a small section of the basement. The rest of the ducting went according to plan for size of pipe. Fast forward a year and the system is working great but the second floor is barely keeping up with the main floor concerning temperature variation. If I leave the fan running continuously there is no difference in temperature. My question is for the times I dont have the fan running could I have my ducts that are passed the choke point increased to help makeup for the restriction in the trunk at the one choke point? Just wondering if reducing friction farther down the pipes help with problems in other areas of the supply.

[Gib's Son]

Air flow will take the path of least resistance. Reducing friction loss anywhere in the system should result in increased air flow but you will need a way to reduce air flow to the outlets with the least resistance to force the air to the ones you want more airflow. I assume your registers have balancing dampers?

[grizzzlle]

If when you say balancing dampeners you are refering to the kind of registers that you can open and close then yes.

[Gib's Son]

It's hard to say how much of an improvement enlarging your ducts would yield. It all depends upon what your static pressure is now and what the final static would be once the ducts are enlarged and you close down others.

Have you tried closing some of the registers down nearest to the indoor unit? Don't close them off, just close them off a little and see if that helps.

[motoguy128]

I fidn balancign at the registers only works if you have fairly low airflow (low velocity) in the ducts already. If it's high at all, it jsut gets noisy.

I woudl first try getting some lower restriction grills. They cost a lot more than the cheap stamped steel grills you normally see, but flow a lot more air. You can even remove the damper altogether. The second is to add dampers on the branch ductwork near the main trunk on those branches that have the most ro too much airflow. IF some supplies are short on air, it means others have too much.

I balanced my home this way with excellent results using both lower restriction grills and adding a damper to one branch that had too much flow.

[Gib's Son]

I fidn balancign at the registers only works if you have fairly low airflow (low velocity) in the ducts already. If it's high at all, it jsut gets noisy.

I woudl first try getting some lower restriction grills. They cost a lot more than the cheap stamped steel grills you normally see, but flow a lot more air. You can even remove the damper altogether. The second is to add dampers on the branch ductwork near the main trunk on those branches that have the most ro too much airflow. IF some supplies are short on air, it means others have too much.

I balanced my home this way with excellent results using both lower restriction grills and adding a damper to one branch that had too much flow.

Preaching to the Choir here.....quality grilles. I NEVER EVER install cheap stamped grilles and ALWAYS install balancing dampers at each takeoff. Yeah it cost's more, but in the end it's worth it. As the saying goes, never try and save the owner money.....his savings results in your loss.

[grizzzlle]

Thanks Gibson and Motoguy for your thoughts. I know the grills have been upgraded. When the install was first done cheap grills were used but replaced before the install was complete. I think the cheap ones were used to keep my boy from putting stuff in the holes. It was amazing to compare the cheap ones vs the ones that are currently installed.

As for pressure within the system I am not sure what it is running at but I know it was checked to make sure it was not to much. I really have not messed with closing off or adjusting registers on the main floor. Im afraid of causing to much static pressure and from reading this site it does not sound like that will be good if I do to much. Im thinking I might ask the installing company what it would cost to increase the size of the ducts when they come out for a spring service.

[motoguy128]

IF they are the heavy duty Hart and Cooley or equivalent floor registers, if you want to reduce restriction further, you can remvoe the damper assembly and it basicalyl becomes the same as a floor return grill. I'd say it increase airflow by another 10-20%. Might be worth a try if nessesary.

On the wall registers with moveable vanes, keep the vanes as stright as possible and you will increase flow a little bit too.

[Shophound]

Increasing the duct size downstream of the restriction might reduce pressure drop due to friction, but it could also slow the velocity of air emerging from the second story supply registers. Additionally, the reduced friction might be offset by increased friction at the restriction due to less friction downstream (more air is trying to get through the restriction since there is less resistance downstream)...in the end it could be a wash and money not well spent.

Is there any way to optimize or modify the choke point so it is not as restrictive? How exactly does the duct pass through this point? Is it merely flex duct crammed through a hole in the construction? Or hopefully some form of sheet metal transition that might pose less of a pressure drop?

[grizzzlle]

Thanks for your input shophound. All the trunk in the basement and up past the main floor into the space behind the wall on the second floor is custom made metal square duct. The runs from the trunk to all the regesters is flex and they are short runs. Unfortunately the choke point is in the basement caused by the design of the floor joist in one small area. After the area that chokes it down the installer put the duct back to the size of pipe planned. I cant do much about the choke point without reworking joist or moving the ducting itself. The wife has said no to moving the ducting since it would interfere with her future plans of upgrading the basement. On an initial phone call to the installer it sounds like he agrees with you that up sizing the duct would not solve the problem without first trying to fix the choke point. He recommended spending money on some other option to try and reduce the solar load on the second floor. Im thinking ill have to leave my fan running continuously if im going to be so picky and not fix the choke point itself.