New Duct system to addition off existing furnace (many bends no insulation) (no heat / velocity)

[Jasperjake]

Hi,

I had an HVAC contractor install a duct system off my exisitng furnace to an addition I added to my house. The new dust system supplies very little heat and velocity at the registers. The existing registers in my house are between 32-40 celsius. The registers in the addition are at about 25 celsius. I suspect the primary issue is that there are too many bends in these ducts for the distance they travel. Obviously I or the contractor needs to do something, however I think it would help greatly to know where the major problems might be. My thought was to perhaps run the 10" supply straight off the furnace removing most off the 90 bends and insulating the duct since it is run in a hetaed, but uninsuated crawlspace. Thoughts on this, anything alse I could address?

 

[Fitter30]

First need to have some that now what a ductulator is. What makes u think your furnace can handle the existing load? Anyone can guess but throwing duct up doesn't make sense. You haven't described the house or the furnace or air handler.

 

[Sponsor]

 

[Jasperjake]

First need to have some that now what a ductulator is. What makes u think your furnace can handle the existing load? Anyone can guess but throwing duct up doesn't make sense. You haven't described the house or the furnace or air handler.

The exisitng furnace is a 10yr od goodman 90000 BTU 93% efficient two stage. No AC. The existing house is 800 sf basement half conditioned, half uninsulated crawlspace (where the furnace and new ducts are), 1000sf main, 600sf 2nd storey. All exisitng construction is 1970, addition is an extra 600 sf. triple pane windows, double stud walls with 12" of insulation. Prior to having the ducts installed I was able to heat the whole addition to room temp with a 2000 watt heater at -20C. Furnace did not struggle with heating loads before addition. I agree about the ductulator, I had assumed the contractor would get it close, that is why I hired it out. Apparently, the ducts were included, the heat was not.

 

[WorthFlorida]

The blower speed in the furnance can be increased with DIP switches. The owners manual will have info on it.

This might be your model. See page 30

Goodman GMV9 Installation Instructions Manual (Page 30 of 40) | ManualsLib

www.manualslib.com

As of now if you can close off the the added work, go back with the electric heater, or increase the blower speed to get more heat into the rooms until the duct work is done correctly.

 

[Jasperjake]

The blower speed in the furnance can be increased with DIP switches. The owners manual will have info on it.

This might be your model. See page 30

Goodman GMV9 Installation Instructions Manual (Page 30 of 40) | ManualsLib

www.manualslib.com

As of now if you can close off the the added work, go back with the electric heater, or increase the blower speed to get more heat into the rooms until the duct work is done correctly.

Yeah I upped the blower speed from low to medium, helped a little. But of course raising the blower speed means the ducts in the ret od the house heat up faster. Kind of robbing peter to pay paul. Not sure yet what ductwork done correctly means? Appreciate all the insights.

 

[WorthFlorida]

.....Not sure yet what ductwork done correctly means? Appreciate all the insights.

The duct work is sized for proper air flow and minimal elbows. The new addition, was a return line added? The poor air flow is that with the door open and a clear path to the return? Florida requires jump vents from a bedroom to the hallway where the return generally is. It allow for proper airflow with the bedroom door closed.

I found this site looking up what length does a 90º elbow added to equivalent length, 30'.

Tech Tip #2: Equivalent Lengths - Southwark Metal Mfg. Co.

Equivalent lengths of duct fittings are important to the system designer for proper airflow distribution.and ease of installation.

www.southwarkmetal.com

 

[Jasperjake]

Appreciate all the information. I think I have decided on how to proceed. It seems to me that I have to try to equalize the register temperatures and velocities in the addition with the ones in the exisitng house. I don't think the return is as big as an issue, as has been pointed out once the entry to the addition has been opened into the exisitng house the pressure should equalize between the addition and the existing house. Unless I equalize the heat output in the addition with the exisitng house any such remedies such as increasing blower speed will only heat the house faster than the addition. This is how I think I should proceed

1. I need to rerun the new 10" round supply to the addition straight off the furnace plenum right down the middle of my exisitng basement as opposed to on the exterior wall. This will result in a duct with (2) perhaps (3) 45's as opposed to a duct with (4) 90's and (2) 45's. This should in theory reduce the equivalent length of the duct by about 80'. (20' per 90?)
2. I need to insualte the duct in the exisitng basement (storage room). This room is a 5' crawl with uninsulated frost walls. The furnace dumps cold air into this space and the colder the weather gets outside the colder that space gets. Additionaly, moving the duct off the exterior wall should reduce heat loss from the duct even more. The duct has a 25' - 30' run through this space, so I can see how it would lose heat. With the short run times that are typical with forced air furnaces (10 min max) I think much of the heat that runs during that time is working hard to heat that large duct up just in time for the furnace to shut down. Once the duct enters the new addition crawlspace which has insulated frost walls, I don't see insualtion as being necessary.
3. If the above two changes don't increase velocity and heat output to where it is closely matching the existing house, then I will add dampers to my existing three trunk runs in the house right at or near to the furnace plenum so I can better balance the old ducts with the new.
4. If necessary, if I feel like punishing myself with more work, I think it might help to space the branch runs in the new addition crawlspace more evenly (24" min. apart) off of the 10" supply trunk, right now they look like this

2 vents, 1 to bedroom, 1 to end of crawlspace are right beside one another

And the three suppy branches at the end of the 10" supply trunk look like this, they are all ganged very closely right at the end of the trunk. Perhaps moving them back to a min. of 24" off the end of the supply trunk might help.

This will hopefully, keep most of the exisitng duct work salvageable. Thoughts, before I begin?