Air flow issue on one side of house, cannot figure out cause.

Hi everyone, I'm having an issue with my furnace or ductwork that I'm stumped on. My first floor is divided into three large duct zones that are supplied off my furnace. Two of them have a great deal of pressure while the third does not. I have closed dampeners on two zones and made sure the one with little flow is open. Even with this it still seems uneven. What seems strange is the farthest vent from the furnace on the good flow vents is still much higher than the whole duct zone with little flow. Any help would be appreciated.

I have tried closing dampeners on the high flow sides.

Sealing any and all leaks.

Looked for any obstructions.

It just seems that more air is going to the other two ducts.

Replies might be a little slow with the Holiday but I’m sure the heating pros will show up and offer some help. When you say 3 zones do they all start at the same point on the furnace? I don’t think you are confusing a cold air return duct with a hot air supply but I have to ask the question? How many cold air returns do you have? Your second floor heat how does it branch off the duct in question?

One option would be a duct booster fan but that might be a band-aid fix.

Ductwork is sort of like an electrical circuit where you should measure current (CFM) and voltage (pressure in inches of water column).
The air handler is the power source and the ducts, depending on their cross-sectional area and length, offer resistance to flow.

Sounds like you need to rent a manometer and anemometer and drill some holes in your ductwork for pressure readings. The readings should narrow down the problem area.

The engineering toolbox site has formulas and calculators for what readings you should expect for round or rectangular ducts.
IIRC a typical design goal is 100 CFM for each output register but measuring CFM from a duct requires several readings because the flow is less nearer to the duct surfaces and higher in the center.

Taking the electrical analogy a step further elbows and turns would be resistors.

There is nothing wrong with instruments, measurements and mathematics but I think a problem like this is better looked at with common sense at least in the beginning. Be aware that duct size, duct length and number of turns all diminish flow. Compare the good runs to the bad and you should be able to estimate if you should be having an issue.

Is this a problem that just started or has it always been like this? Also keep in mind what you feel as wind force coming out can be misleading as to the amount of air coming out. The size of the register at the end will change that. Is there any chance that something caused a blockage inside this duct?

And to have airflow in a room you need the air to exit the room. Maybe an intake duct is plugged.
You might be able to test this idea by opening and closing doors in the house. This may reroute the air flow paths.

I saw an online article where they used a huge plastic bag to measure CFM. Seriously! They said in some cases it was better than instruments.
7.5 gallons per cubic foot at 100 cubic feet gives you 750 gallons so a 13 gallon plastic bag would fill in 1 second at 100 CFM.
Get the biggest bag of thin plastic you can find to measure this unless you have a stopwatch.

The zones each have a separate connection for return flow, so three. It looks like the return for the zone that doesn't have the best flow isn't complete. The return doesn't run all the way to the wall with the vent on the first floor. The return is just sucking under the floor. And I guess passively pulling from the vent on the first floor.

One of the main trunks of ductwork supplying heat comes off the furnace and does two 90 degree angles runs 15 feet and the does two more 90 degree turns, to drop 10 inches, and then Ts into the ductwork which then branches at 90 degrees into the 6" branch ducts that supply the floor registers. Could this be the issue? I'm looking at the other zones and they run off at 90 degrees at one point before exiting through the registers. Is there a calculator??


As for the second floor, it turns out we have a second furnace in the attic space that heats it and was missed by our inspector. I just found it hidden in a space which I guess would be considered a third floor. Anyway that floor works great.

We purchased our house in the spring and didn't have the heat running until October. This HVAC system has had issues since we bought it. Every time I fix something I find a new / old problem that needs attention.

I think the Manual D does ductwork design, with the Manual J doing the heat load calculation.

You might almost do well hiring an HVAC consultant, a member of ASHRAE. He'd write a prescription for your house and then you could get at least five bids from HVAC contractors for each to bid on the same job.
Opinions are divided whether to tell each bidder that you are getting other bids. If I do tell a bidder and the smile disappears from his/her face I disqualify that bidder.
The contract should probably specify the max temp differences from room to room and the max air speed from the registers (to keep the noise down). Women are comfortable in different temps than men and this region of comfort is shown on some psychrometric charts.
http://en.wikipedia.org/wiki/Psychrometrics

I had a customer whose front door didn't close properly because the house had changed shape because of some flooding. I recommended she find a structural/civil engineer and then find companies who would do the work. They put some serious I beams in her basement for reinforcement and I'd think contractors would just have to guess at what was required.