Help with forced air balancing

bphillips921

I have a new house (5 months old). It's a 2-story with a separate HVAC unit and ducts on the 2nd floor. The great room has a 2-story ceiling so I get some air movement from the main floor to the 2nd floor. All the bedrooms are on the 2nd floor.

I have some large temp variations in the 2nd-floor hallway and individual bedrooms. The thermostat in the hall will show 75 degrees while one bedroom shows 70 degrees and another shows 68 degrees (I have remote wireless sensors in those two bedrooms). I'm assuming the hall is warmer due to the heat rising from the main floor via the large (20'x30') 2-story great room (we keep the main floor at 74). I put the wireless remote sensors in the bedrooms and then had the thermostat average those two temperatures as the target temp. My problem is that the two bedrooms with the remote sensors have a pretty wide temp variation. If I leave both air registers open the temp in the two rooms varies by 5 degrees. If I close both registers in the hotter room I can get that closer to 2.5 degrees but that creates an obnoxious whistling noise through the registers and creates large swings (6 degrees or more) in the colder bedroom since it's getting all the heat but the temperature sensors are being average for the target.

I'm trying to get the HVAC installer back to look at it, but I'm having a little trouble getting him to respond. I'm just looking for some second opinions in case he says there is nothing that can be done.

Can something like this be fixed with load balancing? And can that be done without creating the whistle in the registers?

ChrisJ

Registers are really not meant for balancing, dampers in the ductwork are and will produce far less noise.

Please provide pictures of the entire system or as much as possible. Showing the registers, ductwork, overall layout etc.

Jamie Hall

Yes, you really do need dampers in the ductwork. But the other thing which is sometimes overlooked is return air. You have registers going into those bedrooms, but how does air get out? If the ways that air can return and get back to the heating unit are not adequate, it's almost impossible to get even heat.

offdutytech

Unfortunately I've seen this in my area time and time again that the return air was not properly sized.I would have an contractor come over and take a few measurements at the fan. At this point we are assuming that the furnace is sized correctly for the load of the house. I would have them take the following readings.
1. Ask them to measure the static pressure across the fan to ensure that it is inline with manufacture specs. (typical 0.50" W.C. to 1.0" W.C. This is found on the nameplate of the furnace). Make sure you have a clean filter on hand. Check the pressure drop across the filter. 

2. Take the temp rise across the heat exchanger. This is also found on the nameplate of the furnace. You would like the temp rise pretty much in the middle of the specified range. 

3. Check the gas pressure at the manifold. Again this is found on the furnace nameplate. 

4. Check to make sure that fan speed is set correctly for heat mode. (if this is an option for your furnace).

5. Check the installation manual to see if your furnace requires both the side and bottom return panels cut out. Some manufactures require this based on the BTU and CFM requirements. 

Based on the above information your HVAC person should be able to determine if the furnace is working correctly or if there are some ductwork issues that need to be resolved. I might have another company come over and take the measurements for you so have a second set of eyes. The hard facts won't lie in this situation. 

Derheatmeister

Proper Air balancing requires training and tools.
It could be a simple issue with your room doors in need of cutting to a maladjusted damper,improper duct sizing .....
Many things to consider such as Delta Tee across the HX..
IMO just installing electric dampers can create larger problems and actually Crack a HX if not implemented correctly,this could cause a Carbon monoxide issue..
For a professional Assessment i would get in touch with NCI and see if they can recommend someone in your area..

bphillips921

Thanks for the comments guys. To answer a few questions

- the house is ~1,900 sq ft main floor + ~1,900 sq ft basement fed by one 70K BTU furnace (zoned per floor). There is an additional 70K BTU furnace with separate ducts handling about 1,650 sq ft upstairs where I have my problem.
- I've long suspected the return registers. Each room has a single return register in the ceiling that is connected to the return trunk by flex duct (I think 8") EXCEPT the room that gets too much air. For that he cut a 14.5x8 register into the drywall in a stud bay opposite the furnace then cut the other side of the drywall connecting it to the return trunk.
- There are no dampers on the supply ducts.

I ended up wrapping my GC into the conversation and he said he'd get a new HVAC guy involved, so maybe this will finally get fixed.

Jamie Hall

Air is even lazier than water. That return opening through the wall has almost three times the area of those little round flex ducts -- and no length at all. Guess where all the return air for that system is coming from? And therefore where all the output is going?

This isn't rocket science... you need better returns.

STEVEusaPA

Proper balancing can be easily achieved when the proper furnace is selected and the ductwork has been properly designed.
Properly commissioning the system after installation is rarely performed.
If the above aren’t done, balancing could be pretty tough.
Time to do some math and testing.