HVAC...balancing returns?

Hi All, I was going to post this in the silencer thread to try and steer it back to a focused discussion, but it's been locked so here I go with a new thread. I've got my return and send HVAC lines going into my room, and back to my furnace, but I'm finding that the return is too strong....lots of air motion, and naturally air noise. The problem, I beleive stems from the return for my studio being mounted right at the root of the main return. I haven't built a silencer box for this line yet (I was going to wait since I have easy access to theis line in my furnace room). My plan is to build a baffle box using Johns Manville superduct. Now, what I'd like to know is how can I lower the air handling noise, reduce the bleed thru of studio generated noise into the HVAC return, and better balance the return flow? Seems like these are all sort of contrary objectives... I understand that for air rush noise I need to work at large opening sizes to slow the flow, but how do I control the return air pressure (I guess with maybe a mechanically adjustable baffle at the connection to the return ait main trunc)? Looking for help here....and trying to keep any discussion 'on topic'.. Cheers Kris

This is a mighty fine standalone thread. 8)

drfrankencopter wrote:

Hi All, I was going to post this in the silencer thread to try and steer it back to a focused discussion, but it's been locked so here I go with a new thread. I've got my return and send HVAC lines going into my room, and back to my furnace, but I'm finding that the return is too strong....lots of air motion, and naturally air noise. The problem, I beleive stems from the return for my studio being mounted right at the root of the main return. I haven't built a silencer box for this line yet (I was going to wait since I have easy access to theis line in my furnace room). My plan is to build a baffle box using Johns Manville superduct. Now, what I'd like to know is how can I lower the air handling noise, reduce the bleed thru of studio generated noise into the HVAC return, and better balance the return flow? Seems like these are all sort of contrary objectives... I understand that for air rush noise I need to work at large opening sizes to slow the flow, but how do I control the return air pressure (I guess with maybe a mechanically adjustable baffle at the connection to the return ait main trunc)? Looking for help here....and trying to keep any discussion 'on topic'.. Cheers Kris

Kris, 1st = a couple of questions....... explain in as much detail as you can exactly how you ducted your system - sketches if yu can...... do you have fresh air intake into the system or the room? Sincerely, Rod

Thanks Rod, I'll try my best.... The sends(there are 2) for my room are seperate feeds from the main trunc. They drop into the room from the ceiling. They each have a set of home built baffles to try and quiet the bleed from the studio room from getting into the HVAC. The ductwork itself is nothing special...it's straight steel (I know, not optimal, but time pressures forced me to not change those. Air handling noise from these vents is not objectionable as it currently stands, and the amount of heat coming out of the vents is quite acceptable. The return is brought in through a hole low in the wall (right near the floor), and ducted straight to the main return trunc with 8" diameter steel pipe. I've shown a photo below. Right now the return is WAY too powerful. It sucks all the dust out of the room (and since it's under construction that was alot of dust....had to vacuum the filter out). My plan is to replace the round duct with a rectangular one built from Superduct, and use fabric wrapped 703 as baffles for the return. I think I need to install an adjustable baffle in there though to prevent the return pressure from being too low...I want to allow for that line to be balanced, as given it's close proximity to my furnace/blower it's seeing the lowest pressure on the return side. Now, fresh air. Currently all that I have is the fresh air line that feeds the furnace itself. My plan was to see if my connection to the main HVAC system was sufficiently quiet (in terms of noise transmitted throught the house), and if it provided sufficient air exchanges to keep the 'stale air' from building up in the studio. If either of these things aren't met by my current desig, then the plan was to install a Heat Recovery Ventillator for the studio space, and seal up the conenctions to the main HVAC system. Cheers, Kris

drfrankencopter wrote:

The return is brought in through a hole low in the wall (right near the floor), and ducted straight to the main return trunc with 8" diameter steel pipe. I've shown a photo below. Right now the return is WAY too powerful. It sucks all the dust out of the room (and since it's under construction that was alot of dust....had to vacuum the filter out). My plan is to replace the round duct with a rectangular one built from Superduct, and use fabric wrapped 703 as baffles for the return.

Kris, what size is your supply line - and how is it ducted?

The 2 supply lines are 5" diameter straight steel runs, with the baffles that I built in-line. They come into the room via standard registers (4X12"). Cheers, Kris

drfrankencopter wrote:

The 2 supply lines are 5" diameter straight steel runs, with the baffles that I built in-line. They come into the room via standard registers (4X12"). Cheers, Kris

OK- that makes sense- you have a total of 36.27 si of supply running free and full - and then you have a return of 50.27si free air trying to return to your system........ so what happens is you create a build up of static pressure in the supply - which increases the retun air rate even more....... See - this is why I always over size evenrything for studios- you won't see anything less than a 10" round feeding a little room that is only getting 100 cfm if Iminvolved in the project.... and the supply and returns should alwys be the same size...... Yes - a balancing damper on the return (although odd - they usually always go on the supply) can be used to slow the rate down to match......... you should try to place it close to the unit - and let any air noise it makes dissapear with the baffles you built. Rod

Thanks for the information Rod.... I was stuck with the pipe sizes used for the send lines, as a result of all the other lines feeding the rest of my house, and the requirement to fit them in between joists. Fortunately, the air rush noise on the send is quite low...and manageable for all but the quietest of sources. I may need to turn off the furnace/fan when recording solo acoustic guitar, but I don't anticipate doing that often. Mostly I'll be working with the loud stuff! So, the plan is then the following:

Install an adjustable baffle near where the return line conncts to the main return trunc Build a duct/baffle combination out of Superduct and 703 (fabric covered to keep dust/debris down). Adjust the baffle to best balance the flow between send and return Pray it all works well and that I don't need to install a HRV

Cheers, Kris

drfrankencopter wrote:

So, the plan is then the following:

Install an adjustable baffle near where the return line conncts to the main return trunc Build a duct/baffle combination out of Superduct and 703 (fabric covered to keep dust/debris down). Adjust the baffle to best balance the flow between send and return Pray it all works well and that I don't need to install a HRV

Cheers, Kris

Kris, 1st off - I am not talking a baffle here - It's a balancing damper....... there's a difference...... and I have to apologize - but it has to go on the supply duct - not the return duct - placing it on the return duct will only make things worse...... you have to slow down the supply until the return can quietly keep up......... Unfortunately that might not leave you enough air - but it's the ony way it will work...... Sorry about the brain fart.......... sincerely, Rod

Yeah, I meant a damper too....had a brain fart as well. As far as dampers on the supplys, they're in place....but now that the ceiling is up they're not easily adjusted...doable, but not easy. So my return noise has little to do with proximity to the return line, and more to do with an excess of supply air? Maybe a 2nd return is in order....so I can keep up with the supply air. I have a feeling that I'll need all that supply air as my room is about 3500 cubic feet in volume. Cheers, Kris