Technical > Home Improvement > HVAC: Splitting flex duct lines

HVAC: Splitting flex duct lines

Taking the one 8" line feeding your bedroom and splitting it into 2 lines to feed 2 registers isn't going to get you anything. You could certainly come off the main trunk with another flex line and feed a 2nd register in your bedroom. But without a return, you still aren't going to get as much airflow as you want.

Why can't you hook the return back up, but hook it back up properly before the filter? This would reduce noise, and increase airflow.

reply to Glenn
Are you now "starving" the airflow system by your action of removing the return?
Starving the return side effectively reduces airflow in ALL the supplies throughout the house AND increases noise.

reply to Glenn
Without a way for air to escape the bedroom and return to the unit to be cooled again it just isn't going to work well, you need circulating air flow in a home, not just air being blown into a closed off room.

Instead consider getting something to suppress the noise from the register. There are various products from acoustic covers to liners for ducts.

reply to Glenn
I don't know if what I did is good, or even OKAY, but here goes:

Similar issue as you, my bedrooms were warmer than the other rooms. The bedrooms are on the top floor, so the air is warmer by default, and they don't get so much air from the blower as the main floor rooms have much shorter branches.

What I did was find rooms that were colder than the average house temperature (Colder than the thermostat setting).

Then in those colder rooms, I either blocked off the vent completely (Completely open room getting cooled) or partially flipped the damper in the register.

This forced more air in the desired rooms and works.

I'm sure some people here will frown upon my suggestion because of the increased static pressure, but I'm not sure pressure really increases at the blower. I just balanced out WHERE the air goes

As for the return, well, my bedroom doors are about 1 inch off the floor, so there is plenty of space for air to move out.
But if you can access the duct for the return vent, I'd cover it in sound proofing material like Roxul Safe'n'Sound.

reply to Glenn
In order for a room to be cooled, you need air flow. The amount of air coming in equals the amount of air going out. Right now the air is only going out through your door's gap, that's your bottleneck.
2x6" (226 sq in) vs 1x8" (201 sq in) won't make any significant difference. You are still forcing the air to go through a ~1"x30" = 30sq in opening - if not less - under the door.
Install the return properly. If needed add a return properly before the filter and open it as needed. You can minimize the noise by having a longer and twisting return duct, at the expense of poorer air flow - but still better than with no return.
A booster fan will also help - but there is a bit of added noise.

reply to Glenn
Removing the return is a big no no, it directly pulls the hot air from the ceiling, even if you double the cfm to the bedroom, your not going to match the effectiveness the return provides and splitting the same flex isn't going to add any cooling

Next best thing is make sure you have at least R40 insulation in the ceiling above the room, this will reduce the heat load, that's 14" depth

What did you do with return register? that's potentially a hole right inot the attics superheated air

If it were my home, I would restore the return and find other ways to mitigate the noise

And as another poster mentioned, you can close off/ restrict other registers in the house to increase flow to the bedroom

reply to Glenn
Sounds like this is more trouble than it's worth.

The house is a raised ranch and was retrofitted with A/C at some point. The master bedroom isn't all that big. One of the HVAC guys explained the setup; they probably had a return in the master bedroom so you could shut the door and the system would function just fine. All the other bedrooms have one supply. There's a main return in the hallway.

I don't beleive there's a way to hook the master bedroom return up before the filter. The return in the hallway has the filter mounted level with the ceiling. There's a flex back to the fan system to reduce noise. The old master bedroom return bypassed the flexback (IIRC), hence the noise. Or it could have just been the volume of air that was being sucked through a small louvered opening in the ceiling.

I removed the master bedroom return at the ductwork near the fan unit. I removed the flexduct, installed an endcap over the protruding circular duct, used some sheet metal screws, then foil tape. The register that fed the return is now closed.

Maybe the easiest thing here would be to shut another register off on another room...as mentioned.
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I ski at Mount Snow

Just get a longer piece of flex duct and hook the return up again. Use a piece that's 10 feet or so longer than you need, so that you can arrange the extra in an S-shape in the attic. That will attenuate the fan noise quite a bit at the return.

You also mentioned that this return "bypasses the filter". You need to install a return fixture that accepts a filter. Letting your AHU suck in unfiltered air is not a good thing.

I added a return to our master bedroom a few years ago because the a/c would actually cause the walls and windows to make popping noises when it turned on and off while the door was closed. I installed a huge return fixture that uses the same size filter as the main return, but kept the duct size much smaller than the main return. This way I only have to keep one size of filter "in stock" and the filter in the bedroom lasts about a year before it needs replaced, simply because it is far oversize for the job.

reply to Glenn
Maybe the old return was too small and that's why you had so much noise with it.
I added a return in our MBR so the doors didn't have to be open at night. Made all the difference in the world on temp.
You must have air movement to heat or cool.
What size is the old return vent?

reply to Glenn
Guys he doesn't necessarily NEED a return vent in his bedroom.

Most old forced air systems only have two or three big return vents in central parts of the home.

Unless his bedroom door is nearly air tight, air will easily flow OUT of his room.

At the very least, he can keep his bedroom door slightly open and see if it improves things. If it does improve things it might just be because the heat generated by the people sleeping inside is superior to the cold air being pushed in.

Glenn, first thing first. Keep your bedroom door open and turn on the blower. Then go compare by "feel" how much air flows through your bedroom register vs. registers closer to the blower.

If it's easily noticeable that pressure is lower in your bedroom than elsewhere, you know where ONE problem is.

reply to Glenn
The old return is basically a supply; same exact register that's used throughout the house.

Since it's just my wife and I, we leave the master beedroom door open. So no worries about airflow.

I was just looking into this since that room was getting a little warmer this year, due to the loss of a tree in the front of the house.
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I ski at Mount Snow

If you are going to come and ask for advice atleast take the advice and stop trying to shoot it down. Put that Damn return back and be done with it. The return was there for a reason and putting it back will most likely fix the problem. I find it hard to believe reputable company would hook up a return directly with out a filter. You need to change out the grill in the bedroom for a return air grill that will increase the size of the face opening and will also add a filter at the bedroom.

Return won't make any difference if they are leaving the door open. The problem was not caused by the removal of the vent but instead by the removal of a tree. Perhaps they should just put the Damn tree back?

Yeah that should only take about 20 years if you take care of it

reply to Glenn
If the supply is in the right place, and you have airflow across the bed, I would simply look to the source of the heat (the window) and try to deal with that. Insulated drapes or shades that are pulled during the day, or reflective film if you can deal with the looks of it. Or an outside awning, or shutters, or even solar screening material (which you could take-down again in the winter).

reply to Glenn
I defense of the tree removal...

It was a Magnolia tree that was hit hard by the late October 2011 snowstorm. Even if I had left it up, it's shading abilities had been greatly hampered.

Thanks for the advice everyone.
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I ski at Mount Snow

reply to Glenn

Which is exactly why I told him to put the return back and stop second guessing the advice everyone had already given him. Door closed or not the return was there for a reason, how close is your main return to that room could be a factor.

My house for instance has 3 returns one of which is in a bedroom and it's not because that room needs the extra flow. It's because the 4ton a/c system requires a certain amount of return air. I also included to replace the return register with a true return air grill that was bigger and also had the ability to house a filter as that should help with the noise.

Yes the tree probably has something to do with the problem too but you have two issues one of which is easily fixed.

I would need a very small return that accepts a small filter. The bedroom return is hooked to an 8" flex duct. To they make small returns? Say 12"x12"? Idealy, I'd like to remove the current register, which has no filter and plop in a new register...without having to cut up the bedroom ceiling.
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I ski at Mount Snow