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My house was built in '61 and was 100% electric with no air conditioning. At a later point (we think 60s-70s) the electric baseboard heating was removed and a forced air system was retrofitted. We use that same basic system with a newer (2010) furnace and central air conditioning system today. The house is basically a 2000 sq. ft. ranch with a full basement.

The round duct is 6". The floor registers in the bedrooms are 2"x12" and are "plumbed" in from below like this:

bedroom vent form below

I am no HVAC person but that feels like it would constrain the airflow at that juncture and that I could get more air through if I was to replace that with a 3"x12" or 4"x12" transition? Or is that all I could expect out of the feeder pipe?

I know that there is supposed to be a balance in the AC system but I just feel like this is very restrictive. The room is also closest to the AC and is consistently the hottest in the house.

I would like to make the existing system work better so I am open to any suggestions. But if need be we could do something like adding a in min split although that will be a bit ugly.

There are two vents into the room like the one pictured each on the N and W walls and then a 4x10 return on the south wall. We always keep the door open since it is a half-broken pocket door with a broken track. But fixing that is a different question.

If you look at it from above the house is shaped like a capital T with the bedrooms lined up across the top. The bedroom in question is the "west wing" and as such gets all the late afternoon sun so I am certain that solar load is playing a big part directly and indirectly via the attic.

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    Heard of "Solar Gain"? 300 BTU per square foot of solar energy blasting at your roof and walls. It causes sunward rooms to be much hotter than other rooms. But if you mess with the ducting system to combat solar gain, then in winter, you still get the solar gain, but you also get way too much heat.
    – Harper - Reinstate Monica
    May 30 at 20:14
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    I would concentrate on the return. You cannot put more air in a room unless you also get it out. Also check, sometimes filters or sent things are placed in the furnace ducts. Also be sure the damper is open for that duct.
    – Gil
    May 31 at 21:47
  • How 'bout some tape.
    – Mazura
    May 31 at 22:29
  • The room is also closest to the AC [condenser, outside? or the air handler?] - A bigger blower isn't a thing. It's unlikely but possible they don't have it hooked to the high speed lead on a call for AC. - There's already two ducts, but if they both tee off of the same 6" branch, then there's little reason to be bigger than x2". Duct booster fans are BS. Zoning would do it, but my god. W/o more pics all I can say is turning veins, or make that branch a trunk (meaning you do square as far as pos, then tap off a 6" round per register, in spiral if you don't like ugly).
    – Mazura
    Jun 1 at 20:35
  • "We have been here for 4 years now and setting all the dampers and closing off whole registers I still can't get the cold air where it needs to be" - Leave the door open but block off the return. Reasoning: negative pressure is pulling air from the attic. Or block off other returns elsewhere, to get the return in that room to do its job. When the AC freezes up, that was too much blocking off. Don't use pleated filters (the white ones) unless they're 4" thick. Use fiberglass. Those keep dust out of the unit, not make clean air for you. That's what TFPs are for.
    – Mazura
    Jun 1 at 20:46

6 Answers 6

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Can you increase the size of the floor register? Almost certainly. Cutting a bigger hole and installing a bigger register will reduce the restriction, moving the pinch point upstream, probably to that supply duct. With no other changes, this will increase the amount of flow available in that room, at least a bit. It will also adjust system balance.

Will this improve cooling? Maybe. That's the part of the question where experience and system engineering plays in. If there's other duct problems, or there isn't a nearby return, or the room you want to cool gets more solar load, it's not going to improve significantly just by changing a register and boot out.

With that said, if it's easy and cheap to do, it's probably a good spot to start. Since you have a damper in this line, you can always use the damper to crank it back if it turns out that increasing the size of the register was a bad idea.

There's also a very quick and easy way to try this on: take the register out of the boot and try it that way for a couple of days. You'll have to be careful not to accidentally step in the hole in the floor, of course, but in the meantime it will reduce the airflow restriction from such a small register and likely give you an idea of what the additional airflow will do to your system. If it gives you results that you want, consider changing the ducting on a more permanent basis. If it doesn't, you will have to look elsewhere for improvements.

Speaking of those dampers: all open isn't necessarily the best setup. Assuming your system still gets enough airflow with some of the dampers closed down, you may well have to partly close dampers to some rooms during each season. When I moved in to my current house, the previous owners had never closed any of the dampers. The heat was pretty inconsistent between rooms. After a week or so of tuning, we found a new pattern of damper adjustments that balanced the temperature pretty well without increasing the restriction too much. When we switch from heating to cooling, all that goes out the window, and we need a totally new set of damper settings to get the temperatures even again. It would be great if the systems didn't need this sort of adjustment or did it automatically. The reality is that many systems require seasonal damper adjustment changes for the best comfort.

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    "it's annoying the system needs adjustment" yeah, it's almost like some outside malevolent being is capriciously blasting some parts of your house, but not others with extra heat. I wonder if it has anything to do with what the goths refer to as "that yellow hurty thing"? :)
    – Harper - Reinstate Monica
    May 30 at 20:23
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    @Harper-ReinstateMonica I am still disappointed that it's so expensive to have a system which can adjust this stuff itself. Auto climate on a car is a few hundred dollar option and adds multiple electronically controlled dampers along with multiple temperature sensors. Yet trying to get even a second temperature sensor in your home can be a headache, depending on your system. Forget zoning unless you want to really pour dollars in to the system.
    – KMJ
    May 30 at 22:30
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    yeah I had autoclimate on a '76 Caprice. But yes, the answer is zone control, so it really boils back to the same reason we're still doing lame stuff like using bang-bang controls in the 21st century. "It's cheaper". A few years later, OP could've avoided the ductopus and had a mini-split head in each room, and then you have zone control. Because why would a 21st century maker of a 21st century product not have that :)
    – Harper - Reinstate Monica
    May 30 at 23:12
  • @KMJ I think "smart" vents hold promise, but currently everything I've seen feels like overpriced IoT buzzword junk. Maybe in 3-5 years we'll be able to buy a set of vents, stick one in each room, and have instant room-level zone balancing. A man can dream... in the meantime I'm tempted to try building a system myself with arduinos/rpi.
    – mbrig
    May 31 at 16:53
  • @mbrig I can imagine it to require substantial amount of analysis on the system, where you have everything open, and start closing the one with the greatest deviation and analyze the outcome. From there, start mapping the change relation between each vent and every other room's changes, and then mathematically calculate one where all the rooms end up being even.
    – Nelson
    Jun 1 at 8:30
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The area of the duct should be determined, ( Pi x R sq.) and then compared to the open area of the floor opening.

If the floor opening is a 2x12 that's 24". If the duct is 4" in diameter the area is 12.566. The floor opening is already almost twice the size as the duct. There should be no benefit in increasing the size of the floor register.

Perhaps you need to install a duct fan to force more air into the bedroom. Or close off some of the other dampers to get the balance you want.

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Other answers have adequately addressed your question, but I want to make you aware of another remedy for stubbornly inadequate airflow to a single room.

If the larger floor register doesn't help, if adjusting dampers still leaves you too hot, if there's no chance of adding another return and if the room's duct attaches to the main duct at a 90 degree angle --

Install a duct scoop

See the image below (credit). A sheet metal scoop oriented with the open side facing the oncoming air flow in the main duct dramatically increases the airflow into the side duct.

Be prepared for an increase in noise, which will be proportional to the increase in airflow. If you get too much airflow, you can judiciously trim the scoop with tin snips to make it smaller by degrees until the flow is just right.

e

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  • Turing veins, +1. While were at it (making the resister a x4 instead of x2, let's get a boot with an actual radius in the elbow, or transition to square duct sans the boot and use a real, short turn elbow. If it's awesome with the register off, then that's what you'd get out of a x4 instead of x2.
    – Mazura
    May 31 at 22:17
  • obviously this will reduce airflow to the other rooms
    – user253751
    Jun 1 at 23:01
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That's a fairly cumbersome project. To estimate the benefit, consider the cross-sectional areas of the duct and the boot:

  • 6" Duct: 28.27 sq. in.
  • 2x12 register: 24 sq. in.

That's a reduction of ~15% for a 6" duct. Not much. You can easily compensate for that with no actual sawing whatsoever. If you have a 5" duct there is no restriction.

Of course, your register presents some reduction as well, but I can't see that. If it's very restrictive, look for one that isn't.

I'd look at balancing the home by partially closing registers (or duct dampers) elsewhere, usually in rooms on lower levels and on the shady side of the house. An additional benefit of this strategy is that it can be reversed in winter, when the other areas of the house need more HVAC supply.

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    Your figures are off a little because the free area of the register is smaller than the overall size of the boot, so there's yet one more reduction step. A 2x12 register might only have 17 sqin of free area. limaregister.com/products/800/stamped-floor-register for reference from one manufacturer. To match that 6" duct in free area you'll need a 4x10 or 4x12.
    – KMJ
    May 30 at 22:40
  • The only place 2" goes is the toe kick of a cabinet. Anywhere with 2" floor registers hasn't been touched in at least 30y.
    – Mazura
    May 31 at 22:27
  • @Mazura I've got one in a back bathroom as well, and it's overkill for such a tiny room.
    – KMJ
    May 31 at 22:33
  • That's why, "balance the system". +1. "overkill" is not a thing in hvac unless your AC works so well it can't drop the moisture out of the air before it gets too cold.
    – Mazura
    May 31 at 22:38
  • @Mazura In the world of overkill would it make sense to upsize all those vents and then install a beefier furnace blower? Is that even a thing? We have been here for 4 years now and setting all the dampers and closing off whole registers I still can't get the cold air where it needs to be
    – Ukko
    Jun 1 at 14:19
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The thing to try first is clued in by:

I do have duct dampers and I have them fully open to the bedrooms.

So, the damper to the room with cooling problems should be fully open.

The dampers to other rooms that are not too hot should be closed somewhat, which will drive more flow to the room with cooling problems, (without even changing its duct) and increase the run-time until the thermostat location is "satisfied" unless the thermostat is in the room with cooling problems (in which case the rest of the house would be too cold, normally.)

How much to close the dampers is figured out by how comfortable the rooms fed by each are after adjustment and how comfort in the currently too hot room is improved. And that is "balancing" the system. Many people find duct dampers and think that fully opening them all is the way to best results, and it almost never is.

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  • Very true, the dampers mostly control zones of the house in this system. A given branch serves both the house and the basement so I have closed all the basement registers and opened all the upstairs ones. I basically invert that configuration for the winter. I might need to make a detailed diagram...
    – Ukko
    Jun 1 at 14:25
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You could try installing a fan to suck/push more air into the room:

https://www.lowes.com/pl/Register-boosters-Registers-grilles-Heating-cooling/4294512213

They are called register boosters.

enter image description here

You could also close registers in unused rooms forcing more air through the open registers (As Isherwood pointed out).

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