Add Registers or add a return?

[Mike N]

We have 2 zone heat / AC in our house and the issue I have is with the upstairs. The configuration is typical with a single register over each window in each of the rooms, however one of the rooms is significantly larger and has a high vaulted ceiling compared to the other rooms, even with the volume of 2 to 3 times the other rooms it only has 2 registers. The large room consistently lags the other rooms even if you close the register vanes in all all the other rooms, so much so that the other bedrooms become ice boxes while the large room is still uncomfortably hot. The heat / AC unit is in the roof space and is within 10-15 feet of the large room so I can add vents relatively easily. Based on the relative room volumes I think I need to add 2 or maybe 3 more registers to the large room and I can do this by running new flexible ductwork directly from the heater plenum. So for you HVAC experts here is my question. There is only a single air return for the upstairs system and it is in the upstairs hallway located by the thermostat, is this contributing to the problem? Do I need to add a return to the large room? Can it be in the ceiling? I am planning on adding the extra registers as it is not that hard, adding the return would be more challenging and I might contract that out if I needed to have a return added. Thanks for your input.

 

[pattenp]

I don't think the return is the problem. It sounds like the supply ducting to the larger room is undersized. Adding additional ducts may help. You need to use a fan anemometer to measure the air volume coming out of the current vents with all the vents wide open to get an idea as to how much less the air flow is in the larger room.

Edit: Before you do any thing you need to check for duct air loss due to leaks.

You will probably need to call in a HVAC company to get things balanced correctly.


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[ShawnJ]

Are you closing your bedroom door? If so, is there a large enough gap under the door to allow the air to move (3/4")? If the door is closed and there is no bottom gap, the air flow may be too restricted.

 

[Milton Shaw]

Is the return high or low. For cooling purposes the return should pick up hot air from the ceiling area, not low close to the floor. Exactly the opposite for heating. Also is the large room close to the stairs so that all the cold air from it goes downstairs as cold always goes to the lowest level.

 

[CNGsaves]

Is the return high or low. For cooling purposes the return should pick up hot air from the ceiling area, not low close to the floor. Exactly the opposite for heating. Also is the large room close to the stairs so that all the cold air from it goes downstairs as cold always goes to the lowest level.

Above is how my HVAC guy explained to me for normal setup. However, the OP has 2nd story system where the registers are in ceiling above the windows.

My guess is the big upstairs room needs more of everything: supply, return at bottom since registers for supply are at top (thus to churn / turnover the air in room), and finally more insulation on top of that cathedral ceiling.

 

[rsanter]

You need to look at the ducts and be sure that the ones to the large room are not crimped or folded or crushed in any way. It would be a good idea to do an air balance test to measure air flow to each of the registers.
Based on this you can decide to up the duct and register size to the big room or add a duct and register if it is called for.

Do a duct blaster test and look for duct leakage. Average home in the USA has about 30% leakage in the ducting. Seal up al the duct joints really well and seal the place the register duct meets with the drywall. For this you can use cheap silicone to fill the gap

Side note on return
Returns are always sized to the minimum and once the filter gets any little clogged it will reduce efficiency of the entire system. If you run the fancy pleated or heppa filters it will restrict and reduce efficiency. I generally recommend upping the return size or just put a second one next to the first one. This will save you energy in the long run

 

[tgb]

2 story houses are always a problem with cooling since it is warmer upstairs anyway. Returns in every room is really the way to go, but the filter grills are cheaper and easier that's why in tract homes they always use them. If you add returns you will have to filter the system at the unit instead of the grills, ceiling returns are fine and you will be much more comfortable with returns in all the bedrooms rather than just trying to throw more supply at your system.

 

[Mike N]

Thanks for the replies. The house is relatively new, less than 4 years old, and I can see almost all the flexible ducting in the roof space and it all looks in very good shape with no crimps or leaks. I don't have a flow meter but the airflow is good out of all the registers, subjective I know. Currently the return is filtered at the ceiling return in the upstairs hallway, the return line looks to be about 18" or so in diameter. I'm guessing as it has insulation on the outside. We don't normally have the door to that room closed but I'm looking at the door to the room as I am typing this and I think I need to trim another 1/2" or so off the bottom to allow a better return. There is a ceiling fan in the room that we run pretty much full time in the summer to circulate the air but there is no doubt that the hot air tends to sit in the high ceiling area.

So based on the consensus of the replies I am going to run another two lines to new registers in that room and see how much that helps the problem. I checked quickly on line and I think I should be able to add the runs for a little more than $100 in materials but it will mean spending a few hot and sweaty hours in the roof space getting insulation stuck all over me.

If that doesn't improve things enough I will get the same company back that did the original install and have them figure it out. I still have a sneaking suspicion that the fix is going to be to add another return in the ceiling of that room. It would have to be one with a filter which would then be a challenge every time you needed to change the filter.

 

[pattenp]

If the flex lines to the large room are long runs and in the hot roof crawl space you may be having a large amount of heat gain. Do you have a temperature probe to check the air temp at the registers to get a comparison to some of the other rooms?

 

[Mike N]

I think I might have a temp probe that works with my multimeter I will check. The AC unit is actually as close as or closer to that room than any of the others so I don't think heat gain in the roof space is likely to be the issue.

 

[Mike N]

Added another 6" dia run from the plenum to the center of the ceiling. Made a noticeable difference but I think I will need to add another run to get the desirable results when it is 90+ outside. Not pleasant work, it was hot and sweaty and I ended up with insulation clinging all over but at about $65 total for the fittings, hose and grille I'm happy with the results.

 

[brihvac]

A lot of the local townships around here will not let you use flex. Everything has to be hard ducted. I prefer to use hard duct because you don't lose as much CFM on the runs like you do with flex. I also do a return in every room. Just makes for a better job and no call backs

 

[Mike N]

A lot of the local townships around here will not let you use flex. Everything has to be hard ducted. I prefer to use hard duct because you don't lose as much CFM on the runs like you do with flex. I also do a return in every room. Just makes for a better job and no call backs

I hear what you are saying. Around me things are a lot different to what I was familiar with when I lived in NJ and PA. Flex duct is used almost exclusively here. There's no way you could ever clean the flex duct if it needed it. Even the returns are big *** flex duct.