Central returns vs a return in each room

9 years ago

We are building a new house and reviewing HVAC bids this week. Their scopes all seem to include a couple of central returns for the main level and one for the 2nd floor.

Our current home has returns in each room and one big filter at the airhandler where those returns all connect.

Is one method better than the other?

Featured Answer

udarrell

9 years ago

last modified: 9 years ago

How to Optimize the Supply Air CFM Volume to & from the
Rooms for balancing airflow to all rooms & upstairs rooms; in cooling mode, the
Return-Air should also all come from upstairs.

You want to greatly reduce the airflow ‘resistance’ on
the return-side to produce more static pressure and CFM airflow on the
supply-side at the diffusers. Any airflow needlessly restricted on the Return-Air
side reduces the air availability to the blower wheel blades, which ‘reduces’
the blower wheel performance and the airflow through the evaporator coil and to
& from all the rooms.

You always want the highest static pressure on the
supply-side at the diffusers to the rooms, while at the same time, as much as
possible, reducing as much ‘resistance’ or pressure drops in the rest of the
supply-side, which also includes low pressure drop fittings in all of the
supply side ductwork to optimize pressure at the supply-side diffusers.

You should also always have branch duct dampers installed so
you can adjust the airflow balance, for example to 2nd floor areas, where that
is a problem.

Reducing all the return air ‘resistance’ possible on the
return-side making any return ductwork as large as optimally possible, plus install
large & open return-air grilles & filter grille areas as large as possible to reduce all return-side resistance and pressure drops possible, which will optimize supply-air CFM both to and from the rooms.

If U use 1" deep
filters I’d use Purolator model P312 filters; can't see through them & have
low resistance & good filtering qualities; can order them over the
Internet. Extra large Return-Air filter rack areas are always needed, even RA filter cabinets with 2 large slanted filters can be used.

Very low airflow with some conditions, could lead to the
freezing over of the evaporator coils resulting in possible severe compressor
damages… If indoor airflow drops off, shut the outdoor condenser/compressor off immediately…!

Also, always check the temp-split of the air off the outdoor
condenser compared to the outdoor temp, the 'Expanded Cooling Data' will show
you what the temp-split should be at existing conditions.

CFM x WB Delta difference on enthalpy chart x 4.5 @sea level =Total operating Btu/hr Capacity...

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Comments (10)

mike_home
9 years ago
Having a return in every room is the best. This works well with bedrooms where the doors are usually kept closed at night. But it may not be practical to have many returns. It depends how the duct work is set up in your new house.
dbrad thanked mike_home
ionized_gw
9 years ago
Imbalances in air flow cause pressure differentials between indoors and outdoors. Pressure differentials cause air to be sucked in and blown out of your living space through what ever small leaks are available. That makes you blower into an uncontrolled power vent, not good. To eliminate pressure differentials the best way to duct airflow is with ducts inside the home's envelope, not in the attic or outdoor crawlspaces. Individual returns are best for similar reasons. It costs more.
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energy_rater_la
9 years ago
or for instance if bed rooms are off a hallway, transfer grills or jump ducts will provide return air pathway to return in hall.

lots of ways to skin a cat, all dependent upon your layout & needs.

best of luck
weedmeister
9 years ago
"all dependent upon your layout & needs."

And your budget.
dbrad
Original Author
9 years ago
Okay, so more returns = better = more $$. That makes sense I guess. I'm just surprised given that our current home with individual returns in each room is a tract house, while our new custom build is twice as large as our current house in both size and budget. Could it be the I-joist floor system that is steering them toward spec'ing fewer large returns?
Bruce in Northern Virginia
9 years ago
Where is your house located? It seems that when A/C is priority the builder includes more returns, but may not have the same priority for heating.

I would push the builder and/or pay extra to get returns in every bedroom and at least one large return for each main living area, or area where you may have temperature extremes.

For example, if you have large south-facing windows in your dining room (lots of heat load) and it is not near the main return for that floor, you may need a dedicated return to keep the air mixing well. Also if you plan to sleep with bedroom doors closed, you need a return in each bedroom.

Transfer grills and jump ducts are band-aid solutions for an older home. A new home should be designed with an efficient return air ducting system right from the start.

Bruce
dbrad thanked Bruce in Northern Virginia
dbrad
Original Author
9 years ago
Bruce, we're in upstate SC - and definitely prioritize A/C around here.

Thanks all - I'll start asking some additional questions.
tigerdunes
9 years ago
more returns equal better airflow and better comfort....central returns are cheap shortcut...

IMO
wishiwereintheup
9 years ago
That makes sense I guess. I'm just surprised given that our current
home with individual returns in each room is a tract house, while our
new custom build is twice as large as our current house in both size and
budget.

In my late-50's, same size, cookie cutter tract home subdivision, the forced air systems vary in this regard. The home next to me has individual returns at the floor. The house across from me has a central return. The house on the other side of me has individual returns at the ceiling level. (All the homes did not have central AC originally.) Don't know if that's just a matter of different contractors with different styles or the original owners paid for something extra.