HVAC returns are fewer than supplies!

[zolotiyeruki]

I'm finishing our basement, and one of the first tasks is addressing the ductwork.  I've rerouted some, I've repaired some, and I've sealed a whole lot.  It finally occurred to me to count the number of supplies and returns, and I was rather dismayed at what I found:  There are 22 6" supply ducts, but only 12 6" returns.  This means that the air handler is pulling air wherever it can, through every crack and pinhole in the return ductwork, and that also means that when it's running, there's a nice breeze blowing down the basement stairs under the door.

I'm surprised at the huge discrepancy between the number of supplies and returns.  Is it normal to have nearly twice as many supplies as returns?  Or did the original builder just get lazy because there weren't convenient/available joist bays to run additional returns?  What options are there for fixing this (if it needs fixing)?

[lthenderson]

A few things. First, I think the amount of airflow resistance is used to balance out returns versus supplies, not square area of the lines. Many times returns are shorter runs and have less bends and thus not as many are needed to equal the supplies. Second, things have changed over time. Back when a lot of houses had ductwork installed, the inefficient systems weren't really bothered by differences in the airflow through supplies and returns. Modern systems are more sensitive. Finally I think it is probably optimum that supply is slightly greater than return air so that fresh air is pulled in through the intake and an interior positive pressure is created to keep out things like allergens and cold drafts in the winter. I guess I wouldn't be too worried if things have been working.

[Sibley]

I have an old house (1919) and it was retro fitted for central heat/air. Quick count, I've got 11 supplies and 5 returns, and I honestly am not sure that the returns are actually feeding air to the furnace. So, it could be worse.

If it's working decently, don't sweat it.

[Agg97]

I'm surprised at the huge discrepancy between the number of supplies and returns.  Is it normal to have nearly twice as many supplies as returns?  Or did the original builder just get lazy because there weren't convenient/available joist bays to run additional returns?  What options are there for fixing this (if it needs fixing)?

Pretty normal, yes.  The idea is that your supply ductwork has dampers, usually at the registers themselves, to balance airflow.  On the return side, those dampers aren't necessary so you can use 100% of the capacity of the return air ductwork.  A 6" duct has the capacity to handle ~100 CFM before becoming loud or causing too much air turbulence.  So, 12 6" returns is approximately 1200 CFM, or 3 tons of cooling.  Is that the size you have?

Agg97

[zolotiyeruki]

Nope, our A/C unit is 5 tons.  Waaay bigger :)

[Fishindude]

What you describe is a pretty normal installation, no need to be concerned.
On a real cheap simple install, it's not unusual to have supply runs only to the individual rooms and a single large return vent located centrally somewhere on the floor.

[zolotiyeruki]

What you describe is a pretty normal installation, no need to be concerned.
On a real cheap simple install, it's not unusual to have supply runs only to the individual rooms and a single large return vent located centrally somewhere on the floor.

Yeah, our last house (1500 sq foot ranch in TX) was like that.  I wish there was an easier way to add returns from the bedrooms upstairs now, since the entire house is on a single thermostat, and a couple of the bedrooms have trouble maintaining temperature.

[BudgetSlasher]

As has been mentioned, more goes into a properly designed HVAC layout than just duct sizing. I don't recall all of the rules and calculations I looked up when making some changes to my HVAC (and I will probably look up again when I want to make more changes), but I do recall that most residential system I have seen are not correct; that is for a variety of reason, some were just rule of thumbed from the beginning, some had sloppy add ons, some had intentional compromises made.

A couple things.

you have 22 6 inch supplies? (how big is your house? I only have 10 in a 3,200 sq/ft house that needs 74,000 BTU of heating).

How big is the duct coming out of the air handler? If the main trunk is not 24"x24" (roughly the same cross section area as 22 6 inch ducts) then that is trunk is your smallest point. (unless you have the dreaded duct-o-pus). Those supplies may be larger to accommodate resistance encountered from turns (or because that was what the builder was used to working with).

Ductwork return sizing is based on CFM needs, which in turn is based on how much heating or cooling the system provides. You should be able to find out how much return register area you need if you know your heating/cooling. Then you can figure out the duct sizing needs based on run length and number of turns. If you have more supply ducts than returns, but enough return area, I doubt it will be a problem (the air will just be spread out).

If I recall correctly there is a test you can do to ensure your supply and return are balanced. It requires taking static pressures in the supply and return (this is the first google link that appears to be dealing with it: https://www.primexvents.com/static-pressure-testing-hvac/). Basically too low a pressure on the return side and there is not enough return flow, too much pressure on the supply side and not enough supplies (I don't think you have to worry).