Basement cold air return

[Jered S]

I have a new finished basement that has plenty of supply air ducts on the ceiling. The return air duct is also on the ceiling. I question this. It makes sense to me to have the cold air return duct on the floor as to pull the cool air off the floor and draw the warm air down off the ceiling therefore warming the rooms more completely. Am I right?

Alberta. Cold climate. No AC. Insulated basement. Open concept basement with one big bedroom, bathroom, and two utility rooms. The HE furnace is in a utility room with a door you can close.

I really want to add a damper on the existing return duct on the ceiling. As well as tie into the cold air at the furnace in the utility room thru a wall and run a short return line at the floor into the open room. Damper it as well. Open the floor one in the winter. Close it off and open the ceiling one in the summer. Also I should probably put one in the bedroom as well as that door is shut most of the time.

Makes sense? Is there anyone who utilizes a ceiling cold air return as well as a floor one and damper them depending on the season? Please help. Thanks everyone.

 

[Dana]

Are the ducts & air handler up in the attic, above the insulation? (That's a system-efficiency disaster in a place like Alberta!)

Randomly hacking on duct work changes the flow characteristics every where. It really needs to be re-designed, with supply ducts properly sized for the room loads, and a properly sized return path for every supply register/room. Putting the return plenum in the basement clearly makes sense, even if the supply ducts are in the ceiling. But if the supply ducts are above the insulation you might consider starting over completely.

If you simply damper down the return ducts to force a different return path you're likely to unbalance the system (if it was ever properly balanced in the first place), which creates room-to-room pressure differences whenever the air handler is running. Those pressure differences use "the great outdoors" as part of the return path, adding to the heat load, and making for uncomfortably dry indoor air in winter.

 

[Sponsor]

 

[Jered S]

Hey Dana thanks for the reply. Not sure if you received my response or not. There is a cold air return in my basement. It's quite comfortable down there but I feel lit could be better if I changed the cold air return from in the bulkhead down to floor level.
I wasn't going to hack into anything. I was going to add another return at floor level in the area where the current return is. Damper both of them and use them depending on the season. In the winter when the warm air is on the ceiling I would damper shut the ceiling return and open the floor return to pull the warm air down thus better circulation. And in the summer months I would only use the fan on my furnace to circulate the air. Damper closed the floor return and open the ceiling return to pull the warm air only to circulate better.
Is this concept worth my time to do? Thanks again.

 

[Dana]

Any time you increase the impedance of the return path it risks unbalancing the system. A competent pro who cared about system balancing and efficiency would use a manometer to measure the room to room pressure differences under different damper settings, or the pressure across closed doors in rooms with supply registers with a dampered-down (or no) return. Ideally there would be no more than 3 pascals (0.012 water inches) pressure between adjacent rooms under all operating conditions, which puts an upper bound on how much air handler driven infiltration there is. (The < 3Pa number is a requirement for Energy Star homes.) If there's more pressure difference than that the system is a bit unbalanced, and tweaking one room can have unintended consequences for other rooms.

Most hand-held manometers used by the HVAC trades only have 0.01 w.i. resolution, so they would shoot for zero, accept a reading 0.01, but not 0.02. If it's 0.02" or higher, opening up some return path (be it door cuts jump ducts, or tweaking a balancing vane) would be called for. If you're the type who loves owning tools even if they're only rarely used, there are some hand held manometers with 0.01" resolution that cost less than USD$150, even some versions in the USD$50 range (eg RISEPRO HT-189o) that will probably work well enough for your purposes, even if a pro might want something nicer.

It's not a bad idea to measure (and correct) your room pressures and correct things, whether you monkey with return-duct dampers or not, but definitely necessary if you start playing with it. The sad fact of the home HVAC industry is that most duct systems are hacked at both the design and installation stages, and rarely commissioned with flow & pressure measurements to verify that as-installed it meets the Manual-D duct designers' specs.

Running an air handler to circulate air is pretty inefficient, and heats up the house with the air handler's power use. Small fans used only in rooms when occupied heat up the house less. A night-time ventilation cooling strategy using a whole house fan and some open windows will usually work in your climate.

Are the ducts above the attic insulation, or is there a service cavity below the pressure & insulation boundary at the attic floor?