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Ductwork Advice

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Sep 15, 2013, 08:09 PM
Ductwork Advice
Hello All,

I bought a two floor house with central heating and air. There are output vents on the floors of the rooms upstairs and the ceiling of the basement. However, there are no return registers downstairs.

I was thinking that adding a return register in the basement would help keep the house more evenly heated/cooled. It would cycle the coolest air at the basement floor, which currently doesn't cycle.

To do this seems relatively simple... the furnace unit is in the basement, with the return ducting 13.5" away from an existing 14x6" grill at the basement floor. That grill isn't connected to any ductwork... It just provides some quite limited ventilation to the furnace room.

My problem is how to convert the 14x6" register down to a 4" diameter connection so that I'm still providing adequate return airflow from the existing registers upstairs. With only 13.5" to make that reduction, is this possible? Can I get something custom fabricated (where?)? Or maybe I use a 14x6 => 8" to 6" reduction with a restriction plate within the 14x6" attachment piece.

What do you all think? Sorry for the book of a question!
Sep 15, 2013, 10:38 PM
You can make up any size of duct reducer you need out of sheet metal. Easy enough to do as long as you don't mind slicing your fingers up a little.

However, there is a reason for no return in the basement. IF you only have one thermostat and it's located on the main living floor (as it should be) then you are better off returning the basement air up the stairway, into the main floor living space and then back down into the unit via the return grill and ductwork. Basically, if you only have one T-stat then you want to do everything you can to mix the air temperature from the two different floors.

IF you really want to balance the temperatures between the basement and main floor, you'll have to invest in a zone damper system. This will add a thermostat to the basement area as well as it's own return. The zone system will work very well to keep both the basement and upstairs living space at desired temperatures. Of course, you'll need to spend several thousand dollars to do it.

Without a zone sytem (or a split, two-unit system) your basement will always remain cooler in both summer and winter.

Sep 15, 2013, 10:53 PM
Hi Jaybee,

Thanks for the reply. I still don't understand why it's better to return the air up the stairs even with one thermostat. The thermostat is in the primary living area (upstairs), and in order to do as you say I end up pumping lots of air conditioned air into the basement just to ensure air circulation. In other words, I'm wasting lots of energy cooling an area that doesn't need to be cooled. If I could instead open a return in the basement and close all the vents in the basement, I still use the staircase to transfer airflow, but now I'm only expending energy to cool the upstairs... relying upon cool air falling to cool the basement. Sounds much much more efficient to me.

In the winter, I can close the return register and reopen the basement vents, again giving me optimal heating of the basement.

This is a poor-man's dual zone system, where I physically control the vents in the same way a computer would.

Am I missing something?
Sep 16, 2013, 08:59 AM
You have perfectly described the "poor mans zone system" - using dampers to control airflow. And you are doing it correctly, in that you are physically feeling the temperature in the basement and manually adjusting the dampers accordingly. It can work.......OK, but you will always have a temperature difference between the basement and upstairs.

The thing is, your system will only cycle on and off after getting temperature input from the thermostat location. How well this works will vary by the season.

In summer, adjusting the dampers works well enough. The basement will naturally stay cooler, so all you need to do is to close most of the airflow off to the basement and it will remain comfortably cool.

However, in the winter it becomes a problem. Here too, the basement will remain cooler than the upstairs area - only now you want to make it warmer. So you open all your ducts to allow as much warm air as possible in. But, the system will only run when the upstairs falls below the set temperature. If you keep the basement isolated via it's own return air run, then the upstairs will warm up faster and the system will turn off - while the basement is still cool. If the cooler basement air is returned to the upstairs it will mix with the warmer upstairs air, causing the system to stay on a little longer which gets more warm air into the basement.

A single thermostat, multi-floor system will never work perfectly but the short story is that if you try to contain the basement airflow with a balanced supply and return for the basement only then it will work fine for cooling but not very well for heating.

Sep 16, 2013, 10:14 AM
Thanks, that makes sense.

I'm sure I could fab up the necessary duct work, but I think it would be better worth my time/money/frustration to pay someone to make it for me. Any suggestions on where I could have that done?

Sep 16, 2013, 11:20 AM
(sorry this turned into the Great American Novelette.)

any HVAC shop will make up the ductwork. they can also adjust the system for correct balance. there is, yes, a cost involved.

hit your home center. I don't see much ductwork at Big Orange, while there is a better selection at an upper midwest chain, Menards. duct typically comes in rectangular half-sections which you pop together at the work site, or in round duct which needs to be snapped together. once it's assembled, it's pretty much for keeps. things like round sweeps are best bought prefab.

you will definitely reduce airflow upstairs if you tap the intake duct for the basement. the ducting in the house was designed and built that way for a reason, and I wouldn't make any significant changes without having a pro run the numbers. while you might get much of the airflow back by shifting the furnace motor to its highest speed (many are set one wire below max,) you will increase noise and possibly reduce the maximum temp out the registers. there are only so many BTUs in the furnace design, and they are more spread out with higher airflow over the heat exchanger.

what is your temp upstairs/basement? it is typical for basements to stubbornly resist going above 50 degrees due to the thermal mass of the ground surrounding the basement.

a heated floor in the bathroom will help a lot to make that basement room a little cozier. the system we installed is a 200 watt heater wire, and like all electric heat, shows up noticeably on the power bill, perhaps 10 bucks a month. larger areas than a 7x7 will add additional dollarage.

what we do in our cozy family room from the gut and rebuild is leave a series of fleece throws on the sectional. sit down, snuggle in, and toss on one or two fleeces. it also attracts the cats, which is why the place gets very cozy. we crack the registers a little bit wider in winter.

I found a lot, A LOT, of crummy joints in the ductwork in our gut, and replaced one twisted-out elbow and taped all joints. didn't get around to seaming all the rectangular sections before the drywall went up, but found several gaping interstud return sections that were duly sealed and seamed to stop wild air intake from the closets and such. one huge nasty opening to the main return was fixed up, and airflow was greatly improved.

get after the leaks before you add a basement duct. we had a 6-inch equivalent in the center of the basement for return, and the leakage was probably just as much. we are enjoying the upstairs just as much, and the downstairs can rise to 60 now. the sloppy workmanship violated the original design of the system, which should now be working as it was engineered by the HVAC contractor.

you can also get after window and floor leaks if you take the trim off, caulk the openings around the windows and wall/floor junctions, and put the trim on.

these changes, along with doubling the attic insulation to R35 from an R18 and properly insulating the basement exterior walls with batts, cut our fuel oil usage by two thirds. yes, tank and a half of oil down to between 1/3 and 1/2 tank. there are a LOT of places you can DIY a change without hacking in ducts.

tightening up a house can raise issues with intake combustion air for the furnace, gas heater, etc. don't tighten up the furnace room air leaks. watch your flame color. if it goes yellow, you will need to add a duct to the outside air to insure a blue, efficient flame and reduce CO dangers. you don't want to oversize that, which could lead to frozen plumbing. your annual inspection could lead to a demand that you provide more combustion air. if so, do it, don't argue. there's more to it than just pretty colors, there are safety issues that are urgent if you don't have enough air going up the chimney.

This message has been edited. Last edited by: swschrad,

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