any HOME HVAC contractors on the forum?

[maytag]

Hey all....
My central A/C quit last night. Well, I think it actually froze-up. I haven't looked yet. But I suspect it is part of a larger problem that I'm hoping somebody here knows something about.

Here's the nutshell:
I'm in Salt Lake City area, where we see typical winters with 15 days of <0-d weather, and summers with 15-days of >100-d weather. Home was built in '63. it's a Ranch-style with a finished basement. No insulation in the walls (brick over wood-framing).
It was built with a swamp cooler and forced-air-central heat. There's 3" ducts running to the upstairs floor vents, and 4" to the downstairs. None of them are insulated either. For the heat, this works well; the steel ducts radiate heat which warms the floor above and the ceiling below. Hot air rising means they've focused on getting most of the heated air downstairs.

Here's the problem though: I added central air 3 years ago (along with more attic insulation and really good thermal windows/doors). The effect of all of the uninsulated and undersized ducts is that the basement chills like a freezer, and the upstairs stays too hot. Of course we close all the downstairs vents, and open all the upstairs, but the basement ceiling is very cold to the touch. Last year I added another air return to the upstairs, hoping to get that air moving.

So I'm betting that this weekend I'll be tearing-down the ceiling in the finished basement to replace ducts. But I'm not an HVAC guy. What size ducts should I have? 6" seems common? And what is the best / most cost effective way to run duct? is it to run an insulted/flexible duct? Or is it as good to run steel duct in the joist cavity and then stuff 'glass batts around it before putting the ceiling back up? (I'll be insulating that floor/ceiling anyway)

Can anyone shed some light on this?

[rlux4]

I think I can give you a little input Maytag. I was a steam operating engineeer at UCLA for ten years, and air handling was a major part of my job. First, the size of the duct is determined by the area it needs to supply. 6" would be fine if you're supplying one room in the neighborhood of 300-400 sq. ft. Any bigger than that and you wouldn't get enough volume to keep up with the heat load. This depends on the output of the unit of course. As far as what kind of ducting to use, flexible ducting was designed with cost in mind. It's the cheapest way to go, but also the most inefficient. The most efficient way to move air is through a smooth duct with no curves. This is not possible in most cases though, so any curve should be smooth and gradual (not abrubt). Every curve or ridge in a duct will cause turbulence and slow down air flow. Again, sheet metal ducting is more expensive, but much more effecient.
A local HVAC contractor would do a calculation based on the output of your unit and the size of the room(s) it needs to supply for a minimum amount.
Ron

[maytag]

thanks for the quick & helpful reply, Ron!

because the A/C unit is new, adn the blower / heater it's attached to is relatively new as well (5 yrs ago) I'm not going to change it. I asked the contractor that installed the A/C unit to size it appropriately, but he told me my ducts were too small. the added air-return was his idea.

So I think for now, I'm working with what I've got, system-wise. If I'm cooling smaller bedrooms of less than 200 sq', is 6" duct too much? Is there an important volume / velocity relationship I need to maintain? And If I use 6" throughout and "tune" with the dampers built into the floor registers, is that bad?

[rlux4]

If you're using the dampers to control the volume into each zone, then you're OK. The unit will react to the temperature where the stat is located, so it's just a matter of adjusting each room to comfort level in relation to what's happening there.
It was very common at UCLA to find remodels that had cut big rooms into a bunch of little ones with only the area where the stat was located having control. I spent a lot of my time tweaking the dampers to get each area as close to comfortable as possible. The heat load in each zone will have as much to do with it as the size.
Ron

[baltobernie]

If you can't get it tweaked manually, ask your contractor for an estimate to install a "zoned" damper system. A single-board computer adjusts the airflow to as many zones as you like. Every zone gets its own thermostat. Works great for heat as well as A/C.