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bedmad` 02-08-2010 10:52 PM

Dual stage or 2 furnaces?
 
I am adding second level to a 1100 sq ft house, effectively doubling to 2200 sq ft. I am consideringa dual stage high efficiency furnace and air conditioner, in Saskatchewan, Canada.
I was told that the damper controls can be expensive, and a nightmare to maintain to control air flow to the upper level. Maintenance might require rippign out drywall to get at duct work. Are they reliable?
Would it make sense to have two smaller furnace, one for each level? I wouldn't have to change the lower duct work this way and would have full control on each level. Is it insane to go two furnaces, or just more expensive?

user1007 02-09-2010 03:52 AM

I am not an HVAC person but order the retrofit and replacement of a lot of it. One of my HVAC guys swears by the concept of buying two separate units, as high efficiency as you can afford, neither necessarily rated for the whole 2200sf, and with different performance characteristics between heat and AC.

His whole approach is based on hot air rising and cold air sinking. Your greatest heating challenge is going to be warming up cold air that sinks toward the bottom of the house and your AC challenge is to cool the hot air rising up?

When he can, he puts the major heating plant downstairs (usually the basement) and uses smaller heating capability upstairs to balance it out and back it up when needed. And most people don't want the sleeping areas (usually upstairs) so warm as living rooms, rec rooms, etc. because tucked under grandma's comforter it gets too hot to sleep well. A thermostat on a timer can kick the upstairs temp up a notch around shower time.

He takes just the opposite for AC. The workhorse goes upstairs (usually in the attic) and is backed up by a lesser capability for AC downstairs. Each has its own thermostat(s).

With two units he doesn't have to run as much duct work and is also better able to damper, balance and feather the system. My clients seem to like his approach so I assume their energy bills are within the range they expect. They say the HVAC units do not seem to run as much as single units they were accustomed to do. For some reason I have had to spend a bit more time explaining the concept to get permits though and of course two units mean two sets of inspections. No biggy though but you might want to look into it if this is going to be an issue where you are.

Do hope you are also going to spend the money to upgrade insulation.

hvaclover 02-09-2010 03:55 AM

Quote:

Originally Posted by bedmad` (Post 396904)
I am adding second level to a 1100 sq ft house, effectively doubling to 2200 sq ft. I am consideringa dual stage high efficiency furnace and air conditioner, in Saskatchewan, Canada.
I was told that the damper controls can be expensive, and a nightmare to maintain to control air flow to the upper level. Maintenance might require rippign out drywall to get at duct work. Are they reliable?
Would it make sense to have two smaller furnace, one for each level? I wouldn't have to change the lower duct work this way and would have full control on each level. Is it insane to go two furnaces, or just more expensive?

Two separate systems would be Ideal. Up and down stairs controlled by two sperate stats.
Best way you can go

beenthere 02-09-2010 05:18 AM

Six one way, half a dozen the other way.

2 systems gives you redundancy. If one breaks down, you still have heat or air conditioning depending on the season.

On small homes. Hard to get both systems sized right.

Zoned. The dampers would be installed in the main trunk lines near the furnace. So they wouldn’t be hard to get to.
Using a single 2 stage system. You get better humidity control then with 2 separate systems. Along with a nice long run time in heating mode to prevent a cool draft feeling as soon as the heat shuts off.

Seen many homes with 2 systems. Where the HVAC guy installed the “workhorse” A/C upstairs. And its oversized (that’s why they call it the “workhorse“ ). So it doesn’t run long enough during the milder temp days to dehumidify the house properly. And the downstairs now becomes cool, but has high humidity.
And the stat often has to be set lower then what the people really desire for them to be comfortable.

Same with the heat somewhat. The duct for the first floor is sized close to what the first floor needs. But the furnace is bigger then what the first floor needs. So hot air blasts out of the registers. And can create a cool draft feeling as soon as the heat stops running.

Seen enough zoned systems where it was done wrong that it couldn’t cool both zones at the same time. Or it froze up when only 1 zone was calling.

Either system can be installed to work great. Or work terrible.

Its how good the contractor is, that will determine how your system works.

SULTINI 02-09-2010 06:02 AM

Two independent systems one up one down with thermostats on each level.

Better control over the conditioned area and not the whole house at the same time.

Yes you can run a system with dampers controls etc, however most of these type systems are in commercial buildings or very large square footage situations which most residential HVAC companies don't design unless you are in a Mansion.

In addition if one system goes down you have the other as backup until repairs are made.

yuri 02-09-2010 06:49 PM

2 systems are better than 1 IMO. Easier to get more accurate and comfortable temp control and having 1 work when the other is down is a real bonus on Xmas eve when all the serviceguys are stuck in their driveways. Happens a lot in Wpg when we get a blizzard and strong winds.

hvaclover 02-09-2010 07:19 PM

I remember one ac job that had two furnaces.

The problem with putting in right sized equipment was the upstairs furnace.

It was over sized to begin with and the correct sized evap would have restricted air flow (the air-side opening of the proper tonnage evap was too small).

I called the Armstrong factory service and they said up size the tonnage.
i questioned that and ..lol....they told me it'll be a good back up if the down stairs ever quits.

Home was about 3200 sqft. What should have been 2 1/2 ton up and 2 1/2 ton down turned into an embarrassing over sizing predicament.


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