Who to believe???

[FVHowler]

Just a little background. Our house is 2,000sqft in hot/humid NC. The house is 2 story and is 22 years old with the original 3-ton Carrier heatpump. Just one unit for both floors. The temperature difference can be 8-10 degrees different between the 2 floors. Other than the system is very old, the temperature difference is the reason we are replacing. Oh yeah throw in a goverment check, so its time for a new heat pump.

Two contractors have inspected my house and will be getting me quotes in a few days. They both noticed the small return and supply duct (10") coming from the 2nd floor and saw very little space for a bigger size. The working space is a vertical shaft from the 2nd floor down to the crawl space.

Contractor (A) recommends a dual zone (2 thermostats) system and Contractor (B) says to do two systems (one unit per floor). In fact Contractor (B) refuses to give me a quote on the dual zone because of the small return/supply from the 2nd floor. He is adamant about the dual zone not being able to send enough cool air up stairs when the system is calling for it because of the smallish 10" duct.

I have another contractor coming out next week but kinda confused right now.

Sorry to run long. I described the best I could. Any thoughts on this??

Thanks.

FV

[Shophound]

If you can afford it, you'll likely be happier with one system per floor (two systems total). If there was a higher level of competency among residential installers/service technicians regarding zoning systems, that might be viable option for a one system configuration. In reality, two systems tend to do better in multi-story homes than single systems with no or poorly configured zoning systems do.

[Cold Feet]

A 10" round duct is only good for about 380cfm at normal friction rates. That's enough for 1 ton of heat/cool at most. It will not be good enough for a heat pump system unless the upstairs level is very well insulated and has very good, small, windows.

If you install a second system it will need to be upstairs (or on the roof) so it isn't stuck behind the 10" bottleneck. If you can't fit a system upstairs, you'll need to either have the duct work expanded or look at a ductless multisplit.

[dash]

If ducts to the second floor can be sized properly,one system ,two zones is the way to go!!!!!!!

[BaldLoonie]

But if not, a 2nd system is the only way. We added a 2nd system to a zoned house a couple years ago. The duct to upstairs was too small to move enough air to cool that area, even zoned.

[Airmechanical]

IMO, i don't see a big difference between either choice

no matter what you do, you still need some ductwork on that second floor

but personally, i would have 2 system's, the advantage would be

if 1 system ever broke down, you could sleep in the other system's area



.

[FVHowler]

Thanks for the responses.

There is room for a 2nd air handler in the attic above the 2nd floor. In a 2 unit system, I guess this would eliminate the need for 10" return/supply in the vertical shaft. I assume all you would run up thru is electrical and refrigerant to the 2nd floor attic. This will be a split system of course.

Leaning torwards the two units but still interested in the zone system with one unit (mainly price), if I can find a way to get more air up there without tearing out walls.

[heaterman]

Two systems hands down. More control and more efficient.

[Airmechanical]

Two systems hands down. More control and more efficient.

what are you basing your eficiency numbers on?



.

[motoguy128]

IMO, i don't see a big difference between either choice

no matter what you do, you still need some ductwork on that second floor

but personally, i would have 2 system's, the advantage would be

if 1 system ever broke down, you could sleep in the other system's area



.

OTOH, you've double the chances that one of the systems will go down.

[wahoo]

How many sq. ft is your upper floor? If it's the same as the lower (1000) then the 10" round will not supply enough air to condition the space, even with the zone system. Just use the chase to run lineset and wiring to upper unit, and go with 2 systems. If you can increase size, or add another riser duct, then zoning might work. P.S. If either contractor was the one who put the original system in, don't hire them!

[FVHowler]

The break down of sq ft is like this: (1st Flr 1,118) and (2nd Flr 897) so the upstairs is a bit smaller. If I go with 2 units, should they both be 1.5 ton? You guys are great. Thanks!

[Airmechanical]

sounds like you need a heat calc there

don't forget to factor in global warming


.

[FVHowler]

Okay, wanted to follow up with my original post. The third contractor (C) also recommends a dual zone system. Says he can overcome the 10" upstairs return be enlarging the downstairs return. He did say you may hear air coming through the upstairs vents when the system is calling for more air because of the smallish ductwork its being forced up thru.

So Contractor A and C recommend dual zone, while Contractor B says 2 independent systems is the best choice.

Any additional thoughts? Is Contractor C right about making up the 10" return with enlarging the one downstairs? I want to make the best decision I can afford. Ideally, I'd prefer two separate systems but if a zone system can work, it might be the best at least financially. Anyway, still debating this one.

[Cold Feet]

Larger returns downstairs can make up for inadequate return capacity upstairs provided the house is open enough for return air to find its way downstairs around doors, down the stair case, etc. It'll make the place feel drafty, though.

Larger returns downstairs will not make up for the 10" supply duct to the upper level. You'll need to enlarge the upstairs supply for any zoning system to work.

[kajer]

it all depends on what kind of money you want to spend. Use your exsiting system or buy two new systems.

[yourairman]

10" pipe can handle more than 1 ton of air depending on the velocity. Residential systems have the ability to push way more velocity than what traditional rectangular ductwork allows. If it were my house I would zone it every time. 1 system costs less to maintain than 2. If you compare the total amp draw of two 1 1/2 systems to one 3 ton system the 3 ton wins every time. You would need to make sure the zoned system would be able to modulate the dampers as opposed to dampers that are either open or closed. Meaning the system would come on one time and send just the amount of load needed for each zone.

[FVHowler]

Thanks again or the responses guys.

Yourairman: are you saying 10" pipe (instead of 10" flex duct) is better and more efficient at pushing more air up to the 2nd floor?

BTW: I have the 4th contractor coming out on Thursday and then I will make a decision.

[yourairman]

Rigid pipe always flows more air than flex pipe with a spiral internal sleeve.

[heaterman]

The bulk of your heat load is the second floor. You already know that you are lacking in the duct system area particularly the return side. Without a major revamp in the duct system you are going to try to push all of the available air from a 3 ton system through that deficient duct system. The bulk of the time, the second floor thermostat will be the one in control of your system so you will be running a 3 ton system to satisfy your cooling needs when a 1.5 ton system could do it. If you can afford the two separate systems, that would be my choice. Not only will the demand needs be better met, neither system will be beating itself to death and heaven forbid should one system go down, you will still have some cooling from the other.