Two Seperate A/C units versus a single Two Stage unit

[bat1029]

I have a brand new home being built (3600 Sq.FT., 1800 1st fl. and 1800 2nd fl.) and got estimates from two respected HVAC firms in my area (northern NJ)
One suggested a 4ton unit for second floor and a 3ton unit for first floor using trane units
The other suggested using a Carrier Two Stage 4 ton unit for the whole house Any input would be appreciated

[davesrha]

There is quite a difference in tonnage for the house have a manual J done and know what you need.From our experience living in the south 2 systems would be better. You would be able to keep both levels at your desired temp.

[Irishmist]

First and foremost, the contractor that performs a manual J load calculation is doing his 'homework.' Aside from that, I would also agree with two (2) systems. Balance is easier to achieve, you can place two smaller (typically with higher efficiency ratings than larger equipment) systems in place with smaller ductwork, and as an added bonus, you will have redundancy, i.e. if one system should fail, you can 'retreat' to a conditioned space. Sounds like you are able to get in direct contact with the HVAC contractor, or you are contracting the work directly yourself. With new construction alot of the time, you are relegated to whatever the builder decides. I would definited go with two (2) systems and make sure a good, well thought out air distribution system is in place on each. (suggest NO flex duct!) Good luck. J.

[bat1029]

Actually the contractor using two units is also using hard duct, insulated thru-out the entire house, except for the returns, he said because of noise. The other is using hard duct in attic only, then switching to flex to all rooms

[Freezeking2000]

The contractor that is using one unit will save you thousands a year in electric costs (potentially). A 2 stage unit running continuously will de-humidify better and reduce you electrical consumption tremendously. It will also never shut off on design days which is what you want. It is amazing that you need this much cooling (the 2 system guy). What i find is with the open floor plan and high ceilings in the entry the whole home is like one large zone for cooling since the first floor heat all rises to the upstairs. Get a 15-16 seer+ single and you will cool for less $$$$ every month. If you want the home to be 65 degrees get 2 systems.

[bornriding]

Unless you can seperate the upstairs from downstairs - ie, a door between the two - two systems will cost you more to operate than one system.
Without a seperation, the conditioned air from one system will move into the space of the other system.
When the downstairs system is cooling, it will push the warmer air upstairs, causing the upstairs system to have to turn on to maintain its temp.
When the upstairs system is cooling, the cool air will flow downstairs causing the upstairs to run longer.
One system with properly installed ductwork will keep the home comfortable for less energy usage.

Richard

[theslumlord]

Using a two speed system is more for humidity control under low loads (cloudy, but very humid summer day). When you have two floors, generally the load profile are rather different for both. Its easier to balance a system with two zones.


However, you can't guess the load profile. As a previous poster stated, a load calc should have been done on your home. I just finished up one analyis for an older home in Bloomfield, NJ where the downstairs had a larger load that peaked later in the day than the upstairs. Reason was due to the number of windows in the downstairs, their positions, and a number of other factors.

Two system will almost always cost you more than a single system. Sometimes you save a little on labor costs because a system on each floor might be easier to install. You will have more flexibility and better control of energy costs:: i.e. you can turn up the thermostat upstairs because everyone is downstairs during the day.

[backbeatkeeper]

I agree have a load calc done, but I would also look into zoning with one unit. Doesn't cost as much as two, but depending on the areas your are in at what time of day, you can have seperate zones with one system that can do just as good a job as two, with less cost. Plus if the downstairs is a seperate zone than the upstairs, you can offset the thermostats. Because of heat rising and cold air falling and offset temperatures of the t-stats, sometimes the one zone calling can satisfy the opposite zone.

[turtle]

Originally posted by bat1029
Actually the contractor using two units is also using hard duct, insulated thru-out the entire house, except for the returns, he said because of noise. The other is using hard duct in attic only, then switching to flex to all rooms

This is Turtle.

If you can afford the two seperate system for top and bottom floor do it , if not get you a single type system.

Have all the Flex Duct used in your home be of atleast R-6 insulation nature or rating.

TURTLE

[dash]

One two stage heat and cool ,With Infinity zoning will be the most comfortable,and save more(assuming same SEER,etc) in the long run.

[peytonc2682]

Step 1: Manual J to be sure that the unit is sized properly.

Step 2: if less than 5 tons, use 1 2stage system and zoning. If the load is more than 5 tons, use 2 properly sized systems.

Step 3: metal duct is usually better than flex but if flex is done correctly is makes a good duct system.

Step 4: Be comfortable

[jared]

Have a heat load calculation done on the house...go ahead and get two properly sized systems....with one system you will never be truly comfortable...

[dash]

Originally posted by peytonc2682
Step 1: Manual J to be sure that the unit is sized properly.

Step 2: if less than 5 tons, use 1 2stage system and zoning. If the load is more than 5 tons, use 2 properly sized systems.

Step 3: metal duct is usually better than flex but if flex is done correctly is makes a good duct system.

Step 4: Be comfortable

Great advice.

Step 1.5 Manual D, to size the ducts correctly for the cfms(airflow) determined in Manual J.