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.

[chillbilly]

Sounds like you need to get some accurate load calcs.
What's the budget look like and all that stuff.
I would get dedicated systems for each floor too but that's just a personal selection for me.

[hvac hero]

2 systems is the only way to go. You'll be glad you spent the extra money. I see so many houses around here that have 1 system for 2 floors. Its always a lot warmer upstairs than downstairs. 4 tons for the whole house seems very small.

[dan sw fl]

Originally posted by 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

4-tons is too little for 3,600 S.F. for many days in N.J.
However, if you don't mind 82'F inside temp for many afternoons, go for it. Just make sure you have enough ceiling fans.

How much is your comfort worth?

TWO 3-ton systems should not be that much more $
than ONE zoned 5-ton system.

[wyounger]

A single unit can do two stories just fine, but you need a good zone control system, good duct design, and preferably two-stage equipment to do so.

If the load calc indicates that a single 4 or 5 ton unit can do the job, I wouldn't hesitate to use a Carrier Infinity zoning system. It will usually cost less up front than two systems, take less energy to run, less cost to service (one system instead of two), and with the capacity modulation and variable air volume capabilities, should be most comfortable too. Plus you can control and/or program all the zones from a single point (or multiple points, if you set it up that way). You can put in a single high-end zoned system for the same money (or less) than two mid-range systems.

A single zoned system can often be of slightly less capacity than you would use in twin systems, because the load peaks in different zones often occur at different times. You may not care that the bedrooms are a little warm at 6 pm, for example, because you never go to bed before 10pm. By having one small system work hard, you can get much closer to its theoretical maximum efficiency than by having two systems that are forced to cycle on and off more of the time.

Caveats:
1. Yes, you lose the redundancy of being able to retreat to the other floor should one system fail.
2. Zoning works very well when done properly (like many things, it's much easier to do it wrong than to do it right!) and used with reasonable expectations. It will do fine at maintaining an even temperature between upstairs and downstairs. It can also maintain somewhat different conditions between the two (5-8 degrees). If you're looking at a situation where you may not want to heat or cool one floor for long periods (the classic empty nester with master-on-main-floor setup), though, zoning will not work as well, and then you should absolutely have two separate systems for the two floors.
3. The floorplan will have to allow some space for ductwork to go from one floor to the other; this may not be necessary with two systems.

As for sizing the equipment, don't forget that cooling equipment is rated based on the assumption of an 80 degree indoor temperature. So even if your system designer used the temperature you requested- say 75- for the system to maintain during cooling season, that only can account for how much cooling the structure needs to hold 75. The AC equipment has less capacity when the indoor temp is 75 than it does at 80. So if you want an indoor temperature of less than 80 during cooling season, you need to de-rate the capacity of the cooling equipment by 835 btu, per degree below 80, per 1000 cfm. So for a "five ton" system, which today is typically more like 56,000 BTU at 2000 cfm, if you want 75 degrees, you have to derate the equipment capacity by 835 btu * 5 degrees * 2, or 8350 BTU- more than half a ton. That leaves you with 47,650 in this sample case. So a Manual J can easily say you need four tons, but if you want much less than 80 degrees indoors during cooling season, you will often need 5 tons.

I think this is the primary reason that people mistrust Manual J; they think it will tell them what size equipment to use, don't correct for equipment capacity ratings, and then can't figure out why the system can't hold the desired temperature in the summer. It's not Manual J's fault, they're just not interpreting the results correctly. They may well have done the measurements and calculations perfectly, and still had poor results because of this issue. Manual J tells you what the structure needs; this correction tells you what the equipment can actually do under the intended conditions.

All that said, if after Manual J and correcting for equipment capacity you discover that 5 tons won't be quite enough, then you will need two systems regardless