# Central Air replacemnet - need help



## scfa99 (Jun 7, 2006)

Our 19 year old original ac unit is on its way out.  It runs constantly during warm weather.  House is 2700 sqft center hall, r40 in the attic, new windows, not sure about wall insulation R.  Anyway I had 3 firms come in and give me quotes.

Original unit is a 4 ton, 10 seer unit, running at approx 8 -9 seer (i assume).   First company said 4 ton is correct, he measured all rooms and I assume did a schedule J and spent over an hour at the house.

2nd firm said I need a 3.5 ton unit, just went off of the total sqft of the house.

3rd firm said the current unit is undersized for our house and we need a 5 ton unit, he was in and out.

So who the heck do i believe?  BTW the first firm quoted me an installed price of 5500 for a 14 seer lenox and 6800 for a 15 seer 2 stage lenox.  i almost fell over.  2nd company quoted trane and the price was the same for a 14 seer but its a 3.5 ton.  3rd firm didn't get back to me yet.

Are these prices outrageous or inline with what i should expect?  what seer should I get for a NJ climate?  We plan on staying in the house for 10+ years.  Is lennox a good brand?  Any rec'ds on brand or anything else I need to ask to figure out the correct size?  I know this is a bit off topic but you guys always come through with valuable info.


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## elkimmeg (Jun 7, 2006)

Couple of thoughts: Oversizing systems is poor practice. They have a cycly time needed for optium preformance. Oversizing them cause ineffecient short cycles. Code calls for Seer 13. However due to current whearehouse supplies, Codes have been relaxed 6 months so that current inventory has been sold. Some of the Seer is due to fan speed the same unit could be Seer 13-15 

 You you telling me you have one ceiling reture in the Hall? none in the bedrooms? none on the first floor? I no wonder yours cycles all the time from inadequate returns. Buying a new system,  using the same return  is useless, without correcting your current installation.  ITs not the equipment that failed, it was forced into failure by inadequate ducting instalation.  Do not expect  much improvement with new equipment. I am willing to bet ,if you improved added returns your, current system would probably handle the demand.  That a problem with this industry, when in doubt over kill it solves defeciencies right? No wrong ? Let me ask my experts about sizing or dig into my books and look it up.  It seems HVAC contractors have plenty of feeds  Supply = returns, but unless returns are  located well it is hard to draw and cycle air.  Too many  not  adequately locate and allow for cycle distrobution.
 I have seen attics as hot as 120 degrees and see the unit encased in ice. Why  very poor or inadequate returns?


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## Sandor (Jun 8, 2006)

PLEASE LISTEN.

GO WITH THE SMALLEST UNIT POSSIBLE WITH THE HIGHEST SEER RATING

I have built 15 homes in the last 3 years and have witnessed the problems with oversized AC units.

You would MUCH rather have a 3.5 ton unit that runs 16 hours a day versus a 5 ton unit that does not run long enough to dehumidify the air. Cold, clamy and moldy is what you will get.

Go with the 3.5 ton Trane, 14 Seer. Period. DO NOT EVEN CONSIDER a 5 ton unit for that home you described.

5K bucks is about right.

Bag the Lennox, go Trane, Carrier, or Rudd/Rheem.


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## paulgp602 (Jun 8, 2006)

Also keep in mind that you will not achieve the SEER rating of the compressor without replacing your airhandler with one that is matched to the compressor. When I had my Trane system installed, I went with the XL14i compressor and the variable speed Air handler with the "comfort R" feature. This let me achieve the 14SEER rating. With a single speed air handler, the SEER rating would not have been reached. Check out www.hvac-talk.com
  just be sure not to ask any DIY questions because the forum isnt a DIY site. Good luck.


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## elkimmeg (Jun 8, 2006)

How to size your home for air conditioning? I reviewed the 2004 ASHRAE Handbook. The recommendation are in new construction well insulated  tight homes. about 600 sq ft area per ton of cooling. In older less insulated and tight homes, 400 sq ft floor space per ton. Other recommendations are to over size the exchanger up one size but size the compressor to match. Example a 5-ton exchanger matched with a 4 ton compressor. What this does is allow for a larger capacity for removing humidity and using the longer cycle time of the compressor to augment humidity removal. Your collection area is larger in the exchanger. Another benefit of the larger exchanger is the added capacity to deal with longer duct runs. Sandor is correct, it is better to size the compressor to the exact demand than to oversize and short cycle it. To answer the question which is better 3.5 ton or 4 ton? In this situation both contractors could be right with a 1700 sq ft area. Personally I would use the 3.5-ton compressor and 4-ton exchanger. Do not stop there; make improvements to your return system. My prior post recommended


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## fbelec (Jun 8, 2006)

if you have to replace your whole system take a look at the carrier web site and look at the infinity models they have 16 to 21 seer models. decent compressor warranty 10 years.


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