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Old 06-13-2009, 07:59 PM   #1
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Needing to replace a/c unit...have questions


I just had my a/c unit freeze up for the second time in a couple months. I'm in south Florida and its getting hot so need to get this done soon. Had a guy come out and check it out and I have a leak in the outside unit. He filled it up for me for now, but we don't know how long it will last...now for the questions...I have a 3.5 ton 10 seer unit from 1994. The a/c guy quoted me a couple prices on 2 Rheem units both 4 ton, 14 or 15 seer $2900 and $3250 respectively and 2 Bryant units first a 14 seer 4 ton inside unit and 3.5 ton outside for $3200 and 15.5 seer for $3650. All with new R-410a freon. Not a huge price difference between the four units, but are these good units(brands)? or should I look into other brands, sizes? Oh yeah out home is just under 1500 sq ft. If i'm leaving something out i'm sorry just let me know...Thank for the help.

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Old 06-13-2009, 08:08 PM   #2
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Needing to replace a/c unit...have questions


With only 1500 sq ft I'd stay with a 3 or 3.5 tn for the condenser. Going with a coil of the same size of .5 tn larger for better effenicy is ok.

A 4tn system is too large for 1500 sq ft unless you have a lot of windows and high ceilings. When increasing size the installer needs to be sure that the return and supply ducting will handle cfm

I'd go with the rheem over the bryant..prices seem fair

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Old 06-13-2009, 08:23 PM   #3
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Needing to replace a/c unit...have questions


We have 11 foot ceilings at it highest point sloping down to 8 ft. that covers about 400 to 450 sq ft. The rest of the house has 8 foot flat ceilings. Every room has a good size window in it. The front living room has a huge window that takes up most of the wall and the back "Florida room", that's what they call it down here, has a double window and double doors. Is going from the current 3.5 ton to a 4 ton and huge difference?
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Old 06-13-2009, 08:52 PM   #4
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Needing to replace a/c unit...have questions


Going from a 3.5 to a 4 to me is really not needed. If the 3.5 has been doing fine and cooling fine then no don't do it. Go with a 3.5 condenser and a 4 inside coil. Increasing the coil size will further increase effenicy.

Oversizing will cool a house down fast but won't condition the air. Humidity removal. Most customers want the a/c not to run very long and that's understandable but, the a/c needs to run so that the air is conditioned.

Oversizing..cooling fast not conditioning inside eviroment
Undersizing..a/c constantly runs and still inside enviroment not conditioned

In my area a 1500 sq ft house would get a 3 tn condenser and a 3.5 coil
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Old 06-13-2009, 09:28 PM   #5
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Needing to replace a/c unit...have questions


I'm not sure where you are located, but would I be suggested to go with the 4 because of where I live then? due to the hot, humid summers? The a/c we have cools well, just runs a lot. Been timing after the recharge today and it runs for avg of 7 min and off for 5 min. Does that sound like too much on and off? I thought it should be more of a gap, run more when its on but stay off longer too.

Last edited by jstcruzin; 06-13-2009 at 09:53 PM.
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Old 06-13-2009, 09:34 PM   #6
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Needing to replace a/c unit...have questions


Even if sized perfectly for the sq ft and geographic area, even a new high efficiency unit will short cycle if there is inadequate insulation. When there is temp rise, the thermostat will call for cool and it runs till satisfied.
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Old 06-14-2009, 10:14 AM   #7
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Needing to replace a/c unit...have questions


Thanks guys, any other suggestions? Brands? Sizing? The A/C guy did say he doesnt clean the vents and I should call someone like stanley steamer to clean everything after he's done with the install. Also should he be checking for leaks in all the ducts or does the cleaning company do that? I dont think I have any but better to be safe than sorry.
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Old 06-15-2009, 07:59 AM   #8
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Needing to replace a/c unit...have questions


If your current 3.5 ton system cools and dehumidifies adequately then I would stick with that size. A system that short cycles is more expensive to buy, will cost your more in power, not dehumidify as needed and possibly cause longevity issues.

Get numerous quotes on numerous brands to compare. Be sure you are comfortable with the installer as even excellent systems can be junk if installed improperly. Install is key.

Take advantage of any local, state and fed credits and incentives you can.

I don't think duct cleaning is worth the money.
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Old 06-15-2009, 08:15 AM   #9
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Needing to replace a/c unit...have questions


Agreed with plumber101 3 ton condenser unit and a 3.5 ton coil. better to be a little undersized than over sized(4 ton 2 big).
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Old 06-15-2009, 12:14 PM   #10
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Quote:
Originally Posted by dac122 View Post
If your current 3.5 ton system cools and dehumidifies adequately then I would stick with that size. A system that short cycles is more expensive to buy, will cost your more in power, not dehumidify as needed and possibly cause longevity issues.

Get numerous quotes on numerous brands to compare. Be sure you are comfortable with the installer as even excellent systems can be junk if installed improperly. Install is key.

Take advantage of any local, state and fed credits and incentives you can.

I don't think duct cleaning is worth the money.
What do you mean by short cycles? I have about 7 min on and 5-6 min off right now. Is that considered average run time or is that short? would a larger system do that? I was asking my wife if she thought the a/c turned on and off a lot, she home more than I am, and she says it does more so than at her moms. To me turning on and off every 5-7 min is too much. Would that be form a system that is too big or too small??
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Old 06-15-2009, 01:08 PM   #11
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Needing to replace a/c unit...have questions


Quote:
Originally Posted by jstcruzin View Post
What do you mean by short cycles? I have about 7 min on and 5-6 min off right now. Is that considered average run time or is that short? would a larger system do that? I was asking my wife if she thought the a/c turned on and off a lot, she home more than I am, and she says it does more so than at her moms. To me turning on and off every 5-7 min is too much. Would that be form a system that is too big or too small??
Your total on-off cycle time is 5 per hour and that's acceptable for what most manufacturers like to see on compressors and motors. But to answer the question, there is no exact answer. Of more importance how is your humidity removal? Can yo get down to 50% or so?

Here's a discussion link to help
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Old 06-15-2009, 05:14 PM   #12
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Needing to replace a/c unit...have questions


I dont have any way of measuring the humidity. I have the temp set to 77 and its been about 90-95 the last couple days. Is there any kind of online, DIY calculator I can use to figure out the best unit for me so I dont have to depend on what these companies tell me.

You guys are all telling me a 3 or 3.5 ton is what I should be looking for (I'm likely to believe that) and so far everyone that I call out to my house is telling me to go up to a 4 ton unit from my current 3.5. Only the first guy quoted me for the 3.5 outside unit and the other three he suggested were all 4 ton units also.
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Old 06-15-2009, 05:44 PM   #13
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Needing to replace a/c unit...have questions


If 3.5 tons is cooling your house fine now. then you don't need a 4 ton unit.

Try HVAC CALC, its a sizing program.

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