manual J calculation

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Old 10-12-02, 02:16 PM
pmammenfl
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manual J calculation

We got 3 bids for a new system. According to the manual J we are right on the border between 2.5 and 3 tons. We have a 2.5 ton system now but besides being old and inefficient we can't cool the house below 77. If we want to lower the temp just 2 extra degrees we need the 3 ton. Does this sound right? What are the down sides of the bigger unit? One contractor said because we will have a variable speed we can leave it on low and it will behave like a 2.5 or when it is very hot or there are more people in the house we can crank it up to a higher speed. Is this right?
 
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Old 10-12-02, 03:22 PM
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ToBig

Hey your down south with me.So look at our schools and all the mold.What do they do they put in more AC. Now it gets cold and shuts off but darn it left all the bad old thing called humidity in here and I still feel hot.Not seeing the home or lay out I still say go for the 2.5 with the new blowers you can get one VS and get a unit with an seer about 14 or 15 and you got it made.Id look at the old duct work and make sure its good.Moved into one home
here half the ducts just blowed out in to the attic. ED
 
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Old 10-12-02, 07:35 PM
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Check your duct work and if possable do a blower door test to see how much leakage you have. The original installer probably went low as far as tonnage to save/make money.

I'd install a 3 ton w/variable speed to save energy cost, and get efficiancy.
 
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Old 10-13-02, 08:05 AM
pmammenfl
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I got one quate for American Standard for 2.5 ton. One Quote for Trane for 3 ton and one quote for Ruud for 3 ton. All of these were 14 SEER. I checked the ducts myself last winter and they seemed ok, but I'll call FPL on Monday and get a test. I still don't understand the variable speed thing. I also don't want mold! Thanks for your input Paul
 
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Old 10-13-02, 12:05 PM
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Cooling comfort is produced by lowering of temperature and removing humidity. Air conditioners must run a significant period of time to remove moisture from the air. Oversized unit cycle frequently and remove less moisture. The capacity to remove moisture from a system is measure by "Sensible Heat Factor (SHF) The SHF depends upon the size and construction of the evaporator coil and fan speed. Humid climates normally require a SHF between 0.67 and 0.77.

Check with either the manufacturer or contractor for the SHF for their units. The SHF is a decimal number between 0.5 to 1.0. Lower numbers remove more moisture and higher less. High SHF are for dry climates, where you don't want to remove too much moisture from air that already has a low humidity level.

Thermostats do not provide good comfort control in very humid climates. Manufacturers have developed units controls that respond to both temperature and humidity, and variable-speed blower that are more flexible in providing both cooling and humidity control.

Also, you could have a unit with a low SHF, but if the unit is oversized, it will be ineffective in removing moisture in the air or lowering humidity. Remember, it take much longer to remove moisture in air than it does to drop the temperature in air.
 
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Old 10-20-02, 08:43 PM
pmammenfl
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2.5 or 3.0

Dude, I'm getting a Trane.
sorry, wrong company
Decided to get the trane xl14i at 3.0 ton. the manual J results put me right on the upper limit of 2.5. If I decide to add in the laundry room, currently not under AC, the manual J is definately for the 3.0. Also Manual J calculations assumes a constant air flow at medium or high speed through the air handler. with the variable speed, even if I am oversized, it will run mostly at low speed. The specs on this one says it will pull 24 gallons a day at low speed. That is the highest of all the ones we considered.
A Story.
My brother in NJ put in a heat pump system with a subterranean pipe for the source. It worked great for several years, then suddenly wasn't dehumidifying. It still got cold enough, but wouldn't take out enough water in the hot nj summer. Eventually the installing contractor found the problem. My brother was doing something in the attic and decided it was a good idea to increase the insulation up there. More is always better when it comes to insulation, right?
Not this time. by adding insulation he changed the heat load characteristic, decreasing run time on the heat pump. The fix was to leave one window open in the house.
Paul
 
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Old 10-21-02, 05:49 PM
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Good story

Good idea to go with the Trane 3 ton. The thing with variable speed is just that, variable speed. The motor works on one speed for heat and another for cool. But the motor is also "smart" in that it always delivers the smae CFM (differant of course in cooling and heating).

BAsically, say, your conitioning 10 rooms. The total CFM for all ten is 1000 (100 cfm per room). Now (for what ever reason) you close off the vents in half the rooms. A regular blower doesn't know this and still runs at the same speed and the 5 rooms still being conditioned have all this extra air screaming in an the vent noise more than doubles making alot of noise. That is assuming you don't blow a duct. The variable speed motor would sense the added restriction in the duct and slow itself down to maintain the correct amount of air flow to the room stull being conditioned.
 
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