Oversized air conditioner dilemma


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Old 07-29-15, 10:23 PM
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Oversized air conditioner dilemma

My new Lennox 3.5 ton air conditioner is not running as long as I want it to want it to. It will run on a very hot day for at max 10-12 mins and 5 on least cool days with 4 cycles one too much . now it does a good job most the time keeping the setpoint steady without too much fluctuation at most it goes only one degree higher than the setpoint before turning on. now the fact that the unit is new and is short cycling is giving me a red light that it might be oversized for a 1614 sq ft home in Texas. Humidity has not been an issue till I came home today and I was sweating and relized I had mistakenly hit the fan button my thermoststat. My previous unit was a three ton that constanly ran nonstop. as far as ducts are concerned we added 2 returns and did not much modifications from the previous duct system. questions
1. Is my ducts too small for a 3.5 ton system
2. Is the system itself too big
3. could it be something else
As far as a load calculation goes I think the contractor went by the square footage of the home but I am not sure. We though we were getting a higher efficiency system from the best contractor in town but it has performed way below
 
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Old 07-30-15, 05:45 AM
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Is it a single stage or 2 stage system?

I bought an over sized heat pump (4 ton instead of 2.... and redid my duct work) because I sized it mainly Winter heating, and I'm not having any Summer issues. Now my thermostat is pretty expensive and offers lots of staging flexibility as well as complete CPH customizing. CPH is set way down at 2, stage 2 kick-in temp is set pretty high so it runs on stage one most of the time.

If you have a 2 stage system then all you need is the proper stat for better control. If you don't then the only cheap fix I can think of is moving the thermostat to the hottest location in the house.
 
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Old 07-30-15, 10:07 AM
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How long are the OFF-TIMES?
We need to know both the on-time & the off-time for a complete cycle.

What are the outdoor conditions during the hottest times?
What is the INDOOR %Relative Humidity? Very important...


It will run on a very hot day for at max 10-12 mins and 5 on least cool days with 4 cycles one too much.
We'll use 11-mins ON & 8-mins off.
That's a complete cycle of 19-mins; 11-mins ON / 19-min cycle is .57% runtime; we'll say the 3.5-Ton is only delivering 38,000-Btuh * .57 it is only taking 21,660-Btuh to maintain the T-stat setting on your hottest days.

I'd say it is way over sized...1614-sf / 3.5-Ton is only 461-sf per/ton; normally that's way too much tonnage.
 
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Old 07-30-15, 01:45 PM
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Thanks off times tend to be between 5-10 min we have 98 degree weather with a low of 78. Outdoor humidity tends to be between 36-50 percent at at the day to 91 at night. I think indoor is between 40-50
 
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Old 07-30-15, 02:29 PM
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Thanks off times tend to be between 5-10 min we have 98 degree weather with a low of 78. Outdoor humidity tends to be between 36-50 percent at the day to 91 at night. I think indoor is between 40-50
Well, the estimate of 8-mins off was close enough to get a ballpark average.
You have to actually accurately log the On/OFF Times.

Everyone should be spending a little time logging that data; plus doing the 5-mins of time it takes to do the temps & humidity checks to know if your HVAC unit is performing near well enough.

All you need is a good air-temp probe thermometer (digital reading in tenths preferable) an HVAC Supply might sell you one; and a low cost 'INDOOR Humidity Monitor Gauge' ACU-RITE Digital only $8.94 at Walmart or over the Internet. Check to see if it registers the temperature correctly; if not, take it back for a full refund.

This form is for anyone to use to have us analyze your HVAC system's performance, if we find problems you'll need to call a Contractor to fix the problems. Many systems are functioning way, way below their mfg'ers performance Ratings; the average was found to be less than 60% of their Rated Btuh.

Because very few Techs seem to actually test the Delivered HVAC system's performance It is time for you to run an easy, totally safe, A/C Performance test of your own. No Kids; do not lay anything on top of the outdoor condenser, & hang onto the thermometer so it doesn't drop into the fan!

If we find problems, you will need to contact & qualify a good HVAC Contractor to fix those performance problems.


1) Helpful: condenser 'Tonnage' & SEER Rating of Unit &/or model number: Yours: ______
FIRST; We need to know the 'Indoor' % of Relative Humidity__ %RH
Also, (not critical) however, it's helpful to get outdoor humidity from online weather info, etc., & wind direction ____, & velocity ___mph, & outdoor ___%RH, outdoor-temp ___F:

Weather Forecast by Zip Code

2) TXV or, orifice metering device? __. Only if U know; not critical…

3) Outdoor condenser’s discharge-air-temperature ___F
Subtract Outdoor air temperature: __F

Outdoor Condenser Air-Temp-Split ___F

4) Need the ‘Indoor’ percent of relative humidity - in the middle of the rooms or, at Return-Air inlet grilles ___%RH

5)Subtract Indoor Supply-Air Temperature __F
Return Air Temp __F Subtract Indoor Supply-Air Temperature __F
Indoor temperature-split = __F

“Reply with Quote” Fill the #numbers in where the blanks are. Easy Safe testing of your A/C or heat pump cooling performance for all forum visitors to use.

First, make sure the return air filter is clean, and then you should get a digital probe thermometer that reads in tenths degrees, (though U can use any mercury thermometer) and a low cost percent relative 'humidity gauge 'to check the 'indoor' humidity level.

If you have an air conditioner that was manufactured between 1992 and 2005 it will 'probably be' a 10 or 12 SEER (though some were higher SEER even back then) R-22 refrigerant units.

When the temperature reaches around + 80 to 95F outdoors and the indoor temperature is 80F and the relative humidity indoors' is right around 50% RH the outdoor condenser temperature split should be around 20F above the outdoor temperature.

If the indoor temperature is 75 and the relative humidity is 50% then the air discharge temp-split off the condenser should be around 17F.

The indoor temperature split between the return-air & at the supply air grille closest to the air handler should be 18 to 20F temperature drop with either an 80F or 75F indoor temperature.

This is also the indoor split with the higher SEER units at 13 SEER or above.
 
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Old 07-30-15, 03:10 PM
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Now I check the next smallest size three ton which gave me 538 sq ft/ton and 2.5 ton which is 645 sq ft/ton how do these compare as the numbers are bigger than the 461 that my 3.5 ton is for 1614 sq ft home
 
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Old 07-30-15, 06:20 PM
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Now I check the next smallest size three ton which gave me 538 sq ft/ton and 2.5 ton which is 645 sq ft/ton how do these compare as the numbers are bigger than the 461 that my 3.5 ton is for 1614 sq ft home
Normally the 645-sf per ton would handle the load.
In my home I cool 1300-sf per/ton!
 
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Old 07-30-15, 07:47 PM
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Thanks off times tend to be between 5-10 min we have 98 degree weather with a low of 78. Outdoor humidity tends to be between 36-50 percent at at the day to 91 at night
That's a lot of cycling. I don't know what you have for a thermostat now but honestly I would try one with better control before you go spending big bucks on a new system or new duct work. The worst case is $100 wasted on a stat that doesn't do any better.... you can probably return it if you're careful not to scratch it up. The best case... $100 fixes your problem.

Get a good thermostat with settable cycles per hour (CPH) and set cph down to 2 or so. This will produce fewer cycles and therefore longer running times.
 
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Old 07-31-15, 11:33 AM
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Ok thanks so I have a Lennox CS3000 model 51m34 the problem today I noticed some holes in the wall from where the old one was so I want to know is it that or is the system too big for a house my size condesor model 14acx-041 it is a single stage system
 
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Old 07-31-15, 02:41 PM
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Oh I believe it is oversized. If i whip on over the Alpine home air site and borrow their little size calculator, and punch in your square footage (basement and 2nd floor), with basic insulation, and a texas zip code... the calculator punches out 2.5 tons. Granted their simple little size calculator is a long way away from a serious load calc.... but you do get a ball park number out of it.

Alpine Home Air Products: Contractor-grade furnaces, heaters, air conditioners and indoor-air quality products at wholesale prices.

I don't however think that's the end of the world. You would have been better off with a 2 stage system. It's easier to control an oversized system that way. On the other hand it's going to be a pretty serious expense to rip out what you have and start over again so I would at least try the cheap fix first (thermostat with settable CPH)
 
 

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