Point of diminishing return


Old 06-15-15, 01:55 AM
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Point of diminishing return

As far as SEER rating , what is the point of diminishing return ( straight A/C ) when it comes to capital investment ? Capital investment + maintenance ?

Thanks , :-)
God bless
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Old 06-15-15, 05:19 AM
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Hard question to answer. Electric rates, how many cooling days you have, if your duct has been sized and installed correctly and what do you set your temperature at all play a role in this. Then you have comfort that's something that you can't put a number on.

Im in VA with very low electric rates. I tell people to get 15 SEER with a VS blower.
Old 06-15-15, 06:05 PM
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The biggest problem is bad design-engineering, bad installs & a whole bunch of other factors which renders SEER near meaningless to the whole equation.

Test your A/C to see if it's a bad as major tests have shown units to be: 59% to only 63% delivery of their Rated Btuh!

Simple easy anyone can do ways to check the performance of your central air conditioner so, if needed, you can call an Energy Efficiency HVAC Technician.

If you want me to run a ballpark analysis, (or do it yourself,) of how your system is performing in respect to its 'Nominal Rated Btuh' you/we need at least the following numbers:

Performance Data Collection – Best Time to collect data is Late afternoon around 4:30 pm, when attic is HOT; also when outdoor temps are around 85; 95; 105F or, anywhere in between.

*All you need is a good thermometer (digital reading in tenths preferable) & and indoor decent Humidity Gauge (like an ACU-Rite digital for under $9.00) Walmart or on the Internet...Not fixing anything just testing performance...

1) Helpful; Tonnage & SEER of Unit & outdoor condenser model number: __________________

2) TXV or, orifice metering device? _______. Only if U know…

3) Outdoor condenser’s discharge-air-temperature ______F

Subtract Outdoor air temperature: _______ - from cond. air _______ = Condenser Air-Temp-Split ______F

4) Very Important, Need the ‘Indoor’ percent of relative humidity - away from Supply-Air outlets ______ F

5) Indoor Return-Air Temperature ______ F

Subtract Indoor Supply-Air Temperature ______ F

Indoor temperature-split _______F

Need the above information for troubleshooting & performance analysis.

Example below:
A Goodman 2-Ton 13-SEER condenser, 800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19F.

@ 85-F outdoors; condenser air 103.9-F - 85-F outdoors or around an 18.5F temp-split;

@ Indoor 75-F & 50% RH condenser temp-split is only around 14.9F.

This data will indicate a lot of things to me.
Just use "Reply With Quote" or copy & paste in Reply then fill-in the numbers...
Old 06-16-15, 04:00 AM
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My question was aimed at the equipment . I realize a bad design or install can mess it all up .

Thanks ,
God bless
Old 06-16-15, 09:07 AM
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My question was aimed at the equipment . I realize a bad design or install can mess it all up.
In most situations go by EER Not SEER.

It is important to understand that "equipment ratings are only the potential efficiency of that component of the system under perfect conditions." Over half of the system’s efficiency depends on the efficiency of the ductwork system and quality of the field-installation.

There are some components that will it help elevate these seer ratings.
1. First, is a variable speed and CFM control air handler.
2. Second, is a same tonnage evaporator coil with a thermostatic expansion valve, both will help control humidity at 50% or less levels.
3. Third, it is a room thermostat with an adjustable on/off set point differential up to 3F.
4. Fourth, you could utilize a large 20-inch floor fan to circulate the air when the unit is off, or even when it's on.

SEER Payback needs to be Properly Represented to Consumers!

The SEER Ratings are too nebulous & do not translate to the specific application conditions that each consumer will normally have in their homes. EER is simpler & more reliable when the systems are correctly sized to the load.

From 2008 AHRI Standard 210/240 Performance Rating of Unitary Air Conditioning and Air Source Heat Pump Equipment:

Pages 89 and 90:

SEER calc for a single speed compressor w/fixed speed indoor fan:

SEER = EER (at 82F ambient) x PLF (.5)

Where PLF (.5) = Partial Load Factor = 1 - (.5 x Cd) where Cd is the Cyclic Degradation Coefficient.

Basically, it's the EER at 82F ambient, 80F indoors adjusted by fudge factors.

Pages 89 - 94:

SEER calculation for a single speed compressor and variable speed indoor fan:

SEER = EER over a weighted average range of outdoor "bin" temperatures x fudge factors.

The outdoor bin temps and their weighted averages are as follows:









Total = 100%

Note that the 3 lowest outdoor ambient temps (67F, 72F, and 77F) make up over 66% of the value of the SEER calculation.
The SEER of a system is determined by multiplying the steady state energy efficiency ratio (EER) but measured at conditions of 82F outdoor temperature, indoor 80F Dry Bulb and 67F (about 50% RH indoors) Wet Bulb indoor entering air temperature by the Part Load Factor (PLF) of the system. (The PLF is supplied by the government.)

You would also have to operate your A/C at those percentages of outdoor bin temperatures to make the SEER rating have any real relevancy. IMO; SEER ratings are a lot of irrelevant paperwork potential improbabilities...pay more attention to EER that you can post install test & verify. When condensing units are 'slightly' undersized, the EER is better & more relevant.

The SEER Rating is 'at a particular set of conditions' that are NOT typical design conditions, or at your particular climate zone and home's conditions, therefore, a bit immaterial in many situations.
Old 06-17-15, 03:05 PM
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I am not arguing what you have said .

I was asking about the equipment , the condenser , A-Coil and fan coil unit / AHU / furnace .

Best I remember , window shakers have EER rating . Split and unitary RTU's have SEER rating .

I did not invent SEER ratings .

God bless

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