Hey turtle! Reply to - A/C Job for Darrel to look at.

............Quote............... This is Turtle.
Here is something to look at and why it went this away.
I had a 4 ton 13 seer Goodman condenser and a Arcoaire 5 ton evaperator coil expanion valve with Arcoaire 100k gas furnace set up in my house. I had run it for about 6 or 7 years or so and out of the blue the piston Compressor bite the dirty. I changed out the condenser to a 3.5 ton 8 seer second hand 1982 Rheem condenser till i could get time to get me a matching condenser Ordered at the warehouse. this part of the country no body wants anything above 10 seer and the warehouses don't carry much in the line of 13 or 14 seer.
So i was running a Goodman 13 seer 4 ton condenser and a Arcoaire 5 ton expanion valve evaperator valve type and 5 ton blower gas Arcoaire furnace in the beginning. I was getting about at the time the highest electric bill of $200.00.
Then I just put a 3.5 ton 1982 second hand condenser in the place of the 4 ton 13 seer goodman condenser units and my electric bill dropped down by $50.00 or so. The highest electric bill that year was $150.00 and so has been the case for a couple of years now.
So Why did my cost to operate drop when i only changed out a Goodman 4 ton 13 seer condenser unit to a Rheem 3.5 ton 8 seer condenser unit ? How does going from a 13 SEER to a 8 SEER save money ? This does not seem right here.
TURTLE ..................end quote....................
The larger the temperature shift that that the system is working across, the less efficient it is. Having the largest evaporator and air flow system that you can fit in the house will boost SEER. It will keep suction pressure up. And having the smallest compressor that can deliver the capacity that can keep the house cold, with the evaporator you picked, will boost SEER.
The lower the capacity you operate a certain evaporator at, the more efficient the evaporator and system will be.
What mainly determines the SEER is the Evaporator size (Area/Btu), and air flow over that evaporator (cfm/Btu), and the condenser size (area/Btu) and air flow over the condenser (cfm/Btu). The only major difference between a 4 ton 14 SEER system and a 4 ton 8 seer system is the size of the evaporator. And to a lesser extent, the size of the condenser coil. The compressor is hardly worth talking about. Compressors are all pretty efficient. Motors are a mature technology. To the point that the newer compressors are only a fraction more efficient than the older ones. The compression system changes, but the motor driving them is about the same. You can take a 16 SEER heat pump with a scroll compressor, rip out the factory compressor, and put in a 20 year old reciprocating compressor of the same tonnage, and power consumption will only change by a few %. It will be louder, but that's it.
The SEER rating mainly comes from the evaporator, when it's being operated at it's "rated" tonnage. If you reduce the tonnage that you are operating it at, then you will increase the system's efficiency. If you increase the air flow over the evaporator, then you will increase the system's efficiency.
If you took the old 14 SEER 4 ton condenser and put the reciprocating compressor from the 8 SEER 3.5 ton condenser in it, and use that for your house, then the oversized condenser for the compressor would boost SEER even more.
And if the compressor/condenser doesn't have to run continuously during the hottest days that you experience, then you should put a smaller compressor in, sized so that the unit will run almost continuously during the hottest days. The reduced system capacity will boost the total system (Evaporator) SEER even more.
Example. Lets say that you have a room with 100f^3 of 100F air. Lets say that outside temp is 100F Lets say (for reasons of simplification.) that it take one "Btu" of cooling to change one cubic foot of air one degree.
You have two ways of cooling it down to 75F
One Run 50f^3 of air through the heat exchanger and cool it down to50F. Blow that air back into the room and it will mix with the rest of the air. It will average out to 75F It will take 250 "Btu" to cool the room. "Btu"s will be taken out at 50F
Two Run 100f^3 of air through the heat exchanger and cool it down to 75F Blow that air back into the room, and you are done. It will take 250 "Btu" to cool the room. "Btu"s will be taken out at 75F.
Which is more efficient? Pressure lift of case one. 210.6-98.8 = 111.8PSI Compression ratio 3.897/1.799 = 2.166
Imaginary units of work to cool room. 1 / 2.166 = 0.4617 0.4617 * 111.8 = 51.6 1 - (1 / 2.166) = 0.538 0.538 * 111.8 * 29% = 17.4 51.6 + 17.4 69 units of work
Pressure lift of case two 210.6-146.9 = 63.7 Compression ratio 3.897/2.678 = 1.455
Imaginary units of work to cool room. 1 / 1.455 = 0.687 0.687 * 63.7 = 43.78 1 - (1 / 1.455) = 0.3127 0.3127 * 43.78 * 29% = 4 43.78 + 4 = 47.78 50.63 units BTU correction factor (A R22 compressor with 75F evap temp will have more capacity than the same unit with 50F evap temp) 1.799 / 2.6798 = 67% 50.63 * 67% = 34 34 units of work to do the exact same thing.
It only takes 49% of the energy to cool the room at 75F evap temperature than it does for 50F one.
...Recap.. To maximize EER, and SEER numbers, you have to have the biggest condenser that can be used outside. With the largest air flow that you can run through it. The biggest evaporator that you can fit in the inside. With the largest ducts, and the most air flow that you can push through the system. That will reduce the temperature gap between the condenser and outside air temp, and the evap and inside air temps. That will minimize the temp gap that the compressor has to work across.
If you want to maximize the efficiency of a evaporator and condenser that is already in place, then you have to size the compressor so that there is just enough cooling capacity to cool the house on the hottest days, with the system running all day. Because the lowest system load means that you have the lowest temperature gaps between the condenser, evaporator, and the air they work with.
You get the largest practical evap and condenser, then size the compressor to run continuously under maximum heat load with the evap and condenser you managed to install.
There is limits. After you increase the size of the fan motors for the evaporator and condenser past a certain point in an effort to reduce the size of the compressor even more, then the electrical consumption of the fan motors will start to consume more power than the compressor they are working with, so any expansion of air flow beyond that point, will increase the power draw, and reduce the SEER
Basically, for maximum SEER on a 2 ton system. Buy, and install a the ductwork, and evaporator for a 5 ton 14 SEER unit. Then install the 5 ton condenser, then rip out that 5 ton compressor, and put in a 2 ton compressor in It's place. SEER will be far above the 14 SEER rating that they sell it as.
You may be surprised. You may even have to downsize the compressor even more than you thought. A 2 ton rated compressor in a 5 ton system will have more than a 2 ton capacity because of the increased intake and reduced head pressures. The compressor will even draw less wattage than it would in a 2 ton system. It may be somewhere in the 16 to 18+ SEER range.

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