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I bought an old house with a 21 year old 5 ton Carrier 38CKC060300 (built in January 1999) that is mismatched to a evap coil that matches up a 3-4 ton condenser. Because of this difference in coils, the compressor will pause from time to time. The air comes out of the registers at a cold 42 degrees to a 1,200 sq ft original house and about 600 sq ft of additions (some permitted, some bootlegged). My electric meter shows 6.4kWh when the A/C on with our fridge and wine chiller not running.
I'm curious to know how different things would be with a properly matched and smaller A/C compressor/condenser with a SEER rating around 14-16.
1) Will the air temperature come out of the registers +/- 42F?
2) Will the time it takes to reach a set temperature on the thermostat take longer with a lower tonnage?
3) What energy savings should I expect?
4) How does an A/C with a SEER rating of 16 save so much more energy than my current 21 year old A/C (which I think predates SEER ratings)?
A simple explanation would be appreciated. Thanks in advance.
kWh is useless, you need watts. You can either put a timer and measure the delta in accumulated kWh, and then divide by the elapsed time, or you can get a wattmeter and read instantaneous power draw.
If the compressor and condenser are "oversized" then you'll cycle more frequently. Whether this is theoretical or actually measurable is indeterminate from the info you've provided.
Air temp coming out of registers is as much a function of your ductwork and its installation and the temp. of your attic as of the system performance. Actually, in theory because you have more compressor and condenser capacity than evaporator capacity, you're taking more heat out of the refrigerant at the condenser, so evap. pressure and temp. should be lower hence theoretically lower air outlet temps than with a smaller compressor-condenser set.
Keep in mind that you put heat into the working fluid at the evaporator and compressor (OK, enthalpy, but it's units of heat) and take it out at the condenser.
I don't think there is a "simple" with air conditioning.
1) Will the air temperature come out of the registers +/- 42F?
It should not, unless the return air is 60 to 65 degrees. A 20 degree drop across a coil +- is normal and optimal.
2) Will the time it takes to reach a set temperature on the thermostat take longer with a lower tonnage?
Yes, but more likely because of the lower delta T (change in temp across the evap coil) more than the change in tonnage. Taking a long time to reach set temp is a GOOD thing in humid climates, but has not so much impact in dry ones.
3) What energy savings should I expect?
Some. It will vary depending on a whole bunch of factors that are too complex to nail down to any exact percentage. As a wild mule guess, maybe 40% of current cooling costs. You have a mismatched system, the system is older, it may not be handling humidity well, and who knows if it is properly sized or zoned. Otherwise, if you use AC just to take the curse off, you will save far less than if you run it for a significant portion of the time.
4) How does an A/C with a SEER rating of 16 save so much more energy than my current 21 year old A/C (which I think predates SEER ratings)?
"Four hours a day x 125 days a year = 500 hours 14 SEER: 500 hours x 34.28 cents per hour = $171.40 per year 16 SEER: 500 hours x 30 cents per hour = $150.00 per year The 16 SEER unit will save approximately $21.40 per year in energy costs."
woohoo! a whole $21.40 a year!
when i replaced my AC unit a few years ago, i believe i got a 13 seer unit. the energy savings for a higher seer unit wasnt enough to justify the higher cost of a 16 seer unit.
Thanks for the simple explanations. So energy efficiency comes from a newer and more advanced compressor/fan design and I should expect conditioned air to be not be as cold with a smaller condenser coil compared to my current oversized compressor/condenser (all else being equal). I'll look to upgrading the A/C during this fall/winter in hopes to find a better deal.
If I were you I would just run what you have till it fails. Why do you think you need to fool with it?
I tend to agree. Replacing a system when there is something obvious that makes the possibility of failure imminent makes sense. If the unit is not doing the job, replacement makes sense. Otherwise, all you are doing is starting an expensive clock early.
I recently had a refrigerant leak in one of the spider (distributor) lines in my Heat pump. I crimped the ends to seal, called a local tech who then evacuated, nitrogen filled, brazed, evacuated and refilled. I figure if I get two years out of the repair I break even. Anything longer is just gravy. Energy costs pale in comparison to repair and replacement costs
The tech mentioned that HVAC systems are now the third largest purchase many people make these days. Home, car, HVAC. Food for thought.
If I were you I would just run what you have till it fails. Why do you think you need to fool with it?
Quote:
when i replaced my AC unit a few years ago, i believe i got a 13 seer unit. the energy savings for a higher seer unit wasnt enough to justify the higher cost of a 16 seer unit.
I have 4 units - A 4 ton heat pump, a 4 ton AC, A 2 ton AC and lastly the upstairs unit which was a 3.5 ton AC.
It was starting to not cool properly, to the point where the supply air was only a few degrees above the return air. Ductwork was fine, no leaks, but the unit would run night and day. Got it installed in early July and I just looked at the usage numbers. Replaced the 3.5 with a 4 ton - ductwork would support it. Old SEER was 10, new 16. Coil replacement was the same - 4 ton.
June - 389 hours so around 13 hours per day (last year, 317 hours cooler June)
July - 356 hours so around 11.5 hours per day (last year, 424 hours)
August - 314 hours so around 10 hours per day (last year, 423 hours)
Even accounting for yearly differences it’s clear the new unit is running a lot less. Running amps between the old 3.5 and the new 4 ton are about the same.
So the old unit hadn’t failed but after 20 years it clearly wasn’t keeping up. By the end it couldn’t hold the temp below 80 if it was 90 out. This was the upstairs unit so it runs by far the longest. In contrast the other units each run around 130-180 hours a month in the summer.
Lastly, I went 16 SEER single stage as it (1) was only slightly more expensive than a 14 SEER, and (2) it is listed at 69db so quieter than the other models. My old one was 82db.
But...I’m taking these units one at a time. As long as the others continue to run and keep up I won’t rush to replace them.
The tech mentioned that HVAC systems are now the third largest purchase many people make these days. Home, car, HVAC. Food for thought.
People get lured in by 25 SEER variable everything systems. Unless you live in a climate where it runs all year around it’ll never pay off.
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