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Thread: Summer vs winter A/C delta T
02-16-2009, 02:40 AM #1
Summer vs winter A/C delta T
I live in Houston, TX where we air condition for about 11.5 months a year. During this time of year when the outside conditions are 73°/70%, I can easily get a delta 25°. Occasionally getting as much as delta 27°. Indoor conditions are about 71-72° and 42-45% with the system running. 73-74°/50% with the system off. The air conditioning runs pretty sparsely in February. But it helps to knock the humidity down and drop the temp by a degree or two.
Now in the summertime, it's a different story. Average outdoor conditions in Houston run about 95°/60%. I get a measly delta 14° during July and August. Inside I'm getting 72°/60%. That's with a supply temp of 58° and 72° at the return. (I can't measure the coil before/after temps). During the mornings, I've tried turning the temp down even farther in an effort to get the humidity down and can only get 68°/57%. The system just can't seem to wring out the humidity. It makes sleeping troubling as I have to turn the temp down to get the humidity in check and then sleep under a somewhat thick blanket. I'd much rather sleep under a thin sheet and have it be 75°/40-45%. (actually I'd like 35%, but that's hard to do)
I run the fan in the AUTO position always. (Switching it to ON will send the humidity inside up into the 70% range.) Windows and doors are closed. No fresh air intake on the system.
Now I know dealing with dew points outside in the summer that are in the mid 70's and even touching 80 occasionally, will really give the system a work out. But should deltas really be cut in half during the summer?
Last summer I was getting $330/month electricity bills (1300 sq ft and 17 cents/kwh prices). If I'm not getting below 60% inside all summer long, something seems not right. I don't make a lot of moisture inside. 5 minute showers, no boiling water on the stove, no fish tank, no drying clothes inside, washers/dryers are located outside by the parking lot.
Could it be the location of my unit? It's a package unit on our apartment's flat black tar roof. No shade, direct sunlight all day long. It's a Carrier ZP 3.5 ton model (10 SEER). I know 3.5 tons sounds like a lot for 1300 sq ft, but I live in a top floor apartment that's 40 years old with large single pane windows. During the summer, it will run non-stop from 2PM until about 7PM. Cycling on and off (mostly on) throughout the rest of the night and morning. But even running it for 5 hours straight during the afternoon, I'm left with delta 14 (delta 15-16 if I'm lucky) and 57-60% inside. So I don't think it's an oversized/short cycling issue.
It can't be the condensate drain getting plugged as there is none. The unit simply drips water on to a downward slope that runs to the hole on the side of the unit. I will admit that there isn't a whole lot of water coming from it though.
I guess I'm basically wondering what people in hot/humid climates are getting for deltas during the worst parts of summer. Inside temp/humidity numbers would be helpful too.