Can adjusting t-stat swing save on electric bill?

13 years ago

We live in south Florida, and we recently replaced the badly undersized central air condioner in our home. The new system is a Rheem 15.5 SEER 13 EER 3.5 ton. (upgraded from the 2.5 ton builder model that was in the house when it was built a few years ago, and could NOT adequately cool the home)I have a digital non programmable thermostat with adjustable swing. I had been using it just as it came out of the box, if we set the t stat to 79, it would turn unit on when temp reached 80.1, and turn it off when it reached 78.5. This results on most days in run times of 23-26 mins, and off times of 20-22 mins in between. I recently discovered that I can adjust the swing on the t-stat so that if you set it at 80, it will turn on when temp reaches 82.1, and turn off when temp gets down to 79.0. The new settings usually result in run times of 30-37 mins, and off times of 1 hour or so in between. The change of a few degrees dosen't bother my wife and I, as we both have arthritis, and prefer to be on the warmer side. The new unit does an excellent job of dehumidification, which is our main concern. The home is well insulated, and interior humidity is usually around 40% or so, which is quite comfortable to us. My question is, will I actually save more energy usage with the slightly longer run times, but greatly increased off times? June was the first month we used the system continuously, as we live by the sea and enjoy opening our windows for the breeze on evenings and mornings when it's not too hot, and often leave the patio doors open on mild days. So far it has proved to be as efficient as the contractors advertised, our electric bill was around $35-$40 a month more in june & july than the months when we don't have to use the heat or AC at all. Just wondering if there's any way we can shave a few extra dollars off that.

Dave

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Comments (5)

daddo
13 years ago
Setting the temp higher always saves money.

You have good off times, but on times seem a little long, but that could be relative to outdoor temps.

If your happy- stay with it.
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parrothead_fa
Original Author
13 years ago
We haven't really set the temp higher, it's just that with the greater swing, if we want the end result at 79 degrees, we had to bump it up, otherwise it wasn't turning off until 78 or below.
Dave
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tiger...long time no speak...thanks for the reply... re: andre your story is not surprising. I went from a 4 ton 10 SEER mismatched system AC/gas furnace, made insulation and window replacement improvements, and down in size to a 3 1/2 ton matched 14 SEER dual fuel system. Savings in AC mode non existent. The rule of thumb from Trane and other top line mfgs are 5% savings for every point increase in SEER. Throw the rule of thumb in the trash. As I recall you did go up from a 2 1/2 ton to a three ton. ***Went from 9 SEER to 14 and from a 2.5T to a 3T. Certainly not seeing 30% electric savings! Based on my prior posts here the system is matched and in fact needed to be for the rebate. I can't recall which stat you installed-the 803 or the HW VP IAQ. You should check the setting for CPH-should be two or three. Of course tech should check the electric draw at both the condenser and air handler plus the refrigerant charge. ***It was the base model Trane Stat..I am thinking of upgrading to a programmable Trane stat..we'll see. I wish I had know about the 2 degree setting...now I may need to wait until next summer to see how it affects my bill and comfort level...assuming it is not already set at 2...will check at lunch. what is a normal run time for a cycle with new system? ***I really never timed it. To be honest, except in the kitchen where there is VERY noisy vent (its almost on top the airr handle below) it hard to tell when the sucker is running! It may now be too late to time it unless we get a few 80+ degree days. What cycle time should I be looking for? you may be unhappy on the AC side but I will be surprised if you are not smiling on heat mode side. ***Actually I am happy on the ac side...its never been hot in any part of the house and everything is more uniform...just concerned that the sucker is cost more $$$$ to run than the 20 year old unit....again, I have seen no decrease this summer and the avg temp was waaaay lower this summer. I hope it heats as nicely but now I fear my bill will go up! Also, for all us lucky folk in PA it will go up 30% next year due to a rate increase. I was hoping there would be savings with this unit to help offset that rate increase. I may be wrong but I doubt if the tech finds anything wrong with your system. ***I sort of agree..plus its the sales dude coming out. If setting the stat to a 2 degree swing lowers my bill without decreasing comfort I will be quite happy. I just was not sure if they needed to lower the fan speeds or whatever..though it is a variable speed so mybe thats not possible. OR if getting that extra duct in the base might reduce some air movement upstairs and maybe somehow affect the cycle time tom my advantge...again...just scared about heating bills! if you do change your backup heat strip, I recommend you have two 7.5s wired staged. **I will mention this! I susoect he will have some bs story now about why we should leave the 10KW strip in place. I want what will best keep me warm at the lowest billed $$$$...not sure which that is....
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david_cary
13 years ago
I know that a/c's use electricity when running and not when off. So it is simple math to see if the greater swing is helping. I think it always would - since the beginning of a run is not very efficient. It is just like taking longer trips with the car (although not for the same reasons).

The humidity will be lower - for better or worse.

People sometimes seem to be worried about excessive run times although I am not sure this is warranted. But certainly 30-40 minutes is not excessive - 6 hours might be.

Does your stat measure the run times? - because that would be nice. I have an Infinity stat and would think I could set the swing but haven't tried yet. I can't generally hear it running so measuring run times would require sitting by a vent for a while - not my idea of a good day.
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parrothead_fa
Original Author
13 years ago
Actually, I can easily measure on and off times, because I work from a home office with a vent near my desk, so I arrived at my figures by looking at the clock on the computer when it would start blowing, and when it stopped. The times I quoted were for an average summer day here. (temps between 88-92 degrees) The run time is a bit longer on excessively hot days,(94+)and the off times a bit shorter, and during the hottest part of the afternoon, (4pm-6pm)but for the most part, what i quoted was fairly average.
We're actually quite pleased with the new system. Even though our new home is only several years old, we replaced the existing 2.5 ton unit because it was just too small, and would run the entire time it was turned on, except during the evening when it cooled off below 83 outside, just to keep the house at 79. The resulting electric bills were horrific, so we rarely used it unless it was brutally hot outside. We actually enjoy the Florida weather, we moved here because we like to be warm, it's just that the humidity here on the coast (70-90% in summer) can get tiresome. This way, we get a break from the humidity, keep our house cooler than outside, but not so cool that we get climate shock when we go outdoors, and don't get an electric bill we cant handle. Many evenings, we'll turn off the AC and open windows when it gets down into the 70's at night, and turn it back on around 10 o clock the next morning when it starts getting warm again. Those evenings when there is little breeze, or when it's just super muggy, I have found we can still turn the AC off before we go to bed, and keep all the windows closed, and the house will only get 1 or 2 degrees warmer overnight. I heard that tip on an energy saving forum... so far, so good. I don't mind paying an extra 35-40 bucks a month to be comfortable all summer. The months when we only use the AC some of the days during the month, it dosen't cost us more than $20 as compared to the months when we have to use no heat or AC. Not to mention if everyone used their air conditioning more responsibly, it would go a long way towards lessening our carbon footprint, and place much less strain on the nation's already overburdened power grids.
Dave
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creek_side
13 years ago
We had a 3.5 Rheem in our last house and before that a Carrier unit. The first thing I did after replacing the old thermostat was to set the swing on the new one to four degrees.

Go for it.
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Can adjusting t-stat swing save on electric bill?

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Can adjusting t-stat swing save on electric bill?

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