Thermostat with user selectable on and off temperatures?

parrothead_faJune 20, 2011

Hi, does anyone know where I might be able to find a thermostat that lets you select the on/off temps, or if they even make such a thing? I have found that my wife and I can be comfortable and save the most energy if our air conditioner was to come on at 81 degrees and switch off at 79. this small swing does not bother us, and I have found (by simply manually switching the system on at 81, and off at 79, t stat is located by my desk) that i save the most energy on our electric bill. The system runs long enough that it keeps the humidity in our home very low (18-24%), and yet is off for an hour or more at a time between runs. i know a lot of you may be thinking, 80 degrees in a house comfortable? But with our arthritis, and on a 90 degree day with 80% humidity outside here in south Florida, 80 degrees at 19% humidity feels just fine to us.

since we are home all the time, we don't need and wouldn't use a programmable thermostat, and I've see none that are non programmable that have any sort of swing adjustment at all. I'm reluctant to try a programmable t stat because I find most of them rather confusing, and I'm also reluctant to give up the thermostat we have now, because it works so well. But it has no swing adjustment, and when set at 79, will come on at 80 and off at 79. The reason I'm reluctant to try another t stat is when we got our new AC installed, they gave us a brand new digital thermostat, supposed to be better than the basic digital one we are using. However, this one would short cycle the system rapidly, turing it on for about 5-6 mins, and off for about 5, then back on again. I would have thought the sytem oversized, except that a manual J was done, and that I really wasnt feeling a difference in temp at all. So on a hunch, I put the old t stat back, and the system now generally runs for around 25-35 mins depending on the temperature outside, and is off for between 30 and 40 mins between cycles. So if I got another super sensitive thermostat, adjusting the swing to go on at 79 and off at 81 wouldn't be a bit of help if it turned the system on and off every 5 or 10 mins. Anyone got any ideas, or am I just an old man with too much time on my hands, lol.


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What you call a "swing" is really a differential. Some thermostats do have programmable (sorry about that ) differentials. I do not know which to recommend to you.

    Bookmark   June 21, 2011 at 8:05AM
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Many of the newer thermostats brag about controlling to the nearest degree "for your comfort". I'm like you, in that this tight control leads to short run times and lots of cycles per hour (maybe 10).

    Bookmark   June 21, 2011 at 3:33PM
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There are certainly stats that have a minimum run time and this can be adjustable. But I bet you will not find one that allows a run time as long as you want it.

I hate to say it but if you are at a 50% duty cycle (ie on 30 minutes and then off 30 minutes), on a summer afternoon, then you are oversized for you. It maybe sized right for a lower temp or more people in the house but you would have been better off with a smaller system.

    Bookmark   June 21, 2011 at 7:41PM
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Well, when I say it generally runs around 30-35 mins, and is off for 30 to 40, I'm usually talking about the average days around here, and not at the hottest part of the day. We're usually in the upper 80's here, 87 to maybe 90 most days, we usually have our AC set on 80 (which turns it on at 80 and off at 79) so that's not a huge temperature differential. Its a fairly new home built in '03, and I have improved some of the attic insulation and weatherstripping. During the hottest parts of the afternoon, and on unusually warm days where it gets into the mid 90's, it's not uncommon for it to run an hour or more during a cycle, and only be off for 20 mins or less. The room temp does have an effect too on the cycle times, if we have guests who prefer it cooler, I'll set it to 77 and the cycles are noticably longer. However, I really can't complain about the new system, because it keeps the humidty in the home low, usually around 23% even when its set to 80, we always feel comforatble, and when we need extra performance, such as when we have a houseful of guests, or when we are baking and using the oven a lot, it still keeps the temp right at whatever its set to. We replaced a severely undersized system that was in the home when we bought it, this one would run continuously just to keep the house at 79 whenever the outdoor temps got over 84. If you were using the oven, or on a really hot day, it couldn't keep up and the house would get up to 81 even with it running the entire day. We upsized from 2.5 tons to 3.5 tons, and the new one costs half as much to run as the original one did. I'm just one of those folks who will always try and save a dollar or two wherever I can. I figure if I can shave another 10-15 dollars off the electric bill, that's enough to buy something nice to make a special dinner once a month, or take the wife out for coffee and donuts a couple times.

    Bookmark   June 24, 2011 at 11:40AM
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One of the considerations about your thermostat selection and whether or not you would like a less or more sensitive thermostat is accuracy. It's not that one thermostat is more or less accurate its that each thermostat will respond more or less rapidly than the other. Given enough time to respond your old thermostat may be very accurate, it just happens to be very slow to respond. Your existing thermostat is not real time accurate compared to the newer one that was installed. The newer one is keeping up with the temperature changes quicker than the old one and is therefore more real time accurate. You're wanting a thermostat that is slower in reaction than the newer one so that it doesn't cycle as often.

Ask this question "is the room really at 81 and 79 degrees when the old thermostat is turning on and off?" The answer is most likely (no). The new thermostat is probably turning the unit on and off at temperatures closer to the 79 and 81 range you are setting them at because it is reacting to the real time temperature of the room, while the other one is lagging behind the present room temperatures.

Since I believe this is the case, you could just find a new thermostat that allows you to set the differential and adjust it to a larger setting. In essence you would be getting the same affect as the older t-stat. Try the Robertshaw RS 3110. I know this is programmable but you may have to bight the bullet. Hope this helps.

Here is a link that might be useful: AC Repair

    Bookmark   June 24, 2011 at 3:57PM
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One of the problems with residential AC is the lack of a reservoir.

They sort of rely on the liquid line, but it is not as effective as having an actual reservoir.

An actual reservoir allows the compressor operation to be disconnected from the system demand (within limits).
The compressor runs to fill the reservoir, while the thermostat control metering into the evaporator.

In the fall and spring every residential system is over-sized since they are sized for the hottest part of the season.
This results in wider swings than when the load on the system is larger.

    Bookmark   June 26, 2011 at 9:46AM
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