A/C Thermostat operation

[Rebel1]

The problem I'm having is that the a/c continues running - for as much as an hour - past the past the point that the ambient temperature first matches the set temperature.

Let's say I last set the temperature at 77F and the display says the ambient is 77F; thus, the a/c is off. I now move the set temperature down to 76F. The a/c stays on until the ambient reached below 75, but I don't think that much of a swing is normal.

This Hunter 44110 thermostat has a function called Span. The manual says this: "allows you to adjust the system ON/OFF cycle rate. The default setting is 2 (the system cycles ON or OFF when the temperature is within

1 degree F above and below the set temperature).

I haven't adjusted the Span to 1, mainly because for the nine months I've lived here, there was no excess run time and the temperature swing was very low.

Sounds like I need a new thermostat. Do you agree?

Thanks,

R1

[Art Todesco]

Can't hurt trying to change the "span". I've never had good luck with anything but Honeywell thermostats. The one's I have don't have a "span" setting but don't seem to need it. Well actually they do, but it's hidden in some of the settings like 'hot water or forced air' settings. I had a non Honeywell in my motor home and it suffered from severe cycle-itis. In AC mode you'd sweat before it would turn on and freeze before it turns off. I now have a Honeywell that works great, however, because the motor home uses DC for control and there actually a heat pump and a totally separate furnace, setting it up is kind of weird ... but no cycle-itis, at least.

[Pat]

No, I don't agree. It sounds like the thermostat is working as intended. If it set to 76, I would expect it to start the A/C when the temp rises to 77 and keep running until the temp reaches 75. In other words, a span of 2. That is how mine and many others work. The real question is what changed to make you notice something. Is is possible, the A/C is failing such that it takes a much longer time to cool? As another person said, you can change the span to 1 to make it cycle twice as often. That won't hurt anything, but I suspect you need to have the A/C unit checked out. Buying a new thermostat won't change anything.

Pat

[Stormin Mormon]

Does the temp swing more than a couple degrees? It could be your thermostat, or could be some other part of the system going bad. Seriously, two degree swing is just fine for most folks.

I knew some folks, I visited.It was hot in the house, and they had just returned the defedive thermostat 3rd time, they were on the 4th stat. I got permission to look around, and found a sticky relay on the furnace control board.

You may be asking for too much, with one degree swing.

[songbird]

Rebel1 wrote: ...

no, i think you're misunderstanding. if you want the air to go off one degree higher just bump it up one more degree...

songbird

[Rebel1]

I don't know what happened recently, say within the last week or two. I just noticed that the a/c runs much longer. The temperature at the supply registers is about 58F; just downstream of the evaporator it's

54F, which is the normal 20 degree drop from the return air from the rooms.

I've set the span to 1 degree and test it that way for a day or two.

Thanks to the other two guys as well.

R1

[dpb]

On 07/16/2015 7:50 AM, Rebel1 wrote: ...

It's gotten much hotter outside, maybe???

Have you changed filters (or at least checked they're clean), made sure the condenser fins, etc., are also clear, the outdoor unit is also free of obstructions, etc., etc., etc., ...?

I'm in agreement with the opinion it's unlikely to be the thermostat itself that's the culprit here.

[Stormin Mormon]

If the AC runs longer, changing thermostat won't help.

It is possible the humidity is much higher, and it's also possible the unit needs maintenance.

From here, the thermostat doesn't sound like the problem.

[trader_4]

His problem is the range of swing, not the set temp. Solution is to change the swing parameter from 2 deg swing to 1 deg. IDK why he didn't just try it. If that results in too many start/stops, then I guess it's time for a better thermostat. I've had very good experience with Honeywell VisonPro.

[micky]

Why shoudln't I pile on?

So you didn't have a problem so you didn't change the setting and now you do have a problem but you still don't want to change the setting even though that might be what the setting is for, and are considering buying a new one. Not a good plan, for thermostats and other problems too.

[J Burns]

It sounds OK to me. The display of my thermostat shows when it's calling for heating or cooling. That can be very helpful.

My swing doesn't seem to be adjustable. Sometimes if the thermometer says 79 and I step down the setting to 78, it doesn't always come on. A reading of 79 may mean 78.6, and 0.6F may not be enough to turn it on.

For efficiency, an air conditioner should be sized to run continuously on the hottest, most humid afternoon. When it's 90 and humid, mine may run only a few minutes at a time and cost a dollar or so a day. If it's in the high 90s, and the dew point is high, and the house has been baking in the sun for hours, and there's a breeze to increase infiltration of hot, humid air, it may run continuously.

My thermostat is mounted on an old-fashioned plaster wall. Behind the wall is a closet that may be hotter than the room. That can cause the air conditioner to take longer to satisfy the thermostat.

On hot afternoons when it runs a lot, I may let relative humidity be my guide and step up the thermostat a degree at a time. As long as the air conditioner runs enough to hold down the humidity, a couple of degrees warmer will be fine. If I've let the temperature rise in the afternoon, I can step it down at night so the air conditioner will run enough to control the humidity.

[DerbyDad03]

...snip...

..snip...

You have your stat set at 79? Holy crap!

Anything above 74-75 outside for more than a few hours and our A/C is on and set for 70. On the flip side, winter heat is set for 68. (We'd do 68 in the summer too but even I can see the economic foolishness of that!)

The last few nights have been in the 50's. We have our windows open and a fan drawing air into the bedroom. That's why God invented comforters and snuggling.

Do you turn the heat on when the night temps dip into the 60's?

[trader_4]

I'd say if it's sized that small, you're not going to be very happy. If it's running constantly on the hottest day just to maintain, what happens on days when it's say 10F below the hottest, but the AC has been off or turned way back? It's going to take a very long time to bring the temp down. You need some reasonable amount of reserve.

[clare]

Down to 8C overnight last night, and expecting 31C tomorrow afternoon. Furnace and AC both turned off - dehumuidifier running in the basement when hydro rate is mid - low - never when on high rate.

If the humidity is high and it goes over 31C the AC might get turned on.

[Rebel1]

So far, with the span set to 1, it's not running as long. I can't yet comment of whether the a/c cycles on and off too often.

(With my thermostat I have to press the temperature Up and Down buttons for about three seconds to get to the span adjust mode.)

The indoor filter was thoroughly cleaned just a few weeks ago, after what may have been 15 years without a cleaning. (After clogging to a certain point, the resulting force of the incoming air causes the filter to be released from its retainer, which in turn allows the air to bypass the filter around its edges. Clever design.)

A hour ago, I cleaned the fins in the outdoor unit. We'll see what happens over the next day or so.

One thing I'm missing is the humidity connection. The thermostat responds only to temperature, so why does humidity affect run time?

A common problem happens most nights here in central Florida, 20 miles from the Gulf. During the evening, the outside temperature drops so the a/c doesn't run as often. At the same time, outside humidity increases and the house becomes mildly uncomfortable. At 6:30 am, it not unusual for the outside temp to be 75 and the humidity to be 93%.

Thanks, everyone, for your responses.

R1

[J Burns]

The output temperature can't be below the output dew point. The evaporator has to soak up a lot more heat to bring the dew point down a degree (condensing moisture) than to bring dry air down a degree.

I monitor the output with a thermometer probe in the vent nearest the evaporator. It won't cool the air as many degrees when the air in the house is humid. It is making me more comfortable by removing humidity, but it will take longer to cool the thermostat to the shutoff point.

Some mornings, the indoor temperature is great, but it's very humid. That's uncomfortable and can cause caked salt, caked sugar, and mildew. I may turn down the thermostat to run the AC a little.

[J Burns]

79 in the morning would probably be way too humid, but I may edge it up to 81 in the early evening, when the AC is running enough to control the humidity.

Sometimes I'll run the AC when it's cooler than 74 outside (and inside), if the relative humidity is too high. Trying to _keep_ the house that cool could mean a bigger battle with relative humidity.

I do that whenever possible. The dew point, which may be quite different than what the National Weather Service measures 10 miles from here, determines how far it's feasible to cool the house that way. As outside air cools the interior, relative humidity will rise. When it gets high, the natural fibers in wood, textiles, and paper can absorb a lot of moisture. It can be hard for the AC to get rid of all those gallons of water after I close the windows.

[J Burns]

That happened when the system was new. The house had been empty a couple of weeks when my aunt brought my 90-year-old grandmother home. It was in the mid-80s and humid inside, and a lot hotter outside. For hours, my aunt thought the AC was broken.

However, you're most comfortable while the AC is running. While it's off, humidity increases. At a given thermostat setting, a little AC that keeps running will keep you more comfortable than one with twice the capacity running half the time.

[trader_4]

I find that just running the system for 20 mins or so will take enough humidity out so that it feels a lot more comfortable. During that time, the temp reading may not even move down one degree.

[Art Todesco]

Very good point. I have a Honeywell that has a humidity setting. It allows the AC to run, actually overrun in order to try to achieve better humidity. And because of what you just said, it can do it with only minor temperature change. That's very important where I live because the temp is usually cooler but, because of the rich forestation and lots of streams, the humidity is high. I have a 2 stage compressor and it usually only runs on stage one. In fact, it just turned on now at 8:30 AM; the set point is 77 it currently reads 74 inside. The humidity is

51%. Outside it is 68 degrees but humidity is 91%. That thermostat is a great product, at least for my area.