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Home HVAC programmable thermostat

Old 06-30-2013, 12:12 PM
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Default Home HVAC programmable thermostat

Guys,

I have a programmable thermostat on my HVAC system. I set the target temp to 74 and the AC won't shut off until 73 is displayed.

I have the swing set to .25 , the lowest possible setting to keep the temp as close to 74 as possible.

Am I expecting too much ?
Old 06-30-2013, 12:45 PM
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Originally Posted by BW23 View Post
Guys,

I have a programmable thermostat on my HVAC system. I set the target temp to 74 and the AC won't shut off until 73 is displayed.

I have the swing set to .25 , the lowest possible setting to keep the temp as close to 74 as possible.

Am I expecting too much ?
Yes. I'd bet the accuracy specs are at least +/- 1 degree.
Old 06-30-2013, 01:22 PM
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You may just be looking at a rounding error in the display. If the thermostat displays 73.75 degrees as "73" and 74.0 as "74", that would explain it.

FWIW, I have my digital thermostat set for a 4 degree swing. HVAC equipment is generally more energy efficient and durable if it starts less often and runs longer when it does start. Can't say the temperature swings in my house are even noticeable.
Old 06-30-2013, 01:23 PM
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As above, I think a .25 degree swing is pushing it, your unit must be going off and on quite a bit - why would you try to enforce such a tight tolerance?
Old 06-30-2013, 01:38 PM
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Originally Posted by BW23 View Post
Guys,

I have a programmable thermostat on my HVAC system. I set the target temp to 74 and the AC won't shut off until 73 is displayed.

I have the swing set to .25 , the lowest possible setting to keep the temp as close to 74 as possible.

Am I expecting too much ?
I work on HVAC systems as well as other mechanical items for work, to give you my background. With such a "tight" setting you may be asking your unit to kick on and off too many times (cycles per hour). Each time the compressor, or any other electrical motor is started a slight electrical arc takes place on the windings. When enough of these arcs occur you usually have burned up the motor (over many many years with normal use). I like to keep my house at a constant temperature as well, as to prolong the life of the equipment I would at minimum set the "swing" or on/off total value for 1 deg F. In other words if you have it set for 74, when the home hits 75, have the unit kick on, depending on the thermostat it will kick off at 74 or 73 (all depending on if you can set the lower swing value). Most equipment manufactures recommend at best 3 or 4 cycles per hour tops to prolong the life of the unit. Depending on the thermostat, you can program this safety feature in the installers menu options. I hope this does not open up a can of worms. Just watch the display on the thermostat and go off of that. If you want to use a separate thermometer to check the accuracy that is ok, sometimes you can program a temperature offset in the install settings to make the thermostat more accurate. For normal thermostats design tolerance is usually +/- 1 deg F from an accurate thermometer.
Old 06-30-2013, 02:02 PM
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2 degrees is average. Set at 74, on at 75, off at 73 and leave it alone.
Old 06-30-2013, 03:15 PM
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Agree with Dave^^
Old 06-30-2013, 04:15 PM
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We tried the programmable route and my wife was NEVER satisfied with the comfort level. Always kicking it up or down a couple of degrees to "take the edge off" or something.
When I replaced everything with a new heat pump, I went with a digital non-programmable one.
Surprise! She is still never satisfied.
Old 06-30-2013, 05:32 PM
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Thanks. I set the swing to .25 to try and have the AC shut off at 74 more accurately.

I just reset it to 1.0 and will monitor it. On another note, what temp should the cooled air be coming out of the floor register ?

I'm measuring 50-53 deg but the AC runs constantly to get the temp below 74. it might run for 1 hour to go from 74-73.
Old 06-30-2013, 05:36 PM
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do you have a humidity point set as well?

I believe some units will pull the temp down past the setting to meet a humidity requirement you set.
Old 06-30-2013, 05:38 PM
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Originally Posted by BW23 View Post
Thanks. I set the swing to .25 to try and have the AC shut off at 74 more accurately.

I just reset it to 1.0 and will monitor it. On another note, what temp should the cooled air be coming out of the floor register ?



17 - 19 degrees lower coming out vs. going in. If you have more than 1 return take a temp at each one and average them.



I'm measuring 50-53 deg but the AC runs constantly to get the temp below 74. it might run for 1 hour to go from 74-73.
sounds about right

That's why I'm not a big fan of programmable thermostats except for use with gas heat.
Old 06-30-2013, 05:45 PM
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Originally Posted by BW23 View Post
Thanks. I set the swing to .25 to try and have the AC shut off at 74 more accurately.

I just reset it to 1.0 and will monitor it. On another note, what temp should the cooled air be coming out of the floor register ?

I'm measuring 50-53 deg but the AC runs constantly to get the temp below 74. it might run for 1 hour to go from 74-73.
Depends on the outdoor temp, humidity and other load factors. Plus the type of system you have affects how much it runs. A 2 stage compressor runs longer and more efficient than a single stage.

A properly sized unit will theoretically run 24 hrs a day on the hottest, most humid day. Typically, the air coming out of the register will be 20 to 25 degrees lower than the air entering the return at 50% RH. If you have a programmable thermostat and it's at 84 while you're away, it might take 24 hours to get to 74 and stable humidity.

76 degrees at 40% RH is more comfortable than 74 degrees at 60% RH.
Old 06-30-2013, 06:01 PM
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No RH setting. Thanks for the suggestions.
Old 06-30-2013, 06:33 PM
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Some of us have a sling psychrometer. Or, you could get a humidistat to monitor how your system is preforming.
Old 06-30-2013, 08:02 PM
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20-23 degrees is usually the norm for a single stage compressor. The best place to measure the temp is at the output side of the air handler, if it's hard to get to you can always measure at the grill like you are doing now. The most important thing is to have a clean air filter, and change them frequently. This is often overlooked. The A/C needs lots of air flow to run efficiently, and if it's not getting as much airflow your system will run longer and less efficient, in return running up your power bill. I would recommend a pleated filter as it gives you more surface area, thereby decreasing the amount of restriction that the filter causes. Don't go crazy with the most expensive filter as they will usually cause too much restriction in the system. I would recommend a MERV 6 or 7 rating as long as they are pleated, or a 3M Filtrete "better" filter. Remember that you can have the best system, installed by the best tech, and the most advanced thermostat, but if you are running a dirty filter that all goes out the window.
Old 06-30-2013, 08:25 PM
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.25 deg F around setpoint is unattainable with a compressor and a bang bang contactor on the cool side. THe compressor would be firing on and off constantly. This is called hysteresis and it is undesireable. That is why they allow a few deg F swing to specifically avoid is, or the compressor motor would not last very long.

.25 deg would be attainable on the heat side, if the heat was electric and the fan could run continuously. The output would have to be proportional type control that has three control terms, proportional, intergral and derivative. Way too much info.

Frankly that tight of control is unnecessary for the home.

Be really happy that it works as well as you describe.
Old 07-10-2013, 07:29 AM
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Update
Tech came and checked the outdoor compressor unit and it was running perfect.

We decide to change the programable thermo with a new Honeywell thermo.

Bingo ! The new thermo fixed it ! If I set it for 74, the AC cycles off / on every 10 - 15min as expected.
Old 07-13-2013, 07:14 PM
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Glad you got your trouble fixed
Old 07-14-2013, 03:45 AM
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Originally Posted by BW23 View Post
Honeywell

Old 07-14-2013, 08:25 AM
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Originally Posted by BW23 View Post
Thanks. I set the swing to .25 to try and have the AC shut off at 74 more accurately.

I just reset it to 1.0 and will monitor it. On another note, what temp should the cooled air be coming out of the floor register ?

I'm measuring 50-53 deg but the AC runs constantly to get the temp below 74. it might run for 1 hour to go from 74-73.
Why would you care? If you want it at 74, then set the thing for 75. The reason they allow a swing is for efficiency purposes. If the AC tries to keep it right at 74 with very little swing, the AC would be constantly cycling on and off. That's why too big of an AC can be just as bad as too small of an AC.

Accuracy at the cost of efficiency defeats the whole purpose of having a programmable thermostat. I would increase your swing size.

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