Can it be this simple? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 14 Old 06-21-2018, 11:28 AM Thread Starter
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Can it be this simple?

As we started this years cooling season, I notice that the A/C doesn't turn off when it reaches the set point. For example, the temp reads 79°, and I set the thermostat to 77°. Next think I know, I'm chilly, so I go to check the thermostat and it's got 77° for the set point, but the temperature is reading 75° and it's still running.

Can this be a simple as the thermostat has lost it's mind? Could there be other mysteries. It would be nice if it's as simple as replacing the thermostat, but I'd probably rather not spend $100 on a new one if it doesn't solve the problem. By the way, none of the setting in the current thermostat have been changed since it was installed 10 years ago.

One more question: It's a Carrier Heat pump system with a variable speed fan motor. Will I need to use a Carrier thermostat, or will any Heat Pump thermostat do the job?

Thanks for any guidance
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post #2 of 14 Old 06-21-2018, 12:01 PM
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It's not the thermostat that has lost it's mind it's you.

Mine kinda does the same thing. My house the faces west and in the afternoon the thermostat is set to 72* but the thermostat says it's 75* inside. Right down the hall (10') a different thermometer says it's 72*. I found the heat from the attic is coming down between the walls and heating up the thermostat. Now ain't that a bummer?

Don in Murfreesboro, TN.
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post #3 of 14 Old 06-21-2018, 12:04 PM Thread Starter
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Originally Posted by hawkeye10 View Post
It's not the thermostat that has lost it's mind it's you.

Mine kinda does the same thing. My house the faces west and in the afternoon the thermostat is set to 72* but the thermostat says it's 75* inside. Right down the hall (10') a different thermometer says it's 72*. I found the heat from the attic is coming down between the walls and heating up the thermostat. Now ain't that a bummer?
I hear ya, but shouldn't the temperature and set point within the same thermostat be in synch?
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post #4 of 14 Old 06-21-2018, 12:25 PM
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Quote:
Originally Posted by Quickstep View Post
I hear ya, but shouldn't the temperature and set point within the same thermostat be in synch?
Both of mine do the same thing, IIRC there is something about the system that needs to finish the "cycle" once you have reached the desired temp. In addition on the Nest thermostats I have there is a setting(Airwave) that turns the condenser off "early" then lets the fan only run to finish the cooling cycle.

What I typically see is a degree below the set point. I don't mind it as much in the winter(heat) but in the summer I get downright cold.

Funny thing is I can have the truck AC set to 60 and be quite OK with it, but the house at 73 freezes me out. Guess I need to go back to being a lot heavier... :)

ETA: my systems are Trane with VS motors, no special thermostat needed on mine.

Last edited by shoot summ; 06-21-2018 at 01:21 PM.
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post #5 of 14 Old 06-21-2018, 01:05 PM
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I think they sell aftermarket thermostats, which leads me to think you don't need to match brands.
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post #6 of 14 Old 06-21-2018, 02:30 PM
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Your thermostat had gone bad, unless it has been doing this the whole 10 years. The limit cycle should be in much closer tolerances.


You need a thermostat that has the same numbers of terminals/wires as the old one. I will not swear that most are common, but that is the impression that I have in the back of my mind. Thermostats are very easy to change.



Thermostats



How to check thermostat compatibility.



George
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post #7 of 14 Old 06-21-2018, 04:36 PM
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dust often collects inside the housing and can interfere with mechanical operation. some thermostats are totally solid state.... if you set the temperature with a button vs. a knob, odds are it's all solid state.


sometimes there's a "deadband" adjustment - if set at 76' it clicks 'on' at (example) 76.5' and clicks off at 75.5'
you would have to check the manual for your exact model (a) if you have it and (b) cycle it up/down and see if that helps. it is usually a potentiometer which can also be affected by dust.



otherwise time for new thermostat - the multispeed fan can complicate that issue. probably best to stick with either an OEM replacement or one that explicitly says it is suitable for your system.
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post #8 of 14 Old 06-21-2018, 09:48 PM
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That's kind of picky to ask a thermostat to be that accurate. Two degrees is awful close to the set temperature.
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post #9 of 14 Old 06-22-2018, 02:28 AM
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That's kind of picky to ask a thermostat to be that accurate. Two degrees is awful close to the set temperature.
You may be thinking of the old analog thermostats that had the bimetal coil. Modern digital thermostats should be accurate to within one degree. By accurate, I mean they should turn off the heater when the temperature exactly matches the displayed setting within a degree or so. There are upper and lower thresholds that prevent the system from constantly cycling on and off for fractions of a degree.

Our thermostat stops heating or cooling when the temperature on the display matches the setting. It fires up when the display shows a degree (rarely two degrees) off from the setting.

Most likely, @Quickstep's thermostat needs to be replaced. I have replaced failed thermostats in at least three homes. I bought digital ones at the big home store. They were middle grade models - not too fancy or complex, but with sufficient features to meet our needs.

Fans on modern thermostats usually run slightly longer than the actual heater or air conditioner, to purge the warm or cold air, so it doesn't sit and go to waste in the ducts in the attic (or wherever the ducts are). This is an automatic function of today's thermostats.

Most new thermostats run off the power in the thermostat wiring - no external power needed. In a typical installation, you connect four wires, configure the thermostat for the type of heating and cooling in your home, set your times and temperatures, and you're done.

I just checked our thermostat, and we have a Honeywell RTH7600 model. I don't remember when I had to replace it, but my spouse says that it has been well over five years ago. According to the installation manual, it controls a long list of system types, too much to type here. That includes heat pumps. I don't know about the variable fan speed, but you can ask Honeywell at 800-468-1502 (from the front of the installation guide for wiring help).

Unless you have something very unusual, modern thermostats should work with most home heating systems. They are easy to install and relatively easy to operate. They are designed for ordinary homeowner installation. They practically beg you to call the company first and not return the thermostat to the store if you run into problems.

Placement of the thermostat can make a huge difference. My parents moved their thermostat from the living room to the hallway. The living room location was too close to a lamp, back in the incandescent bulb days. Heat from the lamp affected the thermostat. It was a pain to adjust the thermostat when you turned the lamp on and off, so I helped them move the thermostat.

Thermostats are getting more sophisticated, and arguably easier to use, as manufacturers deploy new technologies that sense when you are home, what temperatures you like and when you like them, etc. In addition, they use more sophisticated programming to improve overall comfort and attempt to save power.

This is a personal statement:

I do not recommend internet-connected thermostats, such as the NEST. For me, this is a privacy and security issue. Others do not seem to care, or do not understand the issues, which are far off-topic for a woodworking forum.

Last edited by Tool Agnostic; 06-22-2018 at 02:33 AM.
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post #10 of 14 Old 06-22-2018, 06:51 AM
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Originally Posted by Steve Neul View Post
That's kind of picky to ask a thermostat to be that accurate. Two degrees is awful close to the set temperature.

The displayed temperature on my thermostat never goes over or under the set temperature. I also would not be happy if it went over or under that much.


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post #11 of 14 Old 06-22-2018, 08:58 AM Thread Starter
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OK, here’s what I think happened.

I got a copy of the installation manual for my thermostat (actually called a Thermidistat) It has a dehumidification feature and when the feature is set to on, and the ambient humidity is 5% higher than the desired humidity, the system will continue to run until the humidity gets within 2% of the humidity set point, or until the temperature drops to below 3 degrees below the temperature set point. In other words, it lets it go below the set point to dehumidify.

I’ve turned off the dehumidification feature for now to see if cooling stops at the set point. Right now, it’s too cool for a good test, but I think this is going to do the trick.

One more thing-
Since the system has a variable speed blower, there’s also a setting that lets the blower run on low so it will dehumidify without too much cooling, but I need to learn more about setting that up before messing with it.
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post #12 of 14 Old 06-22-2018, 10:14 AM
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I think you found your problem. Unless you have a bad humidity problem I would just permanently turn off that feature.



George
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post #13 of 14 Old 06-24-2018, 12:11 AM
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I was going to say to change the batteries.

The humidity feature sounds like I would have wanted in Ohio, Pennsylvania and Florida. In Arizona I would have never needed it and here perhaps one day a year.

Rich
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post #14 of 14 Old 06-24-2018, 08:54 AM Thread Starter
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I do like keeping the humidity down.

It makes it more comfortable and consistent humidity is good for my woodworking projects.
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