A/C unit will not shut off at set temperature

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  #1  
Old 07-26-13, 12:18 PM
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A/C unit will not shut off at set temperature

Ok my dad setup this A/C system many years ago and helped me replace the unit around 7 or 8 years ago. About a year ago the manual thermostat stopped working properly so I purchased a honeywell rth221b digital thermostat. My A/C unit is a FHP EMO48-1VTN unit. Recently the A/C unit stopped turning off at the set temp on the thermostat so I thought the thermostat went bad again. I purchased another honeywell rth221b and installed it but I still have the same issue. I can set the temp to 79 when the room is at 80 degrees but it will turn on but once it reaches 80 degrees it does not shut off. If I press the button on the thermostat to change the temp the A/C shuts off right away. If I set the temp on the thermostat to 80 degrees and the room is 80 degrees the A/C unit does not come on like it should when it goes above 80 degrees. Is there something in the A/C unit that can go bad that affects the thermostat sensing the correct temperatures or what? I have not changed any of the wiring since I first installed the original honeywell rth221b digital as I finally got the unit to work properly at that point in time but now it is not functioning properly. I certainly would appreciate some helpful input with this situation. I searched the internet and could not find any answers for my issue. Thank you
 
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Old 07-26-13, 03:37 PM
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Did not see anywhere to edit my post so here is an update for my situation. When the thermostat is set at 80 degrees and it reaches 81 degrees it turns on but then the temperature on the thermostat stays at 81 degrees forever and the A/C unit never shuts down. Please I need help with how to resolve this issue as I am lost. Is there something on the A/C unit itself that can cause it not to turn of if the thermostat itself is new? Thanks again
 
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Old 07-26-13, 05:37 PM
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Welcome to the forums.

You set the thermostat to 80, the temp reaches 81 and the AC starts up. Does the house get cooler but the thermostat stays at 81 degrees or is the house not getting cooler ?

It sounds like the house is not getting cooler. Is the compressor and condensor fan running outside ? The fan is easy to check and the compressor a little harder. When the outside unit is running properly..... the discharge air from the top of the unit should be fairly warm to hot.
 
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Old 07-26-13, 05:56 PM
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Sorry I should have been more specific. Yes the house keeps getting cooler and cooler. I have a separate double thermostat that reads the air inside my a/c duct and the temp inside the house as well. It keeps going down but the A/C thermostat stays at the same temp forever. Also I have an inside water cooled unit and there is no fan outside the house. The unit is cooled with water that comes from a pump outside my house and I posted the exact model of my unit as well. Hopefully this will give a little more insight. Just to let you know when I purchased the new honeywell thermostat I did not replace the backplate that mounts the thermostat to the wall to begin with. I just now replaced it and set the temp at 80 degrees and this time it actually shut off. I am not 100 percent sure if that was the issue or not and will update you as soon as I can confirm if I am still experiencing the same issue. By the way do you think this is all related to the thermostat itself because as I stated the A/C unit is performing perfectly as far as cooling down the house.
 
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Old 07-26-13, 08:56 PM
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Angry

Ok guys so after replacing the backplate for the thermostat and testing it more I am still having the same issues. The thermostat is reading 82 degrees for the house temp so I set the thermostat to 80 degrees and the thermostat drops down to 81 degrees and just hangs while the house keeps getting much colder than the temp on the thermostat. I know this because I have another thermostat that is reading 77 to 78 degrees while the A/C thermostat is still reading 80 degrees. I purchased a new thermostat and installed it thinking my other had gone bad but after installing the new thermostat I ended up with the same results. I then thought maybe the new thermostat is bad as well so I took it back for an exchange and after installing it I am still experiencing the same issue. Is there anything else that can cause the A/C unit not to turn off when it reaches the set temp on the thermostat? That is what I truly need to know so I can take care of it and if there is absolutely nothing else that can cause this then I guess the thermostat will just not function with my current setup which would make no sense since I had the exact same thermostat installed and working perfectly for almost a year. Please let me know what I can do to resolve this problematic situation and thank you kindly.
 
  #6  
Old 07-27-13, 04:12 AM
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First, quit buying el-cheapo thermostats.... you get what you pay for. A good quality thermostat will cost you at least $70.

Second, have you plugged the hole in the subbase (where the wires come through) to keep temps in the wall from affecting the thermostat reading? Most HW thermostats actually read in tenths of a degree but display whole numbers. SO, if set the thermostat for 80F it should turn on the AC above 80.5F and shut off at 79.5F while displaying 80F the whole time and never changing. That scenario will work in just that way dependent upon your air conditioning system working properly.
 
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Old 07-27-13, 05:08 PM
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Yes I actually filled the hole in the wall with expandable foam so I know that is not the issue and as I previously stated the first hw thermostat that I installed worked flawlessly for almost a year before I ran into this trouble and again that is the reason I asked if there was anything else besides the thermostat that can be causing this issue. When I set the thermostat to 80 degrees the unit does not shut down when it reaches the specified temp of 80 degrees but just keeps on running and getting the house colder. When I press the run button on the thermostat after it has been running for a while it shuts off immediately and within minutes the temperature on the thermostat drops down to 78 degrees but as long as the unit is running it just hangs at 80 degrees and never shuts off. This has me completely baffled for sure and I truly wished I could figure this out because it just started this recently and as I already stated I had the exact same hw thermostat for many months and it worked fine. I see you made a comment about the cheap thermostat that I purchased but again it worked perfectly before so I do not understand why it is not operating the same. Could you also please enlighten on what a more expensive thermostat has in it that makes it work better than the cheaper ones. Oh and 1 last thing you need to know. I am disabled and am on a limited income therefore my funds are very limited and I don't have a boatload of money like some people hence the reason for purchasing a cheaper thermostat. If you have an expensive thermostat laying around that you don't need then by all means I will pay for the shipping. Thanks

Update: I have a 4 ton A/C unit which is a FHP Manufacturing EMO48-1VTN and I live in a 4 bedroom house so what would be a good quality thermostat for my situation. I do not care about being able to program it as I am home all day and keep the A/C at the same temp pretty much year around. Thanks for any helpful input with this issue.
 

Last edited by hvactechfw; 07-28-13 at 07:06 AM. Reason: removed unnecessary comment
  #8  
Old 07-28-13, 07:04 AM
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To me, it sounds like you have air infiltrating near the thermostat that is throwing it off. Is the fan set to on or auto? The reason for more expensive thermostats is that the cheap thermostat are not manufactured to the same quality and fail much more often. NO, I'm not sending you a thermostat!

FYI, your florida heat pump unit manual : http://www.fhp-mfg.com/files/downloa...EM_Install.pdf
 
  #9  
Old 07-28-13, 12:14 PM
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I already stated that I filled all areas behind the thermostat with expandable foam so there is no way for air to get to it except from the inside the home itself and not from behind the wall. I already knew you were not going to send me a thermostat but I wanted you to be aware that not everyone has the money to buy the better products hence the reason they sell cheaper ones. I already had the manual for my unit but that in now way helps to figure out my issue. So once again if you think that it is the thermostat itself that is causing my issue then what would model thermostat would be best for my current situation.
 
  #10  
Old 07-28-13, 12:36 PM
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Since you want to keep bringing up your income..... i feel i might comment since it is slightly lower than what I make working my a$$ off everyday.....and I can afford a decent thermostat when I also have a family to pay for...... I do not at all feel for you being on a government paid for income and having little money to by necessary things....the best non-programmable thermostat for your system is a TH5110 Honeywell focus pro thermostat..... yes I think it is your thermostat as the issue .. question though... did you replace the subbase when also replacing with the current thermostat? I don't know why it would make a difference but it is something to try if you did not replace it.
 
  #11  
Old 07-28-13, 01:52 PM
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I have been trying to analyze your problem. You stated that your second thermostat is reading 77 to 78 degrees while the A/C thermostat is still reading 80 degrees. Is the second thermostat reading the room temperature at the exact same location as the thermostat you are having issues with? Temperatures inside a house will vary greatly depending on the location, ie, radiant heat coming through windows, distance from air vents, proximity to return air intakes, better insulation in a particular area, outside wall exposure and many other factors. That is why thermostat installation manuals state that locating the thermostat in the proper location is very important. My point is that the second thermostat you mention needs to be right next to the one you are having problems with (if that is not the case). Then, if there is a difference between the two temperature readings on the two thermostats it can be a clue your thermostat is bad. But if the thermostat temperature readings are the same it would tell you there is another issue with your a/c not dropping the temperature.
Check out this link: Does Your Thermostat Show the Wrong Temperature?
It shows that hot air can come down the wall from the attic through holes drilled in the top plate. You stated you put foam in the wall where the thermostat is located. You would have to fill the entire wall from the thermostat wire up to the attic with foam to stop the air infiltration from heating up the wall behind the thermostat location.
 

Last edited by firedawgsatx; 07-28-13 at 03:15 PM.
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