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merc2007 02-10-2007 02:43 PM

Heat Pump Thermostats
 
I live in an apartment complex which has a Heat pump for Air/Heat (2stage heat/1stage cool). I've been here for about 10 years and 2 years ago the mechanical thermostat was replaced with an electronic one (Honeywell T8411R). Last summer, during the first cool cycle since replacement I have noticed that the central air does not cool enough to where the thermostat shuts down. To have thermostat shut down I would have to raise the temp somewhere in the upper 70's degrees.
Are these electronic thermostats dependaple as the mechanical ones or is this particular Honeywell model not the right one? can anyone out there explain this or is this my imagination? I even purchased a mechanical one and waiting for the spring to have maintenance replace the electronic one.

As additional info, I just downloaded a PDF file on the Honeywell T8411R Electronic Heat Pump Thermostat Installation Instructions. It has a section "Set Temperature Setpoint". This may be a stupid question but can anyone out there explain how these setpoints work and where they should be set to (i.e. should the setpoint be higher/lower than set temp in cooling and heat seasons). FYI, cool checkpoint was set for 74 degrees. Is this part of explanation for above?

rakes9720 02-12-2007 03:21 PM

Electronic thermostats are very dependable. It is very unlikely that the thermostat is broken. Your heat pump might not be cooling properly. Check to make sure the filter is clean, and the outside unit is clean. Also make sure the all registers are fully open.

Does the unit turn on if you lower the temp. in cool mode? If not, it might be an incorrectly wired thermostat. (O connection if the heat works).

Some thermostats are more sensitive than others, and maybe cooling wasn't working 100% correctly before, but the mechanical thermostat still sensed it, and now this new thermostat doesn't quite sense it.

Have a professional look at your system.

merc2007 02-12-2007 05:54 PM

Thanks for reply Rakes,
The unit does turn on when lowering temp but as stated before apartment does not cool down properly, even though I've set thermostat to 72-74. Let me ask another question; if outdoor ambient temp is in the upper 90's and very humid, how does this affect the Heatpumps output? The unit does eventually turn off when outdoor temps cool down, at night........let's say 10:00PM'ish.

zunk 02-12-2007 06:12 PM

I have a Honeywell Chronotherm III that is over 15 years old and still works like new, except the screen is a bit hard to see. It's possible it doesn't work, but more likely it's not set or wired right. Or your heat pump is having problems. You can always make it run like a manual stat, just go off of program mode.

rakes9720 02-13-2007 12:10 AM

That sounds more like heat pump problem. Yes, when it gets to the upper 90s, the cooling ability of the heat pump is lowered, and heat gets in the home faster as well. A properly sized system will still cool properly in this situation, and since yours worked before, I dont think size is an issue. It is more likely that the heat pump is not providing as much cooling as it used to for many different reasons (Leak in refrigerent line, tree shading heat pump cut down, clogged filter, etc.)

I would say, to rule out the thermostat, place an accurate thermometer near the thermostat on one of those hot days and see if the temp gets down to the set point, but since its not summer...

Does the heat pump heat properly now? (See if it does when the heat pump is running without any electric auxiliary heat, backup heat, or 2nd stage heat)


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