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Old 04-24-2011, 09:54 AM   #1
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Auto-Change Thermostat


I'm thinking about buying a new Honeywell RTH7500D thermostat for my Lennox heating/cooling system in my home. I like the idea that it will "auto change from heat to cool". It also has a 7-day programmable feature that I like. The problem that I am trying to solve is the west wall of the house has lots of windows. Even on a cool sunny day, this side of the house is warm in the afternoon and will warm the rest of the house, so we switch on the AC. But at night, it cools off and we need heat again. This is more of a problem in spring and fall (6 months of the year). Has anyone tried this thermostat? Does it really work like the manufacturer promises?

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Old 04-24-2011, 10:17 AM   #2
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Auto-Change Thermostat


It does work but it is meant more for Commercial applications. You would need to program it to have at least a 3 deg F difference between when the cooling cuts in or every time the missus bakes something the AC will cut in. May be warm in the kitchen and cool in the rest of the house. This can short cycle the unit or run it with a low load which can harm the compressor. I would not recommend it. I would have to see the specs on it to see if it can have a 3-4 deg F deadband/differential for switchover. I would recommend shades/roller blinds or drapes to reduce the heat load/sunlight infiltration.

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Old 04-24-2011, 10:32 AM   #3
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Auto-Change Thermostat


As far as I know. No Honeywell thermostat can be set closer then 3 deadband between change over.
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Old 04-24-2011, 10:36 AM   #4
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Auto-Change Thermostat


I have never had that request in over 30 yrs of doing HVAC in resi. If it still gets cool enough to turn the heat on where I am then most people open their windows during the day to get fresh air and free cooling rather than fire up the AC and burn elec $$. I hate seeing units running with low or minimal loads.
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Old 04-24-2011, 01:59 PM   #5
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Auto-Change Thermostat


We do not setup any thermostats for the available AUTO function.... It usually resulted in a call back when we did........Homeowners tend to not be able to wrap their head around deadband.
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Old 04-24-2011, 02:33 PM   #6
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Auto-Change Thermostat


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Originally Posted by hvactech126 View Post
Homeowners tend to not be able to wrap their head around deadband.
No sure I get it either - Honeywell website says this thermostat has "precise temperature control. It holds +/-1DegF accuracy to maximize comfort." Is this the same thing?

I understand the need to prevent short cycling the compressor. But not sure what "low load" means and why it is bad.
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Old 04-24-2011, 02:41 PM   #7
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Auto-Change Thermostat


+-1 deg F means when on heat it will be accurate to within 1 degF or cool. You need a minimum 3 degF span before it switches modes or it will cool the house 1 deg F shut off then the sun goes behind clouds and house cools 1 deg more and the heat comes on and the cycle repeats itself depending on whether it is cloudy or sunny. Very inefficient and hard on the machinery. In Commercial apps they need auto switching as you cannot rely on burger flippers to change the tstat from one mode to the other or know how or when. Low load creates a problem with low suction pressure = low evap/A coil temp and liquid refrig can travel back to the compressor and damage it. Evap coil can freezeup also.
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Old 04-24-2011, 02:57 PM   #8
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Auto-Change Thermostat


Simply put a single point of reference, ie, the thermostat will not be enough to adaquately decide whether the system should be in cooling mode or heating mode.

In order to do this type of switching you would need 2 points of reference, a interior set point over interior variable and an ambient temperature probe, these in turn would have to be tied to a processor which could provide time over set point deviation as it relates to ambient temperature.

This would ensure that the switch did not happen too soon or happen repeatedly as the cooling and heating modes fight each other correcting set point deviation.

Unless you are willing to spend the money to buy a processor, a couple of thermocouples and bring in someone to do the programming, I would absolutely stay away from this kind of control.

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Old 04-24-2011, 03:30 PM   #9
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Simply Put??? I worked with a Johnson Controls Metasys building operations system with proportional bands etc etc but I doubt he can afford that.
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Old 04-24-2011, 03:38 PM   #10
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You could always buy a Allen Bradley SLC 503 processor, a Allen Bradley Panelview complete with programming software, cables, analog cards, output cards and then rewire, and you could have all this for a mere say $6000, then add the programming time @ $90/hr say 40 plus 4 hrs stand-by time say $4000, then you could have it control real nice.

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Old 04-24-2011, 03:44 PM   #11
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$90/hr? the guys from the big 3 controls companies and the elevator guys charged us about $200-$500. Or else you buy $10-100,OO0 worth of preferred client "maintenance and monthly" service so they drop it to $180 - $480. Now I really am in the wrong business.
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Old 04-24-2011, 03:58 PM   #12
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What can I say, I am an independant contractor with 25 years in the process controls and design, really big air houses!!!!! for industrial spray booths. Temperature and humidity control to +-1* / 1%.

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