Digital Thermostat Technical Question


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Old 10-06-07, 10:59 AM
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Digital Thermostat Technical Question

I replaced my old analog thermostat this spring and noticed my electric bills have increased significantly (almost double for September 2007 compared to September 2006). I know it has been hotter this summer but I think part of the problem may also lie in the thermostat.

My thermostat is a Honeywell, Model RTH3100C and it doesn't have a programmable variance or differential. I notice it attempts to keep the house within a single degree range. What I mean by that is if I have the thermostat set on 79 degrees I notice the heat pump comes on with the inside temp still showing 79 degrees. When I called Honeywell Tech Support they told me this particular thermostat could not be programmed for a variance and when the temperature lowered to 79.1 it would call for the Heat Pump to provide cooling. Then it supposedly kicks off at the top of the 79 degree range, possibly 79.9 degrees.

I know my old analog had either a 2 or 3 degree variance whereby it wouldn't call for cooling or heating until the inside temp was 2-3 degrees different than the actual thermostat setting. My main concern...I feel sure the digital I have with no programmable variance causes additional compressor run time and therefore higher bills? Can someone with technical knowledge and experience speak to my concern? Thanks in advance.

If my assumption is correct, I need to replace the digital I have with one that can be programmed. I welcome any suggestions.
 
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Old 10-06-07, 11:17 AM
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Wink

From what you say there. That on and off is what cost you. It cost a lot less for a AC unit to run longerat a time. Than to start and stop and start and stop all the time. Keep your filter clean every 30 to 90 days. Can check kind of for charge.The big copper line for cold and wet the small copper line warm to hot . At the condenser unit outside.
 
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Old 10-06-07, 11:33 AM
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The main thing, you have said it has been hotter than normal, and it's true most area has been hotter than normal.

I suggest you get a VisionPro t-stat.. TH8320 from the pro. the Pro lines lets the pro set the cycle per hour to make the compressor run longer.. RTH models don't allow that.

However, if your system is oversized (cycles on and off on a very hot day) you are going to have a higher bill.

All Honeywell T-stat are "smart" stat, they learn your home, and turns on the system before the temp changes.. Our body can feel the 2 degree swing! I have a guy from another board mention he feels MUCH better temp wise with the Honeywell than he did whit his old Lux that had the 2˚ swing.
 
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Old 10-06-07, 03:07 PM
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Then from what you all are saying, the cycling on and off, on and off, with a 1 degree differential thermostat causes higher bills. Is that right or wrong.

In other words, a greater differential,for example, 2-3 degrees variance would cause the compressor to run longer each time it came on but would eliminate the frequent cycling on and off and in the long run would save electricity. Is that right or wrong?
 
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Old 10-06-07, 05:42 PM
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Originally Posted by dididodat View Post
Then from what you all are saying, the cycling on and off, on and off, with a 1 degree differential thermostat causes higher bills. Is that right or wrong.
Right, a system that is oversized cycling on and off is going to cost more to run. I don't know what cycle Honeywell uses on your model. On mine, It can be changed from 1 to I think 6 cph. I have mine set at 2. it will cause to run a bit longer.

In other words, a greater differential,for example, 2-3 degrees variance would cause the compressor to run longer each time it came on but would eliminate the frequent cycling on and off and in the long run would save electricity. Is that right or wrong?
Right, the longer run time will cost lost.. That's why it's soo important to have a Manual-J done to the home. But with the 2 to 3 degree swing, you are not going to be comfortable.. As I said earlier, our body can feel the 2 degree swing.
 
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Old 10-07-07, 04:06 PM
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Why Higher Bills

Can you explain to me why cycling on and off for shorter periods of actual run time causes higher bills than less frequent cycling but longer run times in between cycles?
I don't understand that concept.
 
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Old 10-07-07, 06:24 PM
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With the more start and stop, it will take more power to get the compressor started. Takes the compressor about 5 min to get up to full efficiency.

A good example, you car.. You get better mileage driving in town hitting all the stop signs, or going down the highway with the cruise control on?
 
 

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