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  1. #1
    Join Date
    Nov 2010
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    145

    Question Half degree or tenth degree Thermostat?

    Can anyone recommend "the best" accurate single stage, no setback thermostat.... that calls for heat or air on partial degrees... like half or tenth? (IE, ONE THAT OPERATES LIKE A GOOD OLD MERCURY UNIT, AND WHICH COULD CALL FOR HEAT A FRACTION OF A DEGREE? ) I remember the day that digital stats came out, offering setback capability. I ran out and bought one. It wasn't long before I realized that the digitals were pogrammed in full degree increments, so the call for heat occurred at -1 degree and room temp dropped below that before the heat arrived, and cutoff was +1... with overshoot the variation was at least 2.5 to 3 degrees. I ripped it out and put back the mercury unit which could be set to call at fractions of a degree.

    Is there a better or best unit that can be set to call on fractions?

    Is there one that I can set.. or have a pro set... for a certain number of cycles per hour?

    I was unaware of load calculation when I replaced my split system a year ago with a grossly oversized GMH95 and single stage AC (none of 3 contractors made any mention of it). I have a significant problem with short cycles and long periods between them=poor comfort. It occurred to me that a stat that has a narrower call band could make a significant improvement.

    Current Tstat is Honeywell Pro 3000 non programmable.

    Thanks in advance.
    Paul

  2. #2
    Join Date
    Jan 2009
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    I don't think it's possible ot buy a Mercury thermostat anymore. The question you should be asking si whether or not you equipment is sized correctly and is capable of mainting a tight .1 F temp range. IN mild weather, a unit woudl need ot cycle too frequently and for too short of cycles to maintain such a tight temprature. In cooling, mainting a .1F swing without vairable speed equipment would greatly shorten equipment life and reduce efficiency. Compressor shouldn't cycle more than 3 CPH.

    What you want or hte tightest tmeprature control is a thermostat that operate based on cycles per hour. Then you'll need 2 stage or modulating equipment as well.

    Honeywell probably makes the best units in terms of tight temperatures control based on cycles per hour. Another pro might be able ot recommend hte cheapest model that will control based on cylces per hour.

    That being said, you should expect a .5F temp swing most of the summer, except in the hotter weather and the same in milder fall winter and spring conditions when in heating mode.

    Finally, the last factor is system balance and even air distribution and mixing. IF equipment is oversized and/or ductwork undersized, you'll often have hot and cold spots in a home. A circulating fan feature in some thermostats can help balance out room termpatures along with system balancing.

    By just adding a couple dampers and balancing a system I've taken a home with as mcuh as a 5F temperature difference and reducing it to under 1F despite grossely oversized equipment.... all the while increasing overall airflow and reducing noise.

  3. #3
    Join Date
    Nov 2010
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    145

    Thanks Motoguy...

    Thanks for your input. I did not mean that I wanted to control to one tenth... just that I assumed that digital stats might need to have the capability to measure tenths rather than degrees for a finer variation. I was half way through the post when I recalled from earlier discussions that some stats can be programmed for cycles per hour, which gets me to the same place.

    Here is the scope of my problem on the heat side: 1 have a 1900 SF brick 50's construction ranch house with 800 SF finished basement, mostly below grade. My load calc done post installation said 60k btu's of heat at 22 degrees, 72 inside... before I added R19 insulation, which is supposed to drop the loss to 55k. I have a 92k goodman 95% which delivers 89k at 2nd stage and 63k on first stage.

    Today, it is 30 degrees outside, my house is currently in the sun, and the first stage went off at 9:12 A... came on at 9:41 A... and went off again at 9:50. Heat has been on since 6:30AM, so temp is reasonably stabilized now. So: my 63k of heat is cycling 9 minutes on and 29 minutes off. While some improvement may be available by balancing (I have done some of that but do need another damper on a significant room)... I am convinced that a stat that can force CPH will give me greater comfort. I continue to weigh yanking out the 95k furnace and getting a 70k 4 ton variable to replace it... and look to recover the swap out cost in variable motor savings, efficiency.

    Thanks again, Paul

  4. #4
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    Jul 2004
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    I know of no thermostat that has a display reading in less than 1 increments. Some of the communicating t-stats regulate in 1/16 degree increments but do no display them because of the service calls they could generate. Imagine those people who are so anal retentive as to expect the system to maintain at temperature within 1/16 of a degree? The purpose of having such small increments is to allow the t-stat to adjust the burner more finitely. But the human body senses a temperature swing in roughly 2 increments so anything smaller than 1 for the display serves no purpose. That said, Honeywell 45-years ago tried to remove all numbers from their thermostats and just had water heater type increments that showed cool/warm/hot. Those quickly fell by the wayside, never to be seen again, because the end users wanted to see numbers. So the old mechanical and mercury t-stats maintained a set-point +/- 2 around the set-point. Most of today's t-stats are much more accurate than that.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
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    Nov 2010
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    145

    What about a White Rodgers.. adjustable Temp differential?

    Thanks Skipped. Display not showing tenths is just an annoyance as to not knowing what is really going on. I have a fairly accurate digital themometer setting not far from the Tstat and some times they are right on and others times, in excess of a degree apart.

    Again, furnace is cycling only once per hour, and I am cursed with the ability to sense less than a half a degree of drop and feeling "cold", compounded by oversized furnace. My old mercury stat was way more accurate than any digital thermo I have had since. Never felt this "way off" sensation, and my theory appears to be born out in all the digi specs today: + or - 1 degree.

    Am I reading this correctly that White Rodgers (Nee Emerson) allows adjustment of "temperature differential"? Would that be the drop before a heat call is issued... from their manual:?

    Differential (Single Stage) . . . . . . . . . . . . . . . . . . . . Heat 0.6F; Cool 1.2F (adjustable)
    Differential (Heat Pump) . . . . . . . . . . . . . . . . . . . . . Heat 1.2F; Cool 1.2F (adjustable)


    IF I can set the unit to call for a narrower increment, problem solved.

    Thanks, Paul

    QUOTE=skippedover;12450701]I know of no thermostat that has a display reading in less than 1 increments. Some of the communicating t-stats regulate in 1/16 degree increments but do no display them because of the service calls they could generate. Imagine those people who are so anal retentive as to expect the system to maintain at temperature within 1/16 of a degree? The purpose of having such small increments is to allow the t-stat to adjust the burner more finitely. But the human body senses a temperature swing in roughly 2 increments so anything smaller than 1 for the display serves no purpose. That said, Honeywell 45-years ago tried to remove all numbers from their thermostats and just had water heater type increments that showed cool/warm/hot. Those quickly fell by the wayside, never to be seen again, because the end users wanted to see numbers. So the old mechanical and mercury t-stats maintained a set-point +/- 2 around the set-point. Most of today's t-stats are much more accurate than that.[/QUOTE]

  6. #6
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    Most Honeywell thermostats call for heat or cooling, when the temp increase or decreases by .5F. And less, and the average home with a furnace that was even close to proper size would short cycle no matter what. Remember, every minute the furnace isn't running, is a minute your not paying for fuel.
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  7. #7
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    THX

    Quote Originally Posted by beenthere View Post
    Most Honeywell thermostats call for heat or cooling, when the temp increase or decreases by .5F. And less, and the average home with a furnace that was even close to proper size would short cycle no matter what. Remember, every minute the furnace isn't running, is a minute your not paying for fuel.
    Thanks. So the Braeburn 5000 I just ordered (.5deg differential before heat/cool call) may only be an improvement over the HW Pro 3000 with the availability of part time fan circulation.

    I was envisioning it running more times for shorter intervals. I know I lose some efficiency on startup times that way.

  8. #8
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    Small improvement if any. And a lot of shorter run times will use more fuel.
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  9. #9
    Join Date
    Dec 2002
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    Can anyone recommend "the best" accurate single stage, no setback thermostat.... that calls for heat or air on partial degrees... like half or tenth? (IE, ONE THAT OPERATES LIKE A GOOD OLD MERCURY UNIT, AND WHICH COULD CALL FOR HEAT A FRACTION OF A DEGREE? )
    You are kidding I hope.

    So the old mechanical and mercury t-stats maintained a set-point +/- 2 around the set-point. Most of today's t-stats are much more accurate than that.
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  10. #10
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    I'm not sure any t-stat is going to give you total satisfaction if your furnace is over sized. What you're feeling is not so much an increase or decrease in the actual room temperature but more the presence to absence of hot air flowing out of the supply outlets. That's a common problem with warm air systems and the very reason that multi-staging and modulating furnaces have appeared on the market. They cost more but the longer run times really pay dividends in comfort. And that is also why its' so important that a load analysis be done. No sense having a 2-stage if the 1st stage is so big it does the whole job 90% of the time. In that case, might as well have saved some money and just purchased a properly sized 1-stage.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  11. #11
    Join Date
    Nov 2010
    Posts
    145

    Thanks....

    Amen from the Amen choir. I found HVAC-Talk shortly after the oversized system was installed and learned in excruciating detail from the good folks here.... just how screwed I was! I am basically more than covering my heat needs with the 63k first stage. Had I known and done a load calc, I could have had a 69k 2 stage, run the first stage at like 44k.. etc. etc.
    I was also concerned at the time about high failure rate of variable motors... we get a lot of electrical spikes here in Tall Tree Buckhead ATL, so not sure I would have gone variable at that time. Would now do it in a heartbeat, but not ready to take a $$$$$ bit hit.


    Quote Originally Posted by skippedover View Post
    I'm not sure any t-stat is going to give you total satisfaction if your furnace is over sized. What you're feeling is not so much an increase or decrease in the actual room temperature but more the presence to absence of hot air flowing out of the supply outlets. That's a common problem with warm air systems and the very reason that multi-staging and modulating furnaces have appeared on the market. They cost more but the longer run times really pay dividends in comfort. And that is also why its' so important that a load analysis be done. No sense having a 2-stage if the 1st stage is so big it does the whole job 90% of the time. In that case, might as well have saved some money and just purchased a properly sized 1-stage.

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