need a simple thermostat


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Old 10-30-06, 07:25 AM
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need a simple thermostat

I need a 2-wire, battery-powered thermostat that isn't too smart for its own good. My Honeywell CT3400 keeps trying to mess with the cycling rate during recovery from night setback. I have fiddled with the various settings to little avail.

I need a simple programmable thermostat that functions much like an old mercury stat: set it for a temp and it calls for heat continuously until the temp is reached and it is satisfied. Only needs one program with morning/day/evening/night settings.

Problem is, there's so many thermostats out there I could spend the rest of my life trying to find one. Suggestions? Thanks.
 
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Old 10-30-06, 01:57 PM
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Talking Stat

Would the the Rite-Temp 8022C do the trick? You might think about adding the Common wire so the unit keeps working with a weak or dead battery.

I take it the Honeywell and your Tekmar are fighting over who's the boss regarding the boiler cycle rate? I can't find a digital that has a built in heat anticipator function--the Rite-Temps are giving me a bit of overshoot on heat calls.

Pete
 
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Old 10-30-06, 04:36 PM
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exactly. The cheapo LUX TX500 downstairs might be the ticket, however. So far it seems to be behaving, but most of the behavior is when I'm not there to see....
 
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Old 10-30-06, 05:35 PM
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Stat

I've had good luck with the White-Rodgers 80 series stats but I've never used on a system with a Tekmar or similar control package.
 
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Old 10-30-06, 05:44 PM
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short cycling

Xiphias--are you running your Tekmar in auto differential or did you manually set the spread?

Pete
 
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Old 10-30-06, 05:56 PM
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Boiler is a Burnham Revolution. tekmar set to auto-diff per Burnham suggestion. Burnham has been _very_ helpful with other suggestions such as curves, background on the vs3000 internal mixing system and its differential and mixing scheme, etc. Excellent product support and they really know how to interface this boiler with the tekmar. Their suggested is to do the 076 indoor sensor and an 031 timer, but the price is rather steep.

The problem is with this Honeywell stat, and it's only for the first couple hours during recovery. Once going in the morning, and in the evening, it seems ok. But there's something up with the logic, even with the "adaptive intelligent recovery" that isn't right. Could also be the stat is just bad.
 
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Old 10-30-06, 06:11 PM
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031/032

Those timers are extremely pricy, considering I paid under 300 for the 260 on sale last year! I've noticed a variety of cheap lecto-mechanical timers that will provide a dry contact that should work to set the unoccupied night mode input--but with the programmable stats I didn't need to go that far. I am using the indoor sensor, however.

Good luck on getting the recovery issues squared away--some of the finer points for setting up the Tekmar's features are still beyond my limited abilities to comprehend!

Pete
 
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Old 10-30-06, 06:56 PM
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Pricey indeed. I don't have the budget nor am I that into "comfort systems."

Every morning I check the boil target and the outdoor temp. Plotting it in a spreadsheet. (What a geek....) I've been slowly dropping the temps on the boil min and the boil design to try to get on a near-constant circulation curve. Lately it's been consistently warm. Not like I want it to get cold, but it is impeding data collection....

But anyway, need to ditch this stat.

Grady, I'm looking at the White-Rodgers, here:

http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-6325.pdf

and I'm not sure I get what the FA and SL settings really do for the cycle rate (see below from p. 4)

"Select FA or SL (Fast or Slow) Heating Cycle Rate - The
FA setting is frequently used for gas, oil or electric heat. The
SL setting produces a longer heating cycle which is normally
for hot water or steam (hydronic) systems. Both
settings produce very accurate temperature control and
can be set to your personal preference. FA cycles the
system just under 1F and the SL setting cycles at approximately 1.5F."

Does this mean overshoot, how far its allowed to fall below target before firing, what???
 
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Old 10-30-06, 07:08 PM
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Accuracy

My experience with these stats is that they hold within 1 of set point. Worst I have ever seen was -1 & + 2. I always set them at a fast cycle rate & have yet to get a complaint. Keep in mind, the stats I'm replacing are, at best, T-87's (round Honeywell mercury).
 
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Old 11-02-06, 07:30 AM
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bingo

I think we have a winner. I picked up a Lux TX500. At my local Ace hardware, it is the ACE ATX500. Very simple brain, with modest programmability. Basically it starts to make its way to the set temperature at the time you specify. It does not try to figure out on/off cycles if the temp is below the setpoint. It just keeps calling for heat until it hits the setpoint. Exactly what I wanted for this outdoor reset application.

Once at the setpoint, the cycling is controlled by adjusting the allowable temperature swing. (Basically like the W-R discussed above, I suspect.) As the reset curve is tweaked through the heating season, I expect I can tighten up the swing if necessary. Once optimized, the reset curve should match heat loss to heat gain, so the temp should stay pretty steady at near constant circulation.

Radioconnection, if you have overshoot issues, this might be a good stat to check out. That or maybe start messing with the reset curve.

It was $34 at the hardware store, and in New England, there is a rebate program offered by the regional gas companies for $25/thermostat. So I net out at $9. Not bad.

The rebate program applies to a bunch of home heating and improvement stuff. It's at http://www.gasnetworks.com/efficiency/applications.asp if anyone's interested.
 
 

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