Looking for a more sensitive thermostat

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I apologize. I should have said what it was, OIL. Not so common these days. AIUI, it doesn't burn unless the spark transformer is running and providiing a continuous spark, and defintitely, no oil comes out of the nozzle unless the electric pump is running. I have on a couple occasions turned the switch off, and the furnace stops immediately, including the noise of the fire.
I was also unclear in that I would still be using the same thermostat I have now. It's connected to the furnace 24V transformer, of course.
The X-10 switch would be in place of hte 110 votlt AC toggle switch which tunrs off all the power to the furnace (and all the power to the AC except for the 220 volts that goes to the compressor, but is controlled by the part of the furnace control board.) Nothing woudl be changed about the setup except this switch.
Okay, iwrt your concern about it turning off without my turning it off, I could put in a toggle swtich in parallel with the X-10 and if I went out of town in the winter, turn that switch on, so the furnace always had power. In fact I coulld leave the swtich on all winter. It's only in the other three seasons that I open t he windows.
Does this relieve your concenrs or change your warnings at all?

Thanks for raising this. I meant to say something aobut this to the OP. Posted separately.
I read the read the rest and didnt' delete anything that follows.

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micky wrote:

THAT'S THE CRUX OF THE PROBLEM...AIUI!!! We understand almost zero, cause the OP disclosed little. The things you understand aren't the things that will bite you. It's all those little things you don't understand or interpret wrongly. An HVAC system is a SYSTEM. The parts work together. When you start messing with it, unintended consequences occur.
it doesn't burn unless the spark transformer is

My concerns don't need to be relieved, cause I'd never do something like that.
I'll go off on a short rant about interweb advice.
If a person knew what to do, he'd not be here asking the question. He gets conflicting authoritative advice from people who've never seen his system and have no idea how it's configured. If the wrong answer results in your viewing the wrong movie, it's not such a big deal. If the wrong answer sets your house on fire, it IS a BIG DEAL. I don't mean wrong according to accepted practice for a skilled licensed contractor using components approved for the application and inspected by the local building inspector. I mean wrong relative to an unknown system of unknown configuration being modified by a person of questionable skill and knowledge according to random interweb advice... They wouldn't be here if they knew what to do. They also have no way of determining whether a particular chunk of advice will be helpful or harmful in THEIR situation.
While it is statistically unlikely that the place will catch fire, that's small comfort to the person who IS on fire.
Most everything you buy today has multiple safety systems and is thoroughly tested for safety by independent agencies. Yet we still find the need for multiple smoke detectors and the fire department on speed dial.
One has to look at the cost benefit ratio. Do you REALLY want it badly enough to risk life and limb. Is it really that hard to walk down the hall and flip the switch on the thermostat to turn off the system? Just 'cuz it would be cool doesn't mean it's the right thing to do.
Often, the best advice you could give is, "don't mess with stuff that breathes fire and has the (however slim) possibility to burn down the biggest investment you ever made and put Granny in the morgue."
Want a second opinion? Call up your insurance carrier. Tell them you're gonna mess with the control system on your furnace by adding some unreliable stuff you read about on the web. You won't have the result inspected. Ask how that affects your fire insurance. Maybe you can purchase an "idiot" rider. Better to learn the answer now. It's too late when you're standing in a pile of smoldering rubble and the insurance adjuster is shaking his head".
Call me conservative...

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Okay, conservative. Thanks for the advice.

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CraigT wrote:

I have no idea what I'm talking about, so some expert will likely correct me...
It's not about hysteresis in the thermostat. It's about balancing the heat supplied by the furnace with the heat lost thru the walls, etc.
The longer the thermal time constant of your house, the more stable the temperature will be. So, Let the system stabilize at some temperature. For fixed external temperature, plot the internal temperature vs time with the furnace off. That'll give you the thermal time constant of the house (at the location you monitor). You want the cycle time of the furnace to be much shorter than the time constant of the house.
Infiltration can also make a BIG difference. You don't want any, for this calculation...but you do want some for health. It's a tradeoff.
I have a Honeywell TH8000 on a high-efficiency gas furnace. It has a zillion setup functions that I never bothered to understand. I don't know how it is supposed to work, but I can tell you the symptoms.
The temperature readout never changes (after it stabilizes). I thought it was broke, so I put another thermometer next to it. It don't change either (one degree F resolution). A thermometer on the external wall next to the window does change with the house's thermal time constant, local heat loss and differences in air flow.
The thermostat modulates the furnace run time and the cycle time.
I have a PDA that tracks the on-time, cycle-time, and the duty factor for each cycle. It graphs the duty factor. Duty factor is directly proportional to BTU/hour.
The thermostat runs the furnace for about 5 minutes and modulates the cycle time. As the in-out temperature difference increases, the cycle time gets shorter. When the cycle time gets down to about an hour, the on-time starts increasing as the cycle time decreases.
The graph of duty cycle tracks the internal-external temperature difference (for fixed other heat sources). When I turn off my computer and go to bed, the duty factor goes up to account for the 200W of heat that is no longer generated by the computer system. It's also interesting to watch the duty factor track down during the day due to insolation, but with a phase shift due to the thermal time constant of the house.
If I raise the setpoint, the duty factor overshoots and rings as it settles to a new duty factor value consistent with the heat loss at the new internal temperature. Classic feedback control system behavior. It's interesting that the thermostat temperature readout changes rather quickly to the new setpoint value, but it keeps running the furnace on that first cycle. Takes about 10 minutes of extra run time to bring the house up one degree F. The temperature readout does not overshoot, but the duty factor does.
It seems to be more about tracking average heat loss than the hysteresis of the thermostat. The thermostat predicts future behavior based on past behavior and applies minor tweaks based on sensed temperature to keep it at the setpoint. This is not your grandfather's mercury switch thermostat.
Your fuel injected car works the same way. For a given throttle position, yes there are many other inputs too, the computer looks up an injector pulse width in a table and runs with it. The oxygen sensor determines whether the mixture is rich or lean and tweaks the injector on-time up and down slightly around the optimum point to keep the average mixture about right. It doesn't even try to make it exactly right. There isn't any hysteresis required, but the mixture "hunts" around the correct value.
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wrote:

I think, if you make theis thermostat cycle too quickly, you mightt damage the compressor in the summer time, by what Mike called short cycling. . Maybe if you can get it to work for heat, you would at least need one thermostat for the furnace and the current stat for the AC.,

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Any decent thermostat has a lockout period of 5 mins or more during which it will not allow the AC to restart. That gives the pressure time to equalize. So, while cycling the compressor with a differenct thermostat more frequently isn't a good idea and will lead to more wearing over time, it isn't going to just suddenly burn it out. The disadvantages to shorter cycles for the AC are about the same as for the furnace.
For the OP, it would be interesting to know how much the temperature is varying with the existing thermostat. You say you want it to cycle more, but don't give data as to what the temp swings are. Have you measured the actual temps with a thermometer? And where are the objectionable swings in temp occuring? At the thermostat or at some room remote from the thermostat?
I've had a variety of thermostats in my home over the years and I've yet to find one that caused temp swings that were objectionable. I have seen lots of situations where there were temp problems caused by other factors, like unbalanced or oversized systems. Which brings up another issue. The typical thermostat is accurate enough compared to the rest of the system. Meaning that the system is balanced as best as possible so that when the temp at the thermostat is X, it's close to X at all other points in the house as well. That variation is probably more than the hysteresis of the thermostat. So, even if you put a super accurate thermostat on such a system it's questionable if it will do anything to locations rooms farther away from the thermostat. In fact, with shorter run times it might make it worse.
As HomeGuy pointed out, the temp problems are classic with an oversized system, but I guess we can rule that out since you say the system is undersized and takes a long time to raise the house temp up.
As far as thermostats go, the Honeywell VisionPro models let you directly set the number of cycles per hour that the system will run. I know you can set it to at least 5. How exactly that then translates into the turn on/off points isn't spec'd. But it is a totally different approach than other thermostats I've seen.
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This might be something I would be interested in. Thank You.
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Cycles per hour is not something I would like to have. The number of cycles has nothing in common with temperature controlled lab experiments, and might only have to do with efficiency, where too many cycles is a bit less efficient.
You have to read up on thermostats. Most I have used go to a minimum one degree sensitivity. That's only a start. Some have learning modes so the unit actually functions as a true PID controller that initially learns the needed curve to avoid overshoots, and really good ones adds fuzzy logic to add needed programing as it ages. The specs on most furnace stats tell little. I do think some honeywell thermostats do indeed act smart. Drafts and placement on walls trick the thermostat so that needs to be addressed. Multiple zone sensors as well as personal portable remote sensors make temperature more personal and more accurate. The size of the source of heat or cold unit will change the time constants, and of course best to size properly. Even though. The thermostat, say is going to go plus and minus degree adjustment, a good one will be measuring tenths of a degree resolution to make computations.
Greg
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I was looking for an example of one specifying PID controller. Here
http://dticorp.com/catalog/honeywell-t7350a1004-programmable-commercial-thermostat-p-1759.html
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CraigT wrote:

Hmmm, Actually and simply no matter how sensitive the 'stat, electro- mechanical device such s furnace, ac unit won't respond with equal sensitivity. Your system and your house itself need energy audit may be. Up here we can no longer install a furnace with efficiency rating less than 90% by law.
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On 11/24/2011 11:38 PM, CraigT wrote:

I believe you want smaller temperature swings, right? I have electric resistive hot air heat and I bought a Honeywell CT31A1003 thermostat. It is not programmable. It's a simple design and uses two magnets to help it "click" from on to off to on...
There are two screws set from the factory for on and off positions. All you need to do is adjust the screws so the two contact points are closer together. Simple. Now my heat turns on and off with smaller temperature swings. The only tricky part is setting the heat anticipator properly and that took a little tweaking to get it to my satisfaction.
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CraigT wrote:

here ya go, a high dollar one that looks purty and it's smart.... http://www.nest.com/?gclid=CIP1ycOx0qwCFe1dtgodpzdjHg Probably won't work any better than what you have but the economy needs the money, buy one.
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Yes, slightly more stable temps. I am not looking for anything drastic. Like I said, most electronic thermostats have adjustments for sesitivity, I just want one setting more sensitive than the most sensitive setting I have had in the past. I'm really not sure what the settings H1, H2, or H3 mean exactly when it comes to temp. swings. If H1 allows a .5F swing and H2 allows for a 1F swing and H3 allows for a 1.5F swing then I guess what I'm looking for is something less than .5F If H1 means 1F swing, H2 means 1.25F swing, and H3 means a 1.5F swing, I guess I'm looking for a setting which allows for a swing of less than 1F.
I have to believe that there has to be some variation between the manufacturers of thermostats and that there is one out there more suited to me or maybe one with much greater control, but not to the point of causing damage to the system.
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In

You talk a lot about "settings", but what settings? Various (and most) thermosats have a hysterisis control and an on/off temperature setting. The cycle setting is right n the thermostat and the other temp on/off settings are in the furnace. I can't help but wonder if you aren't shot-gunning t here when the answer is rght in front of you if you look at the right access points.
HTH,
Twayne`
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CraigT wrote:

There's a gradient across the room due to losses on the outside walls. There's a gradient top to bottom due to stratification. The air coming out of the heater vent is a LOT hotter than the setpoint. There's infiltration. You probably have a single point temperature measurement for the whole house. Given all this, I'd be AMAZED if you could tell the difference between .5 and 1.0 degrees hysteresis in the thermostat. Methinks you're addressing the wrong problem.
I think the Honeywell VisionPro 8800 will do what you ask. I just don't think it will solve your problem.

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Exactly what I've been questioning all along. We don't even know what the objectionable temp swing is. Any decent thermostat that is properly installed will control the temp at the thermostat to 1.5 deg or so. And if that is objectionable at the thermostat, then what about the rest of the house, where the temp swings are almost always greater? In fact, with short cycleing, which is what he appears to want, the temp fluctuations in the rest of the house might be worse.
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If the room temp is changing that fast, s ounds like the op needs to add more insulation.
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On 11/26/2011 07:52 AM, snipped-for-privacy@optonline.net wrote:
<snip>
We don't even know

Nor do we know anything about the humidity and its variations, which can have a large effect on how warm or cool it _feels_.
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Wouldn't it be a good idea to RTFM?

Still waiting to hear:
A - Exactly what is the objectionable temp swing, ie .5 deg or 3 deg?
B - Is it that swing measured at the thermostat or somewhere else in the house?
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On 11/24/2011 8:38 PM, CraigT wrote:

sounds like your system is too big. You might explore having the blower speed turned down.
--
Steve Barker
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