Looking for a more sensitive thermostat

Yes, I realize what I'm asking for will mean that my furnace/AC will be running less efficiently, but I'm willing to give this up for comfort. I've had two thermostats in two different homes over the last 15 years. One was a Robert Shaw and now I have a Honeywell, both were electronic and when they were set to their most sensitive settings which caused the furnace/AC to cycle more neither cycled as much as I wished they had. Is there a brand or maybe a particular thermostat out there that is known to be more sensitive than others?

Thanks in advance. Craig T

Reply to
CraigT
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Look. Here's what's going on. I'm going to focus primarily on the furnace side for now, not the A/C side.

You have a furnace in your house that probably puts out more BTU's of heat than it should. In other words, your furnace is over-sized for your house.

What this means is that it can over-heat your house and make you uncomfortable just at the point when the thermostat tells it to shut off. Then as your house cools down, you are again uncomfortable by the coolness just at the point when the thermostat tells the furnace to turn on.

To deal with this, you have 3 options - which can be employed either individually or in any combination:

A) have the furnace put out less heat when it's on. I personally dial down the gas input to my furnace so as to modulate the intensity of the flames (exactly what you do with your barbeque) so that my furnace runs longer and this leads to a more even heat for the house. This works well for older furnaces that ARE NOT computer or electronically-controlled (these are typically 30 to 35 years old) and it can also work well for furnaces that have electronic ignition but are otherwise NOT REALLY computer-controlled (25 to 30 years old) and are likely to NOT be condensing (ie - high efficiency) type.

B) the "hysteresis" or the "span" of the thermostat can be changed (in your case - reduced). Some thermostats have the ability to set the temperature spam between when the furnace shuts off and when it comes back on. This is typically either 1, 2 or 3 degrees F. Sometimes this setting is a small switch on the back of the thermostat (ie - not typically or frequently-accessible user adjustable). When you set it to

1, the furnace cycles more frequently, but theoretically will give you a more even temperature over time. If your current thermostat does not allow you to set the SPAN, or if the span setting is fixed at either 2 or 3 degrees F, then you want to look for a thermostat that has an adjustable SPAN, or one that has a fixed span of 1 degree F.

c) run your fan continuously. If your furnace fan is always running, this will lead to a more evenly heated house and can overcome any tendency to over-shoot or under-shoot the desired temperature. The benefits of running your fan continuously in the summer are even greater, and is the preferred way to deal with keeping or maintaining the desired temperature during A/C use.

One final thought: Where you thermostat is located in your house may be the sole reason for your temperature discomfort, or at least play a major role. This has to do with air currents and circulation and there might be a better location for it in the house. An extension of this idea is that the thermal coupling of the thermostat to the wall itself may be influencing how the thermostat is sensing temperature, and placing an insulating layer between the wall and the thermostat may provide the solution you are seeking.

Reply to
Home Guy

My furnace or AC is not oversized, every contractor that has seen my setup has said that I'm close to being undersized. When my units run one of my complaints is that they run for so long, they do not easily overcome the sizing of my house. My house is only 10 years old and as I stated already I have the thermostat already set to its most sensitive setting. I didn't say it but it is on an interior wall. Like I said I want the heating and cooling units to cycle more often for shorter times, I do not want to listen to a fan running 24/7.

What I want to know is if there is a thermostat out there that is more sensitive than the rest or can be set to be so? Like rather than settings of 1,2,or 3 for sensitivity settings it allows for a more sensitive setting like .5?

Reply to
CraigT

Hmmm, Easiest is getting the blower run on low sped all the time kicking into higher speed when heating or cooling. What style house, where is the location? Jow big? My unit in summer or winter runs 2-3 times an hour.We are comfy. Having a wireless 'stat I can move it where I want for better comfort.

Reply to
Tony Hwang

You remind me. I wanted to get a wireless stat, but didn't want to spend the money. I think now I'm just going to get an X-10 that will only turn the furnace house-current on and off. I'd replace the "Emergency" toggle siwtch in the basement with an X-10 on-off switch, which works remotely and locally.

At the very least that will turn the furnace off when I open the window upstairs.

Any real downside to this plan?

Reply to
micky

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.

Reply to
mike

Generally BAD idea. RISK If you have an electric furnace, you're probably ok. If you have gas, I'd worry about it. Makes me nervous to pull the plug on the control system of something that's on fire. If it's a heat pump, I'd worry about short-cycling the system and burning up the compressor. The safety systems expect to have the power on. Yes, a well-designed system will take care of that on power up...a well-designed system... But you can bet the designers/evaluators did not contemplate someone pulling the plug as a routine control input. Their bean counters were much more concerned with taking out that last penny of cost.

X10 There have been many variants of X10. My experience is with the older stuff. It's not secure. Depending on how far between the button and the switch, Whether it's on the same side of the line, noise, etc. it may or may not switch reliably. And it can switch when you least expect it. So if it's 99 reliable, you can expect that it will screw up only once in a hundred times...like once a week??? It's not the things you plan for that hurt you. It's the unexpected that causes the problems. You need to be DARNED sure that nothing bad can happen while you're not home. If you're not there to dial 911, your house will burn further down.

Compare the cost of a proper thermostat with the cost of damage to the system and/or your house.

The local control function works by supplying current when off and sensing whether anything is there. Works well for a lamp. For an electronic control, not so much. What happens when the controller gets just enough juice to fsck things up?

Try this: Hook a CFL lamp up to an appliance module and let it warm up. When you switch off the relay, the local control current charges up the cap in the CFL until the inverter turns on and flashes the lamp. There are online instructions on how to pull a component to disable this function, but you no longer have local control.

If I were gonna cobble together something with X10, I'd use an isolation relay in the control path from the existing thermostat, but before the safety mechanisms in the controller.

Reply to
mike

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.,

Reply to
micky

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.

Reply to
micky

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.

Reply to
trader4

A new thermostat is not going to solve the problem. If the heater is sized that it needs to run a long period to put enough heat into the house, shutting it off sooner will only make it cooler and have it work longer for the next cycle.

Why not tackle the real problem, the noise? That seems to be what is really bothering you. Perhaps you can put in a variable speed blower or otherwise change the speed of what you have. Or change a duct of vent that is making the noise. Or box in and insulate the unit.

Regardless of the thermostat, the burner has to run a given amount of time to produce the heat needed to warm your house. A more sensitive thermostat may break it into smaller periods, but it still has to run the same total time. If, in an hour it now runs 10 on, five off, you may be able to get it to run five on, but it will still have to run the same 40 minutes total.

You may want to consider putting in some supplementary heat in the room you occupy so the furnace runs less keeping the rest of the house warm. A small electric space heater will help and can be noiseless.

Reply to
Ed Pawlowski

Surely the change he talks about wouldn't make the furnace turn off sooner. I think it would make it turn back on sooner.

My old furnace came with a three speed blower, changed by chainging which wire was connected. Unfortunately, it was already set to the slowest speed.

Reply to
micky

CraigT used improper usenet message composition style not only by top-poasting, but also by full-quoting:

For one thing, a 10 year-old house should be super-efficient in terms of insulation, heat loss and being sealed for air leaks and outside air infiltration. This should make it LESS likely that you'd have comfort problems caused by the cycling of your HVAC system.

Second, I'm not exactly sure what you mean when you say that your thermostat is set to it most "sensitive" setting. I'm not aware of a thermostat that has a sensitivity setting, unless this is the SPAN setting I was talking about in my previous post. Please either confirm that, or explain how this sensitivity setting works. If your current thermostat has a SPAN setting, tell us what those settings are, and which of them you've tried.

Not knowing where on earth you are, you could either be using your A/C or your furnace right now. So it would help to know which of those you'd like to fix for the moment.

As well, what type of furnace do you have? Is it forced-air natural gas? Is it a heat pump? Is it electric?

That's fine, but it could still be in a poor location. For example, is it right over or near an air supply duct or vent? Can the sun shine directly on it? Is it in a secluded corner where it can't get a good sense of ambient air temperature?

If your house is only 10 years old, then almost certainly your HVAC fan is multi-speed - or at least it's capable of multi-speed operation if it's connected to an appropriate wall-mounted control switch.

And I have news for you. If you think that you'd be more satisfied with a furnace that goes on and off every few minutes vs having a constant (and possibly quieter low-speed) fan running all the time, then you'd probably be wrong.

I would think it would be ridiculous to have a span of 0.5 degrees F.

Also -> note this:

If your thermostat has a Celcius/Farenheight setting, then it might be that if it's set to Celcius, your SPAN option of 1 would be 1 degree celcius, which is almost 2 degrees farenheight. So set your thermostat to Farenheight so that your "sensitivity" setting of 1 becomes 1 degree farenheight.

If your furnace really is undersized, then it should basically be on all the time (and not be able to bring the house up to the temperature you want). I doubt that's whats really happening, because your system appears to be cycling.

Unless you have a monster house, or your house was improperly built, insulated or ducted, then it's hard to imagine what an undersized furnace looks like for a 10 year-old house. Some ducting jobs can be horrible - wrong size ducts, poor sealing between joints, garbage left in the ductwork while the house was being built. Maybe that's your problem (assuming you have forced-air natural gas furnace that is).

Reply to
Home Guy

Of course - you asked a bunch of contractors and they all say it's undersized. That's because they all want to sell you a new furnace.

This is the standard answer for an HVAC contractor -> "You need a new system".

Reply to
Home Guy

That depends on how cold it is outside. The above problem would only present itself when it's really cold outside.

It used to be that HVAC guys chose to err on the side of possibly making the system too large, since that is less likely to result in bitching. But here in NJ I've seen a trend to err on the other side now with systems that are too small. One house I know of in particular is Energy Star certified, about 7 years old. Not sure exactly what role that plays in it

=A0>Some ducting jobs can be

Yes, I've seen all that too. House I was talking about above has one register in a hallway that you can see is not connected to a duct. And two registers that are virtually right above the furnace put out almost no air. Which is hard to diagnose because the idiot former owner had the basement drywalled. Never saw anything like it. Get this. They drywalled around the furnace, hot water heaters, etc. The guy must have been paid by the hour or something with all the cuts, dips, etc. You or I would just put up a utility room around the that area because it's useless anyhow.

Reply to
trader4

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.

Reply to
Tony Hwang

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...

Reply to
mike

" snipped-for-privacy@optonline.net" used improper usenet message composition style by unnecessarily full-quoting:

The way the OP is describing the situation, it seems that the contractors are telling him that the reason for his environmental discomfort is because is furnace is undersized (which would be a bull-shit reason for such a problem).

You will note that the OP is not posting this question in the middle of winter - and he's not complaining that his home is generally colder than he wants it to be.

And would you expect comfort problems in November with an undersized furnace?

Or in January?

We don't even know where this guy is, or that he has a nat-gas furnace.

Reply to
Home Guy

Christopher Young top-poasted:

It's meant to infer a klownish posting style.

Reply to
Home Guy

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.

Reply to
Tony Miklos

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