Interesting question!

[MOP]

For many years I have used programmable thermostats in my house, was in on a conversation about them one seemingly knowledgeable gent argued against them. His argument is that the heating system actually works harder getting the house back up to temp with programmables, he says you have to realize that the structure and everything in it has to be re-warmed not just the air. We run two cycles, low at night and again low during the day we only heat for about seven hours out of twenty four. Has me thinking there may be merit, does anyone know of any actual testing that has been done.

[gcarter]

He's entitled to his poinion, Programmable thermostats just make things easier.
Ya gotta understand we usually don't heat our house. We have a very efficient rerverse cycle heat pump w/a SEER rating of 19, but unless it gets into the low 30's, it on cool or off.
But we set the program for the way we would have it a particular time anyway. It's nothing to do w/efficiency, just convenience. I don't care if I save $10.00/month.
BTW, I don't know, but it may be more difficult to keep our house at 74* during the day and 67* at night during the 7 month long summer w/daytime temps of 97* and 80* night time temps along w/near 100% humidity than to keep your houses warm.

[DAULEY]

here in wisconsin i keep mine at 64 from 10.30 pm to 6.30 am then 68 the rest of the time aaerage bill is about 180 per month about 2400sq ft also heat garage 45 i have a set back unit but i found that when i program manualy and set temp and use hold temp my bill is about 20 buck less per mt

[McGary911]

I've heard similar statements about heating other things, like pizza ovens for example. Some shops will keep them warm all the time, because it takes more energy to get them back up to operating temp, than to keep it there.

I wonder if there's a rule of thumb? I'm sure there are a lot of variables. Heater efficiency, insulation value, thermal mass, etc.

I don't have automatic thermostats, so I just turn them on if it's cold in the morning. I kind of like it a little chilly in the winter. My GF, less so....

[DickB]

No actual test results, but found this (http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12720):
"You can save around 10% a year on your heating and cooling bills by simply turning your thermostat back 10°–15° for eight hours. You can do this automatically without sacrificing comfort by installing an automatic setback or programmable thermostat."
"A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. In fact, as soon as your house drops below its normal temperature, it will lose energy to the surrounding environment more slowly. The lower the interior temperature, the slower the heat loss. So the longer your house remains at the lower temperature, the more energy you save, because your house has lost less energy than it would have at the higher temperature. The same concept applies to raising your thermostat setting in the summer; a higher interior temperature will slow the flow of heat into your house, saving energy on air conditioning."
Having an engineering background, this makes sense to me from a thermodynamics standpoint.
Here's a link that describes how to run a test of your own:
http://www.development.ohio.gov/cms/uploadedfiles/CDD/OEE/ProgrammableThermostatSavings.pdf

[justleft]

You also have to consider your HVAC type.

Here in the south we have a heatpump with electric coils for E (emergency heat).

My thermostat will turn on the E heat if the temp falls 3 (or maybe 5) degrees below setting.
When those coils kick in you can hear the electric meter spinning. So if
you knock the heat down at night you pay to bring it back up.

We just keep it at 65 and wear an extra layer or two.

[mike o]

In Northern NH would make the toilet seat 2 cold........:kingme:

[Tony]

Phil, I've heard the same theory, and feel it is valid. The key is not to drop the temp down too far. We drop down to 60 at night and during the day (work weekdays), then up to 66 for a few hours in the morning and 68 for the evening.

In my opinion dropping 6 or 8 degrees is sensible, dropping 15 is pretty extreme. The reasoning is as you explained, it cools the "physical plant" so low that it takes too much energy to raise it back up.

I agree that it depends on several factors - your locale and type of heating system being two important ones.

[zelatore]

I've often wondered the same thing but never enough to look for an answer. Our heating unit is a heat pump. And like every heat pump I've dealt with, it's pretty much worthless. To get any heat out of it at all the Emergency elements kick in. At that point, it's just a less powerful/less efficient regular heater.

Since it died a couple years ago though, we've just lived with it. Electric blanket on the bed; down throw on the sofa; small space heater if you need it. But the real heat comes from the wood burning insert.

Of course, the real problem with that is on days like today when we've been gone for a couple days and the house has gotten down to ambient temp (about 40 when I got home). It takes a loooong time for the fire to bring the house temp up!

The real question is which will we upgrade first: the heater or the insert? I'd like too replace the insert with a forced air unit that can accept larger logs and has a provision to draw air from outside. They are supposed to be much more efficient. And certainly cheaper than a new furnace!

But with a new furnace comes a new air con - and it regularly tops 100 here in the summer.

[Marvair]

I have been in the HVAC business for 30+yrs in southeast Texas and this has come up before at the supply houses among technicians. The general idea is to maintain a comfort level while saving energy, nickels. If your system has the capacity to "recover" from temperature reset efficently, go ahead and offset 5 to 7 degrees at night maybe more during the day if no one is there. This applies only to heating and not cooling. In cooling when you offset in climates with high humidity, like Houston, everything in the conditioned space attracts moisture and causes you know what..MOLD. In drier climates 4 or 5 degrees is ok. Hope this helps.

Marvin.

[LuauLounge]

We live in Northern California

We work, so during the summer, no one is home until 5:00. We live next to the Sacramento River, so as soon as the sun goes down, the temperature drops 10 to 15 degrees more than the folks that live 10 miles away.

During the winter, we keep the house at 63 unless we are at home. At night, we set it to the same point.

During the summer, we are in the pool or hangin out outside.

We have a 2000 sq ft house with a 100K furnace (90% efficient, natural gas) and a total of 6 tons of AC.

We can warm it up fast and cool it down fast, which is exactly our lifestyle. May not be the most efficient, but works for us.

[Ghost]

I think Dick B absolutely nailed it above.

In a system with one type of heating (burning gas, oil, resistive heat, whatever), the warmer you set it, the faster the heat flow out, and the more energy and $$ you burn. With no efficiency loss or price penalty in "heating it back up." So, turning it down for ANY period of time saves something.

The other case to consider is a system with more than one type of heating, with two different efficiencies and costs. Like a heat pump with a resistive heat secondary. The secondary is CRAZY expensive compared to the primary. But the primary is probably only capable of maintaining temperature or increasing it VERY slowly. So, maintaining a daily cycle with a 3-7 degree drop nightly puts it too far in a hole for the primary to get back out in a reasonable time. So you'd need secondary resistive heat to climb out every morning . And that secondary costs 3x or more what running the primary does. So you might as well just let the primary hold it at day temperature overnight, or VERY nearly that.

I think this is the difference. Also, the reason why there are "heat pump" thermostats (which cycle on and off with a VERY slight delta in temperature, a fraction of a degree maybe?) versus the traditional ones, which probably cycle on a full degree or more I think.

[Ed Donnelly]

I have a 2 stage heat 1 stage a/c programable thermostat..

It is set for 72 heat 76 a/c and NEVER changes...Some days I don't start work til 11 a.m. One son leaves for school at 9.30 a.m. the other son some days leaves at noon.. Shannon is home by 3 p.m. every day..

Night time is the same. At least 1 kid is studying until 1 a.m. and the wife is up at 5 a.m.

BUT being an old fart I find 72 is cold.. So while I am doing the crossword on the toilet for an hour or so....I have a heated toilet seat...Ed

[DAULEY]

i ran a test this morning to see how long it takes to bring house temp back from 63 to 68
90+ on furnance house well insulated r 60 in attic walls are foamed approx r 15 to 19 not realy sure and good windows temp out side is 10 it took 55 mins

[Tony]

I'd like too replace the insert with a forced air unit that can accept larger logs and has a provision to draw air from outside. They are supposed to be much more efficient. And certainly cheaper than a new furnace!

Any good wood burning system must draw air from outside...that way your home's air pressure is "equalized". Imagine all the gas/air flowing up your flue, well, that has to come from someplace. If you are not using a direct outside air feed for combustion purposes, that air is being pulled into the fire chamber through every little nook, cranny, and crack in your windows, doors, walls, or wherever. You are sucking air into your wood stove that has already been heated...very inefficient!

Almost all wood stoves sold now offer outside air for combustion purposes, usually a 3" or 4" duct that you can run right outside. Also, be sure your entire flue is the same size as the outlet on the stove. Re-line your chimney with 6" or 8" stainless, depending on the size of the stove. Insulated is best, as this prevents your flue gases from cooling and forming into a solid (and dangerous) creosote.

[DickB]

The higher cost of secondary heat in a heat pump is a good point. I wonder if anyone makes a control system that takes this into account, and not switch into high-cost mode if the system could warm back up given more time? We had a "smart" programmable thermostat that would "learn" the conditions and adapt. For example, if we programmed it to warm the house at 6:00 AM, the first time it would start warming at 6, but also track how long it took to reach the target temperature. If it took 15 minutes, the next day it would start at 5:45. It might be possible to run a heat pump earlier and longer to avoid the resistive heat mode.

[Marvair]

The higher cost of secondary heat in a heat pump is a good point. I wonder if anyone makes a control system that takes this into account, and not switch into high-cost mode if the system could warm back up given more time? We had a "smart" programmable thermostat that would "learn" the conditions and adapt. For example, if we programmed it to warm the house at 6:00 AM, the first time it would start warming at 6, but also track how long it took to reach the target temperature. If it took 15 minutes, the next day it would start at 5:45. It might be possible to run a heat pump earlier and longer to avoid the resistive heat mode.


I've added a toggle switch below the thermostat to my second stage circuit disabeling the resistance heat till I ask for it. There are thermostats that do actually target the setpoint and will start before the occupied time slot. I believe all White/Rodgers have that feature, don't quote me on that. I seldome sell these controls, most of the public does'nt use the features and they cause call-backs.

Marvin.

[Tim Morris]

BUT being an old fart I find 72 is cold.. So while I am doing the crossword on the toilet for an hour or so....I have a heated toilet seat...Ed

I should think the air would be warm enough there; what with you talking
to yourself until 11 AM every day.

Do you ever say to yourself "Here's Tim's t-shirt, maybe I should give
it to him"?

[yeller]

The higher cost of secondary heat in a heat pump is a good point. I wonder if anyone makes a control system that takes this into account, and not switch into high-cost mode if the system could warm back up given more time? We had a "smart" programmable thermostat that would "learn" the conditions and adapt. For example, if we programmed it to warm the house at 6:00 AM, the first time it would start warming at 6, but also track how long it took to reach the target temperature. If it took 15 minutes, the next day it would start at 5:45. It might be possible to run a heat pump earlier and longer to avoid the resistive heat mode.This is done in a lot of commercial units and takes into account the outside air temperature as well, (whether it has been rising or falling the last several days), but I don't know if it is available for residential. We never dealt with the residential side.

In any case, the purpose of a programmable thermostat is not to shut the system off completely, but to lower the heating setpoint (or raise the cooling setpoint) during unoccupied times. If you are turning the unit off for short periods of time (less than 1 day), you are probably using more energy to reheat (or recool) the house and would be better off to lower (or raise) the setpoint 4~7 degrees.

If you wanted to get into the physics of it, you can determine where the payback is for turning it off based on time away but there are so many variables involved it's not worth the effort (and beyond almost everyone...myself included). You'd have to take into account the type of heat, efficiency of heating unit, humidity, construction material, direction of building, which floor, etc, etc, etc.

(I was in the HVAC industry for 13yrs dealing with computerized control systems for mostly commercial HVAC systems).

[Phil S]

OPTM concept always interested me. Off-peak-thermal-mass...if I remember right. The idea being that you pre-cool and / or pre-heat thermal "mass" walls (that retained heat / cool) during the night during "off-peak" kilowatt / hour rates from the utility provider thereby having to only run your circulation fan during the "peak" rate hours.

Did those ever work ? Mold issues ? Just curious.

Phil S

[zelatore]

OPTM concept always interested me. Off-peak-thermal-mass...if I remember right. The idea being that you pre-cool and / or pre-heat thermal "mass" walls (that retained heat / cool) during the night during "off-peak" kilowatt / hour rates from the utility provider thereby having to only run your circulation fan during the "peak" rate hours.

Did those ever work ? Mold issues ? Just curious.

Phil S


I can certainly see the logic behind it but I don't know if the average home would be able to effectively take advantage of the technique.

Since I'm cheap and haven't replaced my heater/AC, I use a something along these lines. Basically, in the summer the house gets cool enough at night with the windows open that I can usually go until the early afternoon before it gets as warm in the house as outside, at which point I usually open the windows and turn on the whole-house fan. In the spring/fall when it's warm in the day but cold at night, I reverse the process. It works surprisingly well at keeping the house tolerable. Only on the hottest days (over 100 daytime and 80 nighttime) can I not cool the house to a reasonable level. When that happens we go to the boat where the A/C still works. :biggrin.:

Given that I can sort of 'pre-load' my house using just the basic insulation I have and normal day/night heat cycles I would guess you could do something better if you actually designed for it.

[Ed Donnelly]

Yeller...........Metasys mean anything to you???????????

Tim maybe Thursday THS??????????????????............Ed

[VetteLT193]

I had my programmable set too extreme, so I can see what the guy is saying...

Mine was set so extreme that it was either off, or running for hours on end.

The rule of thumb here in FL is 4-5 degrees max between your hottest and coldest setting.

One of the better ways to save is to get a T-stat with a 'swing' feature. that allows the unit to cycle less because if you set your swing to say, 3 degrees, and your stat is set at 68 degrees heating it turns on at 67 and off at 69. that limits the start ups of the unit amongst other things (cold air in the ducts for example)

[gcarter]

Actually, since it's warm and humid here for so long of a time, most of the time, I'm more interested in the humidity level inside the house than the actual temmperature. I can keep it at about 45% pretty easily. W/a couple of ceiling fans running in the day time, the t/stat can be set at about 74* as long as no one is here. At about 5:00 PM it goes to 70*, at 8:00 it's about 69*, and at bedtime it's 67*. Simultaneously, the humidity will drop to about 40% and it stays very comfortable. I like it cool and dry. Russ stayed here one night and thought I lived in a refrigerator. :wink:

[yeller]

OPTM concept always interested me. Off-peak-thermal-mass...I never heard it called that, but the idea is still used. A couple years back, just before I left the industry, we did the controls for a building heating system designed to take advantage of the storage properties of concrete. Concrete walls/floors with basically a floor radiant hot water heating system. Took a looooonnnnggggg time to heat up, but once up to temp, didn't take much to maintain. Although the system was very simple, it was probably the most difficult one I dealt with in regards to maintaining constant temperatures. The (heat) reaction time was soooo slow it was difficult to tune. With a forced air system I could make adjustments and know in about 15~30 minutes if what I did would work. With the radiant system I'd tweak the program just a bit, then have to wait 1~2 days to find out if it worked. Luckily, I'm the smartest guy I know, so it didn't take me too long to figure it out. :biggrin:

Ed....Metasys is Johnson Controls and I know very little about their product.

....and zelatore, your right. The average house couldn't take advantage of thermal retention simply because there is not enough retention.

[Conquistador_del_mar]

One of the better ways to save is to get a T-stat with a 'swing' feature. that allows the unit to cycle less because if you set your swing to say, 3 degrees, and your stat is set at 68 degrees heating it turns on at 67 and off at 69. that limits the start ups of the unit amongst other things (cold air in the ducts for example)

You guys got me thinking about my system that I feel cycles too much to be efficient. A gas system takes quite a bit of time to heat the combustion chamber before the blower activates and the first blast is cold air, so I would much prefer a greater drop in temperature before it activates instead of holding within a degree. Older thermostats had anticipators which make larger swings possible, but apparently the newer digital ones have CPH settings (cycles per hour). I didn't think my Honeywell Chronotherm III had this capability until I just researched after this thread got me inspired. Here is a link to my solution since mine was set to gas/oil warm air before I changed it. This adjusts (limits) the cycles. :yes: Bill

http://forum.doityourself.com/thermostatic-controls/410865-frequent-cycling-chronotherm-iii.html