Programmable Thermostats = bad?

[SkunkedAgain]

I didn't want to hi-jack the other post about heating/thermostats. It was said that programmable thermostats actually damage your furnace because of the constant cycles. Can someone explain this a little more?

Also it was mentioned that it costs more to bring a house back up to temp than maintain a constant temperature. That's contrary to popular notion but I can understand the argument. I suppose that it depends on the length of time between re-heating. So does anyone know the true positives and negatives for the majority of us who program the t-stats to go down during the day and while we're asleep?

[spearchucker]

I had a new top of the line furnace with a programmable thermostat installed last spring. I don't know how much of it can be attributed to the thermostat, but my gas useage is almost half of what it was last Dec. Probably mostly due to higher efficiency furnace.

I don't believe the stat decreases the life of the furnace. If anything it is going to make it cycle less often.

[BobT]

A programmable thermostat only changes the temperature setpoint for the room. The furnace doesn't know the difference between setting the temperature higher a couple degrees for a few hours during the day or the drop in exterior temperature by 15-20 degrees or more at night. All it knows is that the thermostat has detected a drop in room temperature and is calling for heat.

[BoxMN]

I know that mine has saved me money, and kept the house comfortable for when you wake up and go to sleep, and when gone for the weekend I just "Hold" a lower temp in winter and warmer in summer, and cool thing is when we get back from cabin/gone the house is comfortable when I want it to be.

[solbes]

Ours has saved money as well. There is some truth to changing the setpoint too far. If you drop it 15 degrees for example and it's below zero outside, it would have a real hard time getting back up to temp in the afternoon/evening. Mostly because your walls would be so cold that it takes a long time to bring them back up to temp.

We drop ours 5 degrees during the weekdays and 2 degrees during the night. You're not going to get huge savings, but should see some.

[SkunkedAgain]

This really has a lot to do with Neighbor_Guy's post in the "Not Enough Heat" thread.

What about his claim that programmable thermostats shorten the life of your furnace?

[spearchucker]

I don't think they hurt your furnace, but on an old furnace you might not gain anything.

Most new high efficiency furnaces are made to work in conjunction with a programmable stat. Most are multi-stage gas valves and between the stat and the furnace controller they control the burn rate. My stat uses humidity and inside vs outside temp to decide how hard the furnace needs to work to get up to temp in the house. Unless the temp is below zero the furnace rarely runs at 100%.

[BobT]

Programmable thermostats don't use sophisticated PID control to optimize your heating system. I would suggest that the hardest thing on any heating system is rapid and/or extreme change in temperature.

Your furnace doesn't run at a constant temperature, which would be ideal and only remotely possible using PID control. Instead when the thermostat calls for heat it heats the exchanger from room temperature to a couple hundred degrees until the thermostat tells it to stop. Then it cools back down to room temperature again. This temperature swing causes the heat exchanger to expand and retract with every cycle. I would suggest this is adding stress to the components. From the heating systems we use on our machinery we've learned that the narrower your temperature swing, the better life out of the system.

So you cool your house down by say 5 degrees for the night and then bring it back up in the morning. During the period when it is bringing the house temp up, the exchanger is running hot longer with little swing in temperature. Conversely, when the setpoint drops by 5 degrees in the evening, the furnace remains at room temperature longer and again, little change in exchanger temperature.

I would propose that the opposite is true. That using the automatic thermostat to cool the house a few degrees at night and warm it back up in the morning may actually extend the life of the furnace.

How much is probably minimal and unless your counting minutes probably not measurable to the averge homeowner.

[Macgyver55]

I would propose that the opposite is true. That using the automatic thermostat to cool the house a few degrees at night and warm it back up in the morning may actually extend the life of the furnace.

I'd to tend agree with Bob, we may not be right, but the way I see it, frequent heat cycles tend to cause more problems with autos too. In my line of work, I'd compare it like highway miles versus city miles. A longer "run" is easier on equipment than several on off cycles.

[spearchucker]

Originally Posted By: BobT

I would propose that the opposite is true. That using the automatic thermostat to cool the house a few degrees at night and warm it back up in the morning may actually extend the life of the furnace.

I'd tend agree with Bob, we may not be right, but the way I see it, frequent heat cycles tend to cause more problems with autos too. In my line of work, I'd compare it like highway miles versus city miles. A longer "run" is easier on equipment than several on off cycles.

That is how the newer furnaces are setup. They run longer at a lower BTU and lower fan speed unless it is really cold. This helps avoid the big temperature swings overshooting the set temp and then cooling back down again.

[Neighbor_guy]

A little of what I said was said in jest directed at steve.

And I can see how it could be taken out of context so I will try to explain why I said it.

First if your furnace is a "high efficiency" model and is less than 5 years old, great, hook up a programable, they were made and designed to work together. But on older models, homes that dont hold residual heat well, or those with an air to air exchanger(hole different set of issues), a constant hold heat setting will save you money and maintenance in the long run.

Your furnace does not care if you have your house set at 70 degrees or if you set it for 60 degrees. It will use the same amount of gas to hold temps either way. It is the extended burn to bring your house back up to your "living condition" set point (someplace between 68-72 depending on the lady of the house) that puts excess wear and tear on the older machines.

Look at it like your driving down the highway in your car. If you are holding at say 60mph all the time you get great gas milage, avoid excessive wear on your tires, trany, motor, etc. Now if you are in the same car but slowing down, going threw town 30-40mph catching a few stop lights, then jamming on the gas flat out at the other end of town, and doing it repeatedly your car will take a beating. Milage and the searvice life of everything else goes down.

Your furnace is the same way. They are designed to hold a temp. They do that by allowing your home to roll up and down a bit a degree or two above or below your mark. Say for a 70deg set point it will roll between 69-71deg. If you set your house for 65 (lets just use it) your stat will roll between 64 and 66 (give or take). It uses the same energy to stay at 70 or 65. The only savings you get are the "coast" down from 70 to 65. But the energy it takes to ramp back up, on older units, outweighs the energy saved to allow it to cool off.

Trust me, the energy companies are not going to talk anybody into something that "saves" the consumer and keeps money out of their pockets. Peak and off peak use still applies. When do you suppose people are going to set the furnaces to ramp up to the desired living temp? Durring prime time, before and after school and work. When is the cost per unit of use the most? Before and after school/work.

But all this aside, your energy provider will gladly preach to you about the "savings" of a programable t-stat. Your furnace guy will gladly install one, then he will Happily sell you/install a new state of the art "high efficiency" furnace if yours has a few to many winters behind it. The newest of the new actualy never shut down. The fan itself is always moving air, just at a lower speed. Because if you move the air around you, held at a constant heat, you feel warmer than you do in the same temp room without air movement. Its like running the ceiling fan on low in the winter. Just moving enough air to move it.

In the end, its your house, do what you want. If it makes you feel good to turn the heat down when you sleep or work, fine. But you dont need to. (and run the ceiling fans )

[spearchucker]

Your furnace does not care if you have your house set at 70 degrees or if you set it for 60 degrees. It will use the same amount of gas to hold temps either way. It is the extended burn to bring your house back up to your "living condition" set point (someplace between 68-72 depending on the lady of the house) that puts excess wear and tear on the older machines.

Look at it like your driving down the highway in your car. If you are holding at say 60mph all the time you get great gas milage, avoid excessive wear on your tires, trany, motor, etc. Now if you are in the same car but slowing down, going threw town 30-40mph catching a few stop lights, then jamming on the gas flat out at the other end of town, and doing it repeatedly your car will take a beating. Milage and the searvice life of everything else goes down.

Your furnace is the same way. They are designed to hold a temp. They do that by allowing your home to roll up and down a bit a degree or two above or below your mark. Say for a 70deg set point it will roll between 69-71deg. If you set your house for 65 (lets just use it) your stat will roll between 64 and 66 (give or take). It uses the same energy to stay at 70 or 65. The only savings you get are the "coast" down from 70 to 65. But the energy it takes to ramp back up, on older units, outweighs the energy saved to allow it to cool off.

I agree that a programmable will not save you much on an old furnace and may or may not shorten the life.

I don't agree that setting the stat at a constant 70 is going to use the same fuel that setting at a constant 60 degrees. The higher you are above the outdoor temp, the more fuel you will burn. This is worse on a poorly insulated house. You are right the furnace doesn't know, but I guarantee that your gas bill will.

[deerminator]

I am no expert but I do know we have our upstairs thermostat set at 69 while we are home and down to 65 while we're away and I've noticed a substantial difference in our propane usage. We have in-floor heat on our main level and for that as it would be a much more of a struggle for that system to get back to temperature.

[BobT]

Quote:

Your furnace does not care if you have your house set at 70 degrees or if you set it for 60 degrees. It will use the same amount of gas to hold temps either way.

I don't think that is quite accurate. It takes X BTU's to offset the heat loss in a home. The larger the difference between indoor setpiont and outside air temperature (wind also plays a role), the higher the heat loss and therefore the more BTU required to maintain the setpoint. If what you said was true, we wouldn't need to heat our homes if we never opened a door to the outside.

Quote:

Trust me, the energy companies are not going to talk anybody into something that "saves" the consumer and keeps money out of their pockets. Peak and off peak use still applies.

I don't know how you can say this. My dual-fuel and off-peak electric rate is less than half what I pay or the rest of my electrical energy. The power company offered me the opportunity to install the system. I was already using electric hot water. Yes, I did choose to add the electric heat option when I remodeled and part of my decision was related to the dual-fuel offer so maybe in that sense you are correct that they benefit by offering me the reduced rate.

Quote:

The newest of the new actualy never shut down. The fan itself is always moving air, just at a lower speed. Because if you move the air around you, held at a constant heat, you feel warmer than you do in the same temp room without air movement. Its like running the ceiling fan on low in the winter. Just moving enough air to move it.

If you're talking about forced then again you are mistaken about the heating system never shutting down. At least I'm not aware of any residential forced air system that utilizes PID control to regulate the fuel flow rate, pressure, etc. in order to adjust the fire for optimum continual exchanger temperature. The only thing that continues to run is the fan and as you mentioned, the advantage is similar to using a ceiling fan.

Setting your system up so the fan runs continuously when outside temps are below 40 degrees has been a recommendation since at least the 1960s. I know because the owner's manual for my Lenox fuel oil forced air furnace recommended this and my furnace was older than their books, which went back to 1963. One advantage to circulating the air is that it helps maintain a more uniform indoor temperature. It also reduces the effect on our skin of radiated cold off the walls and windows. Both of these help with indoor comfort.

[Neighbor_guy]

I'm sorry........

I must have forgoten what I do for a living.......

My mistake.......

And for the record. Setting your fan speed to "on" at the t-stat durring the winter heating season reduces its effiency because it over rides the fan to run in high speed(normal heating fan speed being low). Thus not getting the most heat out of the heat exchanger per cubic foot of air moved. If you are going to set the fan to "on" do it durring the spring and fall when you are between heating and cooling seasons. But only if you are upposed to opening a window, freash air is free you know.

And Yes, the newest of the new furnaces have 3 fan speeds H, M, and L. High used durring the cooling cycle, M for heat, and L being constant on, no call for either heat or cool and always moving air . Do some research, they are out there. Or better yet call your energy company. I bet they have a consumer marketing person who would be glad to talk to you about it.

[Coach1310]

Just so I can wrap my head around this....

If I keep my house at a constant 60 degrees I will use the same amount of fuel as if I keep my house at a constant 70 degrees??

If I have a T-Stat with a Auto/On/Off fan switch, it is best kept on Auto during the winter months???

I know little to nothing about this stuff and I just want to make sure I am reading these posts correctly..

[spearchucker]

Neighbor_guy,

This thread has gotten a little off track.

The way the systems you install or service is not the way all of them work.

When my fan is set to "on" it runs at low (this speed is programmable in the furnace controller) when the furnace is not calling for heat. Once heat is needed it ramps up the fan and burner to the needed CFM and BTU based on indoor vs outdoor temp. The gas valve can adjust from 40% to 100% and the fan is a DC motor that is infinitely variable. This doesn't affect the efficiency the least bit. It keeps the temperature even throughout the house so you don't get hot or cool spots. Once the temp set point is reached is goes back to the preset fan speed (low in my case) as long as fan is set to on.

[Macgyver55]

I'm sorry too, but laws of physics show Bob to be correct as far as the difference between the indoor and outdoor temp. Its quite simple, the colder it is outside, the more the furnace will need to produce heat, the more it will run.

Quote:

It takes X BTU's to offset the heat loss in a home. The larger the difference between indoor setpoint and outside air temperature (wind also plays a role), the higher the heat loss and therefore the more BTU required to maintain the setpoint.

If it were not true, it would cost the same to maintain any given temperature any time of the year the furnace was running.

[LightningBG]

I'm sorry........

I must have forgoten what I do for a living.......

My mistake.......

Neighbor, maybe you should look for a new line of work!

From the US Dept of Energy:

"Thermostats and Control Systems

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."

In my own personal experience, I had about 40% lower propane bills on one of my houses by using a programmable thermostat along with keeping my house about 3 degrees colder in general, vs. my neighbor (same house, same builder, same furnace, same weather and wind conditions).

Do the programmable, its well worth it.

[LightningBG]

Look at it like your driving down the highway in your car. If you are holding at say 60mph all the time you get great gas milage, avoid excessive wear on your tires, trany, motor, etc. Now if you are in the same car but slowing down, going threw town 30-40mph catching a few stop lights, then jamming on the gas flat out at the other end of town, and doing it repeatedly your car will take a beating. Milage and the searvice life of everything else goes down.

Your missing the point here. Wear factors aside, MPG has no bearing. The analogy should be comparing Gallons Per Hour (GPH).

Here's an example:

2 cars are headed west on 94 towards Fargo.

Car A starts in Rogers already going 80 mph (or 70 degrees in your house) and holds this speed for the next 2 hours.

Car B starts in Rogers already going 80 mph but immediately coasts down to 60mph (55degree in your house) and holds this speed for 1hour and 50 minutes. At that point they lay on the gas and get back up to 80mph.

After 2 hours they are both going 80 mph (or 70 degrees). BUT car B has used way less fuel. (Sure they haven't traveled as far, which is what is accomplished by going 80 the whole time, but your house isn't accomplishing anything during the day while your at work anyways.

As a side note: We all know is cost less to keep a house at 50 degrees vs. 70 degrees. Otherwise my parents would keep there house at 70 all winter while they are in AZ and I would keep my cabin at a comfy 70 all winter.

And yes, at some point there is a break even point where this doesn't hold true, like if you were going to drop your furnace for 25 minutes while you go walk the dog, but we aren't talking about that situation.

[BobT]

I may be mistaken but when done properly isn't fan speed adjusted to maximize plenum pressure? Too much fan speed over-pressurizes the system and too little can reduce the overall efficiency of a system by not maximizing the exchange of heated air. In every heating system there will be an optimum heat exchange rate. I don't recall the terminology and I may be way off base but maybe "heat density" is an appropriate term? I know we talk of watt-density when we talk about electric systems on our equipment.

Here's some information I found from NDSU regarding temperature setback. Note, this person is not a power company employee and, unless you by default figure he's bought by the energy producers, maybe it can be helpful.

Quote:

Rising energy costs have people questioning whether thermostat setbacks actually save money.

“In most cases, they are cost-effective,” says Carl Pedersen, North Dakota State University Extension Service energy educator. “But there are a few exceptions.”

A thermostat setback occurs when the homeowner turns down the heat for certain periods of time to save money on heating costs. Generally, people turn the heat down at night, when they are sleeping, or during the day when they plan to be gone for several hours.

Residents can turn the temperature down automatically with a programmable thermostat or manually.

People may not use thermostat setbacks as a cost-saving method because of some misconceptions, according to Pedersen. For example, many people believe setbacks are not cost-effective because the heating system has to work harder to heat the house back up to a comfortable temperature.

“This is simply not the case,” Pedersen says. “When the temperature falls below a set level, the system turns on to bring the temperature back above the set level. Once the temperature reaches the set level, it turns off. It is not like the throttle on your favorite automobile, where the harder you push, the harder the motor works. Heating systems are simply on or off.”

Many people also believe that the furnace runs so long to heat the house back up that this offsets any savings. Pedersen says the fuel used to reheat the house is roughly equal to the fuel savings while the home’s temperature was dropping to the setback level. The savings occur while the heating system is at the lower temperature.

“The longer the thermostat is at the lower temperature, the greater the cost savings,” he adds.

According to the U.S. Department of Energy Web site http://www.eere.energy.gov, you can save about 5 percent to 15 percent per year on your heating bill by turning your thermostat back 10 to15 degrees for eight hours. That’s a savings of as much as 1 percent for each degree if the setback period is eight hours long. If your normal heating costs are $1,500 per year, that is a savings of $75 to $225 every year.

To estimate if a setback will save money, you can run a simple test. To start, you need to record the amount of time your heating system runs in one hour at the home’s normal temperature. Next, turn the thermostat down to a desired setback temperature. Give the house time to equalize at the lower temperature and then determine the amount of time the system is running at this setback temperature.

If the heating system runs for 20 minutes an hour at the higher setting and 14 minutes an hour at the lower setting, you will be saving 10 percent on your heating bill each hour the temperature is at the lower setting, To calculate the savings, subtract the minutes the heating system runs at the lower setting from the minutes the system is on at the higher setting (20 minus 14) and divide by 60.

Energy experts do not recommend thermostat setbacks for houses that are running heat pumps. Setting back the thermostat on these systems causes the heat pump to run inefficiently, canceling out any benefits, Pedersen says.

Setbacks also are more difficult for houses with in-floor or steam heating. The difficulty with these systems is their slow response times. Reaching the desired temperature levels with these systems may take a number of hours.

Some manufacturers are beginning to offer systems that track the heat so you can better determine when to set back the temperature. Setbacks are possible with these systems, but determining the proper timing will take some effort by the homeowners, Pedersen says.

[SkunkedAgain]

Wow, this is exactly the type of discussion that I was looking to foster. Lots of good information, opinions, and certainly some expertise and research being displayed.

I've got a 60 year old home with a gas furnace (at the bottom of the efficiency scale) so I've learned a few things. I'm probably not getting as much efficiency out of my programmable thermostat as other folks since the furnace isn't efficient when re-heating. I might look into the fan issue to see how it runs, since I've always kept it on auto. Adding several ceiling fans will be my spring project. Until then...

[spearchucker]

SkunkedAgain,

Depending on the age of your furnace it may be worth looking to replace it. Mine was 34 years old and needed a few hundred in parts. With the rebates from the manufacturer, gas company, electric (for the A/C), and the $1500 tax credit it was pretty reasonable compared to a year ago when I first got a quote. Its amazing how the price drops when they are looking for work. My Dec. gas usage went from 204 therms last year to 133 this year. I will have to compare the next few months to see if the trend continues.

Most of the high end models now have variable speed DC motors on the fans, so they use a lot less electricity when running (600 Watts for the old single speed AC fan vs about 60-75W for the new DC one). You don't even notice it on your electric bill when running the fan constantly. The energy savings is nice, but they are so quiet you can hardly hear it run.

[BobT]

It takes a rather significant improvement in efficiency to offset the cost of replacement but spearchucker is correct that with all the current "go green" rebates, tax incentives, etc., it might be worth considering.

Suppose your furnace is 75% efficient. For every 100 units of fuel you use, only 75 units are actually heating your home and the rest is going up the chimney. If you upgrade to a new one at 95% efficient, you're saving about 20 units per 100. So let's throw some numbers to this.

Hypothetically...

Suppose you use LP gas and it costs $1.00 per gallon.

At 75% efficient your furnace is wasting $.25 per gallon.

At 95%, your furnace is costing you $.05 per gallon.

A difference of $.20 per gallon.

How many gallons will you have to use to pay the cost to replace the old one with the new one.

I had this scenario with my old fuel oil furnace. The service tech told me it was probably working at about 60%. I asked if it was worth upgrading. He told me that the new ones were about 83% but it would cost about $2,500 to install plus the cost of the furnace. When I did the math myself, I calculated that it would take nearly 30 years to save enough in fuel cost to pay for the upgrade.

I eventually was forced into it because the heat exchanger finallly rusted out. The new furnace cost me almost $5,000.00 including installation.

[Tom7227]

Back to the programmable theromstat discussion.

In the back of my mind I keep thinking of something that I think is called Delta T. My understanding of it is that the greater the temp difference between two objects the greater the loss in heat as the hotter tries to match temp with the cooler. I don't know if it is the rate of the loss of heat or the actual amount of heat that is lost.

Anyway, if you apply this principle it seems to me that the colder it gets outside the more rapid the heat loss will be from your home. So if at 72 degrees inside and 0 outside let's say that your home loses 100 units of heat per hour. If it is 72 in your house and 40 outside your house would lose 60 units of heat per hour. If you lowered the temp in your house to 62 when it was 0 outside you would lose less than 100 units of heat, same if it was 40 outside.

If you have the programmable thermostat and lowered the temp for 8 hours while you were sleeping then the thing you would need to know was how many units of heat it would take to raise the temp of your house from 62 to 72. Even if it's less than 800 units of heat you've saved something. From everything I have learned, and what I have experienced in my own home, it's less than the 800 units of heat so I've saved something. That savings occurs every day that I have the heat on, and also when I have the AC on. Given the price of those thermostats I think I come out ahead pretty quickly. And the greater the temp difference between the inside and the outside the greater my savings.

My furnace isn't going to care whether it's raising the temp 2 degrees or 10 degrees, and in fact I suspect it is going to get less wear and tear cause it's going to cycle on and off less often while the temp is at the night setting. When it moves to the morning setting it's going to crank away once while it hits the desired temp.

Have I got this right?

I have a 97 or 98% efficient furance with the DC motor. I have it running all the time. The constant mixing of the air in the house makes it much more comfortable than if it weren't on, and I suspect it also means the unit cycles less often.