Nighttime Setback

Dave_41

I went from "set back" controls to constant circulation in my own home two years ago, designed my own injection system and used outdoor reset on the boiler and on the injection control, I no longer "set back", I maintain a constant 70* and let the warmer OD temps set the system back IN THE DAYTIME when no one is home. Cut my heating COST 35% despite rising fuel costs! Home comfort is uncomparable, there is just no change in the temp of the home. It makes my baseboard heated home as comfortable as any radiant home we have ever done!!! I can now come home to a warm house and stay warm all night and with constant circ "what will freeze?". Yes, this will cost more than a $40 "setback" t-stat, but will pay for itself in the first half of the first season!
This would not of course apply to a forced air system, but our data loggers show that most homes with forced air cool to the set back setting in less than two hours here in northern Wisconsin and cycle repeatedly for almost three hours to regain temp in the morning. I cannot see the savings there. Of course with forced air you do feel wamer when the system is running but feel cold as soon as it shuts off if the mass of the house is still trying to recover.

Dave

Mike T., Swampeast MO

Am again trying to make some sense of the subject.

First, does anyone know of ANY documented study regarding night-time, whole-house residential setback? I'm not talking about the often quoted "up to 1% savings per degree of setback in an eight-hour period" (that is FREQUENTLY misquoted WITHOUT the up to included). I'm talking about REAL studies where insulation, infiltration, type of heat emmitter, speed of recovery, subjective comfort, etc. are included. I'm not talking about large commercial buildings where the extremely sophisticated systems are used and space temperature is generally either beyond the control of the the occupants or highly limited.

Am able to reproduce that 1% per degree per eight hour period with simple numbers and calculations, but they assume that ONLY the air is heated, that the temp nearly instantly drops due to poor insulation and high infiltration, that the heat loss is always at the average outdoor temp and that there are significant periods of poor comfort.

Here's an interesting study regarding the pros & cons of automatic setback thermostats.

Also, MANY sources claim that the idea that it takes more heat to recover from setback than is saved on the way down is incorrect and that MANY studies have shown them essentially identical. I cannot find any of these studies and am VERY interested in seeing them--particularly interested to see if mean radiant temperature as related to comfort is in any way discussed.

gerry gill

mean radiant temp is always

a nagging question in my mind also..seems to me that if the temp of the furnishings etc decreases, they will steal back some heat as one comes out of setback, i think..so does that get factored in?

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Mike T., Swampeast MO

Exactly what I'd like to know.

Systems that rely heavily on radiation and with high mass and/or proportional control are striving to maintain MRT. Deep, daily setback is almost certain to result in a lower average MRT. My personal experience with such a system is that you can use CONSTANT setback in such a system, e.g. keep the air a few degrees cooler than when the MRT is lower.

Unknown

It would stand to reason that it should work something like this, right?:

Say you trigger a setback period at, oh, 9 PM, to recover at 7AM the next day.

From 9PM to some point, the air temperature cools and the heated objects in the room give up their heat to the space.. it coasts downward to a lower temperature. No energy usage at all.

From that point until recovery period, the system maintains a lower temperature, Saving energy.

When the recovery period hits, the system works harder to get the air temperature back up, and will continue to cycle more frequently as heat bleeds into the now cooler objects and mass of the room (raising MRT).

The energy used to get the MRT back up *should* be equivalent to the energy released during the "coast down".. you in effect, stored energy for the coast down. So that period of "no energy usage" *should* be completely offset by the increased load in heating back up... and you should be less comfortable until it does. So much for that warm toasty radiant bathroom at the morning shower!

So then I would say, the only true energy savings you get would be however long the period is from when the mass of the room gives up enough heat that you're no longer "coasting down", to the beginning of the recovery period.

Say you have a 70 degree daytime setpoint and a ten degree nighttime setback; anyone care to take a stab at how long your room will be "coasting down" before you get to and start maintaining the new setback temperature of 60 degrees?

Unknown

It's different in each case

But easy to time. The beginning time is obviously when the setback occurs.

The burner is good enough to come back on at exactly the end of the coast period. From then, on, until the beginning of the recovery cycle, is the savings period, but NOT, as Mike points out, the beginning of the comfort period.

The discomfort period is longer than the fuel saving period.

Noel

Mike T., Swampeast MO

That's how I see it working as well, but there are still three more VERY important variables:

1) The level of insulation

2) The level of infiltration

3) The mass of the heat emitters

For forced air, you can consider the mass of the emitters to be zero. But if you have large radiant panels or big old iron radiators you have a significant mass (thus stored energy) in the emitters.

With quite high mass and very good insulation and tight infiltration control, I daresay that you'd never see a 10° drop in 8 hours. Even my 1903 home with only decent insulation and infiltration control and standing iron, the drop from 66° to 60° will take nearly eight hours in all but the coldest weather.

Mike T., Swampeast MO

Easy with a system controlled by a regular digital thermostat.

Not so easy with condensing/modulating boilers driving proportional or tightly engineered systems.

If you use the built-in setback ability of the Vitodens for instance, you've NOT stopped heat--you've merely reduced the supply temperature to the approximate level required to maintain the lowered space temperature.

Condensing/modulating boilers using truly "smart" thermostats like the Buderus will do something similar.

Such will GREATLY extend the time it takes for space temperature to fall. Any fuel savings will come from lowered supply/return temps that result in a bit more heat extracted from the condensate.

Unknown

I think, in that case

you would see no fuel savings, to go with your period of decreased comfort.

Most people that ask aren't as aware of how much mass they have, but they kind of know they have high infiltration and low insulation.

Money and comfort aren't equal in all houses.

Setback is a means of savings, and greatly, when the money for improvements doesn't exist. I'd hate to minimize that fact.

Noel

Chuckles_3

The saving from setback is due to reduced heat loss to the outside. As first suggested by Newton, to a reasonable approximation the rate of heat loss from a warm body is proportional to the difference between temperature of the body and the temperature of the surroundings. (Hence the concept of "degree-days".)

So it can't possibly be a fixed percentage saving!

Let's take an example: outside 0F, inside 70F regular, set back to 69F for 12 hrs a day. So the savings percentage is 100*((70-69)/70)*(12/24) = 0.7%. However, if the outside is 60F, inside is 70F set back to 69F for 12 hrs, the savings percentage is 100*((10-9)/10)*(12/24) = 5%.

The percentage depends very significantly on the outside temperature, so it's lower in Canada than in Alabama. In Mike T's calculation, it will depend on the average outside temperature he uses.

However, the net therms saved does not actually depend on the outside temperature; both in Canada and Alabama you save the same amount, but as a percentage of the total it works out to be higher in Alabama, because the total is lower there.

NOTE: I've ignored the time it takes to cool by 1F. That will affect the numbers slightly.

Mike T., Swampeast MO

Setback is a means of savings, and greatly, when the money for improvements doesn't exist. I'd hate to minimize that fact.

I won't disagree at all with that statement. Setback certainly did save some in my parent's home. Old, mainly poor insulation, "average" infiltration, very irregular footprint, forced air heat. Upstairs bedrooms and bath were COLD!!!! Bath vanity doors had to be opened in cold weather to prevent freezing. Electric blankets REQUIRED. Morning shower--forget it in the upstairs bath in cold weather!

Took forever to get comfortable in the morning. Home-based business with 2-3 female workers. NONE wore skirts in the winter and they still had to keep an electric radiant heater under their desks...

That's a TOTALLY different situation than the modern heating sytems installed by most Wallsters and the good upgrades to fine OLD systems.

Mike T., Swampeast MO

In Mike T's calculation, it will depend on the average outside temperature he uses.

I used the freezing point of water...

Unknown

The shorter your "coast down" period, the more savings you'll see. Low mass heat and high heat load equals more savings COMPARED to not using setback... obviously the system itself in that case is inefficient to begin with.

If you have a slab on grade house though, or brick, or anything else like that with a high mass, I don't think you'll save at all. you're just releasing heat from the concrete and surfaces of the home, and having to recharge them later.

just a guess though.

Bob W._3

setback

I haven't been doing a night setback because our home is 3 wythe brick with air space, no insulation but lots of mass. Once the brick cools it takes forever to heat up again. Can't see the savings - could be wrong. This year we are going to set the thermostat lower and use the gas space heaters to add a little heat when we are home; I suppose that will be a setback of sorts (pun intended).

Mike T., Swampeast MO

just a guess though

The very reason I'm looking for case studies. Something must back up the up to 1% savings per degree of setback statement. Continue to strike out on the web.

Bill Nye_2

While you slept

While your heating system slept, that little 3/4" copper 90° ell in that drafty basement froze and split. Or the pipe in the wall in that porch addition. Now you can add to your savings by paying the plumber to thaw and repair the pipes. And the carpenter and painter to repair the damaged drywall. NIGHT TIME is not the right time to reset, do it in the day time when the sun is out and no one is home.

I work outside most of the winter if I am inside it is often in an unheated building. When I get home the last thing I want to do is freeze my *&% off in a cold house. What is the point? Really? Don't we suffer enough? A mans home is his castle, why not live like a king and turn the Da&& thermostat up to a livable temperature? Why make your kids suffer?

Save money somewhere else, slow down on the highway, car pool, leave the suv home and take the compact car, bring your lunch instead of going out. How much are you going to save by freezing at home, $50 bucks?

I still hold to lowering the water temperature during "setback" and maintaining the pump operations. Recovery would have to be easier , wouldn't you think? I can't prove a thing but I know I am comfortable, reasonably happy and never had a frozen pipe in my circa 1910 house.

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Bill Nye_2

Mike,

It 's a physics or engineering thing I think. I know they preach it at Viessmann school. That is where I was brainwashed , It sounded good to me and I've been sticking by it for 10 yrs or better. I will try and find it.

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Mike T., Swampeast MO

I did find a study of setback in large, (not single family residence) massive, solid masonary buildings. General conclusion was that slow and steady outperformed deep daily setback. Will try to re-find and post the link tomorrow.

Bill Nye_2

Ask Jeeves

I asked jeeves and got about 9,000 hits. I fell asleep at the key board, must be time for bed.

Try this one: http://www.apsu.edu/blanckh/energy.htm

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Mike T., Swampeast MO

I still hold to lowering the water temperature during "setback" and maintaining the pump operations.

That's the way I'm trying to automatically optimize based on outdoor temperature AND solar gain. The "euro-cave" is comfortable in moderate weather or when you're under the covers. TRV-enabled proportional control allows you to keep your bedrooms cool and baths cozy.

bob young

the last word

Bill Nye, you are the man. truer words were never spoken and you had me laughing my b**ls off !! i agree with your philosophy one hunnert percent.

R. Kalia_6

Yes, but that's not the average winter temperature everywhere, so not everyone will see the same % savings.

Mike T., Swampeast MO

Here's a bit of info regarding Massive Residential Construction

Will try to find the referenced study regarding setback. Conclusions of that study seem interesting but unfortunately it seems related only to "massless" emitters.

Mike T., Swampeast MO

I don't doubt at all that it's based mainly on calculations as opposed to actual case studies. It's the omitted words of "up to" that really bother me.

gerry gill

Mr. Kalia, in summary am i correct

from your example that setback would save more on mild days than on deep winter days?

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R. Kalia_6

No, as I already said, you would save the same amount in therms. But as a percentage it would be more on mild days because you are dividing by a smaller quantity.