Lower temp when not at home?

[free75degrees]

I have programmable thermostats that I lower to 58 degrees in certain zones of my house when they are not used for periods of about 8 hours or longer then raise to 70 degrees when in use. I have heard some people say that this uses more energy due to the work required to get the house back up to temp. They say that I should either not lower it at all or not lower it so much. Based on my limited thermodynamics knowledge I am guessing that this is a myth and that it is more effiecient to lower the temp as much as possible. What do people think?

 

[free75degrees]

Also, let's assume I am burning oil or running of of stored heat so that we can ignore the inneffieciency of an idling fire.

 

[BrownianHeatingTech]

Most "current thought" is that seven degrees is the ideal setback amount, but that's an average. Each system is different. And the time you will be away is also a factor (obviously, the re-heat is a one-time thing, so if you are away for a week, it's trivial compared to the energy that would be used to heat the house at full temp for a week).

The size of the zones being set back is also important. I have one customer who shuts down everything except the master bedroom, each night, and saved a bundle. The more of the house that won't be turned down, the less benefit there is to turning down the unused portion.

In other words, there's no "one" answer. 7-10 degrees seems popular. It would take an adaptive computer system to actually "optimize" something like that.

Joe

 

[Mmaul]

I was told by a HVAC person a few years ago 6* is the minimum so thats was mine is set on. I have no proof to back this up.

 

[muncybob]

I can only say that since buying 2 set back thermostats and splitting the game room addition of my house into a second zone that I am not using any more oil now than before the sizeable addition was put on and not setting back the heat at night. The main part of our house(zone1) sets back at 10:30pm 5* until 6am the next day. Game room area is up to normal temps only first thing in the morning when I log onto my home pc and then it sets back 8* only to be manually kicked backup if I know I'm going to be shooting some pool, etc. It's made me a believer that it works although friends of mine "in the biz" will dispute it but I have documentation of oil usage before and after the addition was put onto the main house.

 

[free75degrees]

I am going to get one of those ARM PC on a board computers like nofossil has and set it up to monitor when the zones go on and off so that I can answer questions like this once and for all. Or maybe I should just send in a request to Mythbusters.

 

[BrownianHeatingTech]

I am going to get one of those ARM PC on a board computers like nofossil has and set it up to monitor when the zones go on and off so that I can answer questions like this once and for all. Or maybe I should just send in a request to Mythbusters.

There's no "once and for all," though - each building is different.

We do have computer systems designed to optimize fuel efficiency and comfort, including setbacks, but they aren't cheap...

Joe

 

[ISeeDeadBTUs]

Only one of my 6 zones has a setback. As far as I am concerned, it only has two benefits:

1)It has a clock and is backlit, which eliminates the need for a clock in that room.
2)When I am away for more than a day and will be counting on oil (No storage at this point) It is great knowing that much of the time I am gone, I am using no oil.

But with radiant heat, I think programable stats are a waste of money.

Lets face it, to tell the heat what to do, you need to know outside temp, outside temp trend, humidity, barometric pressure trend, solar gain, wind speed and direction, etc. Then, some relativly simple VB will allow the system to learn how hot to heat the water and when. All of which requires good data logging. Then, the ability to check and set zones from your cell phone.

I just hope when I get the time to actually write the code, someone other than myself will want to buy it :smirk:

 

[jebatty]

"to tell the heat what to do," you also need to know hormones. Don't forgot the WAP.

 

[Nofossil]

Only one of my 6 zones has a setback. As far as I am concerned, it only has two benefits:

1)It has a clock and is backlit, which eliminates the need for a clock in that room.
2)When I am away for more than a day and will be counting on oil (No storage at this point) It is great knowing that much of the time I am gone, I am using no oil.

But with radiant heat, I think programable stats are a waste of money.

Lets face it, to tell the heat what to do, you need to know outside temp, outside temp trend, humidity, barometric pressure trend, solar gain, wind speed and direction, etc. Then, some relativly simple VB will allow the system to learn how hot to heat the water and when. All of which requires good data logging. Then, the ability to check and set zones from your cell phone.

I just hope when I get the time to actually write the code, someone other than myself will want to buy it :smirk:

Current conditions are not enough - you need to know what's going to happen in the future. I actually have a script in the works for my ARM board that extracts data from an online weather forecast. Not operational yet, but maybe for next year.....

 

[Nofossil]

The better the insulation and the greater the thermal mass, the less benefit you get from setback. However, barring any unusual boiler operating characteristics, the more / lower your setback, the less energy you'll use. Thermodynamics - can't get away from them.

 

[Redox]

Nofo; you are over the top! Predictive controls based on forecast; what a concept. What's next? A WAP algorithm?

Agreed on the insulation and thermal mass factor. In addition, there is the fact that just about any conventional heating system works better and more efficiently when it is running flat out. If, however, you have a modulating burner, it would be better to ride the curve. Since there are now modulating gas furnaces and variable speed heat pumps available, i'm not sure the setback will save much and may end up costing more. In the industrial world, just about everything modulates to match the demand. Is anyone making a small modulating oil burner? Seems to me there would be a huge market for something like this. Maybe in Europe? Helllloooo...., Beckett, anybody payin attention?

Chris

 

[Nofossil]

Modulating is good, but the bottom line is this: the higher your indoor temp, the more heat you lose. A modulating boiler that follows an aggressive setback curve will consume less energy that a modulating boiler that maintains a steady indoor temp.

The ONLY way that setback would use more energy is if you had a heating system that was LESS efficient at high output than at low output. Most are, as you point out, the other way around.

If you were heating with a very efficient primary system that had a low output and a second stage system that was much less efficient, then you might want to be careful that recovery from setback didn't trigger the second stage heating.

Otherwise, it's an old wive's urban legend tale.

 

[free75degrees]

The ONLY way that setback would use more energy is if you had a heating system that was LESS efficient at high output than at low output. Most are, as you point out, the other way around.

Amen. This makes sense, unless we are both missing some other mysterious heat loss.