save money by adjusting thermostat up and down?

[someguy]

Do we really save money by letting our house cool down (in the winter) while we are gone and then heating it up when we come back home? Has anyone read how much we should let it cool down and for how long it is worth it? (Yes, I realize that everyone's house and situation is different.)

 

thanks

[wwat]

Interesting topic, living in Canada with some pretty wild whether fluctuations. I now have a seperate Venstar in the house and workshop and let the workshop get down to 55 when I'm not in there and up to 65 when I enter the shop.

The house temp is determined by a mixture of our comfort and that of our pets so I never let it get below 68 when away and around 70 when home with a high set point of 72.

 

I know this didn't really answer your question but if your house is well insulated I suspect there isn't alot of benefit to letting the house cool down too much over the course of a single work day because it will cost you to heat the rooms and objects back up to the comfort zone again. If you are talking about an extended period away from the house that is another matter.

If your house is not well insulated you certainly might save a few dollars by lowering the temp during the day.

 

In the house I have a heat hi scene and heat low scene and 4 periods throughout the course of a 24 hour day where the setpoints are changed. I've yet to program for summer cooling months.

[IndyMike]

Someguy,

 

Have a look at the excel spreadsheet on the following page: http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=TH

 

Using the baseline temps/set back times it came back with a $224 yearly savings for the KC area.

 

Edit: I took another look at this for my area (northern Indiana). The cooling season numbers look off. I don't think the calculation is taking humidity into account. Humidity has a big impact on the A-Coil efficiency during cooling.

 

Heating numbers look pretty close - make sure to adjust for your local electric/gas costs.

[tahoe]

Another way to look at it is to measure how long it takes to warm the house back up.

 

example: if your heater is small and it takes 10 hours to raise the temp 10 degrees, you wouldn't save anything. From the 8 hour setback data in the chart below, I can remember that every degree of setback saves me 1% in energy.

 

 

Go to this chart:

 

http://www.mge.com/home/saving/thermostat.htm

[oberkc]

This is a pure physics question. Heat loss is due to the difference in temperature between your house and environment. The greater the temperature difference, the greater the energy loss. If you reduce the temperature of your house, you will loose less energy than if you had maintained a higher temperature, all other things being equal. Setting your thermostat back saves energy. I see no way around this fundamental law of thermodynamics.

 

Are there other factors which might sway one from lowering the temperature of your house during periods of inoccupancy? Of course there are. It is hard on some items of the house. Your silverware might not heat up very fast. Your house may have trouble regaining desired temperatures at the right times. Some furnaces use different (and less efficient/economical) sources of energy when they are trying to catch up. There are all sorts of reasons why a constant temperature may be more desirable. Saving energy is just not one of them.

[someguy]

thanks for the help, fellows. interesting stuff.

[apostolakisl]

The biggest problem for me is trying to predict when the house will be occupied. I work all day, the wife works occasionally, and the kids are in school but have random days off and stuff. Unless you have an oversized system (which is very uneconimical), you can't just quickly move the temp, which pretty much rules out occupancy sensors or alarm status as a means of adjusting temp. I suppose you could let it go just 3 or 4 degrees when the alarm is set to away mode since that wouldn't really make the house uncomfortable. For now I just have a sleeping temp set and another temp for the rest of the time.

[someguy]

I have one area completely controlled (via ISY) by time. It heats / cools the bedrooms based upon sleep/nap times.

 

The other zone works off of two insteon motion sensors. When motion is detected, it cools/heats for two hours and then switches to "vacant" mode which is 3-5 degees different.

 

I was just curious whether this really makes sense and it seems it does, from the pure physics of it.

[IndyMike]

Hi Someguy,

 

The question of whether or not to use a "setback" feature on a zoned system is far more complex.

 

I define a Zoned system as one piece of HVAC equipment supplying multiple Zones through a series of dampers. A system that uses more than one piece of HVAC equipment (one on the first floor and another on the second) is not really a zoned system. Multi-equipment systems are completely isolated and, as a result, give you more flexibility in controlling. True Zoned systems share the return line. As a result you are always mixing return air from unused zones.

 

Both of the above systems gain efficiency by using HVAC equipment that is sized to the area it is handling. Perfectly sized equipment will run continuously on the hottest/coldest days. Over sized equipment will cycle on and off and loose efficiency through the heat up/ cool down periods of the equipment.

 

The type of equipment and mode (Heating/Cooling) is a huge factor in whether make use of setback features. Most heat pump systems use multiple stages along with an emergency mode. If your setback feature inadvertently activates these modes you've lost efficiency. There are newer intelligent setback thermostats that are designed to operate heat pumps in set back mode. I have no experience with these.

 

Cooling is likely to require completely different control than heating. Humidity in your home will have a direct impact on the efficiency of the A-coil in your HVAC. If you operating in set back mode allows the humidity level to rise, the equipment will need to work harder to remove that humidity.

 

So what's the answer - monitoring your equipment and your utility bills.

 

1) If your equipment runs continuously when bringing a zone back to temp (from a setback), congratulations it's properly sized and operating near peak efficiency.

2) If the equipment cycles coming out of setback, it's either over sized or the airflow to that zone is insufficient. Check your dampers for restrictions/consider enlarging the zone.

3) Cooling - If you live in a high humidity area, check the RH of the zone when coming out of setback. The A-coil will be more efficient if it is working at a constant 50% RH level rather than cycling from 50% to 70% RH.

4) All of the above is time dependent. Operating in setback for an extended period of time will be more efficient than a steady state temperature. Cycling between setback and normal temps may not be more efficient depending on the times involved. Your utility bill will be your guide.

 

IM

[oberkc]

Unless you have an oversized system (which is very uneconimical),

 

Perfectly sized equipment will run continuously on the hottest/coldest days.

 

This may have been true in the past, all-on, all-off systems, but I understand that the newer systems with variable heating and cooling stages are pretty efficient over their entire range of outputs.

 

If you system is continually cycling on and off while recovering, it is probably doing so even more while maintianing a steady state. This tends to suggest that steady state is less efficient than recovery for a given system.

 

Yes, one must be aware of emergency heat modes, more costly energay at certain times of the day, etc. Like so many answers, it depends......on your system and goals. Saving energy may not save money in some cases.

 

There are definitely many factors involved, as so many point out. Not all of the factors are directly related to energy use. I like the suggestion to watch your bills and decide.