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Is it more cost effective to (1) play with the thermostat by lowering it at times that you're not home or needing the heat and raising it back later or (2) leaving the temperature on a set amount and letting it maintain this heat.

I'm asking because our thermostat has the option to be programmed so that it could be cut down while we're gone or sleep and come back up when we're awake and home. I've heard that it costs more to make these jumps back up than it would be to leave it on a single setting and hold that temp and that it could hurt the overall system.. But our t-stat manual and other places/items advertise this as a cost savings.

In case it matters, I'm only looking at this for heating now around the winter. We have a heat pump with a gas furnace that doubles as the air handler. The gas furnace seems to run as the primary heat source regardless of the outdoor temperature (it's an odd system, we're getting it checked soon).

I've seen this discussion before, but there isn't really an official answer or reasoning to this as it goes both ways on who you ask it seems.

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    I don't think there's an answer to this question, or at least I've never been able to find any actual studies. There are a lot of opinions and theories on the subject, but I've never seen any hard data. – Tester101 Jan 9 '16 at 4:48
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I had my own "green" start up company that I sold off 10 years ago- and I use the same guys to install most new systems I put in. Really for most cases all the data will come back to is the average temperature that a home is kept (given that insulation and other thermal constraints are constant).

Really the most effective thing to do is to always have your heat pump on at a constant that would give you the lowest temperature that you could stand. Most of this comes from the ducts/pipes being used are always heated to level.

There are numerous numerous studies that will show you that if you turn off your heat pump for a large period of time of the day or turn it down dramatically you can save a lot of money. The energy.gov study shows that turning off/down your thermostat for 8 hours a day will save you between 5-20% of your heating bill.

You might say - well that shows you that it cost more to heat up your house because the heat pump is off for 33% of the day but you only save 10%. Now remember the point I made earlier, the cost is really based on the average temperature of your house - not when/how long your heat source is on. So if it is 20F outside and you normally have your thermostat set at 70F but turn it off 8 hours a day you may come back to a house around 59F. Now your average temperature is 66.33% and you can expect savings of 3-5% per degree around ambient temperature so about 10-20% savings which is directly in line with energy.gov stats.

Now the big GOTCHA that I didn't see data on is the ability for the heat pump to source heat from the outside. (Let's take your heat pump being old out of the equation. Most new heat pumps are anywhere between 150-300% efficient. Older ones are all over the place so this may or may not pertain.) If it is below 20 or just too cold and you kick your thermostat off for 8 hours a day and your house gets to 59F when you back... now let's say your heat pump then has a hard time getting the house back up to the desired temperature - and yes you would kick it off when you are at work and when you get home with cold house the temp outside is dropping too.

So the kicker is with a heat pump is you never want to turn it off to the point where you ever need to use the emergency heat. The emergency heat is probably electric and might be close to 100% efficient for new models, but no where near the normal range of efficiency. So putting yourself in a situation where the temp fell so much that it kicks on the emergency heat on a heat pump is the very last thing you want to do.

But even in this case it does not mean you don't turn down your thermostat, it just means you only turn it down to a certain level based on the outside temperatures. (there is a lot more I could have probably added to this but it is already starting to get long)

Physics discussion that is a little more involved on the science side than my answer.

Really good case study by Institute for Research in Construction

There is also a lot of discussion that could be included in maybe a different question that is more specific around:

  • dealing with different types of heat sources (heat pump vs central air - forced vs rad)

  • the thermal properties of your house

  • the way your house deals with heat loss at variable levels of heat differential

  • the inefficiencies of your heating solution (ie is there anything that makes your heat source inefficient and how to limit that)

  • the humidity effects and other climate effects

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  • So you're kind of saying for one point that adjusting the t-stat is only really useful if been done for a long enough period of time? -- Our heat pumps were installed in 2010, and they redid the entire hvac system to the house at that time. -- Our t-stat is programmable and it says that it's smart enough to be able to know when to start heating so as to get the temp back up to say 70 right when we get home. Our emergency heat, hopefully is how it's supposed to be installed, is the gas furnace. – TFK Jan 9 '16 at 14:37
  • I've heard a lot of people quote studies, and talk about studies. However, I've never actually seen any of the studies, as nobody ever seems to link to them. – Tester101 Jan 9 '16 at 15:45
  • @TFK - Well anything helps but obviously the more its off the more you will save. There isn't a whole industry of smart thermostats based on fabrications. On the emergency heat - wow - you have quite the backup. On most heat pumps it is an electrical strip built in. – DMoore Jan 9 '16 at 17:39
  • @Tester101 - I found a lot of scientific studies when we were researching the link to the IRC site is something we actually used when talking with clients. The fact is there are too many variables to discuss. The main three are insulation properties of house, efficiency of heat source, and the average temperature. – DMoore Jan 9 '16 at 23:27
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The amount of heat going out of your house has to be made up by your heating system. When its cooler inside(winter) there is less temperature difference from inside to outside and therefore less pressure to move heat out. Therefore heat loss is lower. Looking at run times and saying savings are uncertain is disingenuous at best, except on a heat pump. Remember that auxiliary heat(electrical resistance heat) costs more than heat pump heat and so a large setback might cost more than it saves on a heat pump(not true on gas heat). Setback does cause less heat loss but only if the temperature goes down in the house.

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