To set back, or not to set back?

[hvac wiz 79]

who the hell wants to wait for a system to recover while being uncomfortable? .. set it for what you like & leave it unless if you plan on going away for extended periods of time. im not one to figure the calculations but couldnt all that time during a hot pulldown every day cost considerably close to what a few cycles an hour would be when your at your desired setpoint during the day? .. i leave mine to cycle as needed & have very low utility bills. granted this is a building constructed in 2011 but still ....

[midhvac]

who the hell wants to wait for a system to recover while being uncomfortable?

LOL, that's a huge part of the variables involved here. I have some customers that are willing to make that sacrifice in comfort to save x $ over the course of an entire cooling season. But those aren't the norm. And this is one reason I mentioned the "intelligent recovery" thing.
EVERYONE'S system is oversized during the vast majority of the cooling season. So they could be saving money the rest of the cooling season if they had an effective way of doing that. And there would be little or no loss in comfort level if the stat was smart enough to ensure it.
But as someone mentioned, some of the electric company's are starting to charge more per kWh during peak demand hours. And that obviously reduces the potential savings.

[CraziFuzzy]

Mine doesnt catch up till after I go to sleep... but Idc cuz by the time it brings it down from 85 to 80 it feels good. Then it spends the rest of the evening bringing it down to 68.

Sounds like you should just keep it at 80 and be satisfied then...

[WeebMan]

Sounds like you should just keep it at 80 and be satisfied then...

Well, I think it has something to do with dehumidifying for 5 degrees that maked 80 feel okay. Plus the fact that theres cool air circulating. Cuz to just set it at 80 doesnt cut it for me.

I didnt realize what an open ended question this was. Lol.

[WeebMan]

And comfort wasnt my reason for asking. I was just wanting to know which was gonna cost less on the electric bill.

[KevinAdv]

I wish I could afford to run mine at 68. Although I don't even know if it will cool that low

[CraziFuzzy]

I didnt realize what an open ended question this was. Lol.

If it was a simple yes or no answer, thermostats wouldn't have any adjustments in them, and they would all be set permanently at the 'right' setting.

[WeebMan]

If it was a simple yes or no answer, thermostats wouldn't have any adjustments in them, and they would all be set permanently at the 'right' setting.

I realise settings are going to vary according to personal preference. I wasn't asking what is the most efficient and comfortable way to operate the system. Just, what is the most efficient? Thats all. Would it cost less to leave my stat on 68 from the beginning to the end of the summer, or is setting it back during the day (like I'm doing) more cost effective.

[midhvac]

I realise settings are going to vary according to personal preference. I wasn't asking what is the most efficient and comfortable way to operate the system. Just, what is the most efficient? Thats all. Would it cost less to leave my stat on 68 from the beginning to the end of the summer, or is setting it back during the day (like I'm doing) more cost effective.

It will always use less kWh per season by setting it back. How much less depends on numerous variables. But if the electric co. is charging you more per kWh at "peak" hours of the day, that must be factored into the savings equation.

[7X]

Strictly speaking from the combustion heating side - I remember quite a while back there was some testing done and conclusions made. I was told years ago that you could save 1%/degree/8hrs. Set back 10 degrees for 8 hours and potentially save 10%. I certainly think it is site specific but as long as you are not cooling the thermal mass in the building (and most buildings will "coast" pretty well for a while), it does not take all that much time or fuel to reheat mostly the air. This is also essentially what DoE states on their EERE website.

I simple analogy for the layperson would be that you did not leave your truck running all night long so it would be warm for you in the morning.

If anyone has any empirical data on this topic, I would be interested in seeing it.

[skippedover]

How do you decrease temps in summer and increase temps in winter? The real answer lays in the equipment design temperatures.

If the design temp is 90°F and at that temperature 35,000 Btu's is just break even with a 74°F indoor temp with 50% RH, then the fact is that there is 35,000 Btu's of energy coming into the home every hour and the unit runs continuously to remove that 35,000 Btu load (both sensible and latent). Now let's run the temperature up to 80°F for the day and set the 74°F for a 5pm arrival home from work. The savings of the setback (setup) temperature of 80°F is comprised of both the dwell time between a call for cooling at 74°F and a call at the new temperature of 80°F, as well as the temperature difference by maintaining a higher temperature during the day. So now the outdoor temperature reaches design conditions at 1pm. How many Btu's of capacity must the system provide to maintain 80°F and 50% RH? Something less than 35,000 Btu's because we're only holding 80°F, not 74°. So let's assume for discussion it's 30,000 Btu's.

Now it's nearing 5pm and time to decrease the temperature to 74°F. Since that is a lower ambient indoors, the relative humidity will begin to increase as the system tries to bring the temperature down. This is where the potential rub comes into play. As the indoor temp comes down, the RH is trying to move in the opposite direction and more of the Btus are being diverted to latent work than sensible work. In all likelihood, the bulk of the 5,000 Btu's saved per hour at 80°F is going to latent heat removal now, thus leaving a system sensible capacity to pull down the temperature to 74°, all while battling a temperature influx of 35,000 Btu's/hour.

So the answer to the question of do I or don't I is really an answer predicated on how much excess capacity there may be in the system at any given outdoor set of conditions up to design temperature. The closer the system is to design size, the less 'excess' capacity is available to return to the occupied set-point. This is just as true for heating as for cooling, except you don't have the diversion of Btu's to latent work. But as the OAT approaches design temp on heating, the amount of excess capacity available to raise temps from set-back is less and less. Many people have discovered this limitation on setback t-stats and just as many have abandoned the use of set-back due to discomfort at near design temperatures. In fact, the DOE has entertained removing set-back t-stats from the rebate program due to lack of proper use and the resulting insufficient energy savings.

The best savings for heat are the systems that actually reference the outdoor temperature and control the output of the heating system accordingly. Thus they provide the necessary Btu's under all conditions without the need to actually vary the room temperature and it's all done without the homeowner/building manager needing to thoroughly understand the needs and limits of the heating equipment. We're now beginning to see this in the variable speed heat pumps that are available, where they can deliver an infinite number of Btu settings from 30% of maximum all the way up to or slightly above their rated maximum, thus allowing for larger systems to be installed for extreme temperatures for heating but still have the smaller outputs needed for proper cooling and latent heat removal.

[54regcab]

Correctly sizing equipment is the way to go. Unfortunately its oversized more often than not, especially on the heat side. I have yet to see a properly operating furnace that can't keep the setpoint, regardless of how cold it gets. The analogy of not letting the truck run all night so it will be warm in the morning is valid in oversized situations..

DOE is removing setback thermostats because they are rarely set correctly, not because they don't work. We are entering a time of variable electricity prices/smartgrid technology. With this, using your house to "store" cool air made with cheap power then raising the setpoint becomes an effective strategy. Once the power peak is over, drop the temperature back down.
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