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.
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Originally Posted by CraziFuzzy
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