Understanding Setback. This is a long question, not a lesson.

Jean-David Beyer

There seems to be disagreement about setback.



There seem to be two considerations, and I understand only one of them.



If I consider a house as a thermally leaky box, then from a heat loss standpoint, you should always use setback when you can tolerate the lower tempreatures, such as at night when you are under warm blankets, preferably with a regular sleeping partner, dogs, or "heater cats." This is because the heat loss will be less as the difference between inside and outside is reduced. Furthermore, the heat loss should not go above the heat loss at the normal temperature as the house recovers from setback. In the simple model I just described, it would not take more heat to recover from the setback than was saved by doing the setback. Yet many people claim that it does take more heat to recover than to never setback at all. With just a high school physics knowledge of the situation, these people are crazy.



But if so many professionals believe this,  <strong>let me play with the idea that they are not crazy.</strong> If it takes more BTUs to recover from setback than it does to just maintain the un-setback temperature, where do they go? In a forced hot air system, I see no where, so perhaps setack is effective for these systems, and you should always do it. Note that the the heat transfer medium here is pretty low mass (the air). In the case of hot water heat, it is relatively high mass, or in the case of gravity circulation, very high mass. Likewise, I imagine the water in a typical steam boiler is fairly high mass too. The only steam boilers I would consider low mass are the ones in Stanley Steamer automobiles, steam irons, or my boiler type humidifier where only a very small part of the water is heated.



With a hot water boiler, at least with mine, the circulator to the house starts as soon as the thermostat calls for heat. The fire starts after the pre-purge interval that is 20 seconds by default. My boiler is low mass (3 quarts of water), and burns up to 80,000 BTU/hour, so it comes up to temperature very fast. If my emitters were all low mass, such as finned tube baseboard, and if I used no outdoor reset, all the heat would go into the house. So I should use setback. For comfort reasons, I do not because the reset is so extreme that recovery takes 4 hours or more in the baseboard zone, and it is ridiculously bad in the concrete slab radiant zone. But if I were willing to put up with that, it still seems as though setack does not cost more than no setback. It should still save fuel.



 I do not know much about steam systems. It seems to me that from a setback standpoint, it should not be all that different. It would be making steam for a shorter portion of the time when setback is in effect. And that should save fuel. During recovery, there is the cost of raising the boiler water back up to the boiling point each time, you recover from setback, but it would do that anyway when running without setback. And this cost should be pretty small compared with boiling the water to steam. 1 calorie per gram to raise the water temperature 1C up to the boilnig point, and 540 calories per gram to boil the hot water to steam. So that is not important either.



So where is the thermal cost of recovery from setback supposed to come from that occurs from setback recovery that is not already present when maintainig the non-setback temperature?

Mike Kusiak_2

Setback and efficiency

First of all let me say I completely agree with you as to the idea that it will never take more net energy to bring the house back up to temperature after setback than to maintain constant temperature. However, it is possible that may take more fuel in the recovery situation.



In the forced hot air case with a non condensing furnace, there probably would be very little difference and setback makes a lot of sense.



But in your situation with a modcon the results may be different. Remember the graph I posted a while back showing the reduction in boiler efficiency as firing rate increased? If you are just maintaining constant temperature, you are probably at low fire and fully condensing with efficiency in the 95-97% range. But if you are recovering from a deep setback, the boiler may go to full fire and the efficiency can easily drop below 90%.



So in this situation it is possible to burn more fuel with setback than just maintaining constant temperature.

Mark Eatherton

A$$umptions...

I think when the DOE came up with the set back suggestions, that they were assuming a LARGE infiltration factor. And I am certain that they did NOT take ODR into consideration in their recommendations. The reason that your homes systems are poor in recovery is because the programmable ODR on your boiler does not speak the same language as the thermostats. The thermostat speaks ON vs. OFF, whereas the boiler thinks digitally (ODR).



My position on this echo's Mikes. It CAN'T cost more energy than it saves, but it CAN cause more fuel consumption at a lower efficiency rate due to recovery.



Personally, if your home (WHAT ever type of distribution system you have) DOES have enough infiltration to show savings with set back, it is an indication that some quality time needs to be spent on caulking the home tighter. Not TOO tight mind you, but it needs tightened up. That will show a great return on the investment than will a set back thermostat, AND it will make for more comfortable occupants, which with a few exceptions is the business that we're in.



Conservation without comfort, is a losing proposition...



Also, don't forget about the difference in heating systems. Forced error systems have little to no effect on the MRT within a given space. A good RADIANT system substantially effects MRT, but has little effect on air temperature. In fact, with a perfect system, you WILL have to maintain a lower air temperature in order to maintain good human comfort when the MRT is elevated. So, what is the net effect on a tight home with a good RADIANT heating system? The minimal loss at night from set back doesn't warrant the slipping clutch syndrome (recovery and acceleration) hence little to no savings from setback IN A GOOD TIGHT RADIANT HOME...



As with any heating question, the only correct answer is "It DEPENDS"...



The only time that a deep set back makes sense in a radiantly heated home is if the space will be un-occupied for an extended period of time. And then, you WILL want someone to turn the temperature up LONG before your arrival.







ME