I finally understand indoor reset

Dan_15

This article was a great primer for me on the basic theory of outdoor reset and indoor reset.

http://www.pmengineer.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2732,21624,00.html

Im sure you pros already know this stuff cold, but I never could get a handle on it. I would love it if anybody could point out other good articles to learn about the subject. My question is this: how does the indoor air sensor that is programed for night setback work? During night setback does the reset curve just get notched down proportionally, or is there something more complicated to it? Can I expect that the system will still be in a constant circulation mode during night setback, but just at a lower temp? I am trying to get a handle around how the indoor air sensor works versus a standard on/off thermostat. More specifically, I am trying to understand the process of recovering from night setback. Will there be any difference in energy consumption using indoor reset versus the on/off thermostat? i.e., if I understand correctly, indoor reset will cause the water in the system to continue circulating all night at a moderate temperature, and may only need a small delta in water temp to recover from setback in the morning; whereas the on/off thermostat will let the water temp drop significantly until a call for heat to maintain setback temp; and then it will blast the water temp in the morning to satisfy the call for heat to recover from setback. It seems like the indoor reset will use less energy than the on/off thermostat. Is this correct?

By the way, I called Buderus techs a couple of times to ask about this question and despite their their reputation I found them to be really standoffish to homeowners, like I was not part of the club. I am definitely not part of the club, but I just want to learn about what I am buying.

Mike T., Swampeast MO

Will Try

Will confine myself here to "real" indoor reset--for example what is used by the new Buderus condensing/modulating boiler.

"Real" indoor reset involves a thermostat (or other sensor)that has TWO-WAY communication with the boiler. A higher-mass system (say tube-in-slab or iron radiators) can achieve a sort of indoor reset if the thermostat is the ONLY thing controlling the firing of the boiler. During periods of lower heat loss, the boiler will simply stop firing before it achieves a high temperature as the high-efficiency emitters have done their job. IMHO there is no such thing as "real" indoor reset with an on-off boiler.

The essence of "real" indoor reset is a vector. A vector is nothing more than a force with both magnitude and direction. The indoor reset controller must know three things: the desired room temp setting; the actual room temp and, most importantly, the rate of change between the two. An indoor reset control will learn over time--VERY similar to the "adaptive recovery" feature in Honeywell thermostats. When teamed with a modulating burner, the indoor reset control will strive to keep the burner firing and the circulator running as much as possible. It does this by varying the fire to keep the actual temperature equal to the desired. Once either desired or actual temperatures vary, the boiler will change its firing rate to a degree that depends on how fast that change is occurring. In this way it can equal (or even surpass) outdoor reset in its ability to perfectly adapt. The biggest difficulty with indoor reset is finding the ideal location for the indoor sensor as it will affect the system as a whole. Poor balance in the emitters will be magnified.

During night setback does the reset curve just get notched down proportionally, or is there something more complicated to it?

Manufacturers do guard their algorithms. Since these are learning programs, I would suggest that the "better" programs look at the desired setback temperature and adjust the supply temperature to the calculated maintenance level. This is VERY different than just stopping the boiler from firing until the space temp drops to the new setting! With deep setback it is HIGHLY likely that the space temperature will NEVER drop to the setback temp! In other words, this becomes "smart" setback! It REFUSES to let you use setback inefficiently!!!! This is a WONDERFUL ability that can come ONLY with indoor feedback!!!!!