Heading Toward Zero

Coming to the last few days of 2010 spending it at the house. Looks like we'll be right at net zero for calendar 2010. December has been colder than average with few days above freezing but a good deal of sun for solar gain. Ambient away energy use was around 20-25kwh/day as we keep the place at 53 when not here. That is on target for my rough formula of 1kwh per HDD.

Since we've been here, we've been using about 65kwh/day of which about 45 is geo heating. We keep it at 69 (I'd be fine with 66-68 but others have differing opinions). That still maintains the 1kwh per HDD. As usual, we've been using the fireplace at night which generally carries the heating load until midnight. Overnight, the geo's runs have been interesting. Its cycling on for 10-15 mins and then off for 5-10 mins and stretches those off cycles out as it gets further into the night. Meaning that, even though the temp is dropping, its mostly bringing the slab up to temp which heats into the morning. Since I turned the geo target temp up to 105 its performed much better at getting to and maintaining 70 when we need it. I haven't actually noticed much more energy usage vs. when it was lower last winter. So our cycles are longer and less frequent when it needs to heat the slab from relative cold (when we're not here and maintaining 53) and shorter and more frequent when keeping the slab up to temp at 70. I also insulated several of the radiant pipes in the mechanical room which is now not as hot and thus transferring that energy into the slab.

One interesting observation was that I thought we were saving a couple of cycles and thus energy by using the fireplace at night. That might not be linear savings. The reason is that the geo doesn't end up running for 6-8 hours at night and the concrete slab gets cool (except around the fireplace). When the temp does drop after midnight, the geo has to cycle longer and more frequently to pick up the slab temp. One night I didn't use the fireplace. When I checked energy use for geo in the morning, I found it was about 20% lower than each of the previous few nights where the temp was consistent around 20 degrees outside. Checking the run pattern, it cycled on notably less and that could only be due to it only having to maintain the slab temp vs. a pickup due to the fireplace keeping it off. Thus, geo and radiant do work if you let them. Now I'm wondering if I should only use the fireplace when we need quick pickup. Doubtful since the aesthetic and entertainment value is still so high.