Enough with the Fluff!!

OK.  So the last several posts have been on the fluffy side and it's high time I try to put that engineering degree to some use (my parents will be so proud).  I know many of you have been wanting more details on the mechanicals and I have been holding off writing about the performance until we had a few good months of operation under our belts, so to speak.  

Here's an overview...we have solar thermal collectors for hot water generation.  The heated glycol from the collectors goes to a 80 gallon Steibel-Eltron tank, that has dual heat exchanger coils.  The bottom coil is for the closed solar loop and the top coil is for the backup heat (also a closed loop), which is provided by a Takagi on-demand water heater.  Hot water in the tank supplies domestic hot water to the house and hot water for the radiant heat system (after going through mixing valves to cool the water prior to entering the PEX water lines).  This system is called an "open system" because drinking water is going through our radiant lines.  The radiant system consists of three loops in our concrete slab on the first floor and "staple-up" radiant tubing on the second floor.  The slab thermostat is a floor sensor that keeps the floor around 69 degrees and the second floor radiant is controlled by a typical thermostat.  We have a third set of radiant loops located deep within the three-foot earthbox under the slab.  The purpose of this set of tubing is two-fold.  First, in the transition months (Spring and Fall) when plenty of sun is available (and thus excess hot water is produced) we send hot water into the earthbox radiant loops, essentially "storing" that heat in the mass where it will passively help to heat the house.  This Spring we found this greatly reduced the amount of time the slab radiant heat turned on (and thus reduced the use of propane).  The second purpose of the earthbox radiant is simply as a heat dump for the system when the tank gets to hot.  Minimum temp for the tank is set at 120 degrees and the max temp is 165 degrees.  We have found that the heat dump will occur once on a really sunny day and only when I'm not using a bunch of hot water (i.e. laundry day).  The dump results in heat going to the earthbox for approximately 20 minutes.   We have NOT seen an increase in the slab temperature (or interior house temperature) more than 1 or 2 degrees due to this, even this week when we had two days over 90 degrees.  This can be attributed to several factors; first, the huge thermal mass of the earthbox and slab, and its ability to absorb the extra heat.  Secondly, we positioned the solar panels to optimize the winter sun (they receive more shading in the summer than winter due to their position next to the house), thus reducing the solar gain and heat generation in the summer.  Lastly, like photovoltaic panels, evacuated tube solar collectors are more efficient in cooler temperatures.  Radiant Floor Company designed the system and calculated the energy loads.  They also supplied all the parts and we hired a heating contractor, Paul Martin, to install the system and the solar collectors.  We did not take advantage of the CT solar thermal rebate program, because we already had a relationship with Paul and did not want to switch to a CT "approved" installer to get the rebate.   Paul is a distributor of  SunMaxx Solar thermal systems.  The evacuated tube solar collectors (hot water generation) were installed in January and it took about 6 weeks to get the control settings and sensors placed properly and get the system running optimally.  Until then, we were burning through an uncomfortable amount of propane.  By the middle of Feb. we were seeing excellent results.  We used about $145 worth of propane in Feb and $90 worth of propane in March, and the last propane fill up was on April 19th for $79.  Since then the propane system has only run briefly on one rainy cold day.  Keep in mind that propane is our backup energy source for hot water for BOTH domestic hot water and space heating in our radiant systems.  We are a family of four.  In a typical year, we would likely use more propane, but it has been unseasonably warm this spring.  All in all we are thrilled with the performance of the house and mechanical/solar systems.  I hope to include some graphics/tables of our energy use in future posts.