Documenting my attempt to cut my energy usage in half.

Monday, June 27, 2011

A Flatlined Hot Water Tank

One of the projects that I've been considering implementing for the house is a water based solar space heater similar to the $2K System that Gary over at BuildItSolar installed.  The basic idea is simple, gather heat from the sun during the day, store that heat in a large water tank, and distribute that heat into the house when needed. One of the issues that needs to be resolved before I jump in and start construction is what to do with all that heat during the summer.   From what I've seen on BuildItSolar and on the SimplySolar yahoo group is that most people use these system for both space heating and as a pre-heater for their hot water.  I would like to use all of the heat during the winter for space heating, but during the summer when I could make hot water, I wasn't sure that I needed it.  I suspected that my ground-source heat pumps with their desuperheaters provided all (or nearly all) of my hot water during the summer.  If that's the case, then my ROI would be significantly lower as the system would only be used during the heating season and would be stagnant for all of the cooling season.

Testing the desuperheaters

To find out if the desuperheaters could handle the entire DHW load I ran a couple of tests.  The first one was simple... turn off the hot water heater at the breaker.   After a short prayer service, the switch was flipped and the hot water heater flatlined at noon on May 24.

The graph above shows that the hot water heater really has been off for the last month.  The tank temperature seemed to be ok as all of the showers continued to be hot.  But to be sure, the second part of the test was to install a temperature sensor on the hot water tank.   The sensor is a DS18B20 and is installed against the inside tank wall underneath the insulation near the upper thermostat.  The sensor readings were spot on with the temperature of the water that I was drawing out of the tank, so now it's just a matter of logging the data every minute and graphing it.

The manual that came with the Water Furnace heat pumps says that the desuperheater circuity kicks out at 130F so I should never see tank temperatures above that.  The 5 day graph above shows that the tank temperature slowly drops during the early morning when the heat pumps run infrequently, but the temperature recovers nicely during the day and if it gets above 90 for any length of time during the day, there's a good chance that I will max out the tank temperature.  The big dip during the day on the 26th was a shower immediately followed by a bath just to see what would happen.

What I've found from this is that my desuperheaters can provide all of my hot water during the summer months as long as some thought is given to when things happen.  So, that brings me back to my original problem.  If I build a water based solar collector, what do I do with it during the summer months?



  1. I have a Water Furnace with a desuperheater and have been wondering how well it actually performs. I'd like to talk about what you use to take all the temperature sensor readings and how you record the data.

  2. I use DS18B20 temp sensors and a home built monitoring system that is detailed here.  The only thing that has changed is that the Parallax microcontrollers have been replaced with Arduino boards.