The Tiburon home originally had natural gas heating in 3 separate zones. It also had all of the hot water used for washing, showering and sinks warmed from natural gas. Finally the cook top stove burners were natural gas. The home is still divided into 2 separate addresses 1 Reserva Ln is the main home and 3 Reserva Ln is the apartment. One of the heating zones/one furnace and one of the 2 hot water heaters were dedicated to the apartment.
Overall natural gas usage for the home including the apartment was between 100 -250 therms depending on time of year; we used more natural gas in the winter with the maximum months being January and February.
We combined the hot water systems into one where solar thermal collectors warm a 120 gallon tank to preheat the water and then follow this with natural gas fired tankless water heaters. This system had a few wrinkles but for the last year has worked flawlessly. The system elements were designed by Heliodyne and the system was installed by SunFirst!
Over the past year we used 148 therms for the entire year. We estimate that the savings are about 88%.
But we expect this to get better still because the heat pumps we installed for home heating in the winter we were unable to control initially when they called for natural gas as a back up heat source. But now that is fixed. Given this change we anticipate that a normal year of natural gas consumption should be below 100 therms for an overall savings of 93%. 100 therms is the equivalent of 2900 KW-hrs so if we make this final target then the home is pretty close to carbon neutral on a overall basis.
Here is the graph for overall natural gas consumption –
As you can see the already low usage drops even further during the sunniest of months from April through October.