In June 2009, my former company, Cedar Mountain Solar, began designing a solar heating retrofit for a residence in the foothills in Placitas, New Mexico, near Albuquerque. This house has approximately 5,000 square feet of living space, which was heated by a propane boiler and a hydronic system embedded in the concrete floors. The building is well-constructed, with good heat retention. It is in a high altitude mountain climate where freezing temperatures and snowstorms are common in winter.
This solar heating retrofit is typical of what I call “Combi 101,” which includes several specific heating system functions (connected with a primary loop): solar heat in combination with a boiler, a domestic water heater (DHW), and radiant-heated floors throughout the house.
By October 2009, 12 SHW collectors had been installed on the roof and the heating system was converted into a solar “combisystem,” with all of the heat sources connected to all of the heat loads. Even though much of the roof is covered with solar collectors, they are mounted in low-profile to reduce their visual impact. The system has been showing good fuel savings for two heating seasons to date. Heating fuel consumption has been reduced by more than half, with the savings estimated at more than $3,000 per year.
The idea of adding solar collectors to a home often proceeds along the same lines: First, homeowners consider a solar water heater with one or two collectors for domestic water heating. Then, they may consider adding heating to a chilly room—maybe more collectors would be worthwhile. Then they consider hydronic baseboards or make connections to heat other rooms. Then, they wonder about solar heating the spa or pool, an ice-melt zone, or some future addition.
When multiple sources of heat are connected to multiple heating jobs, we call them combisystems, since it is a single heating system made up of a combination of different kinds of equipment. When one of the heat sources is solar heat, we call it a “solar combisystem.”
Multiple heat sources and heating loads can be connected in many ways. In the Southwest, the most typical solar-hydronic combisystem includes solar collectors, a gas boiler backup, a domestic water heater, and a hydronic floor. This most basic variant includes only four items: two heat sources and two heat loads. Yet, if you present these requirements to three different solar heating suppliers, you will get three very different designs with heat exchangers, water tanks, tees, motorized valves, and pumps in different locations—and some often cryptic control strategies (or none at all) to complete the confusion. Adding features or changing the heating system requires a redesign with different piping connections, different components, different temperatures, and different controls.