Example project: Combi 101 retrofit in Canoncito, N.M.

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

Existing hydronic (hot water) home heating systems represent a largely untapped market for solar heating installation. Solar retrofits are especially attractive for the following reasons:

• The hot liquid temperatures required by hot water boiler heating systems are directly compatible with the temperatures easily provided by solar thermal collectors.

• Existing hot water radiant masonry warm floors can be used directly for solar heat storage.

• Other important heating system components can be reused, often without modification.

• The whole retrofit operation can sometimes be accomplished in a matter of days (rather than months or years with new construction projects).

Finding somewhere to put the collectors without spoiling the look and feel of the building and surrounding landscape and fitting the new solar-compatible equipment into the old mechanical room are among the biggest challenges with retrofit projects

The example project presented here is a good illustration of how the art and science of solar heating can be applied to a retrofit. The owners of this house already had radiant heated masonry floors heated by propane. Solar heat has become a reasonable alternative to the steadily increasing cost of propane. The new system was designed to provide a substantial reduction in propane consumption and to fit on the house with as small an impact on the existing home design as possible, both outdoors and indoors.

This project makes good use of a typical solar combisystem that I call Combi 101. As I have mentioned in previous articles, a Combi 101 system is the most basic solar home heating system; it includes solar collectors, a backup boiler, domestic hot water tank (DHW) and hydronic radiant heated floor zones, all connected together. This example project fits the Combi 101 profile nicely. The conceptual piping plan used on this job can be seen in Figure 42-1.

The existing house

The house has just over 2,000 square feet of heated space, is single-story, well-constructed, but with high ceilings in the center. (See Figure 42-2) The original hydronic heating system included four heating zones with separate room thermostats and zone valves feeding a total of 1,250 square feet of masonry radiant warm floors.

The only heating fuel available is propane, and the original gas burner was a Lochinvar DDL075 that provided both potable DHW and boiler fluid for the floors (seen on the right in Fig.42-2). This resembles a conventional 75-gallon gas water heater tank, but has a heat exchanger coil built into it, so that boiler fluid can be heated indirectly by the hot potable water in the tank. This was an atmospheric burner with an AFUE burner efficiency rating of only 82 percent, operating at high altitude, with a de-rating for 7,000 feet elevation. The burner required a large flue pipe and large fresh air vents feeding the boiler room.

Roof-mounted solar heat collectors

The house was built with passive solar features (large south windows), so a large portion of the roof had a good southern exposure. The roof was built with a shallow pitch and covered with corrugated metal. Ground mounted collectors were considered, but rejected because of the added difficulty posed by the trees, landscaping and underground piping required.

Figure 42-3 shows the collectors installed in this project. The small modular collectors were chosen for several reasons. The shape of the collectors tends to mimic the existing windows resulting in a softer appearance that is less monolithic than with larger panels. The panels have a low profile that fits below the highest peak of the existing roof, which also helps to moderate the dominating appearance of the panels on the building.

The panels plug together easily side by side to allow a simple flow path using horizontal headers and vertical risers, all hidden inside the collector frames. This allows the easy installation of thermosyphon cooling fins on the back of the collectors, which were used here to prevent overheating during the summer and fall. The panels are the SS16 model made by SolarSkies and have the added benefit for the installers of being easier to lift up a ladder than larger panels.
Existing equipment to keep or replace

When burning propane at high altitude in this home, we determined that a sealed-combustion condensing boiler would provide better fuel economy. We found that a wall-hung condensing boiler would fit in the limited space provided in the mechanical closet; the installer, Jeff Stampfer, chose the Knight WBN106 with an AFUE rating of 95.5%. To control the heat loss from the DHW tank, a Triangle Tube indirect water heater tank was chosen that can be heated by solar or boiler using its tank-in-a-tank heat exchanger, and has no heat loss from a central flue pipe as the original tank did.

The use of a sealed-combustion boiler eliminated the need for the existing large metal flue pipe and fresh air vents that were originally installed from the boiler room up through penetrations in the roof. Since these vent pipes were installed with metal flashing as a permanent weatherproof part of the finished metal roof, it made sense to use them as conduits for our new boiler flue pipes and solar glycol supply and returns. As a result, we did not need to cut any new penetrations in the roof to make these connections.
A number of heating system components were kept and reconnected with little or no modification. This includes the gas supply pipes, the boiler refill and air eliminator parts, the heat distribution tubing and manifolds, including the zone valves, and the existing four conductor thermostat wires.

Results in the mechanical room

Figure 42-4 shows how the final components fit into the existing boiler room. This was a very small closet in the center of the house, so remodeling it and even photographing it were something of a challenge. Our installers from Eldorado Solar did a good job of placing the tank, the boiler, the solar heat exchanger and the new circulator pumps in positions where they could be reached for installation and service. Because the primary loop is mounted horizontally near the ceiling, all the components can be connected with reasonable accessibility in this tiny utility closet. While still a crowded space, this installation would have been much more difficult and much less serviceable without the ceiling mounted primary loop.
The control system for this installation is the SolarLogic Integrated Control, known as the SLIC. It provides comprehensive control over every heating component in the house, including heat storage in the masonry floors and remote access over the Internet. I will describe the control system in more detail in an upcoming column.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.