Alternative backup heat sources

BY BRISTOL STICKNEY
contributing writer

In June of 2009, we began designing a solar heating retrofit for an existing residence in the mountain foothills outside Albuquerque, New Mexico. This house, seen in Figure 32-1, has approximately 5,000 square feet of heated living space, and the entire building is heated with existing hydronic radiant concrete floors. The only heating fuel available is propane. The original conventional “hot water boiler” proved to have a voracious appetite. The owners wanted to cut back on propane consumption and to get better control over the energy efficiency of their home. Fortunately, the building is well constructed, with a reasonably good level of insulation and heat retention.

The large thermal storage capacity of the existing concrete floors allowed us to design a relatively large solar heating system into the building without any additional large water storage tanks, except for one 115-gallon domestic hot water (DHW) tank. Twelve solar collectors, pumped in groups of six, were installed on the house and, by October 2009, the heating system was converted into a solar combisystem and was ready for the coming winter heating season. In the photo, you can see that even though much of the roof is covered with solar panels, they are mounted in low profile to reduce the visual impact on the appearance of the building to a minimum.

Flow-center piping

In many of my previous columns, I have described the use of a primary loop that ties all the heating sources and all the heating loads together in a home heating system like this. I have used the “Combi 101” configuration that includes solar, boiler, DHW and warm mass floors to illustrate this approach. In this retrofit, we use this same approach with a slight twist. The installers preferred using a Caleffi Hydrolink because it eliminates the need for assembly of the primary loop on the job site. The Hydrolink 2+2 model was configured to provide the same piping and heating functions that a primary/secondary system would have done. The result is a piping system that resembles a Combi 101 heating system with a more compact “flow center” and a larger number of collectors (12) and heating zones (8).

SLIC Beta system control and monitoring

This heating system was originally designed with a conventional control system using (8) Venstar 1045 two-stage heating thermostats, (1) Taco SR504 switching relay enclosure, (1) Taco ZVC404 zone thermostat/valve controller, (1) Taco ZVC406 controller, (1) Tekmar 155, (2) Tekmar 152 relays, and sensors. This control package is similar to the Combi 101 controls mentioned in an earlier column, with the added capability of night cooling control functions. Because of the owner’s proficient technical background and willingness to become a Beta test site, we redesigned the control system at the last minute during installation to include a Solar Logic Integrated Control (SLIC) system.

The SLIC solar home heating control system has been under development and Beta testing for a couple of years now. We designed it to replace all the relays and temperature controls mentioned above by a single relay box with built-in software control. From the user’s point of view, it is simple to operate just by turning the room thermostats up or down. But internally, it allows us (and the owner) to monitor the performance of the heating system, measure and record its performance continuously in data files and adjust system settings locally or remotely over the Internet. This has proven to be practical, useful and informative, especially during the first year of operation for this retrofit.

Fuel saving strategies for heating

Twelve 4'x10' collectors are installed, each capable of saving as much as 1/2 gallon of propane on a clear sunny day. The savings achieved in this installation is not only ?from solar heat gain but also from the installation of high efficiency combustion and the addition of heat saving control strategies, which can be monitored and adjusted as needed. Solar priority over the boiler is provided both by the piping configuration and by the control logic. Solar direct heat for the floors has a priority over heat storage in the water tank. Heat storage is provided by the mass floors and controlled by the SLIC controller, using virtual two-stage room thermostats under central software control. Heat storage is also optimized in the DHW water tank and recirculator by software control. The SLIC controller is programmed to save heating fuel in every way possible such as stranded heat recovery and intelligent priority control, based on temperatures and critical loads.

Electrical saving strategies

The opportunities for electrical savings in a heating system, although sometimes small, are always worth considering. Here are some of the steps taken on this job. The boiler circulator is disabled when solar-only temperature is available. Multi-speed circulators are used and set to the lowest speed that is effective for each job. The minimum number of transformers is installed to eliminate the “phantom load” that transformers consume. “Latching” zone valves are used that require no power when fully open or closed.

There is no primary pump; all circulation through the flow center is provided by the secondary pumps that supply flow from the heat sources to the heat loads. Solar circulation for collectors using closed glycol loops can be achieved with very small pumps and/or solar electric PV pumps.
Overheat control and cooling schemes

The SLIC controller is programmed to prevent solar overheating and to maintain safe high limits and comfort temperatures in a variety of ways. The DHW tank is used as a heat accumulator by day and can be cooled through the solar panels by night, if needed.

The solar glycol high limit temperature can be controlled with cool fluid from the garage floor (or other floor zones) when needed. The floors in the warmest rooms in summer can be cooled by night circulation through the solar panels.

Report from the owner

The heating fuel consumption for this house has been carefully recorded by the owner, before and after the solar heating retrofit and dating back ten years. The results show unmitigated success in consistently reducing the heating fuel consumption after the solar retrofit. Between 2004 and 2006, some fuel savings was achieved using thermostat setbacks with the old boiler but, because some of the rooms became uncomfortably cold, the thermostats were raised to around 65°F between 2006 and 2009.)
The owner’s analysis of this data includes some interesting highlights. Propane use has been reduced from about two – three heating degree days per gallon of propane before the retrofit to about five – seven heating DD/gallon after the retrofit. For domestic hot water, propane use is down to 0.6 gallons a day, versus the historic rate of 1.5 gallons/day. The house has typically netted 80 to 110 kWh/day (273 – 375 kBtu/day) of solar heat this winter. Annual propane consumption has dropped by about two-thirds, which amounts to about 1,300 gallons per year saved. This is worth more than $3,000 per year at current local prices and will be worth more as the price of propane goes up.

As an added bonus, the owner has noted that the energy savings documented here was achieved in spite of an increased room temperature setting of 68 F (up from 65 F). So, along with significant fuel savings, the occupants are enjoying increased comfort levels.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 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 any 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., and is involved in training programs for solar heating professionals. For more information visit www.solarlogicllc.com.