A simple solar domestic water heater can easily be switched into solar-only mode by turning off the conventional (backup) heat source. The solar heat then provides as much hot water as the sun allows; on cloudy days, the water temperature may drop below normal settings. This may be acceptable during periods of low occupancy, low hot water usage or when the owner wants to eliminate the consumption of backup fuel for a while. 

Similarly, we can switch a solar-heated building into a solar-only operation, but a typical hydronic solar combisystem is more complex than a simple water heater, so the control system must have quite a bit more intelligence. Not only the water heater, but the kitchen, bathrooms and other solar-heated spaces themselves must be freeze-protected. 

When there is an over-abundance of solar heat, it must be dissipated safely. The conventional heat (boiler) and circulator pumps must be controlled properly so they do not run excessively or in opposition to the solar-only control requirements. 

When in solar-only mode, modern solar heat controllers have become more fail-safe, often providing the capability to prevent solar overheating and offering freeze-protection of some kind. It is especially important when systems are unoccupied and operating without human oversight.

When controlled properly, solar-only operation opens up attractive new possibilities in solar-heated building applications. The idea of heating a building on-demand with only solar can be especially appealing to building owners with intermittent occupancy patterns. This includes second homes, vacation homes, meeting spaces used primarily weekly or monthly, and buildings with predominantly seasonal usage. 

We have been involved in several solar-heated buildings in recent years that were designed to provide solar-only control on demand and by remote control. Let’s take a look at a real-life example: a residence in the ski country of Colorado occupied mostly on weekends. 

This is a 3,780-square-foot house entirely heating with a radiant system to warm the masonry floors. As a recent construction, it is relatively well-insulated, using solar hydronic heat with propane backup fuel. The owners began using the solar-only control settings regularly since the first winter ski season arrived. 

When occupied on weekends, the building uses solar with backup heat to provide average comfort temperatures, but when unoccupied during the week, temperatures are allowed to drift, driven only by available solar heat. The system has been in continuous operation for a little more than five years. The continuous data-logging function provided by the control system allows us to view the results remotely over the Internet. 

Primary Loop Solar Heating System

The heating system in this house is a solar hydronic combisystem. It is installed using a primary loop to connect all the sources of heat to all the heating loads in the house. This allows solar heat and boiler heat to supply both domestic hot water and space heat to the entire home under central control. This heating system is designed following the principles and standards often described in this column. (Please refer to earlier columns for more details.) 

This heating system includes the following familiar major standard features:

Solar heat collectors. A single group (bank) of eight flat-plate panels provides a total of 256 square feet of nominal collector area. A glycol/water mixture is pumped through them by a single circulator pump. It delivers solar heat to the building by day through a brazed-plate heat exchanger attached to the primary loop indoors.

Mod/con boiler. A condensing hot water boiler with a modulating burner provides high-efficiency conventional backup heat to supplement the solar heat. 

DHW indirect tank. The domestic hot water tank is an indirect type (120 gallons), using an internal heat exchanger immersed in the potable water to allow heat from the primary loop (from either solar or boiler) to provide DHW.

Masonry radiant floors. The heated spaces in the house are divided into 10 hydronic heat distribution zones with individual thermostats. Nine of these zones consist of well-insulated masonry radiant floors controlled by separate zone valves. The thermal heat storage capacity of these mass-floors is used directly as solar heat storage for the solar collectors. There is no other solar heat storage system other than the thermal mass of the floors and the DHW tank.

Integrated intelligent control. The Solar Logic Integrated Control (SLIC Gen II) has been installed on this heating system to provide all the required operational functions in a central control unit. This includes zone-by-zone heat accumulation of the masonry floor, freeze and overheat protection, heating load priority management, solar priority, data recording, data display, network connectivity, system adjustment and remote control.

Solar Settings

One of the user adjustments built into the SLIC control system is the solar-only setting. Each room thermostat connected to the SLIC can be set in this mode manually using the “Solar” switch to provide energy savings on a room-by-room basis. A second alternative is to use the “Whole House Mode” available through the computer interface that allows all the room thermostats to be switched to solar mode simultaneously with a single click. 

In this case study, the homeowners have set the Whole House Mode to solar-only during the week when the house is not occupied. This mode of operation has built-in freeze protection. So, the boiler will fire to heat any room that drops to 45 F to keep it from freezing. 

Also, to save on DHW heat during the week, they have set-back the thermostat on the water heater, allowing it to drop as low as 70 F when solar heat is not available. Figure 66-1 shows how the system responds during one week of cold weather in November when solar-only settings are used. 

On weekends during occupancy, the thermostats are turned up a day or so ahead of time by the owner using remote Internet control. The rooms are set at 68 F and the water heater is set at 125 F for standard occupancy. The boiler maintains these minimum settings and the solar heat raises the temperatures higher whenever it can, within a reasonable comfort range. 

During the week when unoccupied, the solar-only mode is activated and the temperatures drift downward, but with a daily temperature boost driven only by the available solar heat.

Energy Savings

The graphs in Figure 66-1 show three days of normal occupancy temperatures followed by four days of solar-only unoccupied settings. Then the controls are returned to regular operation for three days, causing a heat recovery period that lasts for much of the first day, followed by two days of regular activity. Then the solar-only mode begins again. Outdoor temperatures (not shown) dropped below freezing every night during this 10-day period.

Solar heat storage in the floor can be seen as a “bump” in the room temperature each sunny day that goes up by day and down by night. The size of this temperature bump can be adjusted using the control system to match the comfort requirements of the occupants. This slight rise in the room temperature can (and does) prevent the boiler from running, sometimes for the entire 24 hours. 

This is what direct solar heating of the masonry floors is intended to do — reduce the boiler run-time over a 24-hour cycle.

Energy savings provided by the solar-only mode can be confirmed in Figure 66-1 by the fact that the boiler run-time during the four-day solar-only period is virtually zero. We did a quick inspection of the weather data and found that the boiler typically runs for one-quarter of an hour for every “heating degree day” under normal operation at this time of year. 

That translates to a savings of about 39 hours of boiler run-time during those four days. This modulating boiler is capable of consuming around 1/2 to 2 gallons of propane every hour, so significant fuel savings will add up quickly in cold weather.

The heat recovery time is interesting to see in this example. The water heater recovers its normal operating temperature in just a few hours. The masonry floors, however, take about half a day to return to normal. This heat recovery cycle requires about as much heat from the boiler as the house uses typically in one and a half days. 

So, for this house, a good rule of thumb would be to activate the solar-only mode during extended periods of two days or longer (as seen in these data records). Otherwise, the savings may be consumed by the heat recovery warm-up cycle. 

Special thanks to Dr. Fred Milder for providing the data and analysis seen in the graphs above. 

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 in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.