Solar Home Heating Retrofit: Page 3 of 4

Case Study

Inside this Article

Solar thermal collectors provide the majority of domestic hot water and space heating for this Southwestern home.
The home’s thermal mass floors are ideal for heat storage and temperature regulation.
The combisystem may look complex, but to a professional, it’s a simple combination of independent source and load loops.
Basic Solar Combisystem Primary Loop Flow Center
Basic Solar Combisystem Primary Loop Flow Center
Domestic Hot Water Tank
The domestic hot water tank does not have a heat source, but heats through internal exchangers from the primary loop and directly from the backup boiler.
Triangle Tube Propane Boiler
A Triangle Tube propane boiler makes up for what the solar collectors don’t supply.
Caleffi 2+2 Flow Control
The Caleffi 2+2 flow control acts receives and distributes heat from multiple sources.
Expansion Tanks
Expansion tanks allow fluids to expand as they heat.
Two-stage Thermostat
Two-stage thermostats allow custom tuning of the zones to optimize the solar versus boiler heat balance.
The solar home-heating system’s “dashboard” shows vital system information and allows changing the settings to tweak system performance.
Basic Solar Combisystem Primary Loop Flow Center
Domestic Hot Water Tank
Triangle Tube Propane Boiler
Caleffi 2+2 Flow Control
Expansion Tanks
Two-stage Thermostat

For the Placitas system, a hydraulic separator—a “flow center” device that eliminates the need for a primary loop—was used instead of a Combi 101-style primary loop. These devices can be purchased from various plumbing equipment manufacturers. They provide a large, open container that is filled with “boiler fluid” and receives the heat and then provides heat to the other equipment. The Caleffi Hydrolink eliminated the need to assemble a primary loop piece by piece. As seen in the piping diagram, a primary loop consists of tees, valves, elbows and connective piping.  A prefabricated hydraulic separator comes from the manufacturer with many of these parts built in. 

The Hydrolink 2+2 model was configured to provide the same heating functions and advantages of a primary loop system. The result is a piping system that resembles a Combi 101 system with very compact central piping, incorporating a substantial number of collectors (12) and heating zones (8). 

System Control & Monitoring

This retrofit’s controls were originally designed with common equipment such as conventional room thermostats, mechanical relays for switching pumps and zone valves using several differential thermostats, and set-point temperature controls. This is the conventional way solar/boiler heating systems have been controlled. For the Placitas retrofit, the technically proficient owner was willing to test a control system that included a solar logic integrated control (SLIC)—a computer- and Web-based control of our own design. 

The SLIC control system replaces all of the relays and temperature controls with a single box. It is easy to operate using familiar room thermostats and allows both the installers and the owners to monitor and record the heating system’s performance and data, and adjust settings locally or remotely over the Internet. This is great for fine-tuning the balance between comfort and efficiency. 

The fuel efficiency and comfort provided by a solar combisystem is only as good as the control system. There are many ways to save energy through the control system. Features that are not needed are simply turned off at the time of installation. The internal software controls have many functions, such as solar-only and backup-only settings, heat dissipation, and room target-temperature control, plus many other settings subtle and not so subtle that affect system performance and monitoring. 

Fuel-Saving Strategies for Heating

Each 4- by 10-foot collector can produce enough heat to offset up to 0.5 gallons of propane per day. But the savings are not entirely from solar heat gain—other factors include a high-efficiency condensing boiler and heat-saving control strategies. Solar priority over the boiler is guaranteed both by the piping configuration and the control logic. Solar heat for the floors has an adjustable priority over heat storage in the water tank, and is controlled by the SLIC using virtual two-stage room thermostats. (The room thermostats transmit the room temperature and the user’s setpoints to the central control, which implements the two-stage functions.)

Heat storage is also optimized in the DHW tank and DHW recirculator by software control. The SLIC controller is programmed to save heating fuel in every way possible, such as stranded heat recovery—routing hot fluid left in the pipes after a heating cycle is completed to a water-heater tank or some other useful load—and intelligent priority control based on temperatures and critical loads. 

Past and current performance can be reviewed and analyzed at any time. The graph shows an example from two days in October 2009. On Day 1, the room temperature is kept within a comfortable range, and the solar heat is diverted to the water tank after the room warms up in the morning. On Day 2, the weather is even warmer and sunnier, so the room warms up, the water heat gets very hot, and the intelligent heat dissipation kicks in to cool the collectors all afternoon, typically routing heat to the concrete floor in the garage. This verifies that the control system is set correctly; data like this can be viewed at the house or remotely at any time.

Electricity-Saving Strategies

The opportunities for saving electricity in a heating system are sometimes small but worth considering. In this system, circulator pumps are disabled when they are not needed. Multispeed circulators are used and set to the lowest speed that is effective for each job. The number of transformers are limited to eliminate their “phantom load.” “Latching” zone valves are used, which only require power when they change their state. There is no “primary pump”—all circulation through the flow center is provided by the secondary pumps that are smaller and thus require less energy. Solar circulation for collectors using closed glycol loops is achieved with very small pumps that were energized by PV power.

Comments (2)

Fred Golden's picture

This is a interesting and timely project. I am considering designing a home in Portland Oregon, and have considered a 1,000 gallon water tank to store the glycol water mixture, heated with evacuated tube solar collectors.

Wrap 1/2" copper tubing around the tank to pre-heat the domestic hot water (3 parallel tubes to lower presser drop, and store hot water) into a 40 gallon heat pump water heater.

The tank would be uninsulated, sit inside a room with 12" insulated walls, and heat within that room can be directed to the clothes dryer, vented into the garage to warm it, or a vent to the outside can be used to control overheating the tank in the summer time.

12 VDC 10 watt pumps for each zone of heating in the cement floors, and to recirculate domestic hot water back from the farthest bathroom when it's light comes on, until 90F water reaches the bathroom.

It is also good to consider that I can store a lot of heat in the concrete floors too. Due to low electrical prices, back up heat and cooling will be provided with air source heat pumps.

A south facing Trome wall to store heat can also provide cooling in the summer time IF I have a waterfall cascade down the wall, and open vents at the top of the sunroom it would be located inside, to let out the humid air created by the cooling effect of the water falling over the large cement wall. Each pound of water evaporated from the waterfall will absorb 1,040 Btu's of heat from the wall, effectively cooling the wall and living room on one side, sunroom and garden greenhouse on the south side.

I had considered roof mounted solar PV panels, but dual axis trackers would shed snow much better, and mean less walking on the roof. I might even ground mount the solar panels, so they can be cleaned directly after a snow event, and would be easy to cover up during summer vacations.

Fred Golden
San Diego, CA

Scott Pumfrey_2's picture

Hi, Just wondering how this system is working for you?

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