Solar Home Heating Retrofit: Page 4 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

Overheating Control

The SLIC controller is programmed to prevent solar overheating and to maintain safe high limits and comfortable temperatures. Keeping the collectors below 230°F prevents the propylene glycol from breaking down and becoming acidic, corroding the pipes over time.

When the solar heat is not needed and the collector temperature approaches 200°F, several mass floor zones are opened automatically to cool the collectors by 5°F or so. The cooling cycle only takes a few minutes and does not typically contribute any noticeable heat to the floor. (See the graph on day 2 when the cooling cycle occurs five times.) It is most common to use a garage floor, outdoor ice-melt zone, or swimming pool as heat sinks.

When heat in the house is not wanted, the flat-plate solar collectors are used to radiate heat to the night sky. The DHW tank is used as a heat accumulator by day, and can be cooled through the solar collectors by night. This can be very useful when the house is unoccupied and hot water is accumulating in the storage tank. The floors in the warmest rooms in summer can be cooled by night circulation through the collectors as well.

Final Analysis

The homeowner carefully recorded heating fuel consumption, both before and after the solar heating retrofit. Between 2004 and 2006, some fuel savings came from 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 2 to 3 heating degree-days (HDD) per gallon before the retrofit to about 5 to 7 HDD per gallon after the retrofit. For HDD determinations, an outdoor baseline temperature is established (65°F) where it is assumed that no space heating is used.  Whenever the outdoor temperature drops below this baseline, it is assumed that the house will need some heat.  If the average outdoor temperature drops 1°F (to 64°F) for 24 hours, that condition is defined as “1 HDD.” If you know how cold it is in HDDs over a given period of time, and you know how much fuel you used (e.g., in gallons), then you can calculate gallons per HDD, or the inverse: HDD/gallon.  This is a good way to compare the fuel efficiency of your house over any period of time, much the same way automobiles are compared using mpg. 

For domestic hot water, propane use is down to an average of 0.6 gallons a day versus 1.5 gallons per day previously. This past winter, the house netted 273 to 375 kBtu per day of solar heat (80 to 110 kWh per day). Annual propane consumption has dropped by about two-thirds, saving about 1,300 gallons a year. At current local prices, this translates into saving $3,000 per year. The total cost of the retrofit was $57,315. After state and federal tax credits totalling $21,459, the net system cost was $35,856, resulting in a simple return on investment of 10 years.


Bristol Stickney has been designing, manufacturing, repairing, and installing solar hydronic heating systems for more than 30 years. He holds a B.S. in mechanical engineering and is a licensed mechanical contractor in New Mexico. He holds several patents related to solar/hydronic heating and control and is the Chief Technical Officer for SolarLogic, where he develops solar heating control systems and design tools.

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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