Solar Home Heating Retrofit

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

In June 2009, my former company, Cedar Mountain Solar, began designing a solar heating retrofit for a residence in the foothills in Placitas, New Mexico, near Albuquerque. This house has approximately 5,000 square feet of living space, which was heated by a propane boiler and a hydronic system embedded in the concrete floors. The building is well-constructed, with good heat retention. It is in a high altitude mountain climate where freezing temperatures and snowstorms are common in winter.

This solar heating retrofit is typical of what I call “Combi 101,” which includes several specific heating system functions (connected with a primary loop): solar heat in combination with a boiler, a domestic water heater (DHW), and radiant-heated floors throughout the house.

By October 2009, 12 SHW collectors had been installed on the roof and the heating system was converted into a solar “combisystem,” with all of the heat sources connected to all of the heat loads. Even though much of the roof is covered with solar collectors, they are mounted in low-profile to reduce their visual impact. The system has been showing good fuel savings for two heating seasons to date. Heating fuel consumption has been reduced by more than half, with the savings estimated at more than $3,000 per year.

What’s a Combisystem?

The idea of adding solar collectors to a home often proceeds along the same lines: First, homeowners consider a solar water heater with one or two collectors for domestic water heating. Then, they may consider adding heating to a chilly room—maybe more collectors would be worthwhile. Then they consider hydronic baseboards or make connections to heat other rooms. Then, they wonder about solar heating the spa or pool, an ice-melt zone, or some future addition. 

When multiple sources of heat are connected to multiple heating jobs, we call them combisystems, since it is a single heating system made up of a combination of different kinds of equipment. When one of the heat sources is solar heat, we call it a “solar combisystem.” 

System Details

Multiple heat sources and heating loads can be connected in many ways. In the Southwest, the most typical solar-hydronic combisystem includes solar collectors, a gas boiler backup, a domestic water heater, and a hydronic floor. This most basic variant includes only four items: two heat sources and two heat loads. Yet, if you present these requirements to three different solar heating suppliers, you will get three very different designs with heat exchangers, water tanks, tees, motorized valves, and pumps in different locations—and some often cryptic control strategies (or none at all) to complete the confusion. Adding features or changing the heating system requires a redesign with different piping connections, different components, different temperatures, and different controls.


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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