Solar Home Heating Retrofit: Page 2 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

After going through this same design process with many different projects, I decided to standardize the design, making it easier to add, remove, and change components. The key is to make the system modular so that things can be added or removed as the project develops, without requiring re-engineering. I began building all my designs around a “flow center” where all the circulation pumps plug into a “primary loop” with two pipe connections, which can just as easily be unplugged. Making such major alterations with such simplicity is actually a minor revolution for water-heating systems.

The primary loop using closely spaced tees has been popular in commercial buildings for decades, and has proven its worth in residential systems. The schematic shows the basic configuration for both simple and larger systems. This system allows extending the primary loop through attic or crawl spaces to remote areas of a building to pick up or deliver heat from other mechanical rooms—especially useful in many retrofit situations when combining existing and remote heating equipment under one control system. It also allows expansion for additional heating sources and jobs by adding additional double-tee connection points. 

Solar Heating without Large Tanks

Solar hydronic heating systems are commonly designed as if they are very large solar water heaters. Several solar collectors are connected to large heat-storage tanks, and all of the solar heat is put into the water tanks, and then drawn out to meet heating needs. 

But most (or all) large heat-storage tanks can be eliminated when the heat distribution is from radiant-heated masonry floors (see “Thermal Mass in Hydronic Floors” sidebar). High thermal storage in the existing concrete floors allowed a relatively large solar heating system without any additional heat-storage tanks, except for a single 115-gallon domestic hot water (DHW) tank. 

The Details

Temperature regulation. Because the thermal mass of a concrete slab is so large, its temperature can be easily regulated within the range for human comfort. The room temperature can be allowed to drift as much as 8°F from day to night while staying reasonably comfortable. However, comfort range is a personal preference and therefore needs to be controllable room-by-room.

The easiest way to do this, especially in retrofits, is to replace each room’s single-stage thermostat with a two-stage thermostat. As the temperature drops, the first stage calls for heat, but it only delivers solar heat when solar is available. If the room temperature continues to drop, the second stage will then call for heat, which causes the backup boiler to fire (along with solar preheating from storage tanks, if available). The advantage of individual room heat controls is that, for some rooms, a wider daily temperature swing can be tolerated, and this will result in higher heating savings in those rooms.

Two-stage thermostats are adjustable in many ways, and the owner or installer can choose an allowable temperature swing and a low limit to suit the comfort needs of the occupants, zone by zone, to achieve the necessary balance between comfort and energy savings. The room temperatures can be adjusted to drift up and down as little as 1°F or as much as 8°F, depending on how the room is used. The more the room temperature is allowed to drift, the more solar heat is stored and released in the mass floor, resulting in more fuel savings.

If the first stage of a room thermostat has not kicked on, the system sends the solar heat to any other room where stage one is activated. If none of the rooms require stage one heating, then the heat is sent to the water heater, water storage, or pool. If the water from the solar collectors is hot but there’s no use for it, then the heat dissipation cycle is activated (see below).

The key to success with this approach is the substitution of more intelligent controls in place of large water tanks. If done effectively, this can lower the cost of a solar heating installation, while improving the solar thermal system’s efficiency. In our Placitas retrofit, there are eight room thermostats, and all of them include two-stage switching (solar first; boiler second) and programmable temperature swing capability.

Comments (1)

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

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