How the solar heating system works

System Overview
The solar heating system uses ten, 4' x 10' flat plate solar collectors to heat the domestic hot water supply and over 100 tons of sand located under the floor of the house. The sand bed acts as a heat battery, storing the heat and gradually releasing the heat into the house. Because of the large amount of thermal mass located under the floor, the system requires months of solar heating to charge the battery to the point in which heat begins to radiate up and out of the floor.

The advantage of this system is its simplicity and sustainability. The system is entirely powered by the sun and for the most part is maintenance free. There are no electronic control systems or tanks of water that need to be maintained. The system is totally independent of the grid and will continue to operate during power outages. Best of all, the system will greatly reduce heating bills. (Other systems currently in operation in central Wisconsin have reported a 50% reduction in heating consumption and 75% reduction in heating domestic hot water.)

The disadvantage to this system is lack of control. Once the system is charged and heat begins radiating from the floor, there's no way to immediately control the heat and it may require opening a few windows in the house to cool things off on a warm, late winter day.

The system is the primary heat source for the house, but there are times when supplemental heat will be provided by a wood stove and/or a propane fired hydronic heating system.

A Guided Tour

Our tour begins out at the (1.) ten flat plate collectors. Around 10 a.m. on a sunny day, the suns rays begin heating the flat plate collectors and charging the photovoltaic panels. The collectors are mounted at a 52 degree angle to take advantage of the autumn sun. Due to the lag time associated with charging the sand bed, these are the best months to initially charge the floor and the collectors are sloped to be perpendicular (average) to the suns rays during this period.

A heated solution of water and nontoxic glycol begin to make its way into the house from the collectors. Immediately after the heated fluid leaves the solar collectors any air bubbles floating through the fluid are diverted through an (2.) air scoop and vented out of the system through an air vent.

The heated fluid continues on its way through 1" O.D. pex tubing that runs underground to the bridge and into the house. Once in the house, some of the fluid is diverted through a (3.) valve to a heat exchanger that heats a tank of water for the domestic hot water system. (At this time, the domestic hot water system is not installed and all of the fluid travels to the sand bed.) The fluid then continues down towards the supply side of the (5.) manifold. A temperature gauge (4.) is located here to provide measurement of the heat flowing into the sand bed. Notice that the manifolds do not have any valves. In a typical radiant floor system, there are manual or electronic control valves that allow the homeowner to control the amount of heat going into floor. Remember that this system is not heating the slab, but the sand under the slab. There is no reason to add any valves at the manifold.

From the supply manifold the fluid flows through four, 250' coils of 3/4" tubing that is positioned throughout the insulated sand bed. During its journey, heat is conducted through the tubes and into the sand. By the time the fluid exits through the return manifold, it has lost a substantial amount of heat. At this point, the system has completed its mission of dumping BTUs into the sand bed and continues on its way back to the collectors.

Before leaving the house, the fluid is again subjected to another (7.) air scoop and air vent to remove any air bubbles that may have formed along its journey. Two (6.) expansion tanks are also located at this point to allow expansion of the heated fluid. One of the two tanks contains a make-up valve that adds additional water to the system if the pressure drops below 13 pounds of pressure. Most hydronic systems experience very minor water loss due to evaporation at pump seals and air vents. The make-up valve provides additional fluid in case of fluid loss. (At this time, no external water supply is attached to the system. When the indoor plumbing system is installed next year, the water make-up connection will be made.)

The fluid now leaves the house and is routed through 1" pex tubing to the mechanical system located under the bridge. A (8.) temperature and (9.) pressure gauge are located here to provide measurement of the return temperature and system pressure. The system is charged at 18 pounds of pressure at 60 degrees. The system pressure will vary a bit with the fluid temperature.

During the summer, the valves will be positioned to divert the fluid through a (10.) 250' loop of 3/4" pex tubing that is located 2' down underground. This will help keep the system from overheating during the months that the system is only heating domestic hot water.

All closed loop heating systems require a (11.) pressure relief valve that prevents a system from overheating and over pressurizing to a point where a pipe would burst. The system has a 30 pound pressure relief valve installed in case of this.

Next, (12. & 14.) two valves are required to fill and drain fluids for maintenance purposes. Sandwiched in between the two valves is a (13.) flow check valve that prevents backflow from occurring. Thermosiphoning can occur during periods in which the collectors are colder than the coils under the sand bed. A flow check valve prevents this from occurring.

The heart of the system are the two circulators (15.) that provide fluid flow for the entire system. The two circulators are powered by two 75 watt photovoltaic panels with an LCB (Linear Current Booster) in line. The LCB is an electronic device that increases the efficiency of the PV panels. This powers the circulators in partial sun conditions or at times when the sun's rays are not striking the PV panels at a direct angle.

Finally, the fluid returns to the collectors. A (16.) full port valve has been installed at the bank of collectors with the least amount of flow resistance. The valve adds resistance to the bank of collectors to help balance the flow between the two banks. The fluid then flows into the collectors and the sun's heat adds BTUs back into the system.