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Combination of Stiebel Eltron Flat Plate Collectors and Vacuum Tubes for DHW and Radiant Heat in a Home

There are many choices to be made when designing and building a new home. One family, Barry and Cheri Oberpriller, in Leverett, Mass, though, knew exactly where they would start: The house was to be energy efficient.

Collector array consists of 2 Stiebel Eltron SOL 25 Plus flat plate collectors plus 2 Thermomax Mazdon 30-tube vacuum tube collectors.

The design started with a 2200s.f. structure. Walls were constructed with Insulated Concrete Forms (ICF). These Styrofoam concrete forms take the place of traditional removable forms. They are set in place and concrete is poured between. The forms are not removed, but serve as insulation for the walls. ICF building costs are typically greater than for traditional methods, but the very high R-value and lack of air infiltration means the payback period is typically 5 yrs, and can be as little as 3 yrs. Ceilings in the house were also well insulated. The result is an estimated heat loss of approximately 22,000 btu/hr, less than half of the average home in New England of similar size.

Plans next called for designing a heating system and domestic hot water (DHW) system that would use as little fuel as possible. Naturally the Oberprillers turned to solar thermal and radiant heat. During the heating season, solar tanks for solar thermal collectors typically store heat in the 115 to 130°F range. This is an ideal range for the low temperatures of a radiant heat loop, typically 100 to 110°F, as opposed to the higher temperatures needed for hydronic radiators.

The system installer decided to craft a hybrid system, to gain the benefit of both flat plate and evacuated tube collectors. The installation was of 2 Stiebel Eltron SOL 25 Plus flat plate collectors, 2 Thermomax Mazdon 30-tube evacuated tube collectors, and a Stiebel Eltron SBB 600 Plus 160-gallon dual coil solar storage tank. A Weil-McLain wall-hung gas boiler was installed as a back-up. A second storage tank for DHW was also installed.

On cloudy winter days, flat plate collectors typically reach temperatures of 100 to 110°F, not sufficient for a significant portion of the house’s heating needs that day. Evacuated tube collectors, however, will reach temperatures 20 to 30°F warmer. On these days the flat plate collectors work as a pre-heater for the evacuated tubes, allowing the storage of water in the 120 to 130°F range. This eliminates the need for the gas boiler to burn fuel. On sunny winter days the collector combination reaches temperatures of 135 to 150°F, as much energy as a larger evacuated tube array would, at lesser cost. In addition, the flat plates can be used as a heat dump for excess heat from the evacuated tubes if necessary. In the warmest month the evacuated tubes are covered, as the flat plate collectors provide sufficient hot water for the Oberpriller’s needs by themselves.

Oberpriller House, Leverett, MA. Walls were constructed with Insulated Concrete Forms (ICF).

The backup gas boiler was also plumbed into the system in such a way as to reduce its use to a minimum. During the heating system the boiler is bypassed via a 3-way valve if the temperature at the top of solar tank reaches 120°F. The boiler is not back in the loop until the tank temperature drops to 100°F, which the Oberprillers have decided is an adequate temperature for a shower. During the peak solar months of April to November, the indirectly fired DHW tank is bypassed to eliminate standby losses there. The solar tank alone consistently reaches temperatures high enough for both heat and DHW.

The result is that from April to November the system provides 100% of the Oberpriller’s heat and hot water needs. Over an entire year the system provides 50% of their space heating needs and 90% of DHW. Just 160 gallons of liquid propane gas are needed for the house in a year, and that figure includes propane for the kitchen range and the clothes dryer.


System Type: Radiant Heat & Domestic Hot Water
Location: Leverett, Massachusetts
Solar Collectors: Stiebel Eltron SOL 25 Plus and Thermomax Mazdon 30
Solar Storage Tank: Stiebel Eltron SBB 60 Plus
Pump Station: Stiebel Eltron Flowstar
Backup: Weil McLain Ultra Series 3-UE Gas Boiler
Annual System Solar Fraction: 50% Heat & 90% DHW

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