In honor of Willi Hopfner, Lembach, Austria



A week ago, I had the great pleasure of visiting Willi Hopfner’s house in Lembach, Austria.  Willi is a municipal employee in Lembach with an energy passion.  He devotes much of his time and resource to reducing his family’s carbon footprint.  Willi drives a car he’s converted to electric drive with battery storage and an electric scooter. He has solar thermal arrays and a sun following solar PV array.  It’s quite likely that his spacious home is energy positive, that is, it produces more energy in a year than it consumes.  Willi could well serve as a model for many of us; he was awarded the “Energy Star Pioneer Award” this year at a World Sustainable Energy Days banquet last week in Wels, Austria.

I was in Willi’s basement in part to see one of the earliest installations of an ÖkoFEN Smart boiler with an integral Stirling engine based power generator.  Willi is field-testing one of the first installations of the Stirling equipped boiler outside the ÖkoFEN lab.  The 14KW (48,000 BTU) boiler produces enough heat for Willi’s house and domestic hot water needs when his solar thermal array can’t quite do it, but it also produces 1KW of electricity whenever it’s running at full output.  Micro CHP (combined heat and power) is the latest buzz phrase in home energy in Western Europe right now.  Concern about the impending shut-down of many nuclear plants in Germany has distribution of power generation becoming more common.

While in Willi’s basement, I asked Herbert Ortner, founder and co-owner of ÖkoFEN Pelletsheizung, if the large buffer tank in Willi’s system, and in pellet boiler systems in general, leads to increased system efficiency.  Typical of Herbert, he answered, “Well, sometimes yes, sometimes no.”  He went on to say that a very well-insulated buffer tank very well installed with just the right method of return of water to the stratified tank can lead to increased system efficiency.  He followed that quickly with all the reasons that most buffer tanks actually reduce the efficiency of the systems they’re installed in.

Tank insulation must be extremely good to prevent standby losses that quickly defeat any gain possible through buffer tank use.  He talked about the lengths to which ÖkoFEN goes to prevent any air exchange over the actual surface of the buffer tank.

Plumbing to and from the tank must be well insulated to prevent heat losses.  The installation we were looking at had four large diameter water pipes between the boiler and the buffer tank and none were insulated.  Herbert noted that this installation would actually reduce efficiency of the boiler system but that was unimportant in this particular installation.  (Willi’s socks were trying to dry on one pipe.)

Beyond the obvious, the manner in which the tank is configured to create and protect thermal stratification of the water is important.  Water flow from the boiler to the buffer tank that mixes the stratified water can actually increase the number of starts and stops of the boiler, exactly the opposite of the design intent of the buffer tank in the beginning.

I left Willi’s basement with a greater appreciation of the insight of those who’ve been in this trade for a long while and have done the research to know that the real answers are complicated.  I also left it wondering why is it common for us to take near religious stands on such matters when we are only beginning to learn about them.

Do buffer tanks increase the average annual efficiency of pellet boiler systems?  “Well, sometimes yes, sometimes, no.”


Dutch Dresser is the Managing Director of Maine Energy Systems, a manufacturer and distributor of ÖkoFEN boilers and related equipment.