Outdoor Reset Maximizes Heating Efficiency


“Tales from the Sierra” Outdoor Reset by Alan Willi One of our current catch phrases going around is “outdoor reset”. Like all things we don't understand, it sounds a little intimidating until we get familiar with the concept. Outdoor reset adjusts the supply water temperature (or the supply air temperature) in reverse proportion to the outdoor temperature as the outdoor temperature falls. As the supply temperature rises, a typical radiant reset curve (that's the proportion) is 0.6. That means that for every degree the outdoor temperature falls below a set starting point, usually 70° , the supply water temperature increases 0.6 degrees. At 0° outside the supply water temperature (SWT) would be 112° . That is 70° minus 0° = 70; 70 times 0.6° = 42° ; 70° plus 42° = 112° . Previously I’ve written about the 3/1 rule: for every three degrees the boiler water temperature is lowered we save 1% of fuel. A conventional boiler requires 160° supply water temperature to maintain 140° return water temperature (RWT) to prevent flue gas condensation. If we take the 160° supply water temperature for the conventional boiler and subtract the 112° supply water temperature for the Vitodens boiler that is a 48° difference. 48 ¸ 3 = 16% savings with a boiler that would handle 112° SWT, 92° RWT. Now the sales pitch, I visited a radiant job in a brand new home (Beaver Mt. Log Home) heated with HeatLink radiant and a Viessmann Vitodens boiler at 32° outdoor temperature. The supply water temperature was 86° . That is a reset curve of 0.42° instead of the typical 0.6° . At zero degrees outside temperature, the

supply water temperature would be 99.4º. At minus 7° the actual supply water temperature was 104° . Applying the same arithmetic 160° -140° =54° difference. 54 ¸ 3 = 18% actual savings for water temperature. The Vitodens (WB2-11-44 LP) has an AFUE of 94%. A conventional boiler has an efficiency of 87%. Take the 7% AFUE difference and add the 18% supply water temperature difference together and get a 25% savings on your fuel bill. This is why we approach our design from a systems approach where we can utilize our products to their ultimate. That is the way the Vitodens hi-efficiency condensing boiler and comfort of HeatLink radiant systems are designed to operate incredibly well together. Call your Yaun Company sales representative so we can help you design a similar system for your next job.

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Last Updated:02/07/2008