20929 Bridge Street, Southfield, MI  48033
20929 Bridge Street, Southfield, MI 48033
4121 Brockton Drive SE, Grand Rapids, MI 49512
6200 Baron Drive, Bridgeport, MI 48722
6910 Treeline Drive, Suite A, Brecksville, OH 44141
Serving the industry since 1927
Phone: (800) 589-6120 - Fax: (248) 354-3710
www.deppmann.com
November 9, 2009 ~ Monday Morning Minutes:
Boiler Outdoor Reset and its Affect on Condensing Boiler
Efficiencies (part 1) By Brad Notter with edit by Norm Hall
Last week we concluded the discussion about the value of turndown ratios in
condensing boiler applications. Today we will begin with outdoor reset which
also has a positive effect on efficiency.
The design conditions of a boiler system are, in large part, based on outdoor
weather condition. The outdoor design point can range from -12 degrees F in the
Upper Peninsula of Michigan to +2 for Findlay Ohio. The heat loss of your
building is the greatest when the outdoor temp is at your areas outdoor design
temperature. Your boiler plant is usually designed to deliver the maximum
temperature, example 180 degrees, at this condition.
Terminal units such as AHU coils, Fan Coils, and finned tube radiation are
selected to provide the design heat load given the design supply temperature at a
selected ΔT. Your building will stay warm when the heat loss of the building is
equal to or less than the heat output of your terminal devices. Sizing the heating
plant for the outdoor design condition will keep your building warm for those
few coldest days: the rest of the year less load is needed. In effect, the rest of the
year the boiler plant and terminal unit potential are oversized since you need less
heat (BTUs) to stay warm. If the terminal devices in your system are supplied
with a lower supply temperature, giving a lower average temperature, they will
put out lower BTUs.
This lowering of the heating supply water as the outdoor temperature increases is
called Outdoor Reset. This is one method of matching the heat generated to the
heat loss of the building. The rate at which you lower the boiler supply temp as
the outdoor temp increases is called the Reset Ratio. For example if you lower
the boiler supply temp 1 degree for every 1 degree rise of outdoor temperature
would be a 1:1 ratio. If you had an outdoor design temperature of 0 degrees and
had a 1:1 reset ratio, you would only need 150 degree supply water at 30 degrees
outdoor temperature.
Continued on page 2…
Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many
years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions
that result from this information. Please feel free to contact us if you do have any questions.
20929 Bridge Street, Southfield, MI 48033
4121 Brockton Drive SE, Grand Rapids, MI 49512
6200 Baron Drive, Bridgeport, MI 48722
6910 Treeline Drive, Suite A, Brecksville, OH 44141
Serving the industry since 1927
Phone: (800) 589-6120 - Fax: (248) 354-3710
www.deppmann.com
Using outdoor reset in your building allows the heat generated to equal the heat loss, and operating your boiler
at a lower temperature will save you money $$$. The challenge is to make sure the boiler can handle the lower
return temperatures and this is the topic of next week’s discussion.
Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many
years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions
that result from this information. Please feel free to contact us if you do have any questions.
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