[Bldg-sim] Actual Thermal Efficiency of Boilers

[Rich Davis]

Ian,

 

Thanks for the link. 

 

A boiler with no return water / condensate return is the only boiler I can
think of that would operate with 100% make-up water, thus seeing the
incoming water temperatures you cite. In practice I have encountered this
with process steam boilers with no condensate return (incoming temp =
groundwater temp of 50 or 55 F), or pool water heaters that directly heat
the pool water (with incoming water temperatures of 70 F - 80 F), but never
with a commercial space heating boiler. 

 

I have been provided similar misleading high efficiency data from a steam
boiler manufacturer claiming their boiler was as high as 90 % efficient, but
this was only with very cold entering water temperatures.

 

For a new atmospheric boiler I suspect the seasonal efficiency is much
closer to 75% than 65% even with 150 F return water temperatures. I have
seen steady-state combustion efficiency tests of 78% to 80% for these style
boilers, and that is with return water temperatures of 150 F or higher. 

 

Rich Davis

Abacus Resource Management Company

 

 

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Doebber, Ian
Sent: Friday, August 28, 2009 11:20 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Actual Thermal Efficiency of Boilers

 

ASHRAE 90.1 mandates that a Gas-Fired Boiler ? 300,000 Btu/hr & ? 2,500,000
Btu/hr must have a minimum 75% Thermal Efficiency (Et) based upon the Test
Proceedure Hydronics Institute Boiler Standard.  

 

The Hydronics Institute Boiler Standard official title is "BTS-2000 Testing
Standard : Method to Determine Efficiency of Commercial Space Heating
Boilers" published by The Hydronics Institute Division of Air-Conditioning,
Heating, Refrigeration Institute (AHRI).  Can be found at :
http://www.ahrinet.org/ARI/util/showdoc.aspx?doc=1198.   

 

The Testing Standard measures the Thermal Efficiency of the Boiler at steady
state operation, fully loaded maintaining the outlet temperature at 180°F.
Oddly, the inlet water temperature into the Boiler is between 35°F to 80°F
which is much lower than the typical Return Water Temperature of 150°F in a
Building Application.  Consequently, these colder inlet temperatures are
most likely overstating the Thermal Efficiency of the Boiler which is
condensing even if the Boiler is a Non-Condensing Boiler.  Based on various
Manufacturer's data, a Boiler operating with an 80°F Return Water
Temperature achieves a 10% greater Thermal Efficiency compared with
operating at a standard 150°F Return Water Temperature.  Then operating at a
35°F Return Water Temperature, the Thermal Efficiency should be boosted even
further.  

 

Would it be reasonable to assume that a Standard Atmospheric Boiler that
meets the 75% Tc minimum based on ASHRAE 90.1 according to the BTS-2000
Testing Standard using 80°F Return Water Temperature would actually operate
at 65% Tc in the field operating at 150°F Return Water Temperature?

 

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