[Bldg-sim] Actual Thermal Efficiency of Boilers

[James V. Dirkes II, PE]

Dear Ian,

1.	Without looking at the BTS Standard, I suspect the 35 - 80F you
mention below is the delta T between inlet and outlet water.  It would be
ridiculous to test a boiler with 35F inlet water.
2.	A bigger issue, if you are interested in predicting actual operating
efficiency is the "cycling efficiency" of the boiler.  In short, an
accurately sized boiler benefits greatly from a) a high turndown in firing
rate or b) a buffer / storage tank that allows it to operate for long
periods even at low load.  
As far as I know, most larger, high water volume boilers perform reasonably
close to the nameplate efficiency when equipped with good turndown (4:1 or
better). 
Smaller, low water volume boilers (the atmospheric type you are thinking
about) that cycle Off - High or Off-Low-High probably need a buffer tank or
they'll operate at much lower than nameplate efficiency during the non-peak
periods (i.e., 95% of the heating season)
3.	A compounding factor is that all of us engineers routinely oversize
boilers "just to make sure" nobody complains.  This oversizing can be 20-50%
and exacerbates the turndown issue.

Hoping that's helpful.....
 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

 


  _____  

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 2:20 PM
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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