Actual Thermal Efficiency of Boilers

5 posts / 0 new
Last post

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 :

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?

Doebber, Ian2's picture
Joined: 2011-10-02
Reputation: 0

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
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.....

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0


Thanks for you response. Concerning your first question, take a quick look at the standard and you'll see that entering water temperature is required to be between 35?F - 80?F. This is not the delta-T. It doesn't make any sense and I'm wondering why the standard was written this way.


Doebber, Ian2's picture
Joined: 2011-10-02
Reputation: 0

Dear Ian,

I (finally) took a look at the Standard. You're right and, as a result, I
think you are also correct that it's inappropriate to assume that efficiency
is as high as the nameplate when your site's entering water temp is higher
than 80F. By the way, I suspect that water as cold as 35F is part of the
entering water temp range for applications like water or ground source heat
pumps. In those cases, a boiler may be used to boost the water temp.

Your question, and this standard, highlight the importance of understanding
what you are modeling. Good results cannot be obtained with faulty input!

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0


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

Rich Davis's picture
Joined: 2011-10-02
Reputation: 0