[Equest-users] AFUE vs. Default Furnace PLR curves

[Porter, Fred]

All,
I was recently reviewing some heating results from eQ simulations of very normal gas RTUs. (15% OA, NYC area office bldg, typical envelope, two-stage low heat burners.) Both steady state efficiency and AFUE were provided in the manufacturer's sheets and were almost identical (81%) as is typical. The model was over-predicting gas a bit I thought, and I looked at the HSPFs (Heating Seasonal Performance Factors, basically modeled AFUEs) on the SS-Q "Heat Pump" heating summaries. These seemed low, so I ran the model with a 1:1 Furnace EIR f(PLR) curve instead of the default. This led to a 15% reduction in heating gas use. A 15% penalty for cycling, etc. reflects my understanding of AFUEs of furnaces 40 years ago, low-mid 60%s AFUEs, for combustion efficiency just under 80%. with  So the question is:
Is real, in-place AFUE for modern RTUs almost exactly the same as steady state efficiency, and the default curve archaic? The default curve could be based on studies of 1970s era natural-draft furnaces connected to a stack, not an RTU furnace with a power burner located outside w/o any significant stack.
Or
Are the AFUE tests not really indicative of real world performance, and the default degradation factors appropriate?

The default eQUEST/DOE-2.x furnace EIR f(PLR) curve is being enshrined as a required baseline curve in various references; maybe it's not appropriate anymore. Any informed thoughts? Comnet and the T24 ACM do have a factor to derive/increase the input furnace efficiency from the AFUE, but it outputs 82.1% from a AFUE of about 81%, so not even close to balancing the degradation from the PLR curve.

Fred
Fred Porter, BEMP, LEED© AP
Principal Engineer
Sustainability Services
NORESCO
2540 Frontier Ave, Suite 100, Boulder, CO  80301
fporter at noresco.com<mailto:fporter at noresco.com>
www.noresco.com<http://www.noresco.com>


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