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Building Load Coefficient (BLC) or UA Test

The overall building load coefficient (BLC), also known as UA, can be measured using instrumented heating or cooling equipment.  This type of testing is often called “co-heating,” although that term may also refer to metered heating in conjunction with another heat source.  The BLC test described here uses portable electric heaters, controlled to maintain a steady indoor temperature, during steady, cool, nighttime weather.  In hot climates, a chiller may be used for metered cooling during steady, warm nighttime weather, although this approach is more difficult and much less common.  Testing is done at night to minimize interference from solar gains, and testing is done during steady weather to minimize interference from heat storage effects.  Then the BLC is calculated as the heating or cooling rate divided by the indoor-outdoor temperature difference.  The measured BLC will also include any outside air exchange that occurs during the test period.

Typical test apparatus is shown in the figure below.  A portable electric heater is controlled thermostatically by a shielded thermocouple and a datalogger (not shown), while a small fan circulates air to maintain uniform air temperature in the room.  Similar equipment is placed in several locations throughout the house to maintain a uniform, steady indoor temperature.  The datalogger records whole-house electrical energy usage, including the heater, the fan, the datalogger, any other test equipment in use, lights, and any household appliances such as a refrigerator.  Care is taken to disable any energy use outside of the thermal envelope, such as outdoor lighting or a refrigertor in a garage.  Occupants are absent during the test period, to avoid unmeasured heat gains from them.  Heat flow through a slab floor or basement wall, which is not directly related to the indoor-outdoor temperature difference, may be measured with heat flux transducers and separated from the BLC calculation.  See [1] and [2] for further discussion of this test approach, including accounting for solar gains.

Typical apparatus for the BLC (co-heating) test

Typical apparatus for the BLC (co-heating) test.

Bob Hendron, NREL


  1. Subbarao, K.; Burch, J. D.; Hancock, C. E.; Lekov, A. B.; Balcomb, J. D. (1988).  Short-term Energy Monitoring (STEM): Application of the PSTAR Method to a Residence in Fredericksburg, Virginia.  Report No. TR-254-3356.  Golden, CO:  National Renewable Energy Laboratory.
  2. Butler, D.; Dengel, A. (2013). Review of co-heating test methodologies..  NHBC foundation.